Hydrogen permeation in anisotropic Nb–TiNi two-phase alloys formed by forging and rolling

Abstract

The formation of an anisotropic microstructure by forging and rolling of a Nb–TiNi two-phase alloy and the effects of direction and annealing on hydrogen permeability were investigated. After forging and rolling, the granular (Nb, Ti) phase was strongly elongated along the rolling direction (RD) and compressed along the normal direction (ND). Hydrogen permeability along the RD (ND) increased (decreased) dramatically. The hydrogen permeability of this anisotropic microstructure can be explained by the law of mixtures using the hydrogen permeabilities of (Nb, Ti) and TiNi single-phase alloys. The hydrogen permeabilities along RD and ND correspond to parallel- and series-type hydrogen permeability, respectively. The 94-μm-thick RD sample shows a large hydrogen flux of 0.57 mol H2 m−2 s−1 (77 ccH2 cm−2 min−1) without hydrogen embrittlement. Phase boundary between (Nb, Ti) and TiNi phases, aligned parallel to the hydrogen flux, is one of the hydrogen permeation path.