Fracture characteristics of fatigued Ti-6Al-4V ELI as an implant material

Titanium alloys exhibit an excellent combination of properties, including greater strength, lower modulus of elasticity and excellent biocompatibility. Only Ti-6Al-4V ELI and Ti-6Al-7Nb have been standardized for biomaterials in ASTM among various titanium alloys. Ti-6Al-4V ELI is a representative titanium alloy used as an alternative artificial metallic material for failed hard tissue. The implant materials for alternative use to failed hard tissue are often used under cyclic loading. The principal cause of implant failure is fatigue fracture. The fatigue strength is one of the most important properties for implant materials. There is a possibility for the implant materials to be fractured under monotonie loading conditions during fatigue, consequently the prediction of the fatigue life of the implant materials is important. An evaluation of the fracture characteristics of fatigued implant materials will offer information about the residual fatigue life and the residual fracture resistance of these materials. The relationship between fatigue damage and mechanical properties in Ti-6Al-4V ELI with equiaxed α structure were investigated in this study. The tensile strength of fatigued specimens of Ti-6Al-4V ELI with equiaxed α structure increases rapidly within the low-cycle-fatigue region. On the other hand, the elongation of fatigued specimens tested at various fatigue increments decreases rapidly within the low-cycle-fatigue region. The energy absorbed by fatigued specimens during various fatigue steps decreases rapidly in the late stages of the low-cycle-fatigue region. Furthermore, a hardness gradient develops initially from the surface to the inside of the specimens, and then in the later stages of fatigue, the internal hardness is equal to the surface hardness. The sub-structures both near and far from the surface of fatigued specimens, observed after various fatigue steps, show an increased dislocation density. However, at the late stages of low-cycle-fatigue, the dislocation density far from the specimen surface, also increases dramatically.