Changping Tang

Modification of Mg2Si in Mg-Si alloys with neodymium

Abstract The modification effect of neodymium (Nd) on Mg2Si in the hypereutectic Mg-3%Si (mass fraction) alloy was investigated by optical microcopy, scanning electron microscopy and X-ray diffraction. The results indicate that the morphology of the primary Mg2Si transforms from coarse dendrite into fine polygon with increasing Nd content. The average size of the primary Mg2Si significantly decreases to about 10 μm with increasing Nd content up to 1.0%, and then becomes coarser again. The modification and refinement of the primary Mg2Si are mainly attributed to the poisoning effect. The NdMg2 phase in the primary Mg2Si transforms into NdSi and NdSi2 compounds as the Nd content exceeds 3.0%. Therefore, it is reasonable to conclude that the proper Nd (1.0%) addition can effectively modify and refine the primary Mg2Si.

Dynamic recrystallization mechanisms during hot compression of Mg–Gd–Y–Nd–Zr alloy

Hot compression tests were conducted on a homogenized Mg–7Gd–4Y–1Nd–0.5Zr alloy at 450 °C and a strain rate of 2 s−1. Dynamic recrystallization (DRX) mechanisms were investigated by optical microscope (OM), scanning electron microscope (SEM) and transmission electron microscope (TEM) systematically. The crystallographic orientation information is obtained through electron back-scattering diffraction (EBSD). The result shows that the flow stress firstly reaches a peak rapidly followed by declining to a valley, and then increases gradually again when the alloy is compressed to a strain of −1.88. DRX related to {10 2} tensile twins is extensively observed at small strains, resulting in an evident grain refinement. DRX grains first nucleate along the edges of twin boundaries with about 30° 〈0001〉 off the twin parents. While at large strains, conventional continuous DRX (CDRX) is frequently identified by the formation of small DRX grains along the original grain boundaries and the continuously increasing misorientation from the centre of large original grains to the grain boundaries. Evidence of particle-stimulated nucleation (PSN) is also observed in the present alloy.