Phase fraction, texture and strain evolution in superelastic NiTi and NiTi-TiC composites investigated by neutron diffraction

Samples of superelastic NiTi and superelastic NiTi reinforced with 10 vol.% TiC particles were deformed under uniaxial compression to 975 MPa while ne utron diffraction spectra were simultaneously collected. Despite the presen ce of stiff TiC particles, a macroscopic strain of 3% was obtained in the c omposite on loading and was fully recovered on unloading. The diffraction s pectra were analyzed by Rietveld refinement that included a spherical harmo nic description of the texture and a lattice plane (hkl) dependent formulat ion of the elastic strain. The experiments provided bulk, phase-specific me asurements of the evolution of phase fractions, texture and strains during the reversible stress-induced austenite to martensite transformation respon sible for the large recoverable strains. For the composite, Eshelby elastic theory is used to predict the discrete phase strains measured by neutron d iffraction. The observed behavior suggests that the martensite accommodates the mismatch with the transforming austenite (while they co-exist) and the TIC particles (in the case of the composite). (C) 1999 Acta Metallurgica I nc. Published by Elsevier Science Ltd. All rights reserved.