Effect of high and low Nb content in multicomponent Nb-Ni-Ti-Zr-Co alloy on its structure, hardness and hydrogen permeability

In the present work, novel multicomponent Nb-Ni-Ti-Zr-Co alloys were synthesized by arc melting. Two alloys with low and high Nb contents and an equimolar high-entropy alloy (HEA) were produced and compared. The influence of the elemental composition on their microstructure, phase composition, lattice constants, hardness and hydrogen permeability was investigated. The synthesized Nb-Ni-Ti-Zr-Co alloys are characterized by nearequimolar BCC-(Nb, Ni, Ti, Zr, Co) and Nb-rich BCC-Nb(Ni, Ti, Zr, Co) phases. The equimolar Nb20Ni20- Ti20Zr20Co20 alloy had the highest concentration of BCC-(Nb, Ni, Ti, Zr, Co) phase (84 vol %), while the Nb74Ni6Ti9Zr5Co6 alloy had the highest concentration of Nb-rich BCC phase (95 vol %). First-principles study of the lattice parameters for the Nb-Ni-Ti-Zr-Co alloy revealed that the lattice parameter of the BCC phase decreases with Ni and Co addition and increases with Nb, Zr addition. Titanium does not significantly affect the lattice constant of BCC phase for compositions close to equimolar (10-30 at. %). The microhardness of the alloys decreases with increasing Nb content, which is mainly due to the proportion of softer Nb-rich BCC phase and microstructural changes. The synthesized Nb15Ni20Ti15Zr30Co20 and Nb20Ni20Ti20Zr20Co20 alloys demonstrate high hydrogen permeability at 400 degrees C that makes them promising for hydrogen purification membranes. It is shown high-entropy Nb20Ni20Ti20Zr20Co20 alloy exhibit highest resistance to hydrogen embrittlement, while high Nb content alloys are strongly susceptible to embrittlement.