A. E. Craft, D. E. Campbell, B. Aboud, DePuy Orthopaedics, Inc., Warsaw, IN
In an effort to continually investigate alloys with enhanced fatigue properties for orthopaedic applications, two beta-rich alpha-beta titanium casting alloys were evaluated and compared to Ti-6Al-4V ELI (extra-low interstitial; ASTM F136). The alloys had the approximate compositions of Ti-6Al-4V-2Cr-0.5Fe-Si and Ti-6Al-4V-2Cr-1Fe-1Mn. Both alloy castings were subjected to hot isostatic pressing (HIP) plus a variety of heat treatments. Rotating bending fatigue (RBF) specimens were created in smooth and notch (Kt=2.9) forms, and evaluated for each heat treatment condition. RBF specimens were tested until fracture or 10 million cycles (“runout”). The endurance limit (EL) for each alloy and condition was determined as the stress value below which no failures occurred up to 10 million cycles. Biomechanically, 10 million cycles is theorized to simulate 10 years of joint movement in a moderately active person. Smooth and notch EL values for the alloys were found to be similar to or lower than those of Ti-6Al-4V ELI in the same heat treatment conditions. Additional material characterization of hardness, microstructure, and cell culture growth was also performed.
Summary: In an effort to continually investigate alloys with enhanced fatigue properties for orthopaedic applications, two beta-rich titanium casting alloys from FS Precision Tech were evaluated and compared to Ti-6Al-4V ELI (extra-low interstitial; ASTM F136). The first beta-rich titanium alloy, FS-2S, contains aluminum, vanadium, and chromium alloying elements. The second alloy, FS-211, contains aluminum, vanadium, chromium, manganese, and iron alloying elements. Both alloy castings were subjected to hot isostatic pressing (HIP) plus a variety of heat treatments. Rotating bending fatigue (RBF) specimens were created in smooth and notch (Kt=2.9) forms, and evaluated for each heat treatment condition. RBF specimens were tested until fracture or 10 million cycles (“runout”). The endurance limit for each alloy and condition was determined as the stress value below which no failures occurred up to 10 million cycles. Biomechanically, 10 million cycles simulates 10 years of joint movement in a moderately active person. Smooth and notch endurance limit values for FS-2S and FS-211 were found to be slightly lower than those of Ti-6Al-4V ELI in the same heat treatment conditions. Additional material characterization of hardness, microstructure, and cell culture growth was performed for the beta-rich titanium alloys.
