Theoretical investigation of atomic-scale (001) twinned martensite in the NiTi alloy

First-principles calculations using density-functional theory were carried out to optimize the B19' phase crystal structure (space group P21/m) in NiTi alloy by relaxing the Hellmann-Feynman forces. A more stable base centered orthorhombic structure (space group Cmcm) was obtained after the tension had been released. This base centered orthorhombic structure could be regarded as two martensite unit cells twined by the (001) plane with a twin width of 0.9228 nm. Based on this finding, the discrepancy between the geometric theory prediction that the atomic-scale (001) plane twinning cannot exist and experimental observation that the atomic-scale (001) plane twinning does appear in the B2-B19' martensite transformation could be explained. (C) 2007 Elsevier B.V. All rights reserved.