Design of Nb-based multi-phase alloy membranes for high hydrogen permeability and suppressed hydrogen embrittlement

Nb-Hf-Co alloys, consisting of a multi-phase microstructure with primary BCC-(Nb, Hf) and eutectic {BCC-(Nb, Hf) thorn HfCo} phases, are a potential alloys for hydrogen separation and purification. However, the compositional window, comprising of the above phases, has not yet been established because of lack of important information concerning the phase diagrams. To address this need, this study first calculated the phase diagram and related phase equilibria information of this class of alloys, by using the CALPHAD approach, and established a new compositional window suitable for hydrogen permeation. The alloys in this window exhibit very high hydrogen permeability. In particular, Nb45Hf27.5Co27.5 exhibits the highest permeability of 5.32 x 10(-8) mol H-2 m(-1)s(-1)Pa(-0.5) at 673 K, which is the best among all the Nb-based multi-phase alloys. Secondly, the boundary for ductile to brittle transition-hydrogen concentration (DBTC) of the Nb-HfCo multi-phase alloy was determined for the first time and found to occur at around 0.72-0.82 H/M, which reveals the fundamental reasons for brittle failure occurring during hydrogen permeation. From the point of DBTC, a new design concept for Nb-based hydrogen permeable membranes is proposed and the factors affecting their brittle failure are analysed. Thirdly, to verify the feasibility of the above concept, Nb45Hf27.5-xTixCo27.5 (x = 0 ... 10) alloys were designed and comprehensively studied. It is demonstrated that Nb(45)Hf(20)Ti(7.5)Co(27.)5 alloy exhibits excellent hydrogen permeability with enhanced hydrogen embrittlement under appropriate permeation conditions. The significance of the study is that the best composition range for hydrogen-permeable Nb-HfCo alloys is explored, and some novel hydrogen-permeable alloys can be easily developed by analysing their absorption pressure-composition isotherms in combination with the newly explored DBTC.