Effect of heat treatment on mechanical properties of Ti–6Al–4V ELI alloy

Ti–6Al–4V alloy of extra low interstitial (ELI) grade has been used in the biomedical applications because of its high strength-to-weight ratio and excellent biocompatibility. However, its relatively poor wear resistance leading to excessive wear and implant loosening requires proper surface hardening. The body implants are also subjected to a variety of loads at varying strain rates. The objective of this study was to evaluate the hardening behavior, strain rate sensitivity and fracture mechanisms with or without hardened surface layer. It was observed that the hardened surface layer had only a small effect on the strength but reduced the ductility. The yield strength (YS) and ultimate tensile strength (UTS) were higher in the water quenching and aging condition while the ductility was lower, when compared to the as-received condition and air cooling and aging condition. As the strain rate increased both YS and UTS increased and ductility decreased. The strain hardening exponent decreased with increasing strain rate. The strain rate sensitivity evaluated via both the common approach and Lindholm approach decreased as the true strain increased. Fractal dimension of fracture surfaces was observed to be associated with the heat treatment condition, ductility, fracture surface characteristics and roughness. The fractal dimension increased with increasing roughness of fracture surfaces.