DFT calculation of kink boundary migration in LPSO structure

LPSO構造におけるキンク境界移動のDFT計算

板倉 充洋  ; 山口 正剛   ; 江草 大佑*; 阿部 英司*

Itakura, Mitsuhiro; Yamaguchi, Masatake; Egusa, Daisuke*; Abe, Eiji*

Kink boundaries formed in Mg-based long period stacking order (LPSO) alloys play a key role in strengthening of these materials. As the kink structure grows, many high-angle kink boundaries are eventually formed which has inclination angle close to 34 degrees. We show that this peculiar structure is a result of irreversible structural transformation and is energetically stable. We also calculate segregation energies of alloying elements Y and Zn to this boundary. Segregation energy is greater for Zn case and the maximum value is about 0.7 eV. Finally, the critical resolved shear stress for the migration of kink boundary is estimated for a pure-Mg kink and that with saturated with segregation. We show that segregated kink boundary requires very high shear stress about 700 MPa for migration. Kink structure shows apparent asymmetry with respect to the boundary. This asymmetry is induced by irreversible relaxation of the boundary structure. Stress-strain relation for the applied shear stress acting on the kink boundary indicates that without solute atoms, the critical resolved shear stress (CRSS) for boundary migration is about 400 MPa, whereas saturation of solute atoms increases the CRSS to about 700 MPa. In the actual LPSO structure where hard solute clusters prevent local shuffling of atoms, the CRSS for boundary migration should be much higher and expected to be irrelevant to plastic deformation.