Hard X-ray studies of stress-induced phase transformations of superelastic NiTi shape memory alloys under uniaxial load

Abstract

We examined the texture evolution in a superelastic Ni 50.7Ti 49.3 (numbers indicate at.%) alloy under applied uniaxial stress using high-energy synchrotron X-ray diffraction in transmission geometry. Texture information is identified from the intensity variations along Debye–Scherrer rings recorded on area detector diffraction images. The 1 1 0 A austenite plane normals are aligned in the rolling direction and 2 0 0 A is in the transverse direction. Due to the B2–B19′ lattice correspondence, the 1 1 0 A peak splits into four martensite peaks 0 2 0 M, 1 ¯ 1 1 M , 0 0 2 M and 1 1 1 M. The stress-induced martensite is strongly textured from twin variant selection in the stress field with 0 2 0 M aligned in the loading direction while the maxima corresponding to 1 ¯ 1 1 M , 0 0 2 M and 1 1 1 M are at 60°, 67° and 75° from the loading direction. (B19′ unit cell setting: a = 2.87 Å, b = 4.59 Å, c = 4.1 Å, γ = 96.2°). A comparison between the experimental and recalculated distribution densities for the polycrystalline NiTi shows a reasonable agreement. In addition, we compare our experimental results with a micromechanical model which is based on total energy minimization. In this case, we also observe an overall agreement.