Effects of conventional machining on high cycle fatigue behavior of the intermetallic alloy Ti-47Al-2Nb-2Cr (at percent)

The purpose of this work was to test the hypothesis that surface alteration by conventional machining processes will have a strong influence on the high cycle fatigue (HCF) behavior of brittle intermetallic alloys. The near-gamma titanium aluminide alloy Ti-47Al-2Nb-2Cr (at%) was selected as a model for this class of alloys. Grinding and turning under production conditions doubled the near-surface hardness, and turning hardened the alloy to a depth of {dollar}180mu{dollar}m. The heavily deformed outer {dollar}20mu{dollar}m of turned samples recrystallized in less than 1 hour when heated to the anticipated service temperature of {dollar}760spcirc{dollar}C.; Step loaded axial fatigue tests comparing turned samples to an electropolished control group were performed at 23 and {dollar}760spcirc{dollar}C. The initial maximum stress level was 75% of the yield strength, and was increased 20 MPa after every million cycles until failure. Miner's Rule analyses of the step loaded test histories confirmed that the last two stress levels are most damaging. Fatigue behavior was defined by three parameters: the highest stress at which the sample survived a {dollar}10sp6{dollar} cycle block {dollar}(10sp6{dollar} cycle endurance strength, {dollar}rm Ssb{lcub}e{rcub}),{dollar} the maximum stress at failure (fatigue strength, {dollar}rm Ssb{lcub}f{rcub}),{dollar} and the number of cycles at the final stress level {dollar}rm(Nsb{lcub}f{rcub}).{dollar} (Because of the step loaded test method, {dollar}rm Ssb{lcub}e{rcub}{dollar} is 20 MPa less than {dollar}rm Ssb{lcub}f{rcub}.){dollar}; No effect of machining on fatigue strength or initiation sites was found at {dollar}23spcirc{dollar}C. At {dollar}760spcirc{dollar}C, turning improved average strength by 5% and average {dollar}rm Nsb{lcub}f{rcub}{dollar} by about 1.5 orders of magnitude. This improvement was attributed to fine recrystallized grains in the outer 10-30{dollar}mu{dollar}m which suppressed surface crack initiation. Air exposure at {dollar}760spcirc{dollar}C for 24 hours prior to {dollar}23spcirc{dollar}C testing reduced fatigue strength compared to the as-manufactured condition. In all cases, cracks initiated at stress intensities below the long crack thresholds. Stress intensity threshold values were independent of surface condition and test temperature.; These results indicate that surface deformation from conventional machining will not have a strong effect on fatigue strength at low temperatures. However, thermal stability of the deformed layer and environmental effects can be significant variables in fatigue behavior of brittle intermetallic alloys.