Jiawei Yuan

Microstructure and phase compositions of as-cast Mg–3.9Zn–0.6RE (Gd, Y) alloy with different Gd/Y ratios

Abstract 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.

Effects of isothermal homogenization on microstructure evolution of Mg-7Gd-5Y-1MM-0.5Zr alloy

Abstract 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, Mg 24 (GdY) 5 phase, Mg 5 (GdY) phase and Mg 12 MM phase coexisted together in as-cast alloy; the microstructures were largely characterized by α-Mg matrix and gray globular or elliptic ball Mg 12 MM phase, in addition to those with cubic block Mg 24 (GdY) 5 phase after homogenization; the reasonable homogenization regime was maintained at 530 °C for 32 h.

Homogenization heat treatment of Mg-7.68Gd-4.88Y-1.32Nd-0.63Al-0.05Zr alloy

Abstract The microstructures and mechanical properties of the Mg-7.68Gd-4.88Y-1.32Nd-0.63Al-0.05Zr magnesium alloy were investigated both in the as-cast condition and after homogenization heat treatment from 535 to 555 °C in the time range 0–48 h by optical microscopy, scanning electron microscopy and hardness measurement. The as-cast alloy consisted of α-Mg matrix, Mg5(Y0.5Gd0.5) phase which is a eutectic phase, strip of Al2(Y0.6Gd0.4) phase, little Al3Zr and Mg(Y3Gd) phase. With the increasing of homogenization temperature and time, the Mg5(Y0.5Gd0.5) phase was completely dissolved into the matrix. The Al2(Y0.6Gd0.4) phase was almost not dissolved which impeded grain boundaries motion making the grain size almost not changed in the process of homogenization. The optimum homogenization condition was 545 °C/16 h. The tensile strength increased, yield strength decreased and the plasticity improved obviously after 545 °C/16 h homogenization treatment.

Corrosion behavior of extrusion, under-aged, peaked-aged and over-aged WE93 alloys in NaHSO3 solutions

The corrosion behaviors of extrusion, under-aged, peaked-aged and over-aged WE93 alloys in NaHSO3 solutions were investigated by optical microscopy (OM), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The weight loss rates, corrosion morphologies and polarization curves were analyzed. The peak-aged specimen in NaHSO3 salt spray had the highest corrosion rate of 0.9787 mg/cm2/d, while extrusion specimen led to the lowest of 0.2184 mg/cm2/d. The specimens could be arranged in decreasing order of corrosion rates: extrusion<under-aged<over-aged<peak-aged. Polarization curves also gave effective evaluation of corrosion rates in agreement. The NaHSO3 played a great role in the corrosion behaviors of WE93 alloys. The main reasons were that HSO3− developed a reaction: O2+2HSO3−+2e→2HSO42–→2H++2SO42– to generate H+, accelerating the process of corrosion reaction. SO42– stimulated corrosion and led to an autocatalytic type of reaction: Mg2++SO42–+7H2O→MgSO4− 7H2O. XRD characterized that Mg(OH)2 and MgSO4·7H2O were the main corrosion products.

Hot deformation behavior of AZ40 magnesium alloy at elevated temperatures

Hot compression tests on AZ40 magnesium alloy were conducted on a Gleeble 1500d hot simulation testing machine in a deformation temperature range of 330 °C-420 °C 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 kJ/mol, respectively. Deformation twins and cracks located in grain boundaries were generated at 330 °C 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 °C 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 °C and 0.002 s-1.