Microstructure, tensile properties and deformation behaviour of a promising bio-applicable new Ti35Zr15Nb25Ta25 medium entropy alloy (MEA)

This research reports a promising bio-applicable new Ti35Zr15Nb25Ta25 (at.%) medium entropy alloy (MEA) designed with an atomic mismatch value (δ) of 4.0% through replacing 10 at.% Zr with 10 at.% Ti in its parent equiatomic TiZrNbTa MEA (δ = 4.84%). The as-cast MEA shows a single body centred cubic (BCC) phase with ~81% of its solidification grain boundaries (SGBs) having misorientation angles of > 30°. In the as-cast state, the MEA exhibited tensile ductility of 17 ± 0.6% and yield strength of 842 ± 11 MPa, above the minimum ductility−yield strength (10%-759 MPa) requirement for mill-annealed medical-grade Ti–6Al–4V (wt.%) alloy. The oxide film composition and thickness of the MEA are characterized. The MEA is substantially stronger and lighter than the most biocompatible metal, tantalum (Ta), while its Ta content (45.74 wt.% > the critical 40 wt.%) is sufficient to allow the formation of a continuously connected tantalum oxide (Ta2O5) film for it to exploit the bio-advantages of Ta. Both micro-shear bands and twin structures (three types of twin boundaries ∑3, ∑11 and ∑33a) were identified in regions near the tensile fracture surfaces of the MEA, providing insights into the ductility of the alloy in the as-cast state.