A study on the collaborative deformation mechanisms of continuous and discontinuous dynamic recrystallization under low strain rates for the Ti6455x alloy with a low activation energy

To reveal the collaborative dynamic recrystallization mechanisms occurring only at low strain rates, the Ti6455x titanium alloy was designed and prepared by hot deformation. The microstructure, activation energy, and the dynamic recrystallization (DRX) process under different strain rates and temperatures were investigated, with a corresponding mechanism revealed for the first time. Results show that at low strain rates of 0.01 and 0.001 s^-1, as temperature from 1173 to 1223 K, the grain size decreases, jagged bumps appear at the grain boundaries and new small grains are produced. The activation energy of Ti6455x titanium alloy is 128.63 kJ/mol. Quantitative calculations revealed that the thermal deformation mechanisms depend on the dissipation efficiency factor, η. With an increasing value of η, the dynamic softening mechanism changes from dynamic reversion (DRV) to DRV and continuous dynamic recrystallization (CDRX). While with a high η value, CDRX and discontinuous dynamic recrystallization (DDRX) occur at a temperature of 1223 K, and a strain rate of 0.001 s^-1. The collaborative deformation process proceeds because: low activation energy titanium alloys facilitate the rotation of sub-grain boundaries, while dislocations can easily migrate. This condition enables CDRX to occur, leading to the refinement of grains and producing jagged grain boundaries. Then, CDRX provides nucleation sites for DDRX, promoting grain boundary migration and breaking down grains. This mechanism can significantly refine the grain size and improve mechanical properties. It is shown that both the design of low activation energy alloys and controlling hot processing conditions are effective in the development of high-strength titanium alloys.