Microstructural stability and its effect on hydrogen permeability in equiaxed and directionally solidified eutectic Nb30Ti35Co35 alloys

Microstructural stability and hydrogen permeability in dependence of long-time annealing of both equiaxed (EX) and directionally solidified (DS) Nb30Ti35Co35 eutectic alloys are evaluated. After annealing at 1373 K for one week, the initially fully lamellar eutectic microstructure of the EX sample has disappeared and changed to a duplex microstructure. This is accompanied by a decrease in hydrogen permeability from 2.65 × 10−8 to 1.08 × 10−8 mol H2 m−1s−1Pa−0.5 at 673 K. The lower permeability is mainly attributed to the significant reduction of hydrogen diffusivity after annealing. In contrast, DS Nb30Ti35Co35 sample shows higher microstructural stability and much less reduction in hydrogen diffusivity and permeability after annealing. Typically, the hydrogen permeability at 673 K of the annealed DS sample decreases slightly from 3.58 × 10−8 to 3.24 × 10−8 mol H2 m−1s−1Pa−0.5. The present work demonstrates the superiority of the directional solidification technique in fabricating membranes with high hydrogen permeability in both as-solidified and annealed states.