Formation of nanocrystalline and nanolaminate structures in a twinning induced plasticity Ti-12Mo alloy: The role played by {332} 〈113〉 twinning

Grain refinement is an effective strategy to strengthen metastable β titanium alloys. {332}<113> twinning, a typical deformation mode, its role on grain refinement still remains unelucidated. Here, we produced a gradient nanostructured layer with a depth of 800 μm in a metastable twinning induced plasticity β-Ti-12Mo (wt.%) alloy by means of surface mechanical rolling treatment. We found that {332}<113> deformation twinning, mainly dominated the deformation at moderate strain and strain rates, refining coarse grains into submicro-structures. With increasing strain and strain rates, dislocation slip instead of {332}<113> twinning prevailed. These dislocations were first arranged into extended boundaries and then into laminated structures to coordinate deformation. Eventually, shear banding and fragmentation of laminates result in the formation of nanocrystalline with size of 34 ± 8 nm. The nanostructured Ti-12Mo alloy exhibited the high hardness of 4.9 GPa at subsurface. The surface hardness decreased to 4.4 GPa which was mainly caused by the inhibition of stress-induced martensitic transformation α'' and the occurrence of its reversal transformation due to adiabatic heating induced by the high strain rate. This work expands our knowledge on strengthening β titanium alloys by {332}<113> twinning induced grain refinement at different deformation conditions.