A skull-aided technique for directional solidification of Nb-41Ni-40Ti hydrogen permeable alloy

A skull-aided technique (SAT) for directional solidification (DS) of Nb-41Ni-40Ti alloy was developed to avoid the reaction between the alumina crucible and melt. By using SAT in alumina crucibles, DS experiments were performed in a Bridgman-type apparatus at a constant temperature gradient (G=20 K/mm) and growth rates ranging from 3 to 70 μm/s. Continuous and well-aligned Nb-TiNi eutectic microstructure and planar solid/liquid interface were observed at 3-5 μm/s. Higher growth rates resulted in the cellular/dendritic eutectic growth and even the appearance of primary phase. The volume fraction of well-aligned Nb rods (Vwell−Nb) in the eutectic microstructure decreased with increasing growth rate (v), given as Vwell−Nb=11.8+109exp(−v/13.93). No ceramic inclusions were found in DS ingot, which demonstrated that the SAT was very effective in avoiding the crucible-melt reaction for DS of Nb-41Ni-40Ti alloy.