Preparation, microstructure and compressive property of NiTi alloy hollow spheres fabricated by powder metallurgy

Abstract NiTi alloy hollow spheres were prepared using a powder metallurgy at 1100 °C, 1150 °C, 1170 °C and 1200 °C in vacuum sintering furnace. The effects of different sintering temperatures on thin-wall porosities, microstructural evolution and quasi-static compression properties of NiTi alloy hollow spheres were studied. The results showed that NiTi alloy hollow spheres with good surface quality are obtained at 1200 °C. The thin-wall porosity decreases with increasing sintering temperature, while the uniformity of thin-wall thickness increases. The number of pores decreases during sintering process, while the pores with large size grow bigger and become spherify. The NiTi alloy hollow spheres mainly consist of the predominant NiTi phase (B2 and R), and precipitated phases Ti2Ni, Ni3Ti and Ni4Ti3. The size of Ti2Ni increases first and then decreases with increasing sintering temperature, while the Ni3Ti phase continuously and uniformly precipitates at grain boundary. Compressive strength of NiTi alloy hollow spheres increases with increasing sintering temperature, and increases to 189 MPa when sintering temperature reaches to 1200 °C. The loading-unloading curves of hollow spheres revealed that the hollow spheres show superelastic behavior to a certain extent. The fracture characteristics of sintered NiTi alloy hollow spheres were also addressed in this work.