Papers by Author: Jian Meng

Abstract: The microstructures and electrochemical corrosion behaviors of die-cast Mg-4Al-0.4Mn-xPr (x=0, 1, 4 wt.%) alloys have been investigated. Electrochemical behaviors of all alloys are described by open circuit potential test, potentiodynamic polarization test and electrochemical impedance spectroscope in 3.5 wt.% NaCl solution. The results show that the α-Mg grain is refined and the continuous net β phase appears gradually with increasing the content of Pr in the Mg-4Al-0.4Mn alloys. Moreover, the β phase plays a role of galvanic cathode in AM40 alloy and corrosion barrier in Pr-containing alloys, respectively. Electrochemical measurements show that Pr can improve corrosion resistance of Mg-4Al-0.4Mn alloy in the 3.5 wt.% NaCl solution, and corrosion rate decreases with increasing Pr content.

Abstract: The microstructures and mechanical properties of the Mg-5Y-xMM alloys have been investigated, where MM stands for the Ce-rich misch metal. It was found that the additions of the MM led to refinement of the microstructures and improvement of the mechanical properties. The dynamic recrystallization has occurred during hot extrusion. As a result, the mechanical properties of the alloys were greatly improved after hot extrusion. It was indicated that the specimens of the extruded alloy Mg-5Y-3MM displayed a higher tensile strength, and the values of the ultimate tensile strength and yield tensile strength were 260 and 183MPa, respectively.

Abstract: Mg-4Al-6RE-0.3Mn (RE=La, Ce, Pr, Nd, wt.%) magnesium alloys were prepared successfully by the high-pressure die-casting technique. The microstructures and tensile properties in the temperature range from room temperature to 200 °C were investigated. It was confirmed that the Mg-Al-RE-based alloys exhibited an outstanding die-cast character. The cross-section of test bar could be divided into the fine skin region and the relatively coarse interior region by a narrow band with fine structure. Two binary Al-RE (RE=La, Ce, Pr, Nd) phases with the former being the dominant one, Al₁₁RE₃ and Al₂RE, occupied large grain boundary area. All the four alloys exhibited excellent tensile properties until 200 °C, which were mainly attributed to the fine grain size and the strengthening phase Al₁₁RE₃ with high volume fraction, fine acicular/lamellar morphology, "orderly stack” distribution and good thermal stability. The results of this work provided a basis for further investigation of the new heat-resistant high-pressure die-cast magnesium alloys designed to serve at temperature up to 200 °C.

Abstract: The effect of yttrium addition and heat treatment on the mechanical properties and microstructure of AM60 magnesium alloy have been investigated using X-ray phase analysis, microstructure investigation, tensile test, hardness measurement and fracture surfaces analysis. The results showed that the mechanical properties of the alloys were obviously improved with the addition of yttrium no more than 1.0%. The reinforcement of the alloys resulted from the appearance of Al₂Y phase. After solid-solution treatment (T4), the Mg₁₇Al₁₂ phase almost dissolved in Mg matrix, but the rare earth compounds Al₂Y phase was rather stable. The ultimate tensile strength σb was improved, but the yield strength σ_0.2 and elongation δ were only slightly changed. After solid-solution + aging treatment (T6), the Mg₁₇Al₁₂ phase precipitated again and their morphology was changed. The yield strength σ_0.2 was improved.

Abstract: Several Mg-4Al-4La/Ce alloys were developed and their microstructure and mechanical properties were evaluated. The phase compositions of Mg-4Al-4La alloy consist of α-Mg and Al11La3 phases. While two binary Al-RE phases, Al11RE3 and Al2RE (RE=Ce/La), are formed in Mg-4Al-4Ce/La alloy, and Al11Ce3 and Al2Ce are formed in Mg-4Al-4Ce alloy. The optimal tensile properties are obtained in Mg-4Al-4Ce/La alloy, in which the UTS, YS and δ are 250, 149.2 MPa and 12.24% at room temperature, and 157.8, 116 MPa and 27% at 150oC, and 99.1, 118.2 MPa and 23.2% at 250oC, respectively. The mechanics properties of Mg-4Al-4Ce/La alloy are nearly same with commercial AE44. The compression creep is lower than that AJ62 alloy.

Abstract: The relationship between structure, ionic radius and electronegativity and solubility of the various rare-earth elements in Mg was studied. It is found that light RE(La-Sm, Eu, Yb) have more complicated phase relation with Mg but the heavy RE(Gd-Lu, Sc) have the similar crystal structure with magnesium. Also it is found that the less electronegativity difference between Mg and RE is, the more solubility limit of RE in Mg is. The fact of the RE solubility decreased in magnesium with lowering temperature suggests that there is a possibility of Mg supersaturated solid solution formation and it will decomposition during aging. According to the rule, an megnesium alloy with higher strength feature was developed. Their mechanical properties are UTS 347MPa, YTS 290MPa and elongation 12.5% at room temperature.

Abstract: Magnesium (Mg) alloys are becoming one of the key engineering materials for aerospace and automotive industries because of their low density, high specific strength, excellent machinability and good diecastability, etc. In the meantime, conventional Mg alloys are limited for their low strength and creep resistance. Therefore, special attention is given on its applications at high temperature such as the transmission case and the engine block, In the near decades, much effort have been devoted to improving the properties such as strength, ductility, creep resistance of Mg alloys by adding rare earth (RE) elements, and it has been certified that the addition of RE do improve the performances of the Mg alloys. In this paper, we will review the progresses in the investigations of the Mg-RE alloys as a structural material, and also propose its application prospect in future.

Abstract: A new series of magnesium-rare earth master alloys have been developed by the electrowinning method with subsidence cathode in the KCl•NaCl-RECl3 system. The electrolysis conditions were studied. Experimental results showed that the optimum for electrolyte at 850oC and 10~20wt% RECl3 content in the molten salt system. The current efficiency of electrolyte increased with increasing the rare earth content and reach the maximum at the content of 20% RECl3. The current efficiency gradually decreased with increasing cathode current density. The component of master-alloys was analyzed using ICP-MS and chemical method. A series of magnesium-rare earth master-alloy, Mg-Y, Mg-Nd, Mg-Ce, Mg-La, Mg-Nd-rich and Mg-LPC were successfully prepared and the content of rare earth is adjustable between 5 – 20%.

Abstract: The layered manganese oxides LaSr2Mn2-xTxO7 (T=Fe, Cr and Ni) has been prepared. The effect of substitution of transition metal ions for Mn on magnetic and magtoresistance properties has been studied. The compound has a tetragonal symmetry and the lattice parameters a decreases with increasing the doped content but the parameter c increases with x. It indicates that the MnO6 octahedron increase the distortion with x content. With the x increase, their ferromagnetic property was depressed. The Neel temperature TN was depressed gradually from 138K to 89K, 95K and 85K at x=0.3 for T=Fe, Ni and Cr respectively. Their magnetoresistance MR is gradually enhanced for all doped system. It suggests that the introduce of disorder in B-site have an important effect on the magnetic exchange and magnetoresistance because of the weaken of the double-exchange (DE) interaction.