Partitioning of Elastic, Transformation, and Plastic Strains Exhibited by Shape-Memory Nickel-Titanium through Modeling and Neutron Diffraction

Empirical investigations and first principles calculations performed in the years since shape memory alloy (SMA) model development efforts began have unveiled contradictions between the microstructural deformation mechanisms at play within these materials and the phenomenological appearance of SMA deformations, which are used to develop constitutive models. Thus, in this work theoretical calculations, numerical modeling, and neutron diffraction experiments were performed to elucidate relationships between phenomenological appearance and mechanistic activity of SMA deformations, in particular Nickel-Titanium. Numerical methods and improvements were derived to allow for robust finite element implementation of a phenomenological SMA constitutive model. New methodologies were also developed to verify and validate mechanistic SMA constitutive model predictions of microstructure evolution for the first time. In depth neutron diffraction empirical studies investigated in situ non-proportional compression as well as large-deformation uniaxial tension and compression of bulk martensitic NiTi. From these studies, insights were gained as to the partitioning of both macroscopic stresses and strains realized of elasticity, recoverable and deformation twinning, and slip within populations of orientation-specific martensite plates. The implications these empirical findings have toward both the models presented in this work as well as future development of SMA constitutive models are documented.