Atomic scale investigation of grain boundary structure role on intergranular deformation in aluminium

Abstract

The role that grain boundary (GB) structure plays on the plasticity of interfaces with preexisting cracks and on the interface crack dynamics was investigated using molecular dynamics for both <100> and <110> aluminum symmetric tilt grain boundaries (STGBs). In simulations with a crack at the interface, this research shows how the maximum normal strength of the interface correlates with the respective GB energy, the GB misorientation, and the GB structural description. For instance, the normal interface strength for GBs containing 'D' structural unit (SU) or stacking faults in the GB structural description (Σ13 (510) θ=22.6° and Σ97 (940) θ=47.9°) shows a noticeable decrease in interface strength, as compared to other evaluated <100> GBs that contained favored SUs. In the case of <110> interfaces, the presence of the 'E' SU in the GB structure lowers the maximum normal interface strength by ~35%. Further investigation of the deformation at the crack tip in GBs containing the 'E' structure revealed that the 'E' SU underwent atomic shuffling to accommodate intrinsic stacking faults (ISFs) along the interface, which in turn acts as a site for partial dislocation nucleation. Interestingly, regardless of GB misorientation, GB interfaces examined here containing the 'E' structure in their structural period exhibited relatively small variation in maximum normal strength of interface. The GB volume ahead of the crack tip underwent structural rearrangement which, in turn, influenced the crack propagation mechanism. In most GBs, the crack propagation was due to alternating mechanisms of dislocation emission, followed by propagation of dislocation (blunting) and cleavage/crack advance. Moreover, the crack growth rates along the GB interface were strongly influenced by the initial free volume at the interface, i.e., faster crack growth was observed along interfaces with higher initial free volume. A strong asymmetry in crack growth due to differences in available slip planes ahead of both the crack tips (e.g., Σ13 (510), Σ97 (940), Σ11 (113) and Σ27 (552) STGBs) was observed with the crack growth process, with the Σ97 (940) θ=47.9° STGB displaying the largest asymmetry. In most cases, dislocation emission along various available slip planes ahead of the crack tip in <100> STGBs was the dominant plastic event, whereas the <110> STGBs had a partial dislocation emission {111}<112> followed by twin formation for interfaces containing the 'E' SU. Last, there was no direct correlation between any one individual interface constitutive parameters (e.g., free volume, availability of easy slip systems, GB energy, etc.) and interface crack dynamics. Nevertheless, simulation results did indicate an indirect role of these constitutive parameters on atomistic deformation ahead of the crack tip, which significantly alters the interface strength and properties. In summary, the present simulations provide new insight into crack growth along interfaces and provide a physical basis for determining the incipient role between the GB character an...

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