Superplastic constitutive modeling of TA32 alloy with two-phase characteristics

This study proposes a physically-based constitutive model to quantitatively describe both macro-mechanical behavior and dynamic two-phase microstructure evolution of near-α TA32 Ti alloy under superplasticity-favored deformation conditions. The alloy’s microstructure morphology and orientation were observed, encompassing the α to β phase transformation, the steady-state evolution and dynamic coarsening of primary α (α_p) and parent β grain sizes, the simultaneous occurrence of discontinuous dynamic recrystallization (DDRX) and continuous dynamic recrystallization (CDRX) within α phase, and crystallographic orientation stability of the α-Ti lattice. It is demonstrated that grain boundary sliding (GBS) predominates in superplastic deformation. The superplastic constitutive model was developed following the identified two-phase microstructure characteristics and deformation mechanisms, elucidating the relationships among phase fraction, grain size, dynamic recrystallization (DRX) fraction, and dislocation density, while given variations in plastic strain. This model effectively describes the two-phase flow behavior of TA32 alloy during superplastic deformation, considering the dislocation densities of both phases and the effect of phase growth on β grain size. Furthermore, this model was implemented into the VUMAT subroutine to develop a finite element (FE) model enabling accurate prediction of the shape and microstructure distribution in uniaxial tensile specimens. The simulation results show a steady-state grain size and demonstrate excellent predictive capabilities for flow stress and rate-dependent internal state variables both within and outside the calibration range. The superplastic forming (SPF) process of a pyramidal lattice structure was simulated using this model, successfully capturing the evolution of α/β phase characteristics during the forming of geometrically complex TA32 alloy components.