Constitutive behaviour and microstructural evolution in thermally deformed Al–Zn–Mg alloy

Commercial simulation software often lacks comprehensive material data for alloys fabricated through powder metallurgy (PM), creating challenges in developing accurate constitutive models. Establishing a clear understanding of the correlation between microstructure and mechanical behaviour during hot deformation is essential for optimizing alloy performance across diverse processing and service conditions. Thus, this study addresses these challenges by constructing and comparing the traditional Johnson Cook (JC) and Modified Johnson Cook (MJC) models for predicting flow stress. Additionally, the study investigates the correlation between microstructure and mechanical properties during the hot compression of an Al-Zn-Mg PM alloy. Experiments were conducted at various temperatures (300 °C and 500 °C) and strain rates (0.1 s⁻¹ and 0.0001 s⁻¹) to evaluate the effect of compression parameters on microhardness, flow stress and microstructural evolution. Electron Backscatter Diffraction (EBSD) analysis revealed that Kernel Average Misorientation (KAM), high-angle grain boundaries (HAGBs), low-angle grain boundaries (LAGBs), and average grain size are significantly influenced by the deformation conditions. The MJC model, with advanced features like a quadratic strain term and strain-rate-dependent temperature factor, achieves higher accuracy, evidenced by an R value of 0.994, AARE of 3.935%, and RMSE of 1.05 MPa. These results highlight the MJC model’s superiority in capturing complex deformation behaviours. At 300 °C and 0.1 s⁻¹, the microhardness reached 130 HV, with a high LAGB percentage (97.33%) and a fine average grain size of 10.91 μm, indicating a strain-hardened microstructure. Conversely, at 500 °C and 0.0001 s⁻¹, the microhardness decreased to 62 HV due to the dominance of dynamic recrystallization, which increased HAGBs percentage (7%) and grain size (19.72 μm). The Zener-Hollomon parameter and activation energy effectively correlate temperature and strain rate effects on microhardness and stress. Higher Z values indicate restricted grain growth and increased dislocation density, resulting in higher microhardness and stress values.