Fatigue property and failure mechanism of TC4 titanium alloy in the HCF and VHCF region considering different forging processes

Abstract

Based on the self-built three point bending ultrasonic fatigue test system, fatigue behavior of TC4 titanium alloy after different forging processes in high cycle fatigue (HCF) and very high cycle fatigue (VHCF) regimes was discussed in this paper. The experimental results showed that the fatigue S-N curves of TC4 titanium alloy with three different structures presented different characteristics: continuous decline; double platform; linear decline. The fatigue property of TC4 titanium alloy is closely related to the content of primary α phase and β-transformed microstructure. For TC4 titanium alloy, grain refinement and certain volume fraction of the primary α phase contributed to enhancing fatigue property. The fatigue performance of bimodal structure by near β forging was obviously better than two structures by α + β forging in HCF and VHCF regime. It was found that failure mode shifted from the surface at relatively high stress to the subsurface at relatively low stress. For three kinds of structures, crack initiations were observed at the surfaces of specimens in HCF regime. Meanwhile, cracks of the three structures all originated from primary α cleavage planes in the interior. The fatigue life of VHCF is dominated by the crack initiation stage.