Fatigue performance of metastable β titanium alloys: Effects of microstructure and surface finish

Abstract

This investigation examined the role of microstructure and surface finish on the high cycle fatigue (HCF) performance of TIMETAL LCB (Ti-6.8Mo-4.5Fe-1.5Al). The as-received microstructure of LCB consisted of elongated β grains with a semicontinuous grain boundary α layer. In contrast, a fine equiaxed β + spheroidized α LCB microstructure was achieved by hot swaging and solution (recrystallization) anneal. The latter modification of the prior β grain structure, together with the size, morphology, and distribution of the primary α phase, resulted in a significant enhancement in the tensile and HCF properties. Furthermore, prestraining (PS), as would be expected during the fabrication of an automotive coil spring, and prior to aging for 30 min at temperatures between 500 and 550 °C, led to additional increases in tensile strength. In contrast, the HCF performance was always reduced when PS prior to aging was included in the overall processing procedure. Finally, shot-peening and roller-burnishing both resulted in an increased fatigue life in the finite life regimen; however, significant reductions in the 10⁷ cycle fatigue strengths were observed when these procedures were used. These observations have been explained by including the effect of process-induced residual tensile stresses in the fatigue analysis, resulting in subsurface fatigue crack nucleation.