Modeling of Ni4Ti3 precipitation during stress-free and stress-assisted aging of bi-crystalline NiTi shape memory alloys
(1. School of Mechanical and Automotive Engineering, South China University of Technology,
Guangzhou 510640, China;
2. School of Materials Science and Engineering, South China University of Technology,
Guangzhou 510640, China)
Guangzhou 510640, China;
2. School of Materials Science and Engineering, South China University of Technology,
Guangzhou 510640, China)
Abstract: The phase field method was applied to study the microstructure evolution of Ni4Ti3 precipitates during stress-free and stress-assisted aging of bi-crystalline NiTi shape memory alloys (SAMs) with two different initial Ni-contents of 51.5% and 52.5% (mole fraction), respectively. The simulation results show that, during stress-free aging of the NiTi alloy with a low supersaturation of Ni (i.e., Ti-51.5%Ni), the Ni4Ti3 precipitates exhibit a heterogeneous distribution with a high number density of particles at the grain boundary, leaving most of the grain interiors free of precipitates; while for the NiTi alloy with a high supersaturation of Ni (i.e., Ti-52.5%Ni), the Ni4Ti3 precipitates show a homogeneous distribution across the entire simulation system. The stress-assisted aging can give rise to homogeneous distribution of the precipitates, regardless of the initial Ni-content; however, the distribution of variant type within the two grains is heterogeneous.
Key words: NiTi shape memory alloy; Ni4Ti3 precipitate; low-angle grain boundary; martensitic transformation; phase field simulation