Effect of microstructure on creep and creep-fatigue behavior in Ti-6Ali-4V alloy at elevated temperature

The effect of microstructure on creep and creep-fatigue behavior at 773K was studied in the Ti-6Al-4V alloy having three different microstructures. The three types of microstructures prepared using different heat treatment conditions included the equiaxed alpha structure, lenticular alpha structure and bimodal (composed of equiaxed alpha and lenticular alpha) structure. Creep tests were carried out under constant load conditions at 773K in air. Creep-fatigue tests were carried out under total strain controlled conditions using a trapezoidal waveform with hold times of 2min and 10 min at 773K in air. Creep rupture strength of the alloy with equiaxed alpha structure was similar to that of the alloy with lenticular alpha structure and was higher than that of the alloy with bimodal structure. The number of cycles to failure under creep-fatigue condition of the alloy with lenticular alpha structure was lower than that of the other two structures. The effect of microstructure on crack propagation life was small as compared with crack initiation life under creep-fatigue conditions. The fracture mode of the alloy with equiaxed alpha and the bimodal structures was transgranular under creep-fatigue condition. On the other hand, the crack of the alloy with lenticular alpha structure was initiated and propagated at the interface between the alpha layer precipitated at the grain boundary and the lenticular alpha structure.