Hydrogen transport behavior of as-cast, cold rolled and annealed Nb₄₀Ti₃₀Co₃₀ alloy membranes

Cold rolling is presently regarded as the most promising route to a large scale fabrication of hydrogen permeable metal membranes. However, complex hydrogen transport behavior appears for such cold rolled membranes due to potential defect formation and recrystallization. Effects of cold rolling and subsequent annealing on hydrogen dissolution, permeation and diffusion in Nb₄₀Ti₃₀Co₃₀ (at%) alloy membranes have been studied. Membranes reduced to 50% thickness by cold rolling exhibit a significantly lower hydrogen permeability than those with as-cast microstructure. This is attributed to the large decrease in both hydrogen solubility and diffusivity as a consequence of rolling. Subsequent annealing of the cold rolled membrane recovers the hydrogen solubility close to that in as-cast state, and significantly increases the diffusivity, leading to an improved permeability. The modified microstructure in the rolled-annealed membrane ensures a high hydrogen permeation flux and a large embrittlement resistance. Typically, a hydrogen flux of ~21 cc H₂ cm^-2 min^-1 is obtained for a membrane with 130 μm in thickness at a hydrogen pressure difference of 0.7 MPa at 673 K.