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.