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ASM Handbook, Volume 9: Metallography and Microstructures G.F. Vander Voort, editor, p DOI: /asmhba Color images cited in this article appear at end of article. Copyright 2004 ASM
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ASM Handbook, Volume 9: Metallography and Microstructures G.F. Vander Voort, editor, p DOI: /asmhba Color images cited in this article appear at end of article. Copyright 2004 ASM International All rights reserved. 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 METALLOGRAPHY is a complex process with many variables that involve compromises between time, resources, and the end product or purpose of the investigation. For example, a research 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 of manual polishing, 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 describes the fundamentals of titanium metallographic sample preparation. Representative micrographs are also presented for each class of titanium alloys, which include unalloyed titanium, alpha alloys, alpha-beta alloys, and beta titanium alloys. Metallography and metallographic sample preparation of titanium alloys are also described in more detail in Ref 1 and 2. Fig. 1 Cross section through the abrasive saw-cut edge of a Ti-6Al-4V sample. Note there is less than 5 lm depth of disturbed material requiring removal 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. Fig. 2 This micrograph shows the 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. Types of Titanium Alloys Titanium is an allotropic element; that is, it exists in more than one crystallographic form. At room temperature, titanium has a hexagonal Fig. 3 Sample holders for semiautomated polishing machines. (a) Fixed-sample holder with load applied from a central column. (b) Nonfixed specimen mover plate with load applied over one mount Fig. 4 Mount with two specimens for manual polishing or polishing on a semiautomated polisher with a non-fixed specimen mover plate 900 / Metallography and Microstructures of Nonferrous Alloys close-packed (hcp) crystal structure, which is referred to as alpha phase. This structure transforms to a body-centered cubic (bcc) crystal structure, called beta phase, at 883 C (1621 F). Alloying elements generally can be classified as alpha or beta stabilizers. Alpha stabilizers, such as aluminum and oxygen, increase the temperature at which the alpha phase is stable. Beta stabilizers, such as vanadium and molybdenum, result in stability of the beta phase at lower temperatures. This transformation temperature from an alpha-beta phase (or all-alpha phase) to all beta is known as the beta transus temperature. The beta transus is defined as the lowest equilibrium temperature at which the material is 100% beta. Below the beta transus temperature, titanium will be a mixture of b if the material contains some beta stabilizers, or it will be all alpha if it contains no beta stabilizers. The beta transus is important, because processing and heat treatment are often carried out with reference to some incremental temperature above or below the beta transus. Alloying elements that favor the alpha crystal structure and stabilize it by raising the beta transus temperature include aluminum, gallium, germanium, carbon, oxygen, and nitrogen. Two groups of elements stabilize the beta crystal structure by lowering the transformation temperature. The beta isomorphous group consists of elements that are miscible in the beta phase, including molybdenum, vanadium, tantalum, and niobium. The other group forms eutectoid systems with titanium, having eutectoid temperatures as much as 333 C (600 F) below the transformation temperature of unalloyed titanium. The eutectoid group includes manganese, iron, chromium, cobalt, nickel, copper, and silicon. Two other elements that often are alloyed in titanium are tin and zirconium. These elements have extensive solid solubilities in alpha and beta phases. Although they do not strongly promote phase stability, they retard the rates of transformation and are useful as strengthening agents. Alloy Classes. Titanium alloys have generally been classified as alpha alloys, alpha-beta alloys, and beta alloys. Alpha alloys have essentially an all-alpha microstructure. Beta alloys are those alloys from which a small volume of material can be quenched into ice water from above its beta transus without martensitic decomposition of the beta phase. Alpha-beta alloys contain a mixture of alpha and beta phases at room temperature. Within the alpha-beta class, an alloy that contains less than 2 to 3% beta, such as Ti-8Al-1Mo- 1V, may also be referred to as a near-alpha or super-alpha alloy. The principal alloying element in alpha alloys is aluminum (oxygen is the principal alloying element in commercially pure titanium), but certain alpha alloys and most commercially pure (unalloyed) titanium contain small amounts of beta-stabilizing elements. Similarly, beta alloys contain small amounts of alpha-stabilizing elements as strengtheners in addition to the beta stabilizers. The beta alloys can be further broken down into beta and near-beta. This distinction is necessary, because the phase transformations that occur, the reaction kinetics, and the processing could be different if the alloy is a near-beta (lean) alloy, such as Ti-10V-2Fe-3Al, or a rich beta alloy, such as Ti-13V-11Cr-3Al. Further information on the metallurgy, selection, processing, and application of titanium alloys is contained in Ref 3 and in Properties and Selection: Nonferrous Alloys and Special-Purpose Materials, Volume 2 of the ASM Handbook (see, for example, the articles Wrought Titanium and Titanium Alloys and Titanium and Titanium Alloy Castings ). Specimen Preparation Specimen preparation comprises many detailed steps. The first stages of sample preparation are equipment dependant, while the final polish step is driven by the needs of the investigator. Sufficient 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 purpose of the 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 lab where the facilities may not be as elaborate. Sectioning. Common methods for sectioning titanium metallographic samples include the band saw, abrasive cut-off wheel, and slowspeed wafering wheels. Band sawing titanium should be done with slow blade speed using a Fig. 5 Etched with Kroll s reagent for 45 s. Abusively polished example of a Ti-6Al-4V fastener resulting in a smeared and scratched surface. Excessive etching cannot correct poor specimen preparation. Note the severe distortion in microstructure and edge rounding. Fig. 6 A 200 cm (8 in.) wax wheel with relief grooves Fig. 7 Ti-6Al-4V alloy with Widmanstätten 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) four-step method for optimizing removal of deformed material, (b) four-step method for optimizing edge retention, or (c) three-step semiautomated method for optimizing preparation 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 between 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-filled hot-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 Fig. 8 Deformed grain structure from drilling insolution 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 lm. (b) Normal microstructure for comparison Table 1 Etchants for examination of titanium and titanium alloys Etchant Comments Etchant Comments Macroetchants 50 ml HCl, 50 ml H 2O General-purpose etch for b alloys Microetchants (continued) 10 ml HF, 10 ml HNO 3,30 ml lactic acid Chemical polish and etch for most alloys 30 ml HNO 3,3mLHF,67 ml H 2O (slow) to 10 ml HNO 3, 8 ml HF, 82 ml H 2O (fast) Used at room temperature to 55 C (130 F) for 3 5 min. Reveals grain size and surface defects 2 ml HF, 98 ml H 2O 98 ml saturated oxalic acid in H 2O, 2 ml HF Reveals case for most alloys Reveals case (interstitial contamination) for most 15 ml HNO 3,10mLHF,75 ml H 2O Two-stage etch(a) consisting of: (1) 8 ml HF, 10 ml HNO 3,82mLH 2O and (2) 18 g/l (2.4 oz/gal) of NH 4HF 2 (ammonium bifluoride) in H 2O Microetchants 1 3 ml HF, 10 ml HNO 3, 30 ml lactic acid 1 ml HF, 30 ml HNO 3,30 ml lactic acid Kroll s reagent: 1 3 ml HF, 2 6 ml HNO 3,H 2Oto 1000 ml 10 ml HF, 5 ml HNO 3,85 ml H 2O 1mLHF,2mLHNO 3,50 ml H 2O 2,47mLH 2O Etch about 2 min. Reveals flow lines and defects Reveals and b segregation (aluminum segregation) 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 6 g NaOH, 60 ml H 2O, heat to 80 C (180 F), add 10 ml H 2O 2 2 ml HF, 98 ml H 2O, then 1mLHF,2mLHNO 3,97 ml H 2O 10 ml KOH (40%), 5 ml H 2O 2,20mLH 2O 18.5 g benzalkonium chloride, 33 ml ethanol, 40 ml glycerol, 25 ml HF 2mLHF,4mLHNO 3,94 ml H 2 50 ml 10% oxalic acid, 50 ml 0.5% HF with H 2O 10 s with Kroll s, then s with 50 ml 10% oxalic acid, 50 ml 0.5% HF with H 2O alloys Good -b contrast, general microstructures for most alloys General-purpose etch for near- alloys(b) Stains, transformed b General-purpose etch for Ti- Al-Zr and Ti-Si alloys Reveals microstructure in aged Ti-13V-11Cr-3Al Etch 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 C ( 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 C ( 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 machines work best at high pressure 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 lm 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 lm 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. 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 lm 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: A9lm diamond suspension with an alcoholbased lubricant on a woven non-nap cloth 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
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