Both titanium and its alloys have a high strength to density ratio as well as good resistance to corrosion. This makes them invaluable in defense, aerospace, and marine applications. It is also useful in biomedical applications due to its good biocompatibility. Although it is as strong as some steels, it is a fraction of the density of steel.
It quickly becomes apparent, when preparing metallographic samples, that titanium is more difficult to prepare than steel. This is because it is ductile and readily damaged, and, also, has a relatively slow material removal or recovery rate, which is challenging for sample preparation.
Figure 1. Tarnishing of a titanium sample due to incorrect selection of the cutting disc and insufficient cooling during the cutting process. Image Credit: Buehler
As titanium is fairly tough, sectioning can be difficult. Grade 2 is particularly difficult because it is easier to deform. Commercially pure titanium is ductile and softer. If great care is not taken, sectioning can cause twinning and thermal damage. As a result, it is important to be careful during the cutting process.
Cooling and proper selection of the blade is important. Abrasive blades that have bonding strengths that are optimized for titanium and other refractory metals are preferable. Usually, the binder of these blades is designed to break down at the right moment, releasing dull, used abrasives, and exposing sharp, fresh abrasive particles.
This enables abrasive blades to maintain optimum cutting efficiency and prevents blades from plowing into the sample with dull abrasive. Thermal damage can be prevented using a cutter with a robust cooling system. Conservative loads and feed rates should be used as an extra precaution.
Figure 2. As-polished cross section showing damage induced in titanium from improper cutting technique (cross-polarized light). Image Credit: Buehler
Generally, selecting a chemically resistive mounting media with little or no shrinkage makes the preparation of titanium and its alloys easier. Epoxies are best, whether compression or castable mounting methods are used. A hard, mineral-filled, epoxy like EpoMet should be considered when using compression mount. They can resist chemical attack from etchants and tend to have little to no shrinkage.
It is best to mount specimens in a low exotherm castable epoxy like EpoxiCure 2 if the hydride content needs to be measured. These castable epoxies adhere well to specimen edges, are chemically resistive, and have low shrinkage, which provides great edge retention properties.
It is important that specimens are dry and clean before mounting. If they are not, this can cause shrinkage gaps between the mounting media and the specimen. Shrinkage gaps are sites that can collect and disperse contaminates during polishing and grinding. It is time-consuming to remove contamination scratches from titanium.
Grinding and Polishing
Titanium does not abrade particularly easily. Across all stages of preparation, material removal rates are low. Titanium, in particular, grade 2, can be easily deformed during grinding.
Using aggressive grinding to shorten preparation time can cause twinning, smear, and gross deformation. Often, initial grinding steps use silicon carbide (SiC) discs with moderate pressure.
SiC is relatively hard and contains sharp abrasive particles that cleave while grinding, creating new cutting edges as the disc is used. This means that the initial material removal rate is relatively high.
Each disc should only be used for 1-2 minutes. Grinding with SiC discs that are degraded may cause excessive damage. 320-Grit SiC can be used for the initial grinding step if it is properly sectioned.
Titanium is prone to local overheating because of its thermal conductivity (local structural change). Therefore, it is also important to make sure that there is sufficient water cooling during the grinding process.
9-micron polycrystalline diamond suspension should be used on a hard woven cloth after the initial grinding step to recover grinding damage. Typically, hard-woven cloths like UltraPad have higher material removal rates and help maintain flatness.
Polycrystalline diamonds, such as the ones used in MetaDi Supreme, have an increased number of cutting edges. This enables the diamonds to remove damage more quickly, and with minimal subsurface deformation, leaving a better surface finish.
For lots of titanium alloys, like Ti-6AI-4V, a final polish can be completed after the 9-micron stage. For most titanium alloys, this 3-step method is very effective.
Alloys that are more sensitive, such as commercial purity titanium and Grade 2, may require an added 3-micron stage. This 4-step process increases the clarity of grain orientation of titanium alloys that are more sensitive.
As with the other steps in titanium preparation, the final polishing stage takes more time than with most other materials. This is due to the low material removal rate. Buehler recommends polishing using MasterMet colloidal silica mixed with 10% (NH4)2S2O8 solution on a chemically resistant, no-nap polishing pad, for example, ChemoMet. Best results can be achieved by mixing 5 parts Colloidal Silica to 1 part (NH4)2S2O8 solution.
As is the case with many non-cubic materials, microstructural details in titanium can be seen using cross-polarized light on a light optical microscope, without etching. This is a rapid and easy assessment of the polish quality.
Low surface deformation is indicated by a good response to cross-polarized light (sharper detail, more contrast). Higher surface deformation and poor sample preparation are indicated by a bad response to cross-polarized light (lack of sharp detail and poor contrast).
Titanium can be contrasted well with standard etchants like Kroll’s etchant to illustrate the grain structure (Figure 4). It is recommended to let the sample rest for at least half an hour in the air if relatively long polishing times occur in the final polishing.
The material reacts less intensively with the etchant because of the semi-reconstructed passivation layer. Without the passivation layer, the samples are usually over-etched quite rapidly (t ≤ 1 sec).
Figure 3. Contrasting of the titanium grade 2 sample with Kroll’s etchant. Image Credit: Buehler
Table 1. 4-step method for polishing Titanium. Step 3 can be omitted for many alloys. Source: Buehler
||320 Grit SiC
||9 µm Metadi
>> Comp, >< Contra
• Rinse platen last 15-20 seconds
*(optional, for difficult materials only)
** Attack polish For best results mix 5 parts Colloidal Silica to 1 part (NH4)2S2O8 solution.
This information has been sourced, reviewed and adapted from materials provided by Buehler.
For more information on this source, please visit Buehler.