Full-stage prediction of discontinuous dynamic recrystallization of a titanium alloy through a sub-mesh internal state variables method

A sub-mesh internal state variables (ISVs) based cellular automata-crystal plasticity finite element model (CACPFEM) is proposed for the full-stage prediction of discontinuous dynamic recrystallization (DDRX) of a titanium alloy during deformation. The idea of the sub-mesh scale is proposed to break through the mesh size limit in a finite element analysis by considering the typical physical characteristics of a new DDRX nucleus and the computational availability. Physical characteristics of a new DDRX nucleus including extremely small size, new grain orientation, low level dislocation density and strain-free deformation state, can be fully considered in the sub-mesh scale. The subsequent growth of the nucleus during deformation can be well described by introducing an important ISV, the volume fraction of the DDRX grain in the sub-mesh scale. The microstructure evolution of DDRX will be calculated by the previously developed CACPFEM when the DDRX grain reaches the mesh size. By doing this, the full stage of the DDRX evolution during hot deformation can be well predicted. With a titanium alloy (Ti-6Al-2Zr-1Mo-1V) being applied, it can be found that the size of the nucleus plays an important role in slip resistance at early stage. The grain growth velocity of the DDRX grain in sub-mesh scale first increases and then decreases with the dislocation density increasing from a low level. The stress of the DDRX grain increases from zero to a high level that is relevant to its grain orientation. The equivalent plastic strain rate of DDRX during early stage keeps an 'S-type' increasing trend during deformation.