Fatigue Performance of Heat Treated TC21 Ti-Alloy

TC21 is considered a new titanium alloy that is used in aircraft applications as a replacement for the famous Ti-6Al-4V alloy due to its high strength. The effect of single and duplex stage heat treatments on fatigue behavior of TC21 Ti-alloy (Ti-6Al-2Sn-2Zr-3Mo-1Cr-2Nb-0.09Si, wt.%) was investigated. Two heat treatment cycles were applied on as-received TC21 Ti-alloy. The first cycle was called single stage heat treatment (SSHT). The other cycle was named duplex stage heat treatment (DSHT). Typical microstructures of SSHT & DSHT composed of primary equiaxed α phase, residual β phase and secondary α phase (αs). Secondary α phase was precipitated in the residual β phase due to low cooling rate using air cooling and aging treatment. Morphology of α phase does not change after solution treatments, while their volume fraction and grain size were changed. SSHT showed the highest fatigue strength of 868 MPa due to high tensile strength, hardness and existing of high percentages of residual β phase in the microstructure. However, DSHT reported lower fatigue strength of 743 MPa due to increasing grain size of α phase. The fracture surface of fatigue samples showed cleavage ductile fracture mode for both heat treatment cycles.