The Diffusion And Homogenization Research On Mg-xGd-Zr Alloys

The reactive diffusion in Mg-Gd binary system between 450℃-550℃ were investigated by Optical Microscopy (OM), Scanning Electron Microscopy (SEM) and Electron-probe Microscopy Analysis (EPMA). Moreover, the thermodynamics and dynamics in the diffusion were analyzed. Meanwhile the microstructure evolution of Mg-xGd-Zr(x=1、2、3at%) alloys, annealed in different atmospheres, were analyzed by means of OM, SEM, X-ray diffraction(XRD) and Transmisson Electron Microscope(TEM) as well as the composition of the cubic-shaped phase. Furthermore, the formation mechanism of the cubic-shaped phase was proposed.The results of Mg-Gd reactive diffusion show that four different intermetallic layers are formed between 450 ℃ and 550 ℃, and they are Mg5Gd, Mg3Gd, Mg2Gd and MgGd, respectively. The intermetallic layers thickness increase with the increasing annealing temperature and time and it can be described as:1=ktn. The value of n is nearly equal to 0.5 between T=450℃-500℃, which means the reactive diffusion is mainly controlled by grain boundary diffusion at this temperature range. While n=0.36 at T=550℃, which indicates that the grain growth occurs at a certain rate. The diffusion frequency factor and energy were calculated by Gaussian solution and the values are Do=1.49×10-9m2/s and Q=65.39kJ/mol, respectively.In the heat treatment experiments of Mg-xGd-Zr (x=1、2、3at%) alloys, the optimized homogenization system for the different alloys l#-3# are 520℃×6h,520℃×8h and 520℃×12h, respectively. The cubic-shaped phase, which has an FCC structure with the lattice constant a=0.53nm, has high thermal stability and can’t be eliminated at elevated temperature. The Gd content in the cubic-shaped phase is 41.3at%-89.0at% and this is quite different from the intermetallics in Mg-Gd equilibrium phase diagram. Meanwhile an interesting phenomenon was observed when the alloys were reheated after quenching, a number of cubic-shaped particles formed along the grain boundaries in the surface layer (0-2mm) but not in the center(2mm below the surface), which indicates that quenching-induced stress promotes the rare earth element to immigrant to the grain boundaries to form the cubic-shaped particles.The results of alloys annealed in different atmospheres (pure H2, O2, Ar and Air) show that the composition of the cubic-shaped phase is controlled by the atmosphere but they have the same lattice structure and constant, they are GdH2 and Gd2o3 when the alloys annealed in H2 and O2, respectively. The cubic-shaped particles formed in Air atmosphere are mainly Gd2O3, which are mainly formed during annealing and only a few are formed during melting. The cubic-shaped particles are mainly formed by the eutectic decomposition and then oxidized by oxygen. In summary, the cubic-shaped phase in air atmosphere is mainly Gd2O3 and it is prior to be formed in the RE-rich and high energy region, the stress and high temperature has significant contribution to the formation of Gd2O3...