Palladium-free multicomponent high temperature shape memory alloys

One of the challenges is the design of the relatively cheap Pd-free multicomponent hightemperature 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: Af changed from 90 degrees C to 220 degrees C, Ms varied from 35 degrees C to 172 degrees C, while the hysteresis was constant and equal to 45-48 degrees C. Further increase in the Hf concentration increased the Af temperature by 20 degrees C but hardly affected the Ms temperatures. Recoverable strain variation was studied on cooling under stress (in torsion mode) and heating without stress. Strain jumps were observed on gamma(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.