| A dislocation-density based crystal plasticity formulation, which accounts for martensitic microstructures, variant morphologies, orientation relationships (ORs), retained austenite, carbide precipitates, dislocation-density evolution, and dislocation-density grain boundary (GB) interactions, has been developed to investigate large inelastic deformation and failure modes, such as quasi-static and dynamic crack nucleation and growth and diffusion assisted hydrogen embrittlement in high strength martensitic steels.;A dislocation-density GB interaction scheme for dislocation transmission and impedance across martensitic block and packet boundaries has also been developed, and it was incorporated into the dislocation-density based crystal plasticity formulation. This dislocation-density GB interaction scheme was then coupled with a fracture approach to investigate the effects of GB on crack nucleation. The failure criteria are based on resolving stresses onto microstructural fracture planes, such as the cleavage planes of {100} and the hydrogen assisted microstructural fracture planes of {110} in lath martensitic steels. This microstructurally based fracture method utilizes an overlapping finite-element method where failure surfaces are nucleated and propagated as a function of preferred failure planes and orientations, dislocation-density evolution, and martensitic block orientations. For hydrogen diffusion assisted fracture, a pressure dependent form of Fick's Second Law diffusion equation was coupled to the crystal plasticity formulation and the non-linear finite element framework to investigate the effects of martensitic block/packet boundaries and carbide precipitates on hydrogen diffusion and embrittlement, and to understand and predict how hydrogen diffusion affects dislocation-density evolution and subsequent martensitic embrittlement. Stresses along the three cleavage planes and the six hydrogen embrittlement fracture planes were monitored to characterize the competition mechanism between cleavage fracture and hydrogen diffusion assisted fracture along preferential crystallographic planes.;Dislocation-density evolution and accumulation ahead of the crack front had a dominant effect on crack propagation, and large dislocation-density generation ahead of crack front blunted crack and inhibited crack propagation, which significantly increased fracture toughness. GBs and blocks with large misorientations resulted in dislocation-density pile-ups, high local stresses, and this resulted in crack nucleation. Hydrogen diffusion was shown to suppress dislocation density emission, which led to martensitic embrittlement and subsequent fracture at low nominal strains.;Retained austenite resulted in lower interfacial stresses and larger plastic deformations, and also resulted in dislocation-density accumulation around crack fronts for quasi-static and dynamic fracture. This decreased overall strength and increased the toughness of martensitic steels due to enhanced slip activity along austensite-martensite interfaces. Carbide precipitates resulted in higher local stresses and lower plastic deformations. Due to the impedance of dislocation-densities, precipitate interfaces were shown to be the sites of crack nucleation. Size refinement of martensitic blocks and packets increased the frequency of crack deflection at block and packet interfaces, which would increase overall fracture toughness. For the diffusion assisted hydrogen embrittlement cases, GBs and blocks with large misorientations and carbide precipitates impeded dislocationdensities, led to high local tensile pressure and hydrogen accumulation, which locally embrittled martensitic blocks and resulted in crack nucleation. Stress accumulations ahead of crack front resulted in large pressure gradient and hydrogen accumulation, and this led to a significant reduction of the critical fracture stress and accelerated crack growth.;These validated predictions indicate that the collective and interrelated effects of the orientation and size of martensitic blocks and packets, variants and GBs, the distribution of retained austenite and carbide precipitates can be fundamentally optimized at the microstructural scale for failure resistance for quasi-static, dynamic, and hydrogen diffusion assisted crack nucleation and propagation. |
