Thermally Induced Martensitic Transformation of As-cast and As-forged Ni₄₇Ti₄₄Nb₉ Alloy

The characteristics of thermally induced martensitic transformation and the reverse transformation of as-cast and as-forged Ni₄₇Ti₄₄Nb₉ alloy were studied and compared by using differential scanning calorimetry (DSC), X-ray diffraction (XRD), optical microscopy (OM) and scanning electronic microscopy (SEM), and the influence mechanism of microstructure on phase transformation was analyzed. The results show that the as-cast alloy was composed of large primary NiTi phase, eutectic phase (fine NiTi phase and β-Nb phase) and (Ti,Nb)₂Ni phase distributed in the eutectic area. β-Nb phase and (Ti,Nb)₂Ni phase of the alloy after forging diffusely distributed in the matrix of NiTi phase. The phase transformation peaks and reverse transformation peaks of as-cast alloy were wide and flat because of the compositional segregation and the difference of Nb content in primary and eutectic NiTi phases. NiTi phase became fine and the chemical composition became more homogeneous after forging and thus resulted in the narrowness and sharpness of the phase transformation peaks and reverse transformation peaks.