Microstructure characteristics of Ni-43Ti-4al-2Nb-2Hf alloy prepared by conventional casting and directional solidification

To further investigate the microstructure characteristic and solidification mechanism, so as to provide knowledge for the microstructure control of a NiTi-Al based high-temperature structural material, the microstructure of Ni-43Ti-4Al-2Nb-2Hf (at.%) alloy ingots prepared by conventional casting (arc-melting) and directional solidification (DS) at various drawing velocities (2 mm·min^-1, 18 mm·min^-1, 30 mm·min^-1 and 60 mm·min^-1, respectively) was investigated by means of electron probe microanalyses. Experimental results reveal that the microstructures are composed of NiTi matrix phase, β-Nb phase and Ti₂Ni phase for samples obtained by both conventional casting and DS. Conventional casting has an equiaxial structure, while DS has a slender and acicular cellular structure which grows along the [001] orientation preferentially. Small amounts of white β-Nb phase and black Ti₂Ni phase co-exist at the grain boundaries or intercellular regions. With an increase in drawing velocity, the NiTi matrix phase is inclined to grow along (100) and (200) crystallographic planes, and the cellular arm spacing reduce gradually, but the directionality of the solidified structure weakens significantly. The homogeneous dispersion of β-Nb phase and the decrease of Ti₂Ni phase in DS samples are beneficial to improving the mechanical properties. Solidification mechanism analysis indicates that the dark grey NiTi matrix phase initially precipitates from the liquid phase, and then the divorced eutectic reaction takes place, which produces the light gray matrix phase and β-Nb phase. Finally, the peritectic reaction happens, which generates the black Ti₂Ni phase. The complete solidified path of the alloy is L→ NiTi+L → NiTi+ β-Nb+L → NiTi+β-Nb + Ti₂Ni.