This document does not belong to me, but it might benefit others like it benefited me.Full description
Proses extraksi titaniumFull description
Titanium FingerstyleDescrição completa
manual original de torno chino 11" x 26" marca tiranium vendido en maincasa guadalajara
metallography hand book
MAK214E ITU Experiment report
ASM Handbook, Volume 9: Metallography and Microstructures G.F. Vander Voort, editor, p899–917 DOI: 10.1361/asmhba0003779 10.1361/asmhba0003779 Color images cited in this article appear at end of article.
Metallography and Microstructures of Titanium and Its Alloys Luther M. Gammon, Robert D. Briggs, John M. Packard, Kurt W. Batson, Rodney Boyer, and Charles W. Domby, The Boeing Company
METALLOGRAPH METALLO GRAPHY Y is a comple complex x process with many variables that involve compromises between time, resources, and the end product or purpose of the investigation. For example, a re-
search lab may beneﬁt from the more time-consuming method of vibratory polishing, while a production quality-control lab may not require specimen preparation with a vibratory polisher.
A lab for teaching also may beneﬁt from the training experience experience of manual polish polishing, ing, or polishing may be done with semiautomatic polishing machines. With a little forethought and planning, excellent metallographic samples can be produced in a short time for light microscopy of titanium and its alloys. This article descri describes bes the fundam fundamentals entals of titanium metallographic sample preparation. Representative Represe ntative micrographs are also presented for each class of titanium alloys, which include unalloyed titanium, alpha alloys, alpha-beta alloys, and beta titanium alloys. Metallo Metallography graphy and metallo metallographic graphic sample preparation of titanium alloys are also described in more detail in Ref 1 and 2.
Types of Titanium Alloys Fig. 1
Cross section through through the abrasive saw-cut saw-cut edge of a TiTi-6Al-4 6Al-4V V sample. Note there is less than 5 lm dep depth th of dis distur turbed bed ma mater terial ial req requir uiring ing rem remova ovall for proper specimen preparation, seen as a thin layer at the surface. This layer would be deeper in commercially pure titanium and more difﬁcult to discern.
This micrograph shows the impact impact of mounting defects on edge retention. Note the edge rounding near the air bubble and the sharp edge where the mounting material ﬁlled the gap. This shows the importance of good-quality mounting techniques and materials.
Sample holde holders rs for semia semiautom utomated ated polis polishing hing mach machines.(a) ines.(a) Fixed Fixed-samp -sample le holde holderr withload appli applied ed froma centr central al Fig. 3 Sample column. (b) Nonﬁxed specimen mover plate with load applied over one mount
Titanium is an allotropic element; that is, it exists in more than one crystallographic form. At room temperature, titanium has a hexagonal
Mount with two specimens for manual manual polishing or polishing polishing on a semiautomated semiautomated polisher polisher with a non-ﬁxed specimen mover plate
900 / Metallography and Microstructures of Nonferrous Alloys Alloy Classe Classes. s. Titan close-packed (hcp) crystal structure, which is reTitanium ium alloys have generall generally y alpha-beta alloys, ferred to as “alpha” phase. This structure trans- been classiﬁed as alpha alloys, alpha-beta forms for ms to a bod body-c y-cente entered red cubi cubicc (bc (bcc) c) crys crystal tal and beta alloys. Alpha alloys have essentially essentially an all-alp alpha ha mic micros rostru tructur cture. e. Beta allo alloys ys are thos thosee alstructure, called “beta” phase, at 883 C (1621 allloys from which a small volume of material can F). Alloying elements generally can be classiﬁed be quenched into ice water from above its beta as alph alphaa or bet betaa sta stabili bilizers zers.. Alp Alpha ha sta stabili bilizers zers,, transus without martensitic decomposition of the such as aluminum and oxygen, increase the tem- beta phase. Alpha-beta alloys contain a mixture perature at which the alpha phase is stable. Beta of alpha and beta phases at room temperature. stabilizers, such as vanadium and molybdenum, Within the alpha-beta class, an alloy that conTi-8Al-1Mol-1Moresult in stability of the beta phase at lower tem- tains less than 2 to 3% beta, such as Ti-8A peratures. This transformation temperature from 1V, may also be referred to as a “near-alpha” or an alpha-beta phase (or all-alpha phase) to all “super-alpha” alloy. The principal alloying element in alpha alloys beta is known as the beta transus temperature. The beta transus is deﬁned as the lowest equilib- is aluminum (oxygen is the principal alloying rium temperature at which the material is 100% element in commercially pure titanium), but cerbeta. tain alph alphaa allo alloys ys and mos mostt com commer mercial cially ly pur puree Below the beta transus temperature, titanium (unalloyed) titanium contain small amounts of beta-stabilizing bilizing elements. Simila Similarly rly,, beta alloys will be a mixture of b if the material con- beta-sta tains some beta stabilizers, or it will be all alpha contain small amounts of alpha-stabilizing eleif it contains no beta stabilizers. The beta transus ments as strengtheners in addition to the beta is important, because processing and heat treat- stabilizers. ment are often often carried out with reference reference to some The beta alloys can be further broken down incremental temperature above or below the beta into beta and “near-beta.” “near-beta.” This distinction distinction is nectransus. Alloying elements that favor the alpha essary essary,, because the phase transformations transformations that crystal structure and stabilize it by raising the occur, the reaction kinetics, and the processing beta transus temperature include aluminum, gal- could be different different if the the alloy is a near-b near-beta eta (lean) lium, germanium, carbon, oxygen, and nitrogen. alloy, such as Ti-10V-2Fe-3Al, or a rich beta alTwo gro groups ups of elem elements ents stabilize stabilize the beta loy, such as Ti-13V-11Cr-3Al. crystal structure by loweri lowering ng the transformation transformation Furtherr inform Furthe information ation on the metallu metallurgy rgy,, selectemperature. The beta isomorphous group con- tion, processing, and application of titanium alsists of elements that are miscible in the beta loys is contained in Ref 3 and in Properties and Selection:: Nonfer Nonferrou rouss Alloys and SpecialSpecial-PurPurphase, including molybdenum, vanadium, tan- Selection talum, and niobium. The other group forms eu- pose Material s, Volume 2 of the ASM Handbook tectoid systems with titanium, having eutectoid (see, for exampl example, e, the articles “Wrought “Wrought Titatemperatures as much as 333 C (600 F) below nium and Titanium Alloys” and “Titanium and the transformation temperature of unalloyed ti- Titanium Alloy Castings”). tanium.. The eutectoid group includes mangatanium nese, iron, chromium, cobalt, nickel, copper, and silicon sil icon.. Two oth other er elem elementsthat entsthat oft often en are allo alloyed yed Specimen Preparation in tita titaniu nium m are tin and zirconium zirconium.. Thes Thesee elements have extensive solid solubilities in alpha Specim Spe cimen en pre prepar paratio ation n com compri prises ses man many y deand beta phases. Although they do not strongly tailed steps. The ﬁrst stages of sample preparapromote phase stability, they retard the rates of tion are equi equipme pment nt dep dependa endant, nt, whi while le the ﬁna ﬁnall transformation and are useful as strengthening polish step is driven by the needs of the invesagents. tigator.. Sufﬁ tigator Sufﬁcient cient attention must be paid to each step or the quality of the ﬁnished mount may be compromised. The method chosen depends on two factors: the facilities and equipment present and the pur-
pose of the investigation. investigation. There is a large difference between methods used in a research environment, where time may not be as pressing as in a production environment, or in a college instructional structi onal lab where the facilitie facilitiess may not be as elaborate. Sectioning. Common methods for sectioning titanium tita nium meta metallog llograp raphic hic sam sample pless incl include ude the band ban d saw saw,, abr abrasiv asivee cut cut-of -offf whe wheel, el, and slo slowwspeed wafering wheels. Band sawing titanium should be done with slow blade speed using a
Etched Etche d with Kroll’ Kroll’ss reagent for 45 s. Abusively Abusively polished example of a Ti-6Al-4V fastener resulting in a smeared and scratched surface. Excessive etching cannot cann ot corre correct ct poor speci specimen men prepa preparation ration.. Notethe sever severe e distortion in microstructure and edge rounding.
A 200 cm (8 in.) wax wax wheel with relief grooves grooves
Ti-6Al-4V alloy with Widmansta ¨tten alpha in a ¨tten beta matrix after furnace cooling from above the transus. Beta anneal temperature was 1040 C (1900 F). Samples were etched with the oxalic tinting reagent for 15 s after polishing by (a) four-step method for optimizing removal of deformed material, (b) four-step method for optimizing mizin g edge retention, or (c) threethree-step step semiautomated semiautomated method meth od for optimizing optimizing prepa preparation ration time (note the lack of detail in the dark regions). See text for description of polishing procedures. See also Fig. 58 in the article “Selected Color Images” in this Volume for color version.
Metallography and Microstructures of Titanium and Its Alloys / 901 toothed blade and high pressure applied to the workpiece. If a high blade speed and low pressure are applied to the workpiece, damage in the form of cold work will be introduced into the sample, possibly preventing the true microstructure from being observed. With all three cutting methods, sufﬁcient amounts of coolant should be used to prevent the introduction of heat damage into the sample. Abrasive cut-off wheels should be a soft rubber-bonded abrasive type. The erosion of the rubber-bonded wheel will continually provide a fresh cutting surface and prevent titanium debris from loading up on the blade. If a dull band saw blade is used or if an abrasive blade has loaded up with cutting debris, the sample will be damaged from overheating and cold work. Figure 1 shows the edge of a cross section cut with an abrasive cut-off wheel. Mounting. The sample should also be degreased and dried before mounting to ensure adequate adhesion of the mounting media. Careful consideration is also necessary for making a proper metallurgical mount. The ﬁrst consideration is choosing the most appropriate mounting medium. Titanium is a very abrasion-resistant material, and it is essential that the titanium be mounted correctly to produce a quality metallographic sample. The selection of mounting material has a signiﬁcant impact on edge retention and the surface ﬂatness of the mount. Failure to use the proper mounting media may cause rounding of the interface be-
tween the mount and sample, resulting in poor edge retention. It can also cause rounding or faceting of the overall mount surface. (See the article “Mounting of Specimens” in this Volume for more information on mounting and edge retention.) In selecting a mounting material, it is recommended to use a mineral- or glass-ﬁlledhot-compression thermosetting resin. While the costs of the ﬁlled resins are higher than the traditional bakelite or epoxy resins, the performance of ﬁlled resins is superior, as the ﬁller can allow close matching of the abrasive wear resistance of the specimen and the mount. The cost-to-beneﬁt ratio makes ﬁlled resins a good choice when transparency is not needed. When transparency is needed or voids are present in a part with complex shape, it is necessary to use an alternative cold-setting material such as a clear epoxy. This can be vacuum impregnated into sample voids and irregularities such as the gap in Fig. 2. The second consideration is the sample conﬁguration, which refers to the position and number of samples in a mount. The method of polishing can determine the sample conﬁguration, as described in more detail in the article “Mounting of Specimens” in this Volume. In general, there are three types of polishing methods: ● ●
Semiautomatic polishing machine with samples held in a “ﬁxed” sample holder (Fig. 3a) Semiautomatic polishing machine with samples held in a “nonﬁxed” sample holder (Fig. 3b) Manual or hand polishing
Polishing with a ﬁxed-sample holder in a semiautomated machine is achieved by a powerhead that moves the sample holder around the polishing platen. In this method, mounted samples are ﬁxed in place within a rigid sample holder, and central force loading is applied to all specimens in the holder through a centrally located column. In this method, three or six mounted samples must be symmetrically placed in the holder in order to ensure good ﬂatness of the specimens after polishing. The mount surface thus remains ﬂat, as the samples are held in-plane by the sample holder. This method provides optimal edge retention and ﬂatness and is the recommended sample preparation method for operators requiring larger volumes of throughput. With this method, each mount can contain only one specimen. In the semiautomated nonﬁxed (or individual force) method, the specimens sit in a hole in a holder (a thin plate), and a piston comes down and presses each specimen against the working surface. In this case, two or more specimens should always be placed in each mount (Fig. 4). By centering them in each side of the mount, the specimens support the mount so it will not tend to rock back and forth. The result is a ﬂatter sample with better edge retention. This is still not as good as the ﬁxed method. A single specimen should never be mounted in the center of a mount. The result is usually a convex and/or faceted mount surface with poor edge retention. The mount will have the tendency to rock back and forth about the small, hard specimen, rounding the mount surface and degrading the quality of
Table 1 Etchants for examination of titanium and titanium alloys Etchant
Macroetchants 50 mL HCl, 50 mL H2O 30 mL HNO3, 3 mL HF, 67 mL H 2O (slow) to 10 mL HNO3, 8 mL HF, 82 mL H2O (fast) 15 mL HNO3, 10 mL HF, 75 mL H 2O Two-stage etch(a) consisting of: (1) 8 mL HF, 10 mL HNO3, 82 mL H2O and (2) 18 g/L (2.4 oz/gal) of NH4HF2 (ammonium biﬂuoride) in H2O
Fig. 8 Deformed grainstructurefrom drilling in solution
treated and aged Ti-6Al-4V. Solution treatment was at 925 C (1700 F) and aged. Polishing was the fourstep method for edge retention, and it was etched with the oxalic tint etch to reveal the deformed grain structure from drilling. (a) Depth of the cold work as evidenced by the disturbed microstructure to a depth of 310 l m. (b) Normal microstructure for comparison
Microetchants (continued) General-purpose etch for b alloys Used at room temperature to 55 C (130 F) for 3–5 min. Reveals grain size and surface defects Etch about 2 min. Reveals ﬂow lines and defects Reveals and b segregation (aluminum segregation)
Microetchants 1–3 mL HF, 10 mL HNO3, 30 mL lactic acid 1 mL HF, 30 mL HNO3, 30 mL lactic acid Kroll’s reagent: 1–3 mL HF, 2–6 mL HNO3, H2O to 1000 mL 10 mL HF, 5 mL HNO3, 85 mL H 2O 1 mL HF, 2 mL HNO3, 50 mL H 2O2, 47 mL H2O
Reveals hydrides in unalloyed titanium Reveals hydrides in unalloyed titanium General-purpose etch for most alloys General-purpose etch for most alloys Removes etchant stains for most alloys
10 mL HF, 10 mL HNO3, 30 mL lactic acid 2 mL HF, 98 mL H 2O 98 mL saturated oxalic acid in H2O, 2 mL HF 6 g NaOH, 60 mL H2O, heat to 80 C (180 F), add 10 mL H2O2 2 mL HF, 98 mL H 2O, then 1 mL HF, 2 mL HNO3, 97 mL H2O 10 mL KOH (40%), 5 mL H2O2, 20 mL H2O 18.5 g benzalkonium chloride, 33 mL ethanol, 40 mL glycerol, 25 mL HF 2 mL HF, 4 mL HNO3, 94 mL H2 50 mL 10% oxalic acid, 50 mL 0.5% HF with H2O 10 s with Kroll’s, then 10–15 s with 50 mL 10% oxalic acid, 50 mL 0.5% HF with H2O
Chemical polish and etch for most alloys Reveals case for most alloys Reveals case (interstitial contamination) for most alloys Good -b contrast, general microstructures for most alloys General-purpose etch for near- alloys(b) Stains , transformed b General-purpose etch for TiAl-Zr and Ti-Si alloys
Reveals microstructure in aged Ti-13V-11Cr-3Al Etch 12–20 s. Generalpurpose etch for b alloys Brings out aged structure in Ti-10V-2Fe-3Al
(a) Two-stage etch procedure: Degrease (if necessary) and clean, making sure the surface is water-break free. Immerse in solution (1) at 45–55 C (110–135 F) for 2–3 min and rinse thoroughly in clean cold water. Immerse in agitated bath of solution (2) at room temperature for 1–2 min. Rinse thoroughly in clean cold water, rinse thoroughly in clean hot water at 90–100 C (190–210 F), blow dry with clean compressed air. Solutions must be used fresh. (b) First etchant stains phase; second etchant removes stain.
902 / Metallography and Microstructures of Nonferrous Alloys the edge. This convex surface will have an adverse effect on the appearance of the microstructure. A similar-sized mount with two small samples in the holders will reduce the rocking effect, making it possible to prepare a ﬂatter sample. Manual or hand polishing is similar to the semiautomatic nonﬁxed method. Two or more specimens should always be mounted in each sample. The only difference is that the mass of titanium in the mount for hand preparation should be kept to a minimum to facilitate grinding and maintain a uniform applied pressure across the mount. Grinding. The purpose of grinding is to remove the damage caused by the sectioning process. Sectioning methods, such as slow-speed wafering, that do not introduce much damage into the sample do not require extensive grinding and decrease sample preparation time. Semiautomated grinding with a specimen mover plate or the ﬁxed holder can be done with semiautomated polishers using diamond-embedded platens or platens with proprietary coatings designed for applied diamond suspensions. There is a wide assortment of diamond platens on the market to be used with automated grinding. For a 20 to 30 cm (8 to 12 in.) diamondembedded platen, the following parameters should be used. Keep in mind that there are many possible ways to accomplish a grinding operation depending on the complexity of the part, amount of material to be removed, and time available. Various combinations of these steps can be used: ●
● ● ● ●
The speed should be 150 rpm (note: titanium machineswork best at highpressure and low speed). The applied pressure should be 40 to 70 N (9 to 15 lbf) per 38 mm (1.5 in.) diam mount. Grinding step A uses 70 l m or 220 grit diamond. Grinding step B uses 1200 grit diamond. Always use a sufﬁcient amount of coolant to prevent heat damage.
With proper sectioning most ordinary samples can be ground with a single 220 grit ﬁnish followed by a 9 l m diamond suspension on either a grinding platen with a proprietary coating or a woven non-nap silk cloth.
Grinding by hand usually involves the use of silicon-carbide papers. The following parameters should be observed: ● ●
The speed should be kept to no more than 150 rpm. Always use new paper. The maximum paper lifetime is 15 s (or perhaps up to 60 s max in one-time manual grinding). Abrasives quickly lose their cutting ability and smear the sample and introduce cold-work damage. Apply as much pressure as can be controlled when holding the sample to the paper. High pressure and slow speed will produce favorable results. The common grit progression sequence is 120 (or 240), 320, and 600 grit. If the sectioning process produces a ﬁne smooth face, it is possible to start the grinding process with 320 or 600 grit papers, but there must be sufﬁcient material removal to eliminate all cutting damage. Always use sufﬁcient amounts of coolant or water to prevent heat damage.
such as silk or a proprietary platen designed for diamond suspension application is used. A 3 lm diamond suspension on a polyester cloth with an emulsiﬁed oil-based lubricant is used. Speed should be 120 to 150 rpm. Direction of specimen holder rotation should be complementary to the rotation of the platen. Applied force should be 40 to 80 N (9 to 18 lbf) per 38 mm (1.5 in.) diam mount.
Polishing can be broken down into two phases, the intermediate polish and ﬁnal polish. The purpose of polishing is to gradually remove the trace amounts of damage and the surface scratches introduced during the grinding operations. Again, there are numerous methods documented for intermediate and ﬁnal polishing that may ﬁt different operations. The list below discusses of a few of the procedures. An example of abusive polishing is shown in Fig. 5 after etching. Excessive etching cannot correct poor specimen preparation. Intermediate polishing is the bridge step or steps between grinding and 1 l m ﬁnal step or steps. It can be done successfully by either the semiautomated or hand method. The semiautomated method is generally recommended because it is very effective with typical removal rates of 5 lm/min and as much as 25 lm/min with minimal cold work introduced into the sample. It can be utilized both as a ﬁne grinding and a polishing step at the same time. Several semiautomated intermediate polishing parameters have been found effective: ●
A 9 l m diamond suspension with an alcoholbased lubricant on a woven non-nap cloth
Table 2 Typical compositions of microetchants suitable in most applications of titanium metallography Name
Oxalic reagent (tint etch)
Ammonium biﬂuoride (ABF) Lactic hydride reagent
1.5 mL HF 4 mL HNO3 94 mL H2O 20 mL HF 20 g oxalic 98 mL H2O 1 g ammonium biﬂuoride (NH4FHF) 99 mL H2O Mix fresh 5 mL lactic acid and 5 mL stock solution (3 mL HF, 97 mL HNO3)
Fig. 5, 9–11, 13, 14, 16, 35, 37, 56
15 s for Ti-6Al-4V. Do not remove etch products.
Fig. 9–11, 15, 28, 46, 50, 57, 58, 62, 64
Do not remove etch products.
Fig. 9–11, 47–49, 61
Commercially pure titanium hydrides
Coarse lamellar alpha revealed by different etches in Ti-6Al-4V structure after beta anneal at 1040 C (1900 F) and furnace cooling. Preparation was four-step polishing with ﬁnal polish of 16 h on vibratory polisher and 10% alumina slurry. Slightly uncrossedpolarized light for all three etches: (a) ammonium biﬂuoride (ABF) tint etch, 60 s; (b) Kroll’s reagent, 15 s; (c) oxalic tint etch, 60 s. See also Fig. 59 in the article “Selected Color Images” in this Volume for color version.
Metallography and Microstructures of Titanium and Its Alloys / 903 There are two options for intermediate hand polishing; the beeswax platen and the traditional nap cloth diamond method. The nap cloth and diamond option is not recommended where edge retention is critical. The sequence used for hand polishing is typically: 1. A 6 l m diamond slurry on nap cloth is used. 2. A 3 l m diamond slurry on nap cloth is used. 3. Apply as much pressure as possible without rocking the specimen.
pad (neoprene rubber cloth). The wheel speed should be 120 to 150 rpm and the force should be 15 N (3.4 lbf) per 38 mm (1.5 in.) diam mount. Head rotation should be in the complementary direction. For even better edge retention, a Dacron or polyester cloth may be used with the same parametersexcept the force should be 25 to 40 N (5.6 to 9.0 lbf) per 38 mm (1.5 in.) diam mount. The foam pad usually produces a clearer overall microstructure, but with slight edge rounding. Polishing times will vary, with much longer times required for CP titanium than the more highly alloyed materials. The vibratory polisher set up with a short-nap woven synthetic cloth is the preferred method to produce a ﬁnish with the least amount of deformation to the microstructure. However, this process can take a considerable amount of time. A 10% solution of premixed 0.05 lm alumina on a short-nap synthetic cloth may be used. This will optimize removal of deformation from previous steps, but will yield slight edge rounding and require polishing for 8 to 16 h with a weight of only about 0.3 kg (11 oz). More weight will round the edges more. The vibratory polisher set up with a non-nap polyester cloth is the preferred method to produce a microstructure that provides optimal edge retention and little or no deformation to the microstructure. The same suspension is used on a Dacron or polyester non-nap cloth. A weight of 1.0 to 1.5 kg (2.2 to 3.3 lb) is attached to a 38 to 50 mm (1.5 to 2 in.) mount with double-back tape. The polishing time is 1 h. This method works well on hybrid materials containing titanium as well as other materials. It optimizes edge retention.
A recommended intermediate hand-polishing method using a beeswax wheel is an effective, inexpensive method that routinely produces quality samples. About 2.5 mm (0.10 in. of beeswax is cast on the platen into which relief grooves (Fig. 6) are cut at a spacing of 10 to 14 grooves per inch. Polishing is done with a paste made with 5 lm alumina and hydrogen peroxide. While polishing with the paste, drops of 3% hydrogen peroxide (or higher percentage with protective gloves) may be applied to the wheel. Higher percentages of hydrogen peroxide can be more effective. Another way is to polish with the alumina paste, followed by a Kroll’s etch, repeating this cycle until a clean microstructure can be observed. A combination of these steps can prove effective as well. This method has proven very effective for labs without automated polishers. The beeswax wheel would be an excellent choice for the small lab or classroom. Final Polishing. There are three methods for ﬁnal polishing: hand polishing, semiautomated, and vibratory polishing. For hand polishing a 50/ 50 mix of 3% hydrogen peroxide and a 10% solution of a 0.05 lm premixed alumina suspension is used. These suspensions are available Example: Comparison of Polishing Methfrom several suppliers. A nap cloth or a closedcell chemical-resistant foam pad is recom- ods. Micrographs of a beta-annealed Ti-6Al-4V mended. For increased edge retention a Dacron structure are shown in Fig. 7 for three different or polyester cloth is preferred. polishing preparations, as described below. Each A semiautomatic method is used for most tiprocedure is designed for a different purpose. Polishing to Optimize Removal of Deformed tanium alloys as well as commercially pure (CP) titanium when not inspecting for hydrides. The Material (Fig. 7a). In this example, polishing same 50/50 mix of 3% hydrogen peroxide and a was accomplished with a four-step method for diluted 0.05 l m premixed alumina suspension is optimizing removal of deformed material. This used on a closed-cell chemical-resistant foam method is best for overall polish quality:
1. 2. 3. 4.
200 grit diamond-embedded platen 9 l m proprietary grinding platen 3 l m polyester cloth 16 h on vibratory polisher with short-nap synthetic cloth and 10% solution of premixed 0.05 l m alumina suspension
Polishing to Optimize Edge Retention (Fig. 7b). In this example, the same material and etch is used, but polishing was accomplished by the following four-step method for optimizing edge retention:
1. 2. 3. 4.
220 grit diamond-embedded platen 9 l m proprietary grinding platen 3 l m polyester cloth 1 h on vibratory polisher with non-nap polyester cloth and 10% solution of premixed 0.05 lm alumina suspension
Another example is Fig. 8 from Ti-6Al-4V material solution treated at 925 C (1700 F) and aged. It was polished with the four-step method for edge retention. It was etched with the oxalic tint etch to reveal the deformed grain structure from drilling. Figure 8(a) shows the depth of the cold work as evidenced by the disturbed microstructure to a depth of 310 lm. Figure 8(b) shows the normal microstructure for comparison. Polishing to Optimize Preparation Time (Fig. 7c). In this example, the same material and etch is used. Polishing was accomplished with a three-step, semiautomated method for optimizing preparation time: 1. 220 grit diamond-embedded platen 2. 9 l m proprietary grinding platen 3. Closed-cell chemical-resistant foam pad with a 50/50 mix of 3% hydrogen peroxide and a 0.05 l m premixed alumina suspension Total time to prepare sample is less than 20 min. Note that the addition of an intermediate 3 lm polyester cloth step will further improve this process. Also note the lack of detail in the dark regions in Fig. 7(c). Etchants. There are numerous choices of etchants for revealing titanium alloy microstruc-
Ti-6Al-4V plate heated at 885 C (1625 F) for 15 min, air cooled. (a) Ammonium biﬂuoride (ABF) tint etch, 60 s; slightly uncrossed polarized light. (b) Kroll’s reagent, 15 s. (c) Oxalic tint etch, 15 s
904 / Metallography and Microstructures of Nonferrous Alloys ture. Table 1 lists various etchants, and a more extensive listing is also contained in Ref 1. However, this article focuses on three types of etchants that can easily handle nearly all the needs in any laboratory. The three etchants are Kroll’s reagent and two tint etches: an oxalic-acid tint etch and an ammonium biﬂuoride (ABF) tint etch (Table 2). The etching time will vary depending on the alloy and heat treat condition of the sample. It is a good practice to document successful etching practices in the laboratory. When using the tint etchants, it is critical not to swab the specimen after etching. Just wash with warm tap water.
Any swabbing or contact will disturb the surface. The etchant products highlight the grain orientation. The use of tint etchants on specimens where cold work (plastic and elastic deformation) is left from the sample preparation is not recommended. Kroll’s reagent is more forgiving in that case. Comparison of contrast developed by the three etchants is shown for three types of structures or conditions: ●
Coarse lamellar structure after slow furnace cool of T-6Al-4V from beta anneal at 1035 C (1900 F) revealed by Kroll’s reagent and tint etches (Fig. 9) Worked structure in Ti-6Al-4V plate air cooled after heat treatment at 885 C (1625 F) revealed by Kroll’s reagent and tint etches (Fig. 10) Bimodal structure representative of an alpha/ beta forging revealed by Kroll’s reagent and tint etches (Fig. 11). The light phase in Fig. 11(b)—primary alpha in a matrix of transformed beta, a lamellar alpha/beta structure as clearly illustrated in the inset of Fig. 11(c)
ﬁne detail without overetching. However, it is difﬁcult to show etched microstructure in titanium alloy fasteners due to the cold work from rolling the threads. Figures 12 and 13 show the crest area of the thread. Note the lighter structure at the crest where the cold work is greatest. Kroll’s reagent, the most common etchant or reagent used on titanium alloys (Table 2), is used for bringing out the general microstructure in alpha-beta alloys. It is a relatively low-contrast etchant. Figure 16 shows examples of etched microstructures from the same Ti-6Al-4V overaged plate with varying etch severity with (Kroll’s) reagent. Etchant time is a compromise between detail and contrast. The shorter times reveal more detail, while longer etching times result in more contrast. As etching time increases, all detail is lost in the contrast (see Fig. 16d). It is better to underetch than overetch. Figure 16(a) is underetched; however, it has all the detail necessary to analyze the specimen correctly. Figure 16(b) is a good compromise at 15 s. Oxalic acid is a tint etch that stains the microstructure and provides more contrast in the mi-
Figures 12 to 15 are all of a Ti-6Al-4V solution treated and aged fastener with rolled threads. Examples are shown etched with both oxalic acid and Kroll’s reagent. Both etchants show the
Fig. 13 Fig. 12
Oxalic tint etch for 15 s. Ti-6Al-4V fastener solution treated and aged. 1 h vibratory polisher, non-nap polyester cloth and alumina. Note: the mounted parts were vacuum impregnated withhydratedrhodaminedyed epoxy. Note the crest lap, which is typical for a rolled thread.
Etched with Kroll’s reagent for 15 s. Ti-6Al-4V fastener solution treated and aged. 1 h vibratory polisher, non-nap polyester cloth and alumina. Note the crest lap, which is typical for a rolled thread.
Ti-6Al-4V die forging, mill-annealed. (a) Ammonium biﬂuoride (ABF) tint etch,60 seconds; slightly uncrossed polarized light. (b) Kroll’s reagent, 15 s; slightly uncrossed polarized light. (c) Oxalic tint etch, 15 s; slightly uncrossed polarized light. See also Fig. 60 in the article “Selected Color Images” in this Volume for color version.
Fig. 15 Fig. 14
Etched with Kroll’s reagent for 15 s. Ti-6Al-4V fastener solution treated and aged. 1 h vibratory polisher, non-nap polyester cloth and alumina.
Oxalic tint etch for 15 s. Ti-6Al-4V fastener solution treated and aged. 1 h vibratory polisher, non-nap polyester cloth and alumina. Note: This mount was vacuum impregnated with hydrated, rhodamine-dyed epoxy.
Metallography and Microstructures of Titanium and Its Alloys / 905 crostructure in alpha-beta alloys. Etching with oxalic acid requires a better quality polish. It is good for revealing grain orientation, heat effects, and alpha-rich regions formed by exposure to alpha stabilizers such as oxygen or nitrogen. Note: do not remove the etching products or swab during the procedure. Ammonium biﬂuoride (ABF) is another tint etch. It is similar to oxalic acid and also provides more contrast in the microstructure of alpha-beta alloys. Both reagents will reveal grain orienta-
tion. It is most often used for investigating alpha case or alpha-enriched areas in the microstructure. Note: do not remove the etching products or swab during the procedure.
Macroexamination Macrostructural examination of titanium alloys provides useful information about material processing, both melting and metalworking. It is
used for detection of melting defects or anomalies, qualitative assessment of grain reﬁnement and uniformity, as well as determination of grain ﬂow in forged products. Macroetching of titanium alloys is discussed in the article “Macroetching” in this Volume. Four principal defects are to be found in macrosections of ingot, forged billet, or other semiﬁnished product forms. These include high-aluminum defects (HADs or type II defects), high interstitial defects (HIDs, also referred to as type I defects or low-density interstitial defects), beta ﬂecks and high-density inclusions (HDI), Highaluminum defects are areas containing an abnormally high amount of aluminum. These are soft areas in the material (Fig. 17, 18) and are also referred to as “alpha segregation.” Defects referred to as “beta segregation” are sometimes associated with alpha segregation. These are areas in which aluminum is depleted. The high interstitial defects (Fig. 19, 20) are normally high in oxygen and/or nitrogen, which stabilize the alpha phase. These defects are hard and brittle; they are normally associated with porosity, as shown in Fig. 21.
Micrographs from solution treated and overaged Ti-6Al-4V plate after etching with Kroll’s reagent for (a) 5 s, (b) 15 s, (c) 30 s, and (d) 60 s. All specimens polished for 1 h with vibratory polisher, non-nap polyester cloth and alumina. In the severe etch (d), note that ﬁne detail is etched away and the relief is becoming excessive.
Same as Fig. 17. There is a higher volume fraction of more elongatedalpha in theareaof high aluminum content. 50. Courtesy of C. Scholl
Ti-6Al-4V alpha-beta processed billet illustrating macroscopic appearance of a high interstitial defect. See also Fig. 20. Actual size
Ti-6Al-4V alpha-beta processed billet illustrating the macroscopic appearance of a high-aluminum defect. See also Fig. 18. 1.25. Courtesy of C. Scholl
Same as Fig. 19. The high oxygen content results in a region of coarser and more brittle oxygen-stabilized alpha than observed in the bulk material. 100
906 / Metallography and Microstructures of Nonferrous Alloys
Fig. 22 Fig. 21
Ti-8Al-1Mo-1V, as forged. Ingot void (black), surrounded by a layer of oxygen-stabilized alpha (light). The remaining structure consists of elongated alpha grains in a dark matrix of transformed beta. Etchant: Kroll’s reagent (ASTM 192). 25
Beta ﬂecks are regions enriched in a beta-stabilizing element due to segregation during ingot solidiﬁcation. Their occurrence in alpha-beta alloys is uncommon. Flecking becomes more of a problem with beta alloys, which have much higher amounts of beta-stabilizing additions. The problem is most prevalent in iron- and chromium-bearing alloys. This enrichment of a localized region with beta stabilizers lowers the beta transus, locally changing the microstructure and thereby enabling their detection. This microstructural modiﬁcation can take two forms. In alpha-beta alloys, such as Ti-6Al6V-2Sn, vanadium enrichment lowers the beta transus, but is not sufﬁcient to stabilize the beta to room temperature. When working or heat treating the material high in the b phase ﬁeld, the microstructure observed (after cooling back to room temperature) will consist of pri-
Ti-6Al-6V-2Sn forging, solution treated for 11 ⁄ 4 h at 870 C (1600 F), water quenched, and aged 4 h at 575 C (1070 F). Structure: same as in Fig. 22(b), but higher magniﬁcation shows a small amount of light, acicular alpha in the dark “beta ﬂeck.” See also Fig. 24. Etchant: 2 mL HF, 8 mL HNO 3, 90 mL H2O. 200
Ti-6Al-6V-2Sn alpha-beta alloy forging, solution treated, quenched, and aged. Hand forging at 925 C (1700 solution treated for 2 h at 870 C (1600 F), water quenched, aged 4 h at 595 C (1100 F), and air cooled. (a) “Primary” alpha grains (light) in a matrix of transformed beta containing acicular alpha. Kroll’s reagent (ASTM 192). 150. (b) Same structure is the same as in (a), except that alloy segregation has resulted in a dark “beta ﬂeck” (center of micrograph) that shows no light “primary” alpha. See also Fig. 23 and 24. Etchant: Kroll’s reagent (ASTM 192). 75 F),
mary alpha and transformed beta. The beta ﬂeck is a result of the local composition with a higher beta-stabilizer content that results in a local beta
transus lower than that of the bulk material. Beta ﬂecks occur if the temperature is above the transus in the “ﬂecked” region. This condition is ap-
Fig. 24 Ti-6Al-6V-2Sn
b forged billetillustratingmacroscopic appearance of beta ﬂecksthat appear as darkspots. See also Fig. 22 and 23. Etchant: 8 mL HF, 10 mL HNO3, 82 mL H2O, then 18 g/L (2.4 oz/gal) of NH4HF2 in H2O. Less than 1. Courtesy of C. Scholl
Ti-10V-2Fe-3Al pancake forging. (a) Beta forged about 50% alpha-beta ﬁnish forged about 5%, with heat treatment at 750 C (1385 F), 1 h, water quench, 540 C (1000 F), 8 h. (a) Lamellar alpha with a small amount of equiaxed alpha in an aged beta matrix. Etched 10 s with Kroll’s reagent, then 50 mL of 10% oxalic acid, 50 mL of 0.5% HF. 400. Courtesy of R. Boyer. (b) Same as (a), but amount of b ﬁnish forging is 2%. Micrograph illustrates darkened aged beta surrounding a lighter etched beta ﬂeck. See also Fig. 26. Same etch as (a). 50. Courtesy of T. Long
Metallography and Microstructures of Titanium and Its Alloys / 907
Same as Fig. 25(b), but at higher magniﬁcation to demonstrate the reduced amount of alphain the beta ﬂeck. The alpha observed (light) is primary alpha; the alpha that forms upon aging is too ﬁne to resolve. Same etch as Fig. 25(a). 200. Courtesy of T. Long
parent in Fig. 22 and 23. A beta ﬂeck could go undetected if the ﬁnal processing and heat treatment are conducted at a temperature low enough that the beta transus suppression is not sufﬁcient to cause a microstructural perturbation. The effects of beta ﬂecks on properties in such alloys as Ti-6Al-4V and Ti-6Al-6V-2Sn are still in question, but the effect is not a major one. Beta ﬂecks are more of a problem with nearbeta alloys; they are observed macroscopically as shiny spots or ﬂecks. Their appearance is similar in b alloys (Fig. 24). The beta-stabilizer enrichment in the ﬂecked regions of beta alloys, however, is sufﬁcient to stabilize the beta down to room temperature. To guarantee material that will be ﬂeck-free, producers must solution treat samples at a certain temperature below the beta transus, assuring the user that the material will not form beta ﬂecks if heat treated to a temperature up to or below the test temperature. The material will then form alpha, but beta ﬂeck regions will be above or much nearer the transus. Therefore, they will be void of alpha or contain a signiﬁcantly lower volume fraction of alpha upon cooling to room temperature, as shown in Fig. 25 and 26. These regions in beta alloys will be harder, will have higher strength and lower ductility, and will have lower low-cycle fatigue strength than the bulk material. Tree rings (Fig. 27) are another macrostructural anomaly observed in titanium alloy macrosections. This phenomenon represents very minor composition variations that occur during melting. The appearance of tree rings is normally only a cosmetic nuance, not a cause for concern. Grain ﬂow of forgings is useful for evaluating the forging process. For high-quality forgings, in general, the grain ﬂow should conform to the general shape of the part. There should be no forging laps, seams, or areas of grain ﬂow that appear as though they could produce forging laps in subsequent operations. In addition, the part should be uniformly recrystallized and sufﬁciently worked in all areas.
Ti-6Al-2Sn-4Zr-2Mo alpha-beta forged billet macroslice illustrating “tree rings,” which represent minor compositional ﬂuctuations. The slices are from two ingot locations. Etchant unknown. 0.63. Courtesy of W.
Microexamination Bright-ﬁeld illumination reveals microstructure of properly prepared specimens in most cases, but image enhancement can be achieved
in nearly all cases by using just the polarizer (plane-polarized light) or using the analyzer for slightly uncrossed polarized light. This also acts as a neutral density ﬁlter for capturing the digital image. High-quality, strain-free objectives are desirable for any polarized light microscopy,
Variation in appearance with changes in illumination of a Ti-6Al-4V specimen with oxalic tint etch (15 s). Material was beta annealed at 1050 C (1925 F) and furnace cooled. (a) Illuminated and examined with slightly uncrossed (45–129) polarized light. (b) Illuminatedand examinedwith slightly uncrossed(45–139)polarizedlight. (c) Plane-polarized light illumination. (d) Bright-ﬁeld illumination. See also Fig. 61 in the article “Selected Color Images” in this Volume for color version.
908 / Metallography and Microstructures of Nonferrous Alloys Alpha Structures
Ti-6Al-2Sn-4Zr-6Mo, forged at 870 C (1600 F). (a) Solution treated 2 h at 870 C (1600 F), waterquenched, and aged 8 h at 595 C (1100 F), and air cooled. Elongated “primary” alpha grains (light)in agedtransformed beta matrix containing acicular alpha. (b) Solution treated at 915 C (1675 F) instead of at 870 C (1600 F), which reduced the amount of “primary” alpha grains in the b matrix. (c) Solution treated at 930C (1710 F), which reduced the amount of alpha grains and coarsened the acicular alpha in the matrix. (d) Solution treated at 955 C (1750 F), which is above the beta transus. The resulting structure is coarse, acicular alpha (light) and aged transformed beta (dark). All etched with Kroll’s reagent (ASTM 192). 500
while non-strain-free objectives or objectives with a long working distance will compromise the polarized light image. Figure 28 provides
comparative micrographs from the examination of a Ti-6Al-4V specimen with different illumination modes.
Generally, two types of alpha are present: primary alpha and secondary alpha or transformed beta (Fig. 29, 30). The primary alpha is that present during prior hot working, remnants of which persist through heat treatment. The secondary alpha is produced by transformation from beta. This may occur upon cooling from above the beta transus (Fig. 29d) or high within the alphabeta phase ﬁeld (Fig. 29b and c) by aging or by aging of the beta (Fig. 30d). The aged alpha is usually too ﬁne to resolve using light (optical) microscopy. The alpha in these areas has different appearances and may be acicular or lamellar, platelike, serrated, or Widmansta¨tten. Equiaxed alpha grains, such as are shown in Fig. 31 and 32 are usually developed by annealing cold-worked alloys above the recrystallization temperature. Elongated alpha grains (Fig. 33, 34) result from unidirectional working of the metal and are commonly found in longitudinal sections of rolled or extruded alloys. The microstructure of titanium alloys is strongly inﬂuenced by the processing history and heat treatment. The effect of cooling rate on Ti5Al-2.5Sn annealed above the beta transus can be seen in Fig. 35. This is also illustrated for Ti6Al-4V in Fig. 36. As the cooling rate increases, the lamellar alpha (or martensite, depending on the alloy and cooling rate) becomes ﬁner. Coarse and ﬁner lamellar structures in alloy Ti-6Al-4V are also shown, respectively, in Fig. 9 and 28 after furnace cooling from different temperatures above the beta transus. The extent of lamellar alpha in the Ti-10V-2Fe-3Al lean beta alloy is shown in Fig. 37. The structure is completely lamellar alpha (Fig. 37a) when heat treatment is below the beta transus. When heat treated just below the beta transus, a beta structure develops with some residual alpha (Fig. 37b). When heated above the transus and cooled, the structure of Ti-10V-2Fe-3Al is completely beta. The effect of forging temperature is illustrated for Ti-8Al-1Mo-1V in Fig. 38. As the forging
Ti-15V-3Cr-3Al-3Sn cold-rolled strip that has been annealed at 790 C (1450 F) for 10 min and aged at various times to illustrate the progression of aging and what is termed “decorative aging,” a technique used to determine the extent of recrystallization. (a) Not aged. (b) Aged 2 h at 540 C (1000 F). (c) Aged 4 h. (d) Aged 8 h. Grains in center are completely aged (uniform alpha precipitation throughout the grains). An 8 h age results in a fully aged structure. All etched with Kroll’s reagent. All 200. Courtesy of P. Bania
Metallography and Microstructures of Titanium and Its Alloys / 909
Fig. 31 High-purity
(iodide-process) unalloyed titanium sheet, cold rolled, and annealed 1 h at 700 C (1290 F). Equiaxed, recrystallized grains of alpha. Etchant: Kroll’s reagent (ASTM 192). 250
Commercial-purity (99.0%) unalloyed titanium sheet. (a) As-rolled to 1.0 mm (0.040 in.) thickat 760C (1400 Grains of alpha, which have been elongated by cold working. (b) Same as in (a), but annealed 2 h at 700 C (1290 F) andair cooled. Recrystallized alpha grains, particlesof TiH (black), andparticles of beta (alsoblack) stabilized by impurities. (c) Same as in (a), but annealed 1 h at 900 C (1650 F)—just below the beta transus—and air cooled. Recrystallized grains of “primary” alpha and transformed beta containing acicular alpha. (d) Same as in (a), but annealed 2 h at 1000 C (1830 F) and air cooled. Colonies of serrated alpha plates; particles of TiH and retained beta (both black) between the plates of alpha. All etched with Kroll’s reagent (ASTM 192). 250
Ti-6Al-4V plate, recrystallize annealed at 925 (1700 F) 1 h, cooled to 760 C (1400 F) at 50 to 55 C/h (90 to 100 F/h), then air cooled. Equiaxed alpha with intergranular beta. The alpha-alphaboundaries are not deﬁned. Etchant: 50 mL oxalic acid in H2O, 50 mL 1% HF in H2O. 500. Courtesy of J.C. Chesnutt C
temperature increases, the amount of transformed beta increases until the forging temperature is above the beta transus, at which point the structure is 100% transformed beta. The effect of the amount of forging deformation is illustrated for Ti-6Al-2Sn-4Zr-2Mo and Ti-5Al-6Sn2Zr-1Mo-2.5Si, respectively in Fig. 39 and 40. Sufﬁcient working of the cast Widmansta¨tten structure at a temperature below the beta transus causes recrystallization of the lamellar structure to a more equiaxed structure. Sufﬁcient working
and proper heat treatment can produce a completely equiaxed crystal structure (Fig. 32). The microstructural behavior trends will be similar for all-alpha and b alloys. Acicularor lamellar alpha is the most common transformation product formed from beta during cooling. It is a result of nucleation and growth on crystallographic planes of the prior beta matrix. Precipitation normally occurs on multiple variants or orientations of this family of habit planes, as illustrated in Fig. 29(d) and 41. A packet or cluster of acicular alpha grains aligned in the same orientation is referred to as a “colony.” Whencorrelating thistype of microstructure with properties such as fatigue or fracture toughness, colony size is often regarded as an important microstructural feature.The multiple orientationsof alpha have a basketweave appearance characteristic of alpha Widmansta¨tten structure. Lamellar alpha forming from small beta grains also may have a singular orientation (Fig. 42). Under some conditions, the long grains of alpha produced along preferred planes in the beta matrix take on a wide, platelike appearance, as shown in Fig. 35(a). Under other conditions, grains of irregular size and with jagged boundaries, called “serrated alpha,” are produced (Fig. 43).
Alpha Case. Unless heat treatments are performed in an inert atmosphere, oxygen and nitrogen will be absorbed at the surface, stabilize the alpha, and form a hard, brittle layer referred
Ti-6Al-4V, as-forged at 955 C (1750 F), below the beta transus. Elongated alpha (light), caused by low reduction (20%) of a billet that had coarse, platelike alpha, in a matrix of transformed beta containing acicular alpha. Etchant: Kroll’s reagent (ASTM 192). 250
910 / Metallography and Microstructures of Nonferrous Alloys
Ti-5Al-2.5Sn, hot worked below the alpha transus, annealed 30 min at 1175 C (2150 F), which is above the beta transus. (a) Furnace cooled to 790 C (1450 F) in 6 h, and furnace cooled to room temperature in 2 h. Coarse, platelike alpha. Etchant: Kroll’s reagent (ASTM 192). 100. (b) Air cooled from the annealing temperature instead of furnace cooled. The faster cooling rate produced acicular alpha that is ﬁner than the platelike alpha in (a). Prior beta grains are outlined by the alpha that was ﬁrst to transform. Etchant: Kroll’s reagent (ASTM 192). 100. (c) Water quenched from the annealing temperature instead of furnace cooled and shown at a higher magniﬁcation. The rapid cooling produced ﬁne acicular alpha. A prior beta grain boundary can be seen near the center of the micrograph. Etchant: Kroll’s reagent (ASTM 192). 250
Ti-6Al-4V bar, held for 1 h at 1065 C (1950 F), above the beta transus. (a) Furnace cooled. Platelike alpha (light) and intergranular beta (dark). (b) Air cooled. The structure consists of acicular alpha (transformed beta); prior beta grain boundaries. Etchant for both (a) and (b): 10 mL HF, 5 mL HNO3, 85 mL H2O. 250
Effect of heat treatment temperature below, near, and above the transus temperature on etched appearance of lean beta alloy Ti-10V-2Fe-3Al. All specimens were polished with four-step procedure ending up with 16 h on vibratory polisher (10% alumina slurry), etched with Kroll’s reagent for duration noted, and examined under slightly uncrossed polarized light (a) lamellar alpha after air cool (AC) from temperature about 70 C (130 F) below beta transus (730 C, or 1350 F, for 2 h). (b) Heat treated just below the beta transus (788 C, or 1450 F, for 2 h, AC), where almost all of the alpha has gone back i nto solution. One grain in this view contains residual alpha. (c) All-beta structure from beta heat treatment. Duration of etching with Kroll’s reagent: (a) 15 s, (b) and (c) 60 s
Metallography and Microstructures of Titanium and Its Alloys / 911 to as an “alpha case” (Fig. 44, 45). This case is normally removed by chemical milling or machining. A part should not be put into service unless this alpha case has been removed. Figures 46 to 50 show alpha case layers caused by interstitial oxygen migration through the surface. This illustrates the increase in alpha case thickness as the thermal exposure is increased with longer times or higher temperatures. The oxygen migration causes an increase in hardness beyond the visible depth of the alpha case layer. Ti3Al (Alpha-2) Ordered Phase. The alpha phase can decompose to Ti3Al, an ordered
phase, at compositions greater than about 6 wt% Al. This ordered phase is submicron in size and can be observed only by electron microscopy (Fig. 51).
Martensite Martensite is a nonequilibrium supersaturated alpha-type structure produced by diffusionless
Ti-6Al-2Sn-4Zr-2Mo forged ingot. (a) Forged and held 1 h at 1010 C (1850 F), air cooled, heated to 970 C (1775 F), and immediately air cooled. Acicular alpha (transformed beta); prior beta grain boundaries. (b) Same as (a), but reduced 15% by upset forging while at 970 C (1775 F). The structure consists of slightly deformed acicular alpha (transformed beta), boundaries of elongated prior beta grains. Both etched withKroll’s reagent (ASTM192). 100
Ti-5Al-6Sn-2Zr-1Mo-2.5Si forging. (a) Reduced 75% by upset forging starting at 980 C (1800 F), annealed 1 h at 980 C (1800 F), air cooled, and stabilized 2 h at 595 C (1100 F). Fine alpha grains (light); intergranular beta. (b) Same as (a), except upset forged starting at 1150 C (2100 F), which is above the beta transus temperature. Distorted acicular alpha (light constituent); intergranular beta; and boundaries of elongated prior beta grains. Both etched with HF, HNO3, HCl, glycerol (ASTM 193). 100
Ti-8Al-1Mo-1V forging. (a) Forged with a starting temperature of 900 C (1650 F), which is below the normal temperature range for forging this alloy. Structure: equiaxed alpha grains (light) in a matrix of transformed beta (dark). (b) Forged with starting temperature of 1005 C (1840 F), which is within the normal range, and air cooled. Equiaxed grains of “primary” alpha (light) in a matrix of transformed beta (dark) containing ﬁne acicular alpha. (c) Starting temperature for forging was 1095 C (2000 F), whichis above thebeta transus temperature,and the ﬁnished forging was rapidly air cooled. The structure consists of transformed beta containing coarse and ﬁne acicular alpha(light). All etched withKroll’sreagent(ASTM 192). 250
Ti-6Al-5Zr-4Mo-1Cu-O.2Si casting. (a) As-cast. Microstructure: transformed beta containing acicular alpha (light platelets). A thin ﬁlm of alpha phase (light) is evident at the prior beta grain boundaries. (b) Same as (a), but solution treated 1 h in argon at 845 C (1550 F), air cooled, and aged 24 h at 500 C (930 F). Acicular alpha (light) and aged beta; alpha platelets at prior beta grain boundaries. Both etched with 10 mL HF, 30 mL HNO3, 50 mL H2O (ASTM 187). 500
912 / Metallography and Microstructures of Nonferrous Alloys (martensitic) transformation of the beta. There are two types of martensite: , which has a hexagonal crystal structure, and , which has an orthorhombic crystal structure. Martensite can be produced in titanium alloys by quenching (athermal martensite) or by applying external stress (stress-induced martensite). The can be formed athermally or by a stress-assisted transformation (see Fig. 52, 53). However, can be formed only by quenching. Examples of structures are exhibited in Fig. 54 and 55. Aging of the martensite results in its decomposition to b .
Beta Structures In alpha-beta and beta alloys, some equilibrium beta is present at room temperature. A non-
equilibrium, or metastable, beta phase can be produced in alpha-beta alloys that contain enough beta-stabilizing elements to retain the beta phase at room temperature on rapid cooling from high in the b phase ﬁeld. The composition of the alloy must be such that the temperature for the start of martensite formation is depressed to below room temperature. One hundred percent beta can be retained by air cooling beta alloys. The decomposition of this retained beta (or martensite, if it forms) is the basis for heat treating titanium alloys to higher strengths.
Aged Structures The alpha that forms upon aging of retained beta is often too ﬁne to be resolved by optical microscopy, particularly with beta and near-beta
alloys. Aging of martensite results in the formation of equilibrium b, but most aged martensite structures cannot be distinguished from unaged martensite by optical microscopy. Unresolved alpha precipitation is shown in Fig. 56 for alloy Ti-10V-2Fe-3Al with Kroll’s etch. Figures 57 and 58 are light micrographs of cold-rolled and aged Ti-15V-3Cr-3Al-3Sn foil. The white regions indicate there was less or no heat treatment response. Precipitation of alpha during aging of beta results in some darkening of the aged beta structure. The progression of aging response in alloy Ti-15V-3Cr-3Al-3Sn is shown in Fig. 30. Other precipitation products include: ● ● ●
Eutectoid products x phase (Fig. 59), which is a transition phase (potentially resulting in severe embrittlement) Phase splitting
Phase splitting, or phase separation only, occurs in the solute-rich beta alloys; b br b1 where r
Ti-6Al-4V forging. (a) Solution treated 1 h at 955 C (1750 F), air cooled, and annealed 2 h at 705 C (1300 Equiaxed alpha grains (light) in transformed beta matrix (dark) containing coarse, acicular alpha. (b) Same as in (a), except water quenched from the solution treatment (before the anneal) instead of air cooled. Structure is similar to that in (a), but the faster cooling resulted in ﬁner acicular alpha inthe transformed beta.Both etched withKroll’sreagent (ASTM 192). 500
Ti-7Al-2Mo-1V plate, heated to 1010 C (1850 F), which is above the beta transus. Surface layer of white, oxygen-stabilized alpha (alpha case); the remainder of the structure is acicular alpha (transformed beta). Etchant: 2 mL HF, 8 mL HNO 3, 90 mL H2O. 450
Ti-6Al-4V platediffusion-bonded joint(bonded at 925 C, or 1700 F) illustrating bond-line contamination. The white horizontal band is an area of O2 and/or N2 enrichment. An alpha case is also observable on the exterior surface. Etchant: 50 mL H2O, 50 mL 10% oxalic acid, 1 mL HF. 58. Courtesy of J.C. Chesnutt
Unalloyed titanium sheet. Same as Fig. 33, but annealed 2 h at 1000 C (1830 F) and air cooled. Colonies of serrated alpha plates; particles of TiH and retained beta (both black) between the plates of alpha. Etchant: Kroll’s reagent (ASTM 192). 250
Alpha case in Ti-6Al-4V after exposure to 760 (1400 F) for 90 min. Preparation: oxalic tint etch for 60 s, and four-step edge-retention process ending with 1 h on vibratory polisher with a non-nap polyester cloth and alumina. See also Fig. 62 in the article “Selected Color Images” in this Volume for color version. C
Metallography and Microstructures of Titanium and Its Alloys / 913
Alpha case in Ti-6Al-4V after exposure to 885 (1625 F) for 90 min. Preparation: ammonium biﬂuoride tint etch for 60 s, and four-step edge-retention process ending with 1 h on vibratory polisher with a non-nap polyester cloth and alumina. See also Fig. 63 in the article “Selected Color Images” in this Volume for color version.
Alpha case in Ti-SP 700 after exposure to 760 (1400 F) for 90 min. Preparation: ammonium biﬂuoride tint etch for 60 s, and four-step edge-retention process ending with 1 h on vibratory polisher with a non-nap polyester cloth and alumina
br is solute-rich beta and b 1 is solute lean beta. The solute lean beta is designated b (Fig. 60). This is not an important decomposition product from a practical standpoint, because it does not occur in commercial alloys with heat treatments that are used. The x phase and phase splitting can only be observed using electron microscopy.
Other Structures Hydrides. Figure 61 shows hydrides in commercially pure (CP) titanium sheet located at the weld heat-affected zone (HAZ). The black needles in this micrograph are a result of hydrogen migrating to high residual stress areas and forming a titanium hydride. In most cases, because of their brittle nature, hydrides will result in microcracks. Material with this extent of hydrides will be very brittle. Commercially pure titanium is more difﬁcult to polish than more highly alloyed titanium alloys because it is softer and retains more cold work. The same polishing method used for titanium alloys will also work for CP titanium, but longer ﬁnal polishing times may be needed. Do not use hydrogen peroxide or etchpolish to remove residual cold work induced by previous polishing steps. Etching will mask the hydrides. It is best just to add more time to the last two steps. If hand polishing with a 5 lm alumina and beeswax wheel, it can take 20 min. Revealing hydrides in CP titanium with an ABF etch (Fig. 61) requires a perfectly polished surface. The lactic hydride reagent (Table 2) may be more easily applied to reveal the presence of hydrides with etching times ranging from 11 ⁄ 2 to 3 min. A 3 min etch can provide good contrast to reveal hydrides with a light microscope. An etch of 11 ⁄ 2 min may provide subtle contrast under a light microscope, but examination in a scanning electron microscope can easily reveal microcracks associated with hydride cracking. Heat-Affected Zones. Heat damage from machining, sample excision, or preparation is often confused with alpha case. The difference can
Fig. 48 Alpha case in Ti-SP 700(Ti-4.5Al-3V-2Mo-2Fe) after exposure to 900 C (1650 F) for 90 min. Preparation: ammonium biﬂuoride tint etch for 60 s, and four-step edge-retention process ending with 1 h on vibratory polisher with a non-nap polyester cloth and alumina. See also Fig. 64 in the article “Selected Color Images” in this Volume for color version.
be determined by making 10 g Knoop indents since the heat-damaged layer will be softer than the base material but would be harder in an alpha case area. Unlike heat damage from straight thermal exposure (which results in a harder surface), heat damage from mechanical sources during preparation results in a softer surface. In the example of Fig. 62, the layer is softer from heat damage, yielding a larger indent. Another ex-
Alpha case in Ti-10V-2Fe-3Al after exposureto 790 C (1450 F) for 2 h. Preparation: Oxalic tint etch for 60 s, and four-step edge-retention process ending with 1 h on vibratory polisher with a non-nap polyester cloth and alumina
ample is shown in Fig. 63 of a heat-affected zone from a molten ﬁne, which had a 50% knockdown factor on fatigue life. Figures 64 and 65 show heat damage from a lab-induced lightening strike. Note the compromise in etching time in Fig. 64, where the heataffected zone is underetched, while the base material is overetched. Figure 65 shows the effects of intraply arcing and heat-affected zone from a lab-induced lightning strike.
Other Techniques Several metallographic techniques have been developed for speciﬁc purposes, including recrystallization studies and microstructure/fracture topography correlations. Decoration aging
Ti-8Al (with 1800 ppm O2) sheet aged to precipitate the ordered alpha-2 (Ti3Al) phase. The dark-ﬁeld transmission electron micrograph illustrates 2 precipitates (light) in an alpha matrix. 105,600. Courtesy of J.C. Williams
914 / Metallography and Microstructures of Nonferrous Alloys
Fig. 52 Ti-8.5Mo-0.5Si water quenched from1000 C (1830 F). Thin-foil transmissionelectronmicrographillustrating heavily twinned athermal martensite. 5000. Courtesy of J.C. Williams
was developed to study the extent of recrystallization in beta alloys. After recrystallization annealing, the material is given a partial age at a time and temperature appropriate for the alloy of interest. The incompletely recrystallized grains retain some dislocation substructure (stored energy) that accelerates the aging process, resulting in a more rapidly aged grain. These grains then etch darker than the recrystallized ones, making it easy to identify the extent of recrystallization. This effect is illustrated in Fig. 30. Another technique utilizes deep macroetching and thermal etching. The deformed specimen is polished, then subjected to overetching to produce deep grooves at the deformed grain boundaries. Next, the specimen is subjected to the recrystallization cycle of interest in a hard vacuum (106 torr), followed by oil quenching. The material recrystallizes and thermal etching occurs, which differentiates between different grains,because surface atoms evaporate or sublimate at different rates on different crystallographic planes. Different grains will have different crystallographic planes at the exposed surface. The original grain boundaries are observable as ghost boundaries, due to the deep macroetching used previously. Therefore, the recrystallized
Fig. 54 Ti-6Al-2Sn-4Zr-2Mo
forgings, ﬁnish forged starting at 970 C (1775 F), air cooled, machined to 13 mm (0.5 in.) diam test bars, reheated to 995 C (1825 F), the beta transus, held for 1 h, and air cooled. The microstructure is entirely . Etched with Kroll’s reagent (ASTM 192). 100
and original microstructures can be observed simultaneously. This permits studying not only the recrystallized structure, but also the recrystallization nucleation sites. The ghost boundaries can be removed by repolishing and chemically etching. This technique is illustrated in Fig. 66. Figure 66(a) demonstrates the as-deformed structure that has been heavily etched. The specimen was recrystallized at 925 C (1700 F) for 1 h in a vacuum of 106 torr. Recrystallization in vacuum caused thermal etching of the recrystallized grains (Fig. 66b shows recrystallized structure). The prior unrecrystallized structure can still be observed as ghost boundaries remaining from the initial overetching. Subgrain boundaries can be revealed using a relatively simple technique. The specimen is electropolished and viewed in the scanning electron microscope in the backscattered electron mode. The contrast and delineation of subgrains are due to differences in crystallographic orientation. Electropolishing occurs at different rates on different crystallographic planes, similar to the thermal etching phenomenon.
Ti-10V-2Fe-3Al, beta solution treated, water quenched, and strained 5% at room temperature. This Nomarski interference micrograph illustrates deformation-induced martensite in a beta matrix. No etch. 500. Courtesy of J.E. Costa
Several techniques have been developed to observe fracture topography and microstructure simultaneously in the scanning electron microscope using its large depth of ﬁeld. A very simple method involves selective polishing and etching of the fracture face. The fracture face and machined surfaces are ﬁrst masked with a suitable maskant, such as a stop-off lacquer, which can be applied with a small paint brush. Selected areas of the fracture face are left unmasked. The
martensite in Ti-6Al-4V. (a) Light micrograph of bar, held for 1 h at 955 C (1750 F), which is below the beta transus, andwater quenched.Equiaxed“primary” alpha grains(light) ina matrixof (martensite).Etched with 10 mL HF, 5 mL HNO3, 85 mL H2O.250. (b) Thin foil transmission electron micrographof the samemicrostructure as in (a), but at higher magniﬁcation. The large light grains are primary alpha; the darker region is acicular martensite in a beta matrix. 5880. Courtesy of J.C. Williams
Metallography and Microstructures of Titanium and Its Alloys / 915
Fine, unresolved alpha precipitation in light micrograph of aged Ti-10V-2Fe-3Al alloy. The white phase is primary alpha in an aged beta matrix, dark background. Slightly uncrossed polarized light, and a four-step polishing ending up with 16 h on vibratory polisher 10% alumina slurry. Etched with Kroll’s reagent for 15 s (a) and 7 s (b)
specimen is then electropolished, which will affect only the unmasked areas, and etched. Studying the interface between the polished and etched
Structure from cold-rolledand aged foil of beta alloy Ti-15V-3Cr-3Al-3Sn. Oxalic tint etch for 15 s, 1 h vibratory polisher, non-nap polyester cloth and alumina. The white regions indicate there was no or a lesser heat treat response (alpha precipitation).
and the masked areas permits a correlation of microstructural features and fractographic details, as shown in Fig. 67. This technique is useful for correlating general microstructural details, but it may be difﬁcult to pinpoint a speciﬁc area to study. Precision sectioning techniques have also been developed. The area of interest on the fracture face, such as crack origin, is ﬁrst located. The specimen is then cut on a plane perpendicular to the fracture face close to the area of interest. The distance from the cut face to the area of interest is measured. Next, the specimen is placed in a metallurgical mount, then ground and polished the measured distance for metallurgical analysis of the precise area of interest and correlation of microstructure to fractographic features. An example of this technique is shown in Fig. 68. The microstructure and fracture face can be observed simultaneously using the scanning electron microscope by carefully dissolving the mount material. This fatigue specimen had an internal origin at point A, which initiated at an
Fig. 59 A
titanium-iron binary alloy, beta solution treated, water quenched, and aged to form x . The x is the light precipitate in this thin-foil transmission electron micrograph. In alloys where the x has a high lattice misﬁt, the x is cuboidal to minimize elastic strain in the matrix. 320,000. Courtesy of J.C. Williams
Ti-40Nb (at.%), beta solution heat treated at 900 C (1650 F), water quenched, then aged at 400 C (750 F) for 24 h. Thedark precipitate is b (solute lean beta phase) in a solute-enriched beta matrix. Thin-foil transmission electron micrograph. 31,000. Courtesy of J.C. Williams
Structure from cold-rolledand aged foil of beta alloy Ti-15V-3Cr-3Al-3Sn. Oxalic tint etch for 3 s, 1 h vibratory polisher, non-nap polyester cloth and alumina
iron inclusion, as determined in Fig. 68(b) by precision sectioning. The cleavage zone at point C in Fig. 68(a) is due to the TiFe2 zone seen at point C in Fig. 68(b). Below the TiFe2, the structure consists of t ransformed Widmansta¨tten alpha. The section (Fig. 68b) was taken at line AB in Fig. 68(a).
Ammonium biﬂuoride tint etch for 10 s. Commercially pure titanium sheet. Four-steppolishingending upwith 16h onvibratory polisher, 10%alumina slurry
916 / Metallography and Microstructures of Nonferrous Alloys
Oxalic tint etch for 15 s. Ti-15V-3Cr-3Al-3Sn foil heat-affected zone. This is from a lab-induced lightning strike. Note the compromise in etching time. The heat-affected zone is underetched and the base material is overetched. 1 h vibratory polisher, non-nap polyester cloth and alumina
Ti-6Al-4V heat damage. Oxalic tint etch for 15 s. Four-step edge retention process ending up with 1 h on a vibratory polisher and alumina
Ti-6Al-4V plate fatigue specimen with molten ﬁne and heat-affected zone 1 h vibratory polisher with nonnap polyester cloth and alumina
Hybrid Ti-6Al-4V carbon-reinforced polymer composite with arcing and heat damage from a lab-induced lightning strike. Notethe heat-affected zone. The vertical line shows the original surface of the titanium fastener and the extent of intraply arcing. Because of its complex shape, the specimen was vacuum impregnated with hydrated rhodamine-dyed two-part epoxy after sectioning on a wafering saw. This sample was prepared with a ﬁve-step edge retention process; 220 grit, 9 l m, 3 l m silk, 3 l m non-nap polyester and ending up with 1 h on a vibratory polisher with a non-nap polyester cloth and 10% alumina solution.
Metallography and Microstructures of Titanium and Its Alloys / 917
Fig. 67 Fig. 66
Ti-10V-2Fe-3Al deformed at 1150C (2100 F). (a) Etched with 60 mL H2O, 40 mL HNO3, 10 mL HF for 30 min. (b) Etched with 60 mL H2O, 40 mL HNO3, 10 mL HF for 30 min thermally etched at 925 C (1700 F) for 1 h in vacuum (106 torr). Magniﬁcation not given. Courtesy of D. Eylon
Scanning electron micrograph (SEM) image from Ti-6Al-4V beta-annealed fatigued plate specimen. (a) SEM at the polished and etched/unetched fracture topography interface showing microstructure/fracture topography correlation. Secondary cracks are a result of intense slip bands. (b) SEM that illustrates “furrows” or “troughs” that are deﬁned by the lamellar alpha plates. These furrows link up as the crack progresses. Kroll’s reagent. 2000. Courtesy of R. Boyer
Ti-6Al-4V powder metallurgy compact, hot isostatically pressed at 925 C (1700 F), 103 MPa (15 ksi), for 2 h. (a) Scanning electron micrograph. No etch. 80. (b) Optical micrograph. Etchant: Kroll’s reagent. 16. Courtesy of D. Eylon
Color Micrographs of Titanium Alloys / 543
Ti-6Al-4V alloy with Widmansta¨tten alpha in a beta matrix after furnace cooling from above the transus. Beta-anneal temperature was 1040 C (1900 F). Samples were etched with the oxalic tinting reagent for 15 s after polishing by (a) the four-step method for optimizing removal of deformed material, (b) the four-step method for optimizing edge retention, or (c) the three-step semiautomated method for optimizing preparation time (note the lack of detail in the dark regions). See text for description of polishing procedures. Color version of Fig. 7 in the article “Metallography andMicrostructuresof Titaniumand Its Alloys”
Coarse lamellar alpha revealed by different etches in Ti-6Al-4V structure after beta anneal at 1040 C (1900 F) and furnace cooling. Preparation was four-step polishing, with ﬁnal polish of 16 h on vibratory polisher and 10% alumina slurry. Slightly uncrossed polarized light illumination for all three etches. (a) Ammonium biﬂuoride tint etch, 60 s. (b) Kroll’s reagent, 15 s. (c) Oxalic tint etch, 60 s. Color version of Fig. 9 in the article “Metallography and Microstructures of Titanium and Its Alloys”
Ti-6Al-4V die forging, mill-annealed. (a) Ammonium biﬂuoride tint etch, 60 s. Slightly uncrossed polarized light illumination. (b) Kroll’s reagent, 15 s. Slightly uncrossedpolarizedlight illumination.(c) Oxalic tint etch, 15 s. Slightly uncrossed polarized light illumination. Color version of Fig. 11 in the article “Metallography and Microstructures of Titanium and Its Alloys”
544 / Color Micrographs of Titanium Alloys
Variation in appearance with changes in illumination of a Ti-6Al-4V specimen with oxalic tint etch (15 s). Material was beta annealed at 1050 C (1925 F) and furnace cooled. (a) Illuminated and examined with slightly uncrossed (45–129) polarized light. (b) Illuminatedand examinedwith slightly uncrossed(45–139)polarizedlight. (c) Plane-polarized light illumination. (d) Bright-ﬁeld illumination. Color version of Fig. 28 in the article “Metallography and Microstructures of Titanium and Its Alloys”
Alpha case in Ti-6Al-4V after exposure to 760 C (1400 F) for 90 min. Preparation: oxalic tint etch for 60 s, and four-step edge-retention process, ending with 1 h on vibratory polisher with a non-nap polyester cloth and alumina. Color version of Fig. 46 in the article “Metallography and Microstructures of Titaniumand Its Alloys”
Alpha case in Ti-6Al-4V after exposure to 885 C (1625 F) for 90 min. Preparation: ammonium biﬂuoride tint etch for 60 s and four-step edge-retention process, ending with 1 h on vibratory polisher with a non-nap polyester cloth and alumina. Color version of Fig. 47 in the article “Metallography and Microstructures of Titanium and Its Alloys”
Fig. 64 Alpha case in Ti-SP 700 (Ti-4.5Al-3V-2Mo-2Fe) after exposure to 900 C (1650 F) for 90 min. Preparation: ammonium biﬂuoride tint etch for 60 s and four-step edge-retention process, ending with 1 h on vibratory polisher with a nonnap polyester cloth and alumina. Color version of Fig. 48 in the article “Metallography and Microstructures of Titanium and Its Alloys”