Effect of multiple forging process on microstructure and mechanical properties of magnesium alloy AZ80

The magnesium alloy AZ80 billet with ultrafine grains (1-2 mu m) was obtained by multiple forging process if the applied total strain epsilon(x) is larger than a critical strain epsilon(c) (2-2.4), while the mechanical properties of the alloy can be significantly improved. The 7-passed forged specimen has the maximum hardness, yield and ultimate tensile strengths, 87.3 HB, 258.78 MPa and 345.04 MPa, which are 1.43 and 2 times hardness and strength of as-cast specimen, respectively, while the 6-passed forged specimen has the maximum elongation of 7.85%, which is 2.45 times that of as-cast specimen. The formation of new grains during the deformation resulted from a series of strain-induced continuous reactions that are essentially similar to continuous dynamic recrystallization and the intercrossing deformation bands developed in the coarse grains are beneficial for the grain refinement. Tensile-fractured surface of the as-cast alloy is different from that of the forged alloy. The former consists of quasi-cleavage fracture and a little shear fracture, and the latter of ductile dimples. With increasing strain both the density of dimple and the homogeneity of dimple distribution increase.