Modeling of cryo-deformation based on grain size-dependent dislocation evolution

In this paper, a physical-based constitutive model for cryogenic deformation was established by introducing internal variables related to temperature, T and grain size, d. Uniaxial tensile tests and microstructure observations were carried out to reveal macroscopic deformation behavior and corresponding microscopic deformation mechanism. The classical Kocks–Mecking model was modified by distinguishing the significant differences in the dislocation evolution in the grain interior and in the vicinity of the grain boundary. The parameters of the constitutive model were optimized by genetic algorithm (GA). The developed constitutive model was comprehensively validated, including the stress-strain curves and formability indexes at the macro level and the evolution of dislocation density at the micro level by using in-situ digital image correlation (DIC) tests and quasi-in-situ electron backscattered diffraction (EBSD) characterization. The coupling effects of grain size and cryogenic temperature (CT) on the evolution of dislocation are quantitatively analyzed and discussed based on the established constitutive model. The studies show that the constitutive model can effectively address the coupling effects of grain size and CT on the deformation behavior of pure aluminum, and accurately describe the deformation characteristics of heterogeneous sheets with gradient grain size at different temperatures. In addition, parametric analysis shows that the predominant dislocation annihilation in ultra-fine grained (UFG) pure aluminum gradually transitions from the vicinity of the grain boundary to the grain interior with the decrease in temperature, resulting in the significant weakening of the strength-plasticity trade-off relationship at cryogenic temperature. These results deepen the understanding of the grain size-dependent cryo-deformation and inspire a promising idea for the direct manufacture of heterogeneous components with grain size gradients.