Mechanical and functional behavior of a Ni-rich Ni50.3Ti29.7Hf20 high temperature shape memory alloy

Abstract

The mechanical and functional behaviors of a Ni-rich Ni50.3Ti29.7Hf20 high temperature shape memory alloy were investigated through combined ex situ macroscopic experiments and in situ synchrotron X-ray diffraction. Isothermal tension and compression tests were conducted between room temperature and 260 °C, while isobaric thermomechanical cycling experiments were conducted at selected stresses up to 700 MPa. Isothermal testing of the martensite phase revealed no plastic strain up to the test limit of 1 GPa and near-perfect superelastic behavior up to 3% applied strain at temperatures above the austenite finish. Excellent dimensional stability with greater than 2.5% actuation strain without accumulation of noticeable residual strains (at stresses less than or equal to −400 MPa) were observed during isobaric thermal cycling experiments. The absence of residual strain accumulation during thermomechanical cycling was confirmed by the lattice strains, determined from X-ray spectra. Even in the untrained condition, the material exhibited little or no history or path dependence in behavior, consistent with measurements of the bulk texture after thermomechanical cycling using synchrotron X-ray diffraction. Post deformation cycling revealed the limited conditions under which a slight two-way shape memory effect (TWSME) was obtained, with a maximum of 0.34% two-way shape memory strain after thermomechanical cycling under −700 MPa.