Mo and Ta addition in NbTiZr medium entropy alloy to overcome tensile yield strength-ductility trade-off

Abstract

• The 0.1 M addition of Mo and Ta increased yield strength to 1060 ± 18 MPa without compromising ductility. • The improved ductility is attributed to lowered unstable stacking fault energy of {112}<111> slip system in NbTiZr(MoTa) x . • The MEAs in this study showed better wear resistance than traditional alloys. In this study, single-phase NbTiZr and NbTiZr(MoTa) 0.1 medium-entropy alloys (MEAs) were investigated for their use in biomedical implants. The alloys were prepared by arc melting, and were then cold-rolled, annealed, and characterized in terms of phase analysis, mechanical properties, fractography, and wear resistance. Both alloys showed a single body-centered cubic phase with superior mechanical, and tribological properties compared to commercially available biomedical alloys. Mo and Ta-containing MEAs showed higher tensile yield strength (1060 ± 18 MPa)) and higher tensile ductility (∼20%), thus overcoming the strength–ductility trade-off with no signs of transformation-induced plasticity, twinning, or precipitation. The generalized stacking fault energy (GSFE) calculations on the {112}<111> slip system by the first-principles calculations based on density functional theory showed that the addition of less than 0.2 molar fraction of Mo and Ta lowers the GSFE curves. This behavior posits the increase in ductility of the alloy by facilitating slips although strength is also increased by solid solution strengthening. The wear resistance of both alloys against hardened steel surfaces was superior to that of commercial biomedical alloys. Thus, we concluded that NbTiZr(MoTa) 0.1 MEA with good tensile ductility is a potential candidate for biomedical implants.