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

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.