1804 BCC・FCC金属中の変形双晶に関するエネルギー論的考察(J06-1 マイクロ・ナノ構造と変形解析,J06 計算マイクロ・ナノメカニクス)

Abstract

Energy landscapes of (211) <111> deformation twinning in BCC Mo and (111) <112> deformation twinning in FCC Al and Cu are determined using density functional theory for sliding of layers numbering up to 7. In BCC Mo, the minimum thickness of a metastable twin is 2 layers, while twin embryos of 3- and 4 layers are unstable. Starting from 5 layers, the Mo twin can grow in a layer-by-layer fashion. The twin boundary formation and migration energies are found to be 607mJ/m^2 and 40mJ/m^2 respectively, implying that partial dislocations on twin boundaries will have wide cores and high mobilities. The stress to homogeneously nucleate a partial loop on the boundary of a thick twin is determined to be only 1.4 GPa, indicating that once a deformation twin in Mo reaches a critical thickness, which we estimate to be 6 layers, it can grow rather easily. Based on simple defect mechanics considerations, we estimate the condition for runaway defect growth requires twin embryo thickness to be tens of layers. Comparing the twinning energy landscape for Mo with those of Al and Cu, we find the former to have a longer ranged interlayer mechanical coupling, which is due to angular bonding and weaker electron screening in the intervening layers. Between Al and Cu, interactions in the Al are relatively longer ranged.