Effects of Microstructure on the Short Fatigue Crack Initiation and Propagation Characteristics of Biomedical #alpha#/#beta# Titanium Alloys

摘要

This article presents the results of a study of the effects of microstructure on the fatigue strength and the short fatigue crack initiation and propagation characteristics of a biomedical #alpha#/#beta# titanium alloy, Ti-6Al-7Nb. The results are compared to those obtained from a Ti-6Al-4V extra-low interstitial (ELI) alloy. Fatigue crack initiation occurs mainly at primary #alpha# grain boundaries in an equiaxed a structure, whereas, in a Widmanstatten #alpha# structure, initiation occurs within the #alpha# colonies and prior #beta# grains, where #beta# plates are inclined at around 45 deg to the stress-axis direction. In an equiaxed #alpha# structure, the short fatigue crack initiation and propagation life, where the length of the crack (#alpha#) is in a microstructurally short fatigue-crack regime (2a <50 #mu#m), occupies around 50 pct of the total fatigue life. On the other hand, the fatigue crack in a Widmanstatten a structure initiates at very early stages of fatigue, and, therefore, the fatigue crack-initiation life occupies a few percentages of the total fatigue life in an #alpha# structure. Then, the short fatigue crack propagates rapidly and is arrested at the grain boundaries of #alpha# colonies or prior #beta# grains for a relatively long period, until the short crack passes through the boundaries to specimen failure. Therefore, the short fatigue crack-arrest life occupies more than 90 pct of the total fatigue life in a Widmanstatten #alpha# structure. These trends are similar between the Ti-6Al-7Nb and Ti-6Al-4V ELI alloys and biomedical #alpha#/#beta# titanium alloys. The total fatigue life for the Ti-6Al-7Nb alloy with an equiaxed #alpha# structure is changed by the volume fraction of primary #alpha# phase and the cooling rate after solution treatment. By increasing the volume fraction of the primary #alpha# phase from 0 to 70 pct, the fatigue limit of the Ti-6Al-7Nb alloy is raised. Changing the cooling rate after solution treatment by switching from air cooling towrater quenching improves the fatigue limit of the Ti-6Al-7Nb alloy significantly.