A comparative study of the amorphization of NiTi-B2 structure by anti-site defects

Abstract

This paper reports a comparative study of the amorphization of B2 structure of near-equiatomic NiTi and three other equiatomic binary B2 alloy systems, including AlFe, CuZr and FeTi. The study was conducted by means of molecular dynamics simulation on the effects of anti-site defects within the B2 structure. It was found that anti-site defects cause severe random distortions to the B2 structures, lowering their crystalline ordering. Among the four B2 systems studied, equiatomic AlFe and FeTi cannot be fully amorphized by anti-site defects but NiTi and CuZr can become fully amorphous at above a threshold concentration of the defects, as compared to the structure of quenched liquid. It is also found that the threshold anti-site concentration for amorphization decreases with increasing temperature. In comparison between the two, NiTi is easier to be amorphized than CuZr by anti-site defects, both in terms of the threshold anti-site defect concentration required and the temperature range within which it is possible. These findings may help to explain the long-observed phenomenon of the easiness of equiatomic NiTi to become amorphous during cold working and the new observation of third element substitutional diffusion caused amorphization.