Grain refinement in a magnesium alloy AZ31 was studied in multi-directional forging (MDF) with decreasing temperature from 623 K to 423 K and at a strain rate of 3×10−3 s−1. The MDF was carried out up to large cumulative strains with changing the loading direction during decreasing temperature from pass to pass. The structural changes are characterized by the generation of many mutually crossing kink bands at low strain, followed by the development of very fine grains at large strains. The dynamic change in grain size can be expressed by two different power law functions of flow stress for the region of flow stress above or below 100 MPa. The MDF with decreasing temperature can accelerate the uniform development of much finer grains and the improvement in plastic workability, leading to the minimal grain size of 0.23 μm at 423 K. The mechanism of grain refinement is discussed in detail.
