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.