Hydride phase equilibria in V–Ti–Ni alloy membranes

• V70Ti15Ni15 (at.%) comprises a vanadium solid solution plus NiTi and NiTi2.
• Dissolution of Ni and Ti into vanadium solid solution increase critical temperature for BCT β-hydride formation.
• Three VSS hydride phase fields were observed: BCC, BCC + BCT, BCT + BCT.
• NiTi and NiTi2 phases do not stabilise the alloy against brittle failure.

Vanadium is highly permeable to hydrogen which makes it one of the leading alternatives to Pd alloys for hydrogen-selective alloy membrane applications, but it is prone to brittle failure through excessive hydrogen absorption and transitions between the BCC α and BCT β phases. V–Ti–Ni alloys are a prospective class of alloy for hydrogen-selective membrane applications, comprising a highly-permeable vanadium solid solution and several interdendritic Ni–Ti compounds. These Ni–Ti compounds are thought to stabilise the alloy against brittle failure. This hypothesis was investigated through a systematic study of V70Ti15Ni15 by hydrogen absorption and X-ray diffraction under conditions relevant to membrane operation. Dissolved hydrogen concentration in the bulk alloy and component phases, phase identification, thermal and hydrogen-induced expansion, phase quantification and hydride phase transitions under a range of pressures and temperatures have been determined. The vanadium phase passes through three different phase fields (BCC, BCC + BCT, BCT + BCT) during cooling under H2 from 400 to 30 °C. Dissolution of Ni and Ti into the vanadium phase increases the critical temperature for β-hydride formation from <200 to >400 °C. Furthermore, the Ni–Ti phases also exhibit several phase transitions meaning their ability to stabilise the alloy is questionable. We conclude that this alloy is significantly inferior to V with respect to its stability when used as a hydrogen-selective membrane, but the hydride phase transitions suggest potential application for high-temperature hydrogen and thermal energy storage.