Comparative study on microstructure and homogeneity of NiTi shape memory alloy produced by copper boat induction melting and conventional vacuum arc melting

A comparative study was carried out on the microstructures of NiTi shape memory alloy produced by two techniques of copper boat induction melting and vacuum arc melting. The as-solidified and homogenized specimens were characterized by optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with EDS, and differential scanning calorimetry (DSC). The results showed that the microstructure formed in the copper boat method was free of segregation (Ti or Ni-rich precipitates) with a low scatter (<5 K) in phase transition temperatures within the ingot at different positions. Unlike the multi-stage vacuum arc remelting process, a homogeneous microstructure was achieved only after one melting procedure.

► NiTi was produced by vacuum arc melting and copper boat induction melting.
► Ti- and Ni-rich precipitates were observed in VAM as-cast and homogenized samples.
► Eutectic morphology distributed in B2 austenite was observed in CBIM samples.
► The CBIM ingots showed low scatter (<5 K) in phase transition temperatures.
► Ms characteristic temperature of CBIM conformed to early precise measurements.