Microstructure, texture and mechanical properties in an as-cast AZ61 Mg alloy during multi-directional impact forging and subsequent heat treatment

Abstract

Abstract Multi-directional impact forging (MDIF) and subsequent heat treatment were applied to an as-cast AZ61 Mg alloy and the microstructure, texture evolution and mechanical properties were systematically investigated. The microstructural evolution during MDIF was classified into twinning-active and dynamic recrystallization (DRX)-predominant stages. During the early forging passes, a complicated twinning behavior was observed, including {10–12} primary, {10–12}–{10–12} secondary and {10–12}–{10–12}–{10–12} ternary twins with various twin variants, and even detwinning behavior. With further continuous forging, {10–12} twins disappeared leading to the formation of coarse DRXed grains, and fine DRXed grains began to nucleate at serrated grain boundaries of the coarse DRXed grains due to discontinuous DRX mechanism and progressively consumed the coarse DRXed grains, eventually resulting in a bimodal microstructure. Meanwhile, as-cast random texture turned into a special non-basal texture. After subsequent heat treatment at 400 °C for 2 h, the coarse eutectic network completely disappeared with fine β-Mg17Al12 particles uniformly locating at grain boundaries and the non-basal texture stayed stable, thereby resulting in a good balance of strength (ultimate tensile strength of 259 MPa) and ductility (20.6%).