Mechanical and functional behavior of a Ni-rich Ni50.3Ti29.7Hf20Ni_{50.3}Ti_{29.7}Hf_{20} high temperature shape memory alloy

The mechanical and functional behaviors of a Ni-rich Ni50.3Ti29.7Hf20 high temperature shape memoryalloy were investigated through combined ex situ macroscopic experiments and in situ synchrotron X-raydiffraction. Isothermal tension and compression tests were conducted between room temperature and260 C, while isobaric thermomechanical cycling experiments were conducted at selected stresses up to700 MPa. Isothermal testing of the martensite phase revealed no plastic strain up to the test limit of1 GPa and near-perfect superelastic behavior up to 3% applied strain at temperatures above the austenitefinish. Excellent dimensional stability with greater than 2.5% actuation strain without accumulation ofnoticeable residual strains (at stresses less than or equal to 400 MPa) were observed during isobaricthermal cycling experiments. The absence of residual strain accumulation during thermomechanicalcycling was confirmed by the lattice strains, determined from X-ray spectra. Even in the untrainedcondition, the material exhibited little or no history or path dependence in behavior, consistent withmeasurements 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 memoryeffect (TWSME) was obtained, with a maximum of 0.34% two-way shape memory strain after thermomechanicalcycling under 700 MPa