Mechanical and shape memory properties of high temperature Ni-Ti-Hf-Nb alloys

In the scope of developing shape memory alloys for high temperature applications, the Ni-Ti-Hf-Nb system emerged as particular promising due to the superior mechanical properties of its alloys compared to those of the ternary Ni-Ti-Hf system. In particular, the ductility of Ni-Ti-Hf increases significantly by adding Nb due to the formation of a soft eutectic network composed of shape memory matrix phase and a not transforming β-Nb phase at the grain boundaries. Recently, it was demonstrated that by this alloying approach it is possible to obtain small diameter wires using conventional drawing techniques, augmenting the possibilities of a successful industrial commercialization. In addition, it was validated by thermomechanical methods, that the transformation characteristics do not suffer from processing these alloys and remain high and stable after wire-drawing. Despite the encouraging results, the effect of varying Nb content and microstructure morphology on mechanical properties was not studied well yet. This work aims on characterizing the mechanical and shape memory properties of Ni-Ti-Hf-Nb alloys of different compositions by tensile, compression and wire actuation tests. It will be shown by microscopy and fracture analysis how different microstructural morphologies affect the tensile test performance. Compression test on cylindrical samples and wire tensile tests are applied for revealing the shape memory characteristics of the system. Differences in actuation of Hf and Nb containing alloys compared to binary Ni-Ti and ternary Ni-Ti-Hf will be highlighted and discussed in the light of how to exploit the full capacities of the Ni-Ti-Hf-Nb system.