Title: Atomic scale investigation of grain boundary structure role on deformation and crack growth dynamics in Aluminum

Presenter: Ilaksh Adlakha

Affiliation: Arizona State University

Abstract: Here, we present atomistic investigation of the role of grain boundary (GB) character on plastic events at the interface with a preexisting crack and the interface crack dynamics in <100> and <110> aluminum symmetric tilt grain boundaries (STGB) using molecular dynamics. This research shows that the maximum interface normal strength for the <100> GB examined here directly correlates with the respective GB energy. However, in the case of <110> system, we observed an exponential relationship between the interface maximum normal strength and the GB misorientation. Furthermore, the normal interface strength for GBs containing non-favored SU in the GB structural description (S13 (510) ?=22.6° and S97 (940) ?=47.9°) showed a noticeable decrease in their interface strengths as compared to other <100> GBs evaluated that contained favored SU. In the case of <110> interfaces, the presence of ‘E’ SU in the GB structural period lowers the maximum normal interface strength. On further investigation of the deformation at the crack tip in GBs containing ‘E’ structure, it was found that ‘E’ SU underwent atomic shuffling to accommodate ISF along the interface, which acts as sites for the partial dislocation nucleation. Interestingly, regardless of GB misorientation, GB interfaces containing ‘E’ structure in their structural period examined here exhibit constant maximum interface strength. In some cases, 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 the crack growth, due to difference in available slip planes ahead of both the crack tips (in particular the S13 (510), S97 (940), S11 (113) and S27 (552)) was observed with the crack growth process for a S97 (940) ?=47.9° interface displayed greatest asymmetry. In most cases, the nucleation of 111 <110> full dislocation 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 ‘E’ SU. Also, we observed an unanticipated and intriguing process that twinning only occurs in the GB interfaces that were contained ‘E’ SU in GB structural description. Finally, we observed no direct correlation between individual interface constitutive parameters, such as free volume, availability of easy slip systems, the GB energy etc., and interface crack dynamics. However, results presented here indicated an indirect role of these constitutive parameters on the atomistic deformations ahead of the crack tip, which significantly alter the interface strength and properties, and no unique parameter can be used to describe/model the interface behavior. In summary, these new atomistic perspectives provide a physical basis for recognizing the incipient role between the GB character, and interface properties including GB energies as an input to higher scale models.