Mechanical Properties of Bimodal AZ91 Alloy Prepared by Low-Temperature Slow Rate Extrusion and Electrical Pulse Treatment

Low-temperature slow rate extrusion (LTSRE) and electrical pulse treatment (EPT) were used to obtain the AZ91 Mg alloy with bimodal microstructure. Results show that the alloy consists of coarse unrecrystallized grains of 20~60 μm and fine recrystallized grains of ~200 nm. The bimodal grains contribute to the inhomogeneous deformation under LTSRE and the acceleration effect of the static recrystallization of the deformed AZ91 magnesium alloy under EPT. In addition, the growth of the recrystallized grains is effectively restrained due to the notably low recrystallization temperature and short processing time, compared with those of the conventional static recrystallization by heat treatment. Meanwhile, abundant Mg₁₇Al₁₂ phases with an average size of 200 nm and regular shapes are precipitated during EPT. Consequently, the yield strength of 463 MPa and ultimate tensile strength of 527 MPa were acquired for the bimodal AZ91 alloy, which can be primarily attributed to the bimodal microstructure as well as the combined effect of fine grain strengthening, precipitation strengthening and work hardening.