Nb–HfCo alloys with pronounced high hydrogen permeability: A new family of metallic hydrogen permeation membranes

Microstructures and hydrogen permeability of the NbxHf(100−x)/2Co(100−x)/2 (x = 15…40) alloys were investigated. The composition Nb30Hf35Co35 resulted in a fully eutectic microstructure. Alloys with x (20 < x < 30) contain the primary HfCo and the eutectic {bcc-(Nb, Hf) + Bf-HfCo} phases, while those with x (x ≤ 20) additionally contain small fractions of the Hf2Co phase that results from a ternary eutectic reaction. Alloys with x (x > 30) contain the primary bcc-(Nb, Hf) and the eutectic phases, which exhibits pronounced high hydrogen permeability and large resistance to hydrogen embrittlement. The permeability increases with increasing Nb content and increasing volume fraction of the primary bcc-(Nb, Hf) phase. Nb40Hf30Co30 shows the highest permeability, particularly 4.96 × 10−8 mol H2 m−1 s−1 Pa−1/2 at 673 K. This is about 3.5 times higher than that of pure Pd and also the highest value among all the currently reported ones.