Design of hydrogen permeable Nb–Ni–Ti alloys by correlating the microstructures, solidification paths and hydrogen permeability

•A new compositional window for hydrogen permeable Nb–Ni–Ti alloys is established.•A new way to design hydrogen permeable alloys with improved performance is proposed.•Solidification paths in α-Nb phase region were determined by CALPHAD method.•High Nb content and Ni/Ti ratio lead to a high permeability of Nb–Ni–Ti alloys.•Nb55Ni20Ti25 alloy shows the highest permeability of 2.9 × 10−8 mol H2 m−1 s−1 Pa−0.5.

The compositional window in Nb–Ni–Ti alloys leading to the crystallization of primary α-Nb phase and the eutectic (α-Nb + NiTi) phase is of high technical relevance due to its favorable properties with respect to hydrogen permeation. The solidification behavior of Nb–Ni–Ti alloys in the primary α-Nb phase region is investigated to reveal the potential solidification paths. The study is based on the characterization of as-cast microstructures in combination with numerical calculations of solidification paths using the CALPHAD method coupled with a microsegregation model. Four different kinds of solidification paths depending on initial composition and cooling rate are found. Correspondingly, a new compositional window appropriate for hydrogen permeation is established in the primary α-Nb phase region. The variation of the hydrogen permeability of the alloys in this window is surprisingly high. High Nb content and Ni/Ti ratio lead to a high permeability. Nb55Ni20Ti25 shows the highest permeability at 673 K, particularly 2.9 × 10−8 mol H2 m−1 s−1 Pa−1/2. This is about 1.8 times higher than that of pure Pd.