Titanium Specimen Preparation & Testing

Figure 8.1. Alpha at the surface of heat (1038°C, water quench) Ti - 3% Cr alloy after tint etching with Beraha's regeant (polarized light, 500X).
Pre titanium is soft and ductile, but is very easily damaged by twinning in sectioning and grinding. Preparation of commercially pure titanium, which is a popular grade, is very difficult, while preparation of the alloys is somewhat easier.

Some authors have stated that titanium alloys should not be mounted in phenolic resins as the alloys can absorb hydrogen from the resin. Further, it is possible that the heat from mounting could cause hydrides to go into solution. This is also possible with castable resins if the exothermic reaction of polymerization generates excessive heat. If the hydride phase content is a subject of interest, then the specimens must be mounted in a castable resin with a very low exotherm (long curing times favor lower heat generation, and vice versa).
Titanium is very difficult to section and has low grinding and polishing rates. The following practice for titanium and its alloys demonstrates the use of an attack polishing agent added to the final polishing abrasive to obtain the best results, especially for commercially pure titanium, a rather difficult metal to prepare free of deformation for color etching, heat tinting and/or polarized light examination of the grain structure.

Attack polishing solutions added to the abrasive slurry or suspension must be treated with great care to avoid burns. Use good, safe laboratory practices and it is advisable to wear protective gloves. This three step practice could be modified to four steps by adding a 3 or 1µm diamond step, but this is usually unnecessary, see Table 7.1 at the end of this article and Figures 8.1 – 8.1 for expected results.

A number of attack polishing agents have been used. The simplest is a mixture of 10mℓ hydrogen peroxide (30% concentration – avoid skin contact) and 50mℓ colloidal silica. Some metallographers add either a small amount of Kroll’s reagent to this mixture, or a few mℓ of nitric and hydrofluoric acids (avoid contact). These latter additions may cause the suspension to gel. In general, these acid additions do little to improve the action of the hydrogen peroxide (the safer 3% concentration is not effective). Polarized light response of CP titanium can be improved by following this procedure with a brief vibratory polish using colloidal silica.
Table 7.1: 3-Step Methods for Ti Aoolys
Sectioning Abrasive Cutter with a wheel recommended for use on ductile materials
Mounting Compression, typically with EpoMet
Surface Abrasive / Size Load - lbs [N]
Specimen
Base Speed [rpm] Relative Rotation Time [min:sec]
CarbiMet 320[P400] grit Sic
water cooled
6[127] 300 Complimentary Rotation Until Plane
UltraPad 9µm MetaDi Supreme
Diamond*
6[27] 150 Complimentary Rotation 10:00
ChemoMet 0.02 - 0.06µm MasterMet Colloidal Silica** 5[22] 150 Complimentary Rotation 10:00
Platen = Platen Specimen Holder = Specimen Holder * Plus MetaDi Fluid Extender as desired
** Attack polish may bne used, 1 part Ammonium Persulfate solutions (10g Ammonium Persulfate per 100ml distilled water) or 30% Hydrogen Peroxide ro 5 parts silica.
Image & Analysis Grain Size, Measurement & Analysis Applications
Hardness Testing Vickers
You can find more information on metallographic specimen preparation for titanium and other metals in the Buehler SumMet Guide.