Mechanical Properties of Superelastic and Shape - Memory NiTi and NiTi - TiC Composites

The objective of this work was to study materials subjected to external loading where alternative deformation mechanisms are available to generate strains. In the case of shapememory and superelastic NiTi, these mechanisms are twinning and stress-induced phase transformation, respectively. Superelastic NiTi (51.0 at.% Ni) reinforced with 0, 10 and 20 vol.% TiC particles was fabricated by Hot Isostatic Pressing (HIP), heat-treated and deformed under uniaxial compression while neutron diffraction spectra were collected. The experiments yielded in-situ measurements of the reversible austenite to martensite stress-induced transformation in NiTi. A methodology is established to ascertain the evolving discrete phase strains, austenite/martensite volume fractions and texture during stress-induced transformations using Rietveld refinement. Phase fractions and strains are discussed using Eshelby's theory in terms of load transfer in composites where the matrix undergoes a stress-induced phase transformation. Evolution of texture distributions in austenite and martensite are examined as a function of stress and strain by Rietveld refinements using a spherical harmonic texture formulation. The following phenomenological changes are noted from in situ neutron diffraction measurements on superelastic NiTi subjected to stress-cycling : (i) the volume fraction of martensite formed remains almost unchanged (ii) the average phase strain in austenite remains mostly unchanged at intermediate loads but changes for unloaded austenite (iii) the texture in martensite and austenite under load changes significantly resulting in changes in macroscopic stress-strain behavior (iv) the isotropic and anisotropic components of the strain in austenite redistribute themselves. While the mechanical characteristics of stress-cycling have been previously studied, the relevance of this work stems from the fact that such fundamentally significant phenomenological changes are reported for the first time. Mention is made of the work initiated to study the fatigue behavior of shape-memory NiTi and NiTi-TiC composites. The materials were fabricated using HIP and subsequently characterized. Fatigue crack growth experiments were carried out at room temperature and the results reported. A pre-cracked, compact test, shape-memory NiTi specimen was subjected to neutron diffraction measurements under various loaded and unloaded conditions. Suggestions for related future projects are made. Thesis Supervisor: Prof. David C. Dunand Title: AMAX Associate Professor of Materials Engineering