Grain size dependent unloading anelastic behavior and multi-scale model of AA6061 aluminum alloy

The varying grain size distributions in aluminum alloy billets induce distinct responses in their macroscopic plastic deformation and unloading behavior. To investigate these differences and develop unloading models accounting for the initial grain size distribution of the billet for the deformed alloy, AA6061 aluminum alloy billet with a gradient grain size distribution was selected as the test material in this study. Nanoindentation loading-unloading tests were conducted to investigate the effect of initial grain size on the microstructural mechanical behavior and hardness distribution of the alloy. The results indicate that the difference in deformation between the intragranular and intergranular regions under the effect of initial grain size is a key factor determining the unloading springback behavior. Uniaxial tensile and loading-unloading-reloading tests were conducted to investigate the effect of initial grain size on the macroscopic stress-strain response, nonlinear unloading behavior, and fracture mechanisms of the alloy. Based on the nonlinear unloading behavior of aluminum alloys affected by initial grain size, a novel chord modulus model accounting for the effect of initial grain size for the deformed alloy is proposed. Furthermore, a macroscopic phenomenological anelastic strain model considering the synergistic effect of stress, strain, and grain size is established. At the same time, based on the dislocation motion theory and taking into account the effect of the initial grain size of the billet, a microscopic anelastic strain model was developed. The multiscale anelastic strain model developed for alloy deformation by accounting for the effect of grain size in the billet accurately describes the anelastic behavior of the aluminum alloy after deformation.