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In-situ diffraction studies of rate dependence of twinning, phase transformation, and slip in shape memory alloys
Paradise, Paul
Paradise, Paul
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2015
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Abstract
Many advanced materials show strong rate-dependence in their responses, even at time scales considered to be quasi-static. This is especially true for shape memory alloys. While theory is well established for rate dependence in cubic metals due to slip, few attempts have been made to develop similar theories for twinning and phase transformation, and their coupling with slip in low symmetry structures. This absence is largely because little is known about rate dependence in lower symmetry alloys. Toward filling this gap, classical phenomenological formulations of strain-rate sensitivity developed for slip of cubic alloys were applied to new shape memory alloy data to understand if analogous mathematical models could be extended to the more complex inelastic deformations. It was found that the models do not describe relaxation phenomena of shape memory alloys. Thus high-energy X-ray diffraction was employed to elucidate microstructural roles of twinning, phase transformation, and slip during crosshead arrest stress relaxation tests. This thesis documents these empirical results toward first understanding low symmetry twinning in isolation, then competition between twinning and phase transformation, and then the coupling of plasticity with twinning and phase transformation exhibited by monoclinic and phase transforming (cubic to monoclinic) materials during stress-relaxation events of quasi-static loading regimes. These data provide fundamental insights that may be used to develop a new constitutive model for rate dependence in shape memory alloys.
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