Microstructural Modeling of Interfacial Effects on Inelastic Deformation and Fracture in Crystalline Systems

摘要

A dislocation-density based crystalline plasticity framework accounting for dislocation-density interactions on different slip systems, dislocation-density grain boundary (GB) interactions, and microstructural heterogeneities has been developed to investigate the influence of interfaces, such as GBs, carbide/matrix boundaries, and porosity. This framework has been coupled with a new finite-element (FE) based fracture approach that can be used to predict the nucleation and propagation of competing intergranular (IG) and transgranular (TG) failure modes in f.c.c. systems.;A dislocation-density GB interaction scheme, based on a line tension model for dislocation source activation in the presence of a GB obstacle, was developed and coupled to the crystal plasticity framework to understand and predict the effect of GB misorientation on dislocation-density transmission, blockage, and pileups for copper bicrystals, tricystals, and polycrystalline aggregates with random low-angle, random-high angle, and coincident-sitelattice (CSL) GBs. Based on slip system misorientations and the magnitude of residual GB dislocations, an effective Burgers vector for residual GB dislocations was obtained that was inversely related to the energy barrier for dislocation-density transmission for different GB interfaces.;These dislocation-density interactions were also used to postulate a microstructural IG fracture criterion for evolving dislocation-density pileups at GB interfaces to represent the nucleation, growth, and branching of failure surfaces along GB planes and triple junctions (TJs) using overlapping elements. The complex and interrelated effects of dislocation-density evolution, interactions, transmissions, and pileups on multiple active slip systems in TJ regions were investigated in relation to IG fracture for different arrangements of low- and high-angle random and CSL GBs.;A TG fracture criterion was also developed, based on the propagation of cracks along {001} cleavage planes, and it was coupled to the computational framework to understand and predict the influence of GB transmission and pileups on the competition between IG and TG failure. Polycrystalline f.c.c. aggregates with a range of GB misorientation distributions were investigated to elucidate how GB networks and misorientations in polycrystalline aggregates affect the simultaneous nucleation and growth of IG and TG fracture.;In aggregates with a majority of high-angle GBs, low GB transmission and extensive dislocation-density pileups induced the nucleation of IG cracks that propagated along highangle GBs, which is consistent with experimental observations. The propagation and branching behavior of IG cracks in TJ regions was controlled by the evolution and interaction of dislocation-densities, and cracks preferentially grew along high-angle GBs with pileups rather than low-angle GBs. Crack propagation in high-angle GB aggregates was dominated by IG fracture, and local transitions to TG fracture significantly reduced crack propagation rates. Aggregates with a majority of low-angle GBs had much less pileup formation due to alignment of slip systems and slip continuity at GB interfaces. With the lack of dislocationdensity pileups, IG fracture was suppressed, and TG cracks nucleated and propagated due to high stresses on cleavage planes.;The multiple-slip dislocation-density framework, the fracture methodology, and the nonlinear FE approach were also used to investigate the influence of thermo-mechanical process-induced interfaces and defects in additively-manufactured (AM) nickel-base superalloys. Microstructural pores, carbide/matrix interfaces, and unmelted powder were detrimental to the overall fracture toughness, as the high stresses and stress gradients induced by such interfaces promoted failure nucleation and propagation.;These predictions provide a fundamental understanding of how dislocation-density evolution and interactions with interfaces such as GBs, carbide/matrix boundaries, and porosity affect inelastic deformation behavior and the competition between IG and TG failure modes in crystalline materials. The predictions indicate that GB networks and other interfaces, such as those associated with voids and carbides, can be tailored to delay crack nucleation and enhance overall fracture toughness. Hence, these predictions can provides guidelines for failure resistant and durable material systems.