FRACTURE CHARACTERISTICS, MICROSTRUCTURE, AND TISSUE REACTION OF TI-5AL-2.5FE FOR ORTHOPEDIC-SURGERY

The microstructure of Ti-5Al-2.5Fe, which is expected to be used widel y as an implant material not only for artificial hip joints but also f or instrumentations of scoliosis surgery, was variously changed by hea t treatments. The effect of the microstructure on mechanical propertie s, fracture toughness, and rotating-bending fatigue strength in the ai r and simulated body environment, that is, Ringer's solution, was then investigated. Furthermore, the effect of the living body environment on mechanical properties and fracture toughness in Ti-5Al-2.5Fe were i nvestigated on the specimens implanted into rabbit for about 11 months . The data of Ti-5Al-2.5Fe were compared with those of Ti-6Al-4V ELI, which has been used as an implant material mainly for artificial hip j oints, and SUS 316L, which has been used as an implant material for ma ny parts, including the instrumentation of scoliosis surgery. The equi axed alpha structure, which is formed by annealing at a temperature be low beta transus, gives the best balance of strength and ductility in Ti-5Al-2.5Fe. The coarse Widmanstatten alpha structure, which is forme d by solutionizing over beta transus followed by air cooling and aging , gives the greatest fracture toughness in Ti-5Al-2.5Fe. This trend is similar to that reported in Ti-6Al-4V ELI. The rotating-bending fatig ue strength is the greatest in the equiaxed alpha structure, which is formed by solutionizing below beta transus followed by air cooling and aging in Ti-5Al-2.5Fe. Ti-5A1-2.5Fe exhibits much greater rotating-be nding fatigue strength compared with SUS 316L, and equivalent rotating -bending fatigue strength to that of Ti-6Al-4V ELI in both the air and simulated body environments. The rotating-bending fatigue strength of SUS 316L is degraded in the simulated body environment. The corrosion fatigue, therefore, occurs in SUS 316L in the simulated body environm ent. Fatigue strength of Ti-5A1-2.5Fe in the simulated body environmen t is degraded by lowering oxygen content in the simulated body environ ment because the formability of oxide on the specimen surface is consi dered to be lowered comparing with that in air. The mechanical propert y and fracture toughness of Ti-5Al-2.5Fe and Ti-6Al-4V ELI are not cha nged in the living body environment. The hard-surface corrosion layer is, however, formed on the surface of SUS 316L in the living body envi ronment. The Cl peak is detected from the hard-surface corrosion layer by energy-dispersive X-ray (EDX) analysis. These facts suggests a pos sibility for corrosion fatigue to occur in the living body environment when SUS 316L is used. The fibrous connective tissue and new bone for mation are formed beside all metals. There is, however, no big differe nce between tissue morphology around each implant material.