Abstract
The solutionized and quenched depleted U-6 wt% Nb (DU-6Nb) alloy adopts a metastable martensite phase known as α’’. Material in this condition exhibits the shape memory effect (SME), with deformation within the SME regime being accommodated by twinning and detwinning mechanisms. It is mechanistically not well understood how α’’ forms from the intermediate γ0 martensite phase that appears during the quench from the γ (bcc) phase. The fine microstructural scale of the α’’ phase has complicated efforts to characterize it using techniques such as EBSD. Direct experimental measurements of the single-crystal properties are also not feasible, although this knowledge would be valuable due to the elastic and plastic anisotropy of the α’’ phase. Four investigations, each encompassing a distinct technical topic, are presented in this dissertation.
To begin, a review and MATLAB implementation of several continuum-scale theories that are relevant to the prediction of twinned martensite microstructures are presented. These include a group-algebraic approach for generating the martensitic variants, the classical crystallographic model of twinning, the single- and double-shear phenomenological theories of martensite crystallography, and an Eshelby-inclusion-based approach. Applying these theories, it is shown that the γ → γ0 transformation at high Nb content is well described by the classical phenomenological theory. At low Nb content, the direct γ → α’/α’’ transformations do not agree with single-shear predictions, but promising qualitative agreement with experimental observations is achieved if the lattice distortion is accommodated through a combination of {021}α’/α’’ twinning and dislocation slip in γ. The two-step γ → γ0 → α’’ martensitic transformation that occurs in DU-6Nb is well described by a newly proposed, autocatalytic transformation model that accounts for the crystallography and morphology of the intermediate γ0 phase.
Technical challenges posed by the nanotwinned microstructure of martensitic DU-6Nb motivate the next two investigations. First, a study of the unknown, anisotropic elastic stiffness of the α’’ phase is presented. A hierarchical polycrystal model is developed to connect the single-crystal stiffness to in-situ diffraction data and macroscopic elastic moduli that can be measured from polycrystalline specimens. Inverse modeling yields a best-fit anisotropic stiffness tensor for α’’ that gives satisfying agreement with the experimental data. Second, a transmission Kikuchi diffraction (TKD) orientation mapping investigation is reported for the as-quenched α’’ martensite for the first time. TKD is shown to be compatible with the nanotwinned microstructure of the as-quenched phase, and a preliminary study of the twin-related, overlapped Kikuchi patterns is presented.
Finally, the predominantly twinning- and detwinning-accommodated deformation of the martensitic DU-6Nb shape memory alloy is simulated at room temperature using an elastoviscoplastic self-consistent model (EVPSC)-based approach. Ιn-situ diffraction data is employed to calibrate the model parameters and to evaluate the model’s suitability for capturing key features of this alloy’s deformation response. Good agreement with the experimental, macroscopic flow curves and texture evolution is achieved. While the large, heterogeneous lattice strain evolutions observed during in-situ straining experiments are not fully captured, the model does reproduce a rather complex constitutive response using a mechanistic description which is consistent with prior microscopy- and diffraction-based analyses.
