Improvement of microstructure and microhardness of AZ31 Mg alloy sheet by cross-forging-bending repeated deformation with sharply increasing temperature

• FBRD with 90° cross route and increasing temperature effectively refines grain size by 79.3% after four passes, enhancing microhardness to 81.9HV. • Grain refinement is driven by synergistic DDRX, CDRX, and TDRX mechanisms, with {1012} extension twins playing a key role in grain subdivision. • Activation of pyramidal <c+a> slip improves plasticity of AZ31 Mg alloy, addressing its limited formability. • The process demonstrates a promising SPD technique for improving mechanical properties of Mg alloys for lightweight applications.