Effect of Temperature On the Deformation Behavior of B2 Austenite in a Polycrystalline Ni49.9Ti50.1 (at.%) Shape Memory Alloy

Polycrystalline Ni_49.9Ti_50.1 (at.%) shape memory alloy was tension-tested to 18% strain at different temperatures ranging from (165-440) ºC, which is above the stress-free A_f of 105 °C. This temperature dependent deformation of the B2 austenite phase was studied experimentally via ex-situ mechanical testing, in-situ time-of-flight (TOF) neutron diffraction experiments, and hot stage transmission electron microscopy (TEM). At selected temperatures, neutron diffraction experiments were used to follow the evolution of lattice strains, texture, and martensite/austenite phase fractions at different stages of deformation, and TEM was used to determine the deformation modes. It was found that stress-induced martensite formed concurrently with slip and deformation twinning but only at temperatures up to 310 ºC. Above this temperature, no stress induced martensite was observed. At each temperature, (hkl) lattice strains were compared to macro strains and the texture evolution was followed by plotting inverse pole figures from B2 lattice reflections perpendicular to the loading axis at selected strain values. The occurrence of {114}_B2 compound deformation twins confirmed by TEM, along with <001> slip processes were necessary to explain the observed changes in texture during deformation of the B2 phase as predicted by the self-consistent polycrystalline model.