Characteristic deformation and structural changes of ni-ti alloys by molecular dynamics; phase transformation and amorphization, pp. 231-246
Authors: Tomohiro Sato, Ken-ichi Saitoh, Keisuke Kubota and Noboru Shinke
Abstract: In this study, molecular dynamics simulation (MDS) is conducted by using embedded atom method (EAM), which is an interatomic potential function suitable for Ni-Ti system. The EAM potential reproduces the experimental results of the energetic for stable Ni-Ti phases (B2, B19') and the Ti precipitates which are consisted of bct lattices. In the simulation model, we put disc-shaped Ti precipitates inside Ni-Ti parent alloy. Firstly, in the model with the disc-shaped precipitates, martensite is maintained around the interfaces because inevitable local residual strain can be balanced, when the whole simulation model has no strain (completely relaxed). Under tensile loading, in the case with disc precipitates, martensite region propagates from the interface plane. In the propagation, the unit lattices show structurally two pathways to complete transformation. The split into these pathways is strongly depending on the angle between tensile direction and the orientation of the martensitic unitcell. The results suggest that the interface is a preferred site for nucleation of martensite and that there is a certain crystallographic condition for the next variants to form smoothly. Secondly, stress-induced amorphization is calculated by MDS with a simple potential function derived from modified EAM (MEAM) framework. Relation between deformation (elastic and/or plastic) and structural changes into amorphous phase is characterized by the method of common neighbor analysis (CNA).It is found that the amorphization caused by shear deformation is well enhanced especially when slip motion is restricted due to the choice of shear direction. The obtained amorphous is found energetically meta-stable. Besides, it is found that martensite is observed around the amorphous region.