Large Inelastic Strain Deformation and Fracture of Hexagonal Close Packed Crystalline Alloys.

ZIAEI, SHOAYB. Large Inelastic Strain Deformation and Fracture of Hexagonal Close Packed Crystalline Alloys. (Under the direction of Professor Mohammed A. Zikry). A dislocation-density based multiple slip crystalline plasticity formulation, a nonlinear finite-element approach, and a microstructurally-based fracture methodology were developed and used to investigate behavior in hexagonal close packed (h.c.p.) with a focus on zircaloy-2 polycrystalline aggregates. The validated predictive framework can account for the interrelated effects of dislocation-density interactions, generation, and recovery. An energy criterion was formulated and used to identify 63 unique slip system interactions that can result in either junction formation or slip-system annihilation. These dislocation-density interactions, with the interrelated mechanisms due to recovery and generation, were then used to understand and predict why basal planes are strengthening planes and prismatic planes are the dominant toughening planes. Coherent interfaces, between face-centered cubic (f.c.c.) and h.c. p. crystalline systems, and semi-coherent interfaces, between body-centered cubic (b.c.c.) and h.c. p. systems, were developed to understand how large strain deformation and fracture modes in h.c. p. zircaloy aggregates with f.c. c. and b.c.c. hydrides evolve at different scales. Thirty-six unique transformations were derived for both coherent interfaces between f.c. c. and h.c. p. systems, and semi-coherent interfaces between b.c.c. and h.c. p. systems. For the semicoherent interfaces, misfit strains were determined at the interfaces in terms of the deformation-rate tensor, then used these ORs and initial misfit strains with the dislocationdensity crystalline plasticity formulation, the non-linear finite-element and the fracture approach to account for crack nucleation and propagation, and how inelastic slip evolution, stress accumulation, lattice rotation, and adiabatic heating affected nucleation and crack path orientations. The predictions indicate that the physical representation of ORs affect local deformation and fracture behavior due to the interfacial incompatibilities between the matrix and the hydride precipitates. A physically representative framework for twin modes in h.c. p. materials was also formulated to understand and predict the fundamental mechanisms that affect large inelastic strain behavior and fracture. Two twin modes of ( 1121 [1126] and (0001) [1120]) in zircaloy-2, with each mode having 24 unique twin slip systems were incorporated with the dislocation-density crystalline plasticity, the nonlinear finite-element, and the fracture framework to account for crack nucleation and propagation. The predictions indicate that twin modes significantly affect local deformation and fracture behavior, and are, therefore, essential for the accurate representation of behavior at different physical scales in heterogeneous crystalline h.c. p. systems. The validated predictions from this investigation underscore that accurate and physically-based representations, at different physical scales, are needed to account for the collective effects of dislocation-density evolution and interactions on different crystalline systems, twin grains, hydrides, and ORs, such that heterogeneous behavior can be understood, identified, and potentially controlled for failure resistant systems comprised of myriad microstructural mechanisms and crystalline structures. © Copyright 2016 by Shoayb Ziaei All Rights Reserved Large Inelastic Strain Deformation and Fracture of Hexagonal Close Packed Crystalline Alloys