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

Abstract

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=20K/mm) and growth rates ranging from 3 to 70μm/s. Continuous and well-aligned Nb–TiNi eutectic microstructure and planar solid/liquid interface were observed at 3–5μm/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.