Phase field simulation of auto-catalytic growth effect of coherent Ni4Ti3 precipitate in NiTi shape memory alloy

Abstract

The precipitation behavior and distribution of Ni4Ti3 particles in NiTi alloys have a significant influence on the subsequent martensitic transformation, which may consequently affect shape memory effect and superelasticity of NiTi alloys. The latest experimental studies confirmed that in single crystal NiTi alloy, the Ni4Ti3 particles nucleate and grow as an autocatalytic way leading to a step-like configuration. The autocatalytic nucleation known as collective manner has been widely studied in martensitic transformation while causing relatively less attention in diffusion transformation such as precipitation. Previous studies have been launched to investigate the nucleation and orientation issues of Ni4Ti3 precipitate mainly by analyzing the stress or concentration field around the Ni4Ti3 particle. However, the coupling between stress and concentration field is necessary in Ni4Ti3 precipitation due to its diffusion nature, also the coupling and cross influence of each energy in the simulation system should be taken into account. In this paper, the mechanism of autocatalytic effect of Ni4Ti3 precipitate has been studied by means of phase field method, the favorable Ni4Ti3 precipitates array has been investigated through the variation of each energy in the simulation system. The simulation results show that in the initial short time of Ni4Ti3 precipitation, due to the morphological accommodation of Ni4Ti3 and NiTi phases, the system chemical free energy gradually increases, while the elastic and interfacial energy decrease; after the self-accommodation stage, each energy shows the inverse trend compared with the initial stage; in the whole phase transformation process, the system total energy develops toward the lower energy state. In all of the feasible precipitate arrays, it is found that the step-like array formed by same orientation Ni4Ti3 variants has the supreme priority, secondly the horizontal array followed by the edge-face array formed by different orientation variants, the vertical array constituted by same orientation variants was confirmed as the least possible way, these simulation results have deepened and expanded the understanding obtained in previous experiment studies. © Copyright;.