Materials Characterization & Testing: Microstructure Of Titanium And Its Alloys
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.
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).
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.
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.
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.