The microstructure and properties of the Mg-9Y-1MM-0.6Zr alloy were studied by scanning electron microscopy, optical microscopy, transmission electron microscopy, hardness and tensile testing. Homogenization was confirmed, and the solidification model was established. The as-cast alloy was mainly composed of α-Mg and eutectic structures. The suitable homogenization pa- rameters were 535 ℃ for 18h. Most of the eutectic structure dissolved in this process. Only the Mg4.26Y95.74 and Mg12(MM) phases were remained in, and only a few microstructure occurred for prolonged time. The ultimate tensile strength of the alloy increased up to 232 MPa after homogenization, but the yield strength and the elongation were almost as same as the as-cast state. In the as-cast al- loys the cracks started in the eutectic structure, and then intergranular fracture occurred, but in the homogenate alloys residual phases became the source of the cracks and then transcrystalline fracture took place. The most important roles of homogenization were the decomposition of the eutectic structure, furthermore the intergranular fracture turned into a transcrystalline fracture, which was one of the main reasons to increase the strength of the alloys.
Hot compression tests on AZ40 magnesium alloy were conducted on a Gleeble 1500 d hot simulation testing machine in a deformation temperature range of 330 ℃-420 ℃ and a strain rate range of 0.002-2 s^(-1). Hot deformation behaviors were investigated on the basis of the analysis of the flow stressstrain curves, constitutive equation, and processing map. The stress exponent and apparent activation energy were calculated to be 5.821 and 173.96 k J/mol, respectively. Deformation twins and cracks located in grain boundaries were generated at 330 ℃ and 0.02 s^(-1), which are associated with a high strain rate and a limited number of available slip systems. With increasing temperature and decreasing strain rate, the twins disappeared and the degree of dynamic recrystallization increased. The alloy was completely dynamically recrystallized at 420 ℃ and 0.002 s^(-1), with a homogenous grain size of approximately 13.7 μm. The instability domains of the deformation behavior can be recognized by processing maps. By considering the processing maps and characterizing the microstructure, the optimum hot deformation parameters in this experiment were determined to be 420 ℃ and 0.002 s^(-1).
The corrosion behaviors of T5(225℃,6.5 h)and T6(460℃,2 h + 225℃,12 h) peak-aged Mg-7Gd-5Y-lNd-0.5Zr alloys with oxide films were investigated by optical microscope(OM),scanning electron microscopy(SEM),and energy-dispersive spectroscopy(EDS).The weight loss rates and electrochemical tests were also analyzed.The thicknesses of T5 and T6 oxide films are roughly 0.6 and1.0 μm,respectively.The components of oxide films mainly consist of O,Mg,Y,Nd,and Gd,and the T6 oxide film results in surfaces with larger peaks than T5 oxide film.In addition,Y,Nd,and Gd peaks are all higher than those of Mg-7Gd-5YlNd-0.5Zr alloys,but Mg peak is consistently far below than that of the alloys.The specimens could be arranged in decreasing order of corrosion rates and corrosion current densities:T6 oxide film < T5 oxide film < T6 without oxide film < T5 without oxide film.The oxide films are compact to increase the corrosion resistance for Mg-7Gd-5Y-lNd-0.5Zr alloys,which will provide a guiding insight into the corrosion and protection of Mg-RE alloys in atmospheric environments.
Mg–Zn–RE(Gd, Y) alloys with different Gd/Y atomic ratios were prepared by conventional casting, and the microstructure of the alloys was studied by multiple means. Icosahedral quasicrystal phases are observed in all alloys. The different Gd/Y atomic ratios affect the microstructures of the alloys irregularly. The alloy with more Gd has large dendritic structure and more complicated phase composition which are composed of I-phase lamellar eutectic, W-phase divorced eutectic, Mg–RE cuboid particles and Mg–Zn binary phases. Other two alloys show similar microstructures and phase compositions with very thin lamellar eutectics which distribute along the interdendritic region, and the lamellar eutectics are formed by I-phase and Mg. The element contents of the I-phases and Mg–RE phases are partially controlled by the Gd/Y atomic ratio.
Optical microscopy(OM), scanning electronic microscopy(SEM) and X-ray diffraction(XRD) were performed to investigate the influence of homogenization on the microstructures of the Mg-7Gd-5Y-1MM(Ce-rich RE)-0.5Zr magnesium alloy. The results indicated that α-Mg, Mg24(Gd Y)5 phase, Mg5(Gd Y) phase and Mg12 MM phase coexisted together in as-cast alloy; the microstructures were largely characterized by α-Mg matrix and gray globular or elliptic ball Mg12 MM phase, in addition to those with cubic block Mg24(Gd Y)5 phase after homogenization; the reasonable homogenization regime was maintained at 530 oC for 32 h.
