| The major objective of this work has been to develop, within a continuum framework, a microstructurally-based computational approach to investigate dynamic shear strain localization and failure in alloys with heterogeneous microstructures, spanning length scales from the nano to the micron, with specific application to high strength and damage tolerant Al-Cu-Mg-Ag alloys.;Dominant secondary phases, such as dispersoids and precipitates, were identified, and their physical scales and crystallographic orientations were determined. A new crystallographic orientation scheme based on the rational orientation of O and theta' precipitates, was formulated. High strain-rate microstructurally-based finite-element analyses were undertaken, which incorporated a rate-dependent dislocation density-based crystalline plasticity framework that explicitly accounts for the behavior of precipitates, dispersoids, and grain-sizes. These analyses delineated how the nano-sized O and theta' precipitates and the micron-sized Mn-bearing dispersoids affect dynamic shear-strain localization and failure in crystalline materials. The computational predictions, which are consistent with experimental observations, indicate that the O and theta' precipitates result in both the homogenous strengthening and toughening of the alloy. This homogenous deformation inhibits the formation of localized shear bands, as compared with an alloy that would have only Mn-bearing dispersoids. The computational predictions also indicate that the experimentally observed step-ladder deformation of the O precipitates could be the direct consequence of a preferred propagation of dislocation densities along the Al/O interface, resulting in the uniform deformation of the precipitate along its entire length, instead of localizing within a segment of the precipitate, and this collective behavior enhances the homogenous deformation and toughening of the aluminum alloy.;To model the nucleation and propagation of failure at the microstructural scale, under large deformations and dynamic loading conditions, the general finite-deformation theory, related to the decomposition of the deformation gradient, can be tailored to account for displacement incompatibilities and fracture in crystalline solids. Based on this proposed decomposition, a general fracture criterion for finitely deforming crystals, using the integral law of incompatibility, is developed. With this new criterion, computational analyses of single crystals, polycrystals, and microstructural constituents, such as precipitates and dispersoids, were conducted to elucidate the mechanisms of dynamic crack nucleation and growth in Al-Cu-Mg-Ag alloys. The analyses indicate that the newly proposed fracture criterion accurately predicts ductile and brittle failure modes, and intergranular and transgranular fracture modes in polycrystals. It also accounts for the microstructural effects due to dispersoids and precipitates, and their roles in crack branching and arrest. Coarse precipitates promoted the overall dynamic strengthening of the microstructure and improved the uniformity of deformation. They, however, accelerated catastrophic crack propagation, an effect which is amplified by the presence of a pre-crack. On the other hand, large Mn-bearing particles decrease the overall toughness and strain-to-failure, while improving microstructural damage tolerance under dynamic loading, especially in the presence of a pre-crack, in comparison with an alloy that has only precipitates and/or no pre-crack.;As part of the new finite-deformation formulation for crystalline solids, subproblems related to twinning and geometrically necessary dislocation (GND) densities can also be formulated. In this study, the role of GND densities in crack behavior was investigated for single crystals. GND densities were shown to form in loops that are associated with stationary crack tips, but not propagating cracks. GND density loops were also shown to form and annihilate in crack-free single crystals under transient loads, which result in homogenous deformations.;The computational predictions clearly indicate that microstructural effects and mechanisms due to the presence of dispersoids and precipitates have a dominant effect on the dynamic deformation and failure of high strength and damage tolerant alloys. This behavior could only be understood and quantified with the appropriate crystalline descriptions, computational framework, and fracture criterion. |
