Low-temperature processing of Ti-Ni shape memory alloy by E.C.A.P. in shells

Abstract

The possibility of Ti-50.1 at.% Ni SMA processing by the equal-channel angular pressing (ECAP) in core-shell mode at low deformation temperatures was studied in the present work. The ECAP was performed in steel and iron shells in a temperature range from room temperature to 350 °C. Ti-Ni samples with a diameter of 12 mm were successfully subjected to ECAP for one pass at 200 °C in iron shell with a diameter of 20 mm. Lowering of the deformation temperature both to the 100 °C and to the room temperature leads to the samples distraction during the first ECAP pass. Significant deformation hardening of the sample after one pass of the ECAP in core-shell mode at 200 °C is observed. Increase of the structure defectiveness confirmed by the noticeable broadening of forward (from 26 to 103 °C) and reverse (from 20 to 46 °C) martensitic transformations hysteresis. Values of hardness, dislocation yield stress, and ultimate tensile strength after one pass of the ECAP in core-shell mode at 200 °C equals respectively, 275 HV, 1045 and 1200 MPa, that is noticeably higher than the same properties after reference treatment (annealing at 750 °C, 30 min): 176 HV, 520 and 945 MPa.