A new non-equiatomic Ti40Zr25Nb25Ta5Al5 refractory high entropy alloy for potential biomedical applications

A non-equiatomic Ti40Zr25Nb25Ta5Al5 refractory high entropy alloy (RHEA) was designed for potential biomedical applications by replacing 5 at.% Ta with 5 at.% Al in a precursor Ti40Zr25Nb25Ta10 alloy. This simple and low-cost strategy reduced the alloy density from 7.2 to 6.5 g/cm 3 (estimated using the 'rule of mixtures') and increased the yield strength (σ ys ) from 875 to 960 MPa, while still allowing the RHEA to achieve a high tensile ductility of about 17 % in the as-cast condition. Few bio-applicable metallic alloys can achieve such tensile strength-ductility combinations in the as-cast condition. Compared to the mill-annealed medical-grade Ti-6Al-4V (wt.%) alloy, this as-cast RHEA offers significantly higher yield strength and ductility, along with a much lower cytotoxic aluminium content (1.8 wt.%). Furthermore, the as-cast RHEA has a much higher admissible strain (1.12 %) than Ti-6Al-4V (0.75 %) and also exhibits better corrosion resistance in Hank’s solution than Ti-6Al-4V. These evaluation results demonstrate the potential of this Ti40Zr25Nb25Ta5Al5 RHEA for biomedical applications. To elucidate the origin of these attractive properties, the solidification microstructure, tensile deformation mechanisms and surface oxide film of this RHEA were systematically investigated. A series of novel experimental observations were obtained and are discussed in detail. • Replacing 5 at.% Ta with 5 at.% Al in the Ti40Zr25Nb25Ta10 RHEA yields a new strong and ductile RHEA with lower density. • This new Ti40Zr25Nb25Ta5 RHEA exhibited superior yield strength and tensile ductility compared to medical-grade Ti-6Al-4V. • Plastic deformation in this RHEA involved primary and secondary shear bands, along with prominent dislocation loops. • This RHEA further exhibited superior elastic admissible strain and corrosion resistance compared to Ti-6Al-4V.