Microstructure and phase transformation of TiNi alloy with addition of third element Sn

The microstructure and phase transformation behavior of Ti47.5Ni47.5Sn5 and Ti45Ni45Sn10 alloys have been studied by means of optical microscopy, electron probe X-ray micro-analysis, X-ray diffraction analysis, transmission electron microscopy and electrical resistance versus temperature curve measurement. The microstructure of as-cast Ti-Ni-Sn alloys consists of eutectic structure and TiNi matrix with 5at% Sn solid-soluted in the TiNi matrix of Ti47.5Ni47.5Sn5 alloy whereas 7at% Sn solid-soluted in the TiNi matrix of Ti45Ni45Sn10 alloy. XRD results indicate that the crystal structure of the matrix in Ti47.5Ni47.5Sn5 and Ti45Ni45Sn10 alloys at room temperature is B2 structure with the lattice parameter a0=0.3029 nm and a0=0.3032 nm, respectively. The eutectic structure consists of TiNi matrix and TisSns phase, and the TiNi matrix in the eutectic structure has the similar composition to that of the primary TiNi matrix. A few Ni is found to be contained in Ti5Sn3 phase. TEM observations show that the eutectic structure in Ti-Ni-Sn alloys exhibit band-like and triangle-like morphologies. The Ti47.5Ni47.5Sn5 and Ti45Ni45Sn10 alloys would undergo martensitic transformation during cooling to -140°C - -130°C without precursory R phase transformation.