Titanium and its alloys have become quite important commercially over the past 50 years due to their low density, good strength-to-weight ratio, excellent corrosion resistance and good mechanical properties. On the negative side, the alloys are expensive to produce.
Titanium, like iron, is allotropic and this produces many heat treatment similarities with steels. Moreover, the influences of alloying elements are assessed in like manner regarding their ability to stabilize the low temperature phase, alpha, or the high temperature phase, beta. Like steels, Ti and its alloys are generally characterized by their stable room temperature phases - alpha alloys, alpha-beta alloys and beta alloys, but with two additional categories: near alpha and near beta.
Titanium and its alloys are more difficult to prepare for metallographic examination than steels. As for all refractory metals, titanium and its alloys have much lower grinding and polishing rates than steels. Deformation twinning can be induced in alpha alloys by overly aggressive sectioning and grinding procedures. It is safest to mount relatively pure Ti specimens, especially those from service in a hydrogen-containing environment, in castable ("cold") resins rather than using hot compression mounting due to the potential for altering the hydride content and morphology. However, these resins must be used in such a way as to minimize the heat of polymerization. Elimination of smearing and scratches during polishing can be difficult.
Early mechanical preparation techniques [1-5] tended to be rather long, involving procedures nearly always incorporating an attack polishing solution in the last step or last two steps. Some of the more commonly used attack polishing solutions are summarized in . The problems associated with obtaining well-prepared surfaces have prompted considerable interest in electropolishing procedures [3-5, 7, 8]. The inherent danger of some of these electrolytes has prompted interest in chemical polishing procedures . Electrolytic and chemical polishing solutions for Ti and Ti alloys are also summarized in . Mechanical polishing methods for titanium and its alloys continued to rely upon these older procedures into the 1970s  and 1980s . Perhaps the first publication of a modern approach for preparing titanium was that of Springer and Ahmed  in 1984. This was a three-step procedure, assuming that the planar grinding step can be performed with 320-grit SiC paper, which may not always be possible. If the specimens are sectioned using a wafering blade or an abrasive blade of the proper bond strength, which produce a smooth surface with minimal damage, then 320-grit SiC paper may be used. If a rougher surface with greater damage was produced, such as would result from use of a power hacksaw, then grinding must commence with a coarser grit paper in order to remove the damage in a reasonable time. Grinding and polishing rates of Ti are much lower than for many other metals and alloys.
Specimen PreparationAlthough Ti and its alloys can be readily sectioned using band saws, power hacksaws and similar machine-shop tools, these devices produced a great deal of damage. Figure 1 demonstrates the substantial depth of damage that can be produced when sectioning commercial purity (CP) titanium. If the left edge was chosen for the plane-of-polish, then at least 200µm must be ground away to get through the sectioning damage. This damage will be difficult to remove in rough grinding, as the grinding rate is very low. Consequently, to obtain perfect surfaces, section Ti and its alloys with only laboratory abrasive saws or precision saws using blades designed for metallography (avoid using blades made for production machining).
Strictly speaking, any mounting compound can be used for Ti and its alloys. If specimens of Ti used in applications where hydrogen can be picked up are to be mounted, it is best to use a low-viscosity epoxy resin and a conductive mounting approach to minimize the exotherm during polymerization. If the heat involved in polymerization is substantial, titanium hydrides could be dissolved. Specimens never placed in service are unlikely to contain hydrides, and more freedom of choice in mounting is pos-sible. To minimize the heat of polymerization, wrap aluminum foil, as used in cooking, around a block of steel or copper (a heat sink). Then, glue a phenolic ring form (a cylinder) to the foil to create a mold. Place the specimen inside the ring form and add the epoxy. If a low-viscosity epoxy resin is used, which cures slowly, the exotherm during curing will be <10°C above room tem-perature. If a plastic or silicone rubber mold is used with the same epoxy, the exotherm will be higher. The faster the epoxy cures, the higher the exotherm. Acrylic resins cure in less than 10 minutes and the exotherm is very high - high enough to burn your fingers if you touch the mold while it is curing. That is not "cold" mounting! Mounting of your specimens facilitates specimen identification, simplifies automation and yields far better edge retention than unmounted specimens. But choose a resin that does not produce shrinkage gaps.
b) ID of tube mounted in fast-curing EpoKwick resin with a polymeric mold.
c) ID of a tube mounted in a press at 150°C using EpoMet thermosetting resin.
For step 3, I use a psa-backed, napped MicroCloth® pad (synthetic suede) with the same load, rpm, time and rotation direction using MasterMet® colloidal silica as the abrasive. Mix 5 parts colloidal silica with 1 part hydrogen peroxide (30% concentration - avoid skin contact) as the attack-polish agent. Contra rotation works best when the head speed is under 100 rpm. The machines used for these experiments have a 60-rpm head speed. This helps to keep the abrasives on the cloth surface. If the head and platen both rotate in the same direction (called "complementary" rotation), centrifugal force throws the abrasive and the lubricant off the surface almost as fast as you add it. A rule for polishing is: keep the polishing surfaces uniformly covered with abrasive and lubricant to minimize smearing, pullout and deformation. After step 3, clean the specimen and holder. I stop adding any abrasive at least 20 seconds before the 10 minute polishing cycle ends. With 10 seconds remaining, direct the water jet onto the polishing cloth to clean both the cloth and the specimens. Colloidal silica is more difficult to remove from specimens than other abrasives. Table 1 summarizes the three-step preparation method.
Examination of CP Ti is actually more effective with polarized light in the as-polished condition, when using a properly prepared specimen, than with bright field illumination after etching. Figure 4 shows the microstructure of CP Ti in bright field after etching with Kroll's reagent. The grain structure is reasonably well delineated, but details are not as good as using polarized light on an as-polished specimen. Color etching with a modification of Weck's reagent also produces better grain structure development than Kroll's reagent (Fig. 5). Weck's reagent for Ti contains: 100mL water, 50mL ethanol and 2g NH4F·HF. This composition will produce white "butterfly-shaped" artifacts in the color image, which can be eliminated using only 25mL ethanol. Etch by immersion until the surface is colored, usually about 15-25 seconds. Coloration is enhanced with examination using polarized light and a sensitive tint filter. It is often helpful to move slightly off the crossed position.
A few variants of the attack polishing solution have been tried. Leonhardt  uses a mixture of 150mL colloidal silica, 150mL water, 30mL H2O2 (30%), 1-5mL HF and 1-5mL HNO3. Results with this attack polishing additive to the abrasive were equiva-lent to the one used. Buchheit  added 5mL of a 20% aqueous CrO3 solution to 30mL of an alumina slurry. To try this, but us-ing colloidal silica instead, 10mL of the 20% CrO3 solution was added to 75mL of colloidal silica. This also produced excellent results. When using these attack polishing solutions, care must be taken in handling, mixing and using these additives as they contain very strong oxidizers and acids. Avoid physical contact with the ingredients and the prepared attack polishing abrasives.
MicrostructuresQuality control laboratories frequently check lots of titanium for the presence of an alpha case at the surface due to oxygen pick-up during heat treatment. Oxygen is an alpha stabilizer and the case is detrimental to machining, mechanical properties and service life. Good edge retention is important for this work and mounting is necessary. Edge retention is highly dependent upon elimination of shrinkage gaps between the specimen and the mount. EpoMet resin gives superb results but requires a mounting press. Of the cast resins, epoxy works best. The three-step method, despite step 3 being 10 minutes on a napped cloth, gives perfect results using either EpoMet resin or an epoxy resin. The specimens are perfectly flat coming into step 3. As long as the pressure is kept at 6 lbs, and not lower, flatness is not impaired. Figure 6 illustrates alpha case in an experimental Ti alloy prepared using the three-step method.
Alpha-beta alloys respond perfectly to the three-step method, as they are easier to prepare than the alpha alloys. Figure 7 shows the microstructure of an alpha-beta alloy, Ti6242, after alpha-beta forging and alpha-beta annealing compared to the same alloy after beta forging and beta annealing. The beta transus temperature for this alloy is 995°C ± 15°C. Forging and annealing below the beta transus results in a fine grained alpha-beta microstructure (primary alpha and transformed beta) while forging and annealing above the beta transus results in a coarse grained basket weave alpha-beta microstructure.
Beta alloys can also be prepared easily with the three-step method. Figure 10 illustrates the microstructure of two beta alloys, Ti-5333 and Beta C.