Multi-scale fatigue failure features of titanium alloys with equiaxed or bimodal microstructures from low-cycle to very-high-cycle loading numbers

Abstract

Fatigue failure types and their characteristics of titanium alloys with equiaxed or bimodal microstructures were systematically studied in low-cycle, high-cycle and very-high-cycle regimes. Based on the fractography, there are multi-scale features closely related to the behavior of crack initiation and early growth in specific microstructure under different loading cycles. At macro-scale, the presence and location of crack initiation with a rough area (RA) are dominated by the external loads and the number of equiaxed α grains in local microstructure domain. At micro-scale, facets are the most prominent features as the mean stress and the failure life increase. There is a trade-off between facets and the granular RA surface in very-high-cycle fatigue (VHCF) under stress ratio R from a positive to a negative value. At nano-scale, due to numerous cyclic pressing, the microstructure underneath the fracture surface is refined to form nanograins and shaping the granules within RA region, which keeps a relatively high VHCF strength at R = −1. As mean stress increases, the fatigue resistance dramatically degrades in VHCF under R > 0, because the RA morphology changes from granules to facets.