Microstructural characteristics of directionally solidified Ni-43Ti-4Al-2Nb-2Hf alloy

By liquid metal cooling (LMC) process, the Ni-43Ti-4Al-2Nb-2Hf (atomic fraction, %) alloy was directionally solidified (DS) at different casting temperatures (1350, 1450, 1550 °C) and a constant withdrawal rate of 6mm/min. Compared to the as-cast alloy, DS process has no effect on the phase composition, but significantly influences the phase morphologies. After DS, the NiTi phase preferentially grows along [100] orientation, and the macrostructure becomes the columnar crystals morphology with the [100] growth direction. Owing to the micro- segregation difference of Nb and Hf, the contrasts of primary NiTi phase become lighter from the center of dendrites to the edge. White Nb-rich Ti ₂Ni phases distribute along growth direction in the light-contrast interdendritic regions, accompanied by Ti ₂Ni precipitates. Actually, the finally solidified Ti ₂Ni phase is the quasi-eutectic structure composed of Nb-rich Ti ₂Ni and Ti ₂Ni phase. The increases of casting temperature make the dendrites finer, also, Nb-rich Ti ₂Ni more dispersively distributed.