Effects of orientation on the shape memory behavior of Ni₅₁Ti₄₉ single crystals

The orientation dependence of the shape memory responses of [001], [011] and [111]-oriented Ni₅₁Ti₄₉ single crystals and Ni_50.8Ti_49.2 polycrystals are systematically studied in compression. Thermal cycling under constant stress and superelasticity experiments were conducted on both homogenized and aged (1.5 h at 500 °C) samples to understand the effects of precipitation on their shape memory behavior. It has been found that recoverable strain, temperature and stress hysteresis, Classius-Clapeyron slopes, and critical stress for plastic deformation are highly orientation dependent. The recoverable strains were 3, 2.65 and 2.38% under 400 MPa in [001], [011] and [111] orientations, respectively. After aging, the recoverable strains were 3.19, 2.78 and 2.11% under 400 MPa in [001], [011] and [111] orientations, respectively. Above 400 MPa, irrecoverable strain increased considerably for [011] and [111]-oriented samples. In [001] orientation, very narrow temperature hysteresis of 10 °C, recoverable strain of 3.03% and total strain of 3.65% were observed under ultra high compressive stress level of 1500 MPa. Moreover, [001] orientation showed perfect superelasticity with total strain of 7% and a large superelastic window (∆T=140 °C). The transformation temperatures are lower in homogenized samples than the aged samples due to decreased Ni-concentration in matrix by the formation of Ni-rich precipitates.