Evaluation of the microstructure and property of TiNi SMA prepared using VIM in BaZrO3 crucible

Abstract

The contamination from the crucible has been recognized as the main drawback of using vacuum induction melting (VIM) method to prepare TiNi shape memory alloys (SMAs). In the present work, we evaluated the feasibility of utilizing the highly-stable BaZrO3 crucible to prepare TiNi SMAs, whose microstructure and mechanical properties were investigated by High-energy X-ray Diffraction (HE-XRD), analytical Transmission Electron Microscopy (TEM), Differential Scanning Calorimetry (DSC) and tensile test. It was found that there were about 0.035 wt % oxygen and 0.039 wt % zirconium remained in the TiNi alloy after melting in BaZrO3 crucible, which fulfills the ASTM standard on biomedical TiNi alloys. Oxygen from the dissolution of BaZrO3 could form Ti4Ni2Ox phase, and zirconium exists as solute atoms in both of the matrix and second phase. After the thermomechanical processing and vacuum annealing at 850 °C, the alloy achieved 37.63% of fracture elongation and 880 MPa of tensile strength. The transformational strain and martensitic yield strain of the alloy reached about 8% and 21% respectively.