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 degrees C, as opposed to that at -200 degrees C. The same HCF fatigue loading at 25 degrees 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. (C) 2019 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.