High-cycle-fatigue induced continuous grain growth in ultrafine-grained titanium

The cyclic deformation behaviour and microstructural stability of severe plastic deformation processed bulk nanostructured (ultrafine-grained, UFG) commercially pure cp-Ti were investigated by using in situ neutron diffraction combined with R = −1 high-cycle-fatigue (HCF) loading at room and cryogenic temperatures. The UFG microstructure was created by equal channel angular pressing (ECAP) and multi-direction forging (MDF). A considerable continuous grain growth was revealed by neutron diffraction for MDF cp-Ti fatigued at 25 °C, as opposed to that at −200 °C. The same HCF fatigue loading at 25 °C only caused very limited grain growth for ECAP cp-Ti. Transmission electron microscopy confirmed the grain growth. Further confirmation of the room-temperature HCF fatigue-induced grain growth was obtained by transmission Kikuchi diffraction based analysis. Novel insights into fatigue induced grain growth mechanism in UFG cp-Ti are thus provided: (i) the thermally activated process plays an important role in grain growth during the room-temperature HCF fatigue; (ii) Continuous dynamic recrystallisation is responsible for the grain growth and dislocation slip or twinning is not essential to trigger such a grain growth; (iii) the anisotropic grain growth behaviour in {0002} grain family can be reconciled by accepting that these grains accumulated highly stored energy during initial severe plastic deformation and the subsequent recrystallisation nucleation occurred at these highly deformed regions.