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

摘要

We examined the texture evolution in a superelastic Ni_(50.7)Ti_(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 110_A austenite plane normals are aligned in the rolling direction and 200_A is in the transverse direction. Due to the B2-B19' lattice correspondence, the 110_A peak splits into four martensite peaks 020_M, 111_M, 002_M and 111_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 111_M, 0 0 2_M and 111_M are at 60°, 67° and 75° from the loading direction. (B19' unit cell setting: a = 2.87 A, b = 4.59 A, c = 4.1 A, 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.