Implications of grain size distribution, precipitate evolution and texture development on tensile properties in hard plate hot forged and annealed Mg–Zn–Ca–Mn alloy

Abstract

A cast and homogenized Mg–4Zn–0.5Ca–0.16Mn (wt.%) alloy is hot forged at different temperatures (523–623K) employing a new ‘hard plate hot forging’ technique. During the forging operation, the coarse Ca2Mg6Zn3 phase facilitates dynamic recrystallization (DRX) through particle stimulated nucleation (PSN) process. This phenomenon eventually leads to the evolution of bimodal grain size distribution and basal texture in all the forged specimens. The specimen forged at 573K exhibits higher DRX fraction as compared to the specimens deformed at lower (523K) or higher (623K) temperature, due to the pronounced PSN effect originated from the higher fraction of Ca2Mg6Zn3 phase. The texture intensity in the specimen forged at 573K is stronger due to the higher fraction of DRX grains, since the DRX grains exhibit typical basal texture. This specimen exhibits a combination of high room temperature strength (YS ~259 MPa, UTS ~304 MPa) and moderate ductility (eu ~4.2%, ef ~6.5%) owing to the higher basal texture intensity, bimodal grain size distribution, higher fraction of second phase and the presence of nano-sized Mn precipitates. Following annealing of this specimen at 623K for 5 min, an excellent strength-ductility balance (YS ~142 MPa, UTS ~241 MPa, eu ~15%, ef ~19%) is achieved. Such improvement in the tensile properties following annealing has been attributed to the combined influence of the retention of grain size heterogeneity, the weakening of basal texture and presence of nano-sized Mn as well as β1' precipitates.