Analysis of neutron diffraction spectra acquired in situ during stress-induced transformations in superelastic NiTi

Neutron diffraction spectra were obtained during various stages of a revers ible stress-induced austenite to martensite phase transformation in superel astic NiTi. This was accomplished by neutron diffraction measurements on bu lk polycrystalline NiTi samples simultaneously subjected to mechanical load ing. Analysis of the data was carried out using individual lattice plane (h kl) reflections as well as by Rietveld refinement. In the Rietveld procedur e, strains in austenite were described in terms of an isotropic (hkl indepe ndent) and an anisotropic (hkl dependent) component. At higher stresses, au stenite lattice plane reflections exhibited nonlinear and dissimilar elasti c responses which may be attributed to the transformation. The texture evol ution is significant in both austenite and martensite phases during the tra nsformation and two approaches were used to describe this evolving texture, i.e., an ellipsoidal model due to March-Dollase and a generalized spherica l-harmonic approach. The respective predictions of the phase fraction evolu tion as a function of applied stress were compared. A methodology is thus e stablished to quantify the discrete phase strains, phase volume fractions, and texture during such transformations. (C) 1999 American Institute of Phy sics. [S0021-8979(99)09118-5].