Formation ability, thermal/mechanical properties and hydrogen permeability of high entropy TiZrHf0.5Nb0.5CoNiCu amorphous and amorphous plus B2 alloys

High entropy TiZrHf_0.5Nb_0.5CoNiCu alloys with amorphous (Am) and Am+B2 mixed phases were synthesized in the melt-spun ribbons with different thicknesses of 32 μm to 102 μm. The as-spun structure consists of an Am phase for the ribbons with thicknesses below 80 μm and changes to Am+B2 phases for the ribbons with larger thicknesses. The B2 phase has a spherical morphology and its diameter and volume fraction are 0.5–5 μm and approximately 5% for the ribbon with a thickness of 102 μm. No difference in alloy composition between amorphous and B2 phases is recognized. The Am phase crystallizes through two stages: Am→Am'+B2→B2+Cu₁₀Zr₇+bcc-Nb. The B2 precipitates have extremely fine particle sizes of approximately 20–30 nm, and their volume fraction is as large as approximately 60–70%. The tensile yield and fracture strengths of the amorphous+B2 phase ribbon (102 µm in thickness) are 950 and 1523 MPa, respectively, and its plastic elongation is 1.64%, indicating that remarkable strain-hardening occurs for the mixed phase alloy. The reason for the strain-hardening seems to originate from the strain-induced precipitations of B19′ in B2 phase and B2 and B19′ in Am matrix as well as at the Am/B2 interface. The highest hydrogen permeability for the Am alloy sheet of 32 μm in thickness was 7.00 × 10⁻⁹ mol m⁻¹ s⁻¹ Pa^−1/2 at 673 K, indicating that the hydrogen permeation amount in a unit time is comparable to that for the commercial Pd–Ag alloy sheet with a thickness of 100 μm. The knowledge that the HE Am and Am+B2 alloys exhibit good tensile mechanical properties with distinct strain-hardening caused by the strain-induced precipitation as well as rather good hydrogen permeation ability encourages the future practical use of HE Am and Am+B2 mixed phase alloys.