Palladium-free multicomponent high temperature shape memory alloys

Abstract

One of the challenges is the design of the relatively cheap Pd-free multicomponent high-temperature shape memory alloys. This paper presents the route for solving this problem by an increasing the Hf concentration in Ti–Hf–Zr–Ni–Cu–Co alloys. It was found that an increase in the Hf concentration from 23 to 32 at.% increased the transformation temperatures: A_f changed from 90 °C to 220 °C, M_s varied from 35 °C to 172 °C, while the hysteresis was constant and equal to 45–48 °C. Further increase in the Hf concentration increased the A_f temperature by 20 °C but hardly affected the M_s temperatures. Recoverable strain variation was studied on cooling under stress (in torsion mode) and heating without stress. Strain jumps were observed on γ(T) curves which were attributed to a large martensite plate appeared on cooling or disappeared on heating within large grain, which was accompanied by the abrupt strain variation. An increase in the Hf concentration decreased recoverable strain, as well as the strain up to failure. This was due to an increase in the size and volume of the brittle [Ti]₂[Ni] and [Hf]₂[Ni] precipitates. A maximum recoverable strain of 5 % was found in the alloy with 23 at.% of Hf, whereas the recoverable strain did not exceed 1 % in the alloys with 32 and 38 at.% of Hf.