New insights on the deformation mechanism of fretting fatigue in Ti-6Al-4V

Fretting fatigue (FF) in the dual-phase Ti-6Al-4 V (Ti-64) alloy is a critical failure mode in engineering applications. Despite extensive research on macroscopic fatigue life prediction, the underlying microscopic deformation mechanisms remain poorly understood. In this study, a comprehensive FF investigation was conducted using customised fretting fatigue testing equipment and finite element simulations. The microstructural evolution of regions susceptible to FF crack initiation and propagation was systematically analysed, providing new insights into the deformation mechanisms. For comparison, low-cycle fatigue tests, referred to here as plain fatigue (PF), were also performed. A unique stratified structure was observed at FF crack initiation sites, consisting of (i) an outermost recrystallized nanocrystalline layer, (ii) an oxidation layer, (iii) a deformation twinning layer, and (iv) an underlying zone with a high density of dislocations. Notably, the formation of TiO, accelerated by oxygen transport through microcracks, plays a crucial role in crack initiation and propagation due to volumetric expansion effects. Additionally, the simultaneous occurrence of both extension and compression deformation twinning, along with a high density of 〈c + a〉 dislocations, provides direct experimental evidence of severe microplasticity in the slip region. The selected FF simulation parameters were directly correlated with the experimentally observed crack initiation and propagation, enabling the development of a predictive model for both FF and PF lifetimes. These findings contribute to a deeper understanding of FF deformation mechanisms and are expected to enhance the fatigue life prediction and material design of Ti-64 in critical engineering applications.