Hard X-ray studies of stress-induced phase transformations of superelastic NiTi shape memory alloys under uniaxial load
Materials Science and Engineering a-Structural Materials Properties Microstructure and Processing 481: 414-419
We examined the texture evolution in a superelastic Ni50.7Ti49.3 (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 132-B19' lattice correspondence, the 110(A) peak splits into four martensite peaks 020(M), (1) over bar 11(M), 002(M) and 111(M). The stress-induced martensite is strongly textured from twin variant selection in the stress field with 020(M) aligned in the loading direction while the maxima corresponding to (1) over bar 11(M), 0 0 2m and 111(M) are at 60 degrees, 67 degrees and 75 degrees from the loading direction. (B19' unit cell setting: a = 2.87 angstrom, b = 4.59 angstrom, c = 4.1 angstrom, gamma = 96.2 degrees). 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. (C) 2007 Published by Elsevier B.V.