Asymmetric ability of grain boundaries to generate dislocations under tensile or compressive loadings

In order to investigate the effect of structural units on the ability of grain boundaries to generate dislocations, tensile and compressive loading tests were performed on aluminum bicrystals with $⟨112⟩$ tilt grain boundaries using molecular-dynamics simulations. Five favored boundaries were found for $⟨112⟩$ boundaries in an energy minimization analysis. Each structure could be identified using only its single structural unit period: A unit for Σ1 with θ=0^∘, B unit for Σ11 with θ=62.96^∘, C unit for Σ5 with θ=90^∘, E unit for Σ7 with θ=120^∘, and D unit for Σ3 with θ=180^∘. The favored boundaries all show an energy cusp except for the Σ7 boundary. Other boundaries could be represented by a combination of these five structural units. When Σ11 with $|B|$ period showing a minimum energy cusp was used as a reference structure, the core of 2/11[3̄ 1 1̄] or 2/11[3 1̄ 1] dislocations calculated for the displacement sift complete lattice of Σ11 were introduced at interfaces having larger or smaller misorientation angles than Σ11, respectively. Each grain-boundary dislocation corresponds to A and C units, respectively. For example, Σ21 with θ=44.42^∘ and Σ15 with θ=78.46^∘ consists of $|BBA|$ and $|BBC|$ periods, respectively. When the Σ15 bicrystal was loaded in tension, lattice dislocations having the 1/2[1 1̄ 0] core were emitted from C units and then the C units change their structure to the B units. Therefore, the Σ15 boundary can change its structure to the energetically stable Σ11 structure by emitting dislocations from C units. For Σ21, the same structural change occurred when the Σ21 bicrystal was deformed in compression. The simulation results show that there are tilt grain boundaries having the strong anisotropic strength and that the ability of grain boundaries to generate dislocations is closely related to their structural units.