Ti-Pt high-temperature shape memory alloy subjected to severe plastic deformationTuesday (15.05.2018) 19:51 - 19:54 Part of:
Bulk nanocrystalline shape memory alloys (SMA) can be processed using methods of severe plastic deformation (SPD). The small grain size as well as the high density of SPD-induced defects can significantly impact the phase stability and functional properties of SMA. In the present work, Ti-Pt high-temperature shape memory alloys (Ms =1283 K, Mf = 1279 K, As =1309 K, and Af = 1321 K) were subjected to SPD using high-pressure torsion (HPT). At room temperature, both XRD and synchrotron diffraction of the initial undeformed alloys showed orthorhombic B19 martensite. (111) type I twins were observed by transmission electron microscopy (TEM). TEM also showed a small volume fraction of a long-period stacking-ordered martensitic structure but no residual B2 austenite. HPT was carried out using discs of the initial material (diameter of 6 mm, thickness of about 0.6 mm) applying 5 and 80 rotations at a pressure of 6 GPa. After HPT, the specimens had a thickness of about 0.5 mm. TEM specimens were taken from samples with 80 rotations at a fixed value r = 2 mm (nominal strain of about 2000). During SPD a strong variation of the local degree of plastic strain might occur both along the radial direction as well as along a direction parallel to the surface normal of the HPT disc. Therefore, TEM specimens were taken from areas with different distances to the free surface of the HPT disc. In the case of the specimens taken near the surface, nanocrystals embedded in an amorphous matrix were observed. Most of the nanocrystals had a B2 austenitic lattice structure. Contrary to that, specimens taken at a distance of about 40 and 250 micrometer from the surface did not contain amorphous phase. A strongly fragmented structure of subgrains with a size up to about 200 nm was observed. Selected area electron diffraction (SAD) was carried out to analyse the lattice structure. Most of the grains contained severely deformed B19 martensite while some weak reflections observed by SAD and XRD indicate the presence of a minor volume fraction of a strain induced phase with a L10 lattice structure. Using synchrotron experiments some weak additional reflections were observed, indicating the occurrence of another strain induced phase. During heating recovery and grain growth occurred at temperatures far lower than the reverse martensite to austenite transformation Therefore, after HPT, the transformation temperatures agreed with those of the undeformed material.