Gradient microstructure and superior strength–ductility synergy of AZ61 magnesium alloy bars processed by radial forging with different deformation temperatures

Gradient microstructure modification is a cost-efficient strategy for high strength without compromising ductility, which is urgently needed in the fundamental science of engineering materials. In this study, heterogeneous structures of AZ61 alloy bars with anisotropic gradients (with different grain size distributions from the surface to the center) were observed to exhibit strong strength-ductility synergies under different deformation temperatures. The results reveal that the grain refinement process under medium-low temperature deformation conditions (≤ 350 °C) consists of four transition stages along the radial direction, i.e., twin activations and deformation band formations, dislocation cells and pile-ups, ultrafine sub-grains, and randomly orientated quasi-micron grains. Different deformation temperatures have a great influence on twin activations and deformation band formations, and the high temperature can easily provoke the initiation of non-basal slip. The deformation bands were determined as a primary nucleation site due to their highly unstable dislocation hindrance ability. Analysis in combination with the Radial forging (RF) deformation process, the differences of dynamic precipitates can be attributed to microstructural difference and solubility limit of Al at different temperatures. By summarizing the tensile test results, the sample forged at 350 °C exhibited the best strength–ductility synergy, exhibiting the highest elongation (EL) of 23.2% with a 251 MPa yield strength (YS) and 394 MPa ultimate tensile strength (UTS) in center region, and combined with the highest strength value of 256 MPa YS and 420 MPa UTS in the center region, while the EL was slightly degraded to 19.8%.