Role of deformation heterogeneity in the superplasticity of Ti6321 titanium alloy with a bimodal microstructure

Abstract

Superplastic forming offers a promising approach for producing critical ultra-thick titanium alloy components for marine applications. Near-α Ti6321 titanium alloy with a bimodal microstructure provides excellent mechanical performance and is therefore widely used in marine engineering. However, the influence of microstructural heterogeneity on its superplastic behavior and the underlying deformation mechanisms remains insufficiently understood. In this work, interrupted high-temperature tensile tests combined with systematic microstructural characterization were performed. The results reveal that deformation is accommodated by distinct yet coupled mechanisms in equiaxed α and lamellar α regions. In equiaxed α, plastic flow is predominantly governed by strong dynamic recovery and continuous dynamic recrystallization (DRX), with discontinuous DRX activated at intermediate strains. In contrast, lamellar α undergoes pronounced strain- and orientation-dependent dynamic globularization and more extensive DRX, particularly in Type II/III colonies. A distinct DRX mechanism, epitaxial recrystallization, is identified in lamellar α at low strains, where newly formed α grains preserve the local Burgers orientation relationship with adjacent β and form semi-coherent, low-misfit α/β interfaces. Two-beam TEM observations reveal extensive <a> and <c+a> dislocation activity, consistent with the in-grain misorientation axes results showing that the preferred Taylor axes progressively shift from <0001> toward <01¯12> and <uvt0> with increasing strain. The co-activation of multiple slip variants gradually alters the local misorientation axes away from those established by preceding slip, thereby modifying local grain orientations through slip-induced lattice rotation and promoting increasingly dispersed slip activity. The synergistic interplay of heterogeneous slip and multiple DRX mechanisms promotes the transformation of hard-slip-dominated lamellar α into a fine-grained equiaxed microstructure. This microstructural homogenization facilitates multidirectional soft slip and grain boundary sliding, thereby underpinning the superior superplastic response of the alloy.