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Multiscale modeling of the competition between intergranular and transgranular ductile fracture in Al alloys
Abstract
The competition between intergranular and transgranular ductile fracture in Al alloys with precipitate free zones is investigated using a multiscale FE based approach. The solid is represented by discrete grains. Each grain is made of harder interior core and softer grain boundary layers, each region being discretized with many elements and represented by different hardening and damage related parameters. The material behaviour is given by an advanced micromechanical damage model for the transition to the smallest scale. This model relies on the extension of the Gurson model by Gologanu while introducing a new void rotation law and a new generalized void coalescence model. Homogenous biaxial loading, tensile testing with necking and crack propagation from a precrack are simulated to investigate the relationships between the hardening law, microstructure parameters, stress state, fracture mechanisms and fracture resistance expressed by a fracture strain or a JR curve.
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The growth of γ′-precipitate free zones (PFZs) along grain boundaries in polycrystals of the γ′-strengthened nickel-based superalloy NIMONIC PE16 and the ensuing effects on the yield strength, the ultimate tensile strength, and the plastic strain to fracture have been experimentally investigated. If the width of PFZs exceeds a well-defined limit, they lower the yield strength drastically. This result is interpreted on the basis of in situ transmission electron microscopic observations of the dislocation processes operating in the PFZs: the ease with which dislocations are generated in the PFZ and the resulting dislocation pile-ups lead to the reduction of the yield strength. Experimental observations of possible effects of PFZs on the appearance of the stress versus strain-curves and/or on the fracture mode were inconclusive: the observed effects may as well have been caused by the carbide particles formed at the grain boundaries.