A unified internal state variable material model for Ti2AlNb-alloy and its applications in hot gas forming

The high-temperature deformation behavior of Ti2AlNb sheet was studied by the uniaxial tensile tests at the temperatures from 910 degrees C to 1000 degrees C with the strain rates 0.0004-0.1 s(-1). The microstructure evolutions of the as-received specimen and the hot deformation specimens were investigated by the electron back-scattered electron image and electron back-scattered diffraction. The flow stress and microstructure evolution are modeled via a set of internal-state variable unified constitutive equations based on the changes of dislocation density, dynamic recrystallization, grain size, phase volume fraction, globularization softening, damage evolution and deformation heating. The model constants were determined by the GA-based global optimization in MATLAB soft. The flow stress of the internal-state variable unified model was more precise than the flow stress of the Fields-Backofen model under the multistep strain rate conditions. The Ti2AlNb cup-shape parts were produced by the hot gas forming and the microstructures were predicted by the software ABAQUS using the VUMAT subroutine. The simulation predictions of microstructure evolution, i.e. dislocation density and materials damage, for the Ti2AlNb based alloy during hot gas forming process are in consistence with experimental results.