Mechanism of work hardening and softening behavior of AZ31 magnesium alloy sheets with hard plate accumulative roll bonding

• In this study, the work hardening and softening behavior of AZ31 magnesium sheets after HP-ARB process was investigated for the first time. At 350 °C, the dynamic balance between work hardening rate and softening rate is achieved. • With the increase of rolling temperature, the average grain size first decreases and then increases. After 350 °C, the minimum grain size is 4.32 µm, the tensile strength reaches 307 MPa and the elongation is 12.73 %. • The degree of work hardening and softening is affected by the interaction between dislocation density and grain size. The joint action of grain boundary strengthening and grain refinement makes the strength and plasticity of the sheets after 350 HP-ARB increase synergistically. • 350HP-ARB pyramidal 〈a〉 and 〈 c + a 〉 have higher slip opening degrees, and more grain boundary density makes the sheets have higher strength, and the fine grain region promotes dislocation intergranular slip, which significantly improves the elongation of the sheets. In this paper, the work hardening and softening behavior of AZ31 magnesium alloy sheets by hard plate accumulative roll bonding (HP-ARB) process in a specific temperature range was studied for the first time, and the cyclic stress relaxation test, EBSD, TEM and other characterization methods were used. When the rolling temperature is 350 °C, the grain size of magnesium sheets is refined to 4.32 (±0.36) µm on average, and it shows an excellent combination of strength and plasticity. The tensile strength reaches 307 (±8.52) MPa and the elongation is 12.73 (±0.84) %. At this time, the curve of work hardening rate decreases smoothly and the degree of hardening is the lowest, and the amplitude of stress drop Δσ p in work softening test is the smallest with the increase of cycle times, which shows that the well coordination between work hardening and softening behavior has been achieved. Research has found that the combined effect of grain boundary strengthening and fine grain strengthening enhances the yield and tensile strength of magnesium sheets after three passes HP-ARB process at 350 °C. This is attributed to the high degree of dislocation slip opening in the pyramidal surface 〈a〉 and 〈 c + a 〉, which not only coordinates the c-axis strain of the entire grain, but also promotes the slip transfer of dislocations in the fine-grained region, significantly improving the elongation of the sheets. This study provides a new idea for the forming and manufacturing of high performance magnesium alloy sheets.