Extra work hardening and activation of softening mechanisms induced by α-β interactions of a metastable-β Ti alloy during subtransus processing

The property differences between equiaxed α particles and the β matrix is a dominative factor for the microstructural evolution of metastable-β Ti alloys with bimodal microstructures during subtransus processing. For example, the deformation mismatching between two phases will result in extra work hardening and thus impacting the activation of softening mechanisms within β phase. However, these special mechanisms are hitherto rarely clarified due to the lack of understanding into the interactions between α/β phases in a wide subtransus processing range. In this work, the extra work hardening and activation of softening mechanisms induced by the α-β interactions in a wide subtransus range of 745 °C–820 °C and 0.001 s−1–1 s−1 of a bimodal metastable-β Ti-55531 alloy were studied. Result indicated that variant softening mechanisms including the dynamic recovery, continuous dynamic recrystallization, and discontinuous dynamic recrystallization were activated within β matrix under different processing domains. The fraction, size, and deformability of α particles dominated the extra work hardening and activation of softening mechanisms within β phase by controlling the local storage of geometrically necessary dislocations, formation of subgrains, nucleation, and boundary migration. Moreover, the adiabatic temperature rise under high strain rates indirectly influenced the work hardening activation of softening mechanisms. It is achieved not only by influencing the deformation mismatching, but also by inducing a newly found mechanism of selective α→β phase transformation. Furthermore, microstructural evolution mechanism maps of metastable-β Ti alloys in a wide subtransus range were established for the instruction on the microstructure controlling like the distribution and orientation of α phase, and the subgrain size and uniformity of β phase, etc.