Influence of Ni4Ti3 precipitates on phase transformation ofNiTi shape memory alloy
(1. College of Mechanical and Electrical Engineering, Harbin Engineering University, Harbin 150001, China;
2. Hubei Water Resources Research Institute, Wuhan 430070, China;
3. College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China)
2. Hubei Water Resources Research Institute, Wuhan 430070, China;
3. College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China)
Abstract: NiTi shape memory alloy samples were aged for 2 h at 573, 723 and 873 K, respectively. Two R-phase variants are observed in the NiTi samples aged at 573 and 723 K, where the orientation relationship between the two R-phase variants and the B2 matrix is determined. In the NiTi samples aged at 573 and 723 K, fine and homogeneous Ni4Ti3 precipitates are coherent with the B2 austenite matrix. The Ni4Ti3 particles precipitate in the grain interior and at the grain boundaries, where the heterogeneous Ni4Ti3 precipitates are coherent, semi-coherent and incoherent with the B2 matrix in the NiTi sample aged at 873 K. As for the NiTi sample aged at 873 K, one-stage phase transformation from B19'''' martensite to B2 austenite occurs on heating, but two-stage phase transformation of B2-R-B19'''' arises on cooling. The NiTi sample aged at 723 K shows two-stage phase transformation of B2-R-B19'''' on cooling as well, but exhibits two-stage phase transformation of B19''''-R-B2 on heating. The NiTi sample aged at 573 K exhibits three-stage transformation on cooling due to local stress inhomogeneity and local composition inhomogeneity around the Ni4Ti3 precipitates.
Key words: shape memory alloy; NiTi alloy; martensitic transformation; heat treatment; microstructure