A strong and ductile biocompatible Ti40Zr25Nb25Ta5Mo5 high entropy alloy

Abstract

A non-equiatomic high entropy alloy (HEA) Ti40Nb25Zr25Ta5Mo5 (at.%) was designed by replacing 5% Ta with 5% Mo in a precursor Ti40Nb25Zr25Ta10 HEA for improved mechanical properties and reduced density. The Ti40Nb25Zr25Ta5Mo5 HEA fabricated by copper mould casting exhibited a nearly equiaxed grain structure (grain size: 96 ± 16 μm), but within each equiaxed grain, dendritic structures are discernible in the backscattered electron imaging mode. These dendrites are enriched in Ta, Nb and Mo, while the inter-dendritic regions contain higher Zr and Ti. The as-cast HEA achieved tensile yield strength (σ0.2) of 976 ± 8 MPa and strain to fracture (εf) of 16.80 ± 1.86%, well above the minimum requirements of σ0.2 and εf for medical-grade titanium alloy Ti–6Al–4V (759 MPa and 10%). The formation of microscale and nanoscale shear bands and the presence of significant dislocations in the shear band boundary region played a key role in allowing this as-cast HEA to achieve the excellent combination of tensile strength and ductility. Furthermore, the HEA exhibited a lower wear rate, lower coefficient of friction and much higher corrosion resistance in Hank's solution (electrochemical assessments) than Ti–6Al–4V, making it a promising alternative biomedical alloy.