Neutron diffraction measurement of stress-induced transformation in superelastic NiTi

The formation of stress-induced martensite in superelastic NiTi was studied by neutron diffraction during uniaxial compressive loading and unloading. The respective phase fractions were determined as a function of the applied stresses using a Rietveld refinement with a March-Dollase texture formulation. Before loading, the specimen was fully austenitic. At the highest applied stress of -625 MPa, about 90% of the austenitic phase had transformed to martensite, with a concomitant macroscopic strain of -2.8%. Upon unloading, all of the stress-induced martensite reverted to austenite and the totality of the macroscopic strain was recovered. The propensity for various austenitic crystallographic orientations to transform at different stresses was determined and qualitative observation of this incipient texture in the austenite and of the inherent texture in the nascent martensite are reported. (C) 1996 American Institute of Physics.