Papers by Author: Xin Ying Teng

Abstract: Magnesium alloys have been widely applied in many fields, because of their high strength-to-weight ratio. However, magnesium alloys have high chemical activity and are easily corroded. The poor corrosion resistance of magnesium alloys greatly limits its further application. In this paper, the zinc phosphate conversion coatings were prepared on the surface of AZ91D magnesium alloys. Nano-zinc oxide was the source of zinc and the zinc phosphate conversion coatings were prepared by the given process: 1.25 g/L NaNO₃, 3 g/L C₆H₈O₇ H₂O, 2.5 g/L NaF, 5.5 g/L ZnO, 12.5 mL/L H₃PO₄, reaction temperature 50°C, reaction for 30 minutes. The full immersion uniform corrosion test was conducted for the fabricated coatings. The morphology and composition of corrosion in different corrosion stages were characterized by XRD, SEM and other microscopies. The results showed that: (1) the corrosion process of the conversion coatings could be divided into three stages: the dissolution of the conversion coatings, the corrosion of the matrix and the deposition of insoluble matter; (2) XRD analysis and other methods found that the pine-needle magnesium oxychloride compounds were formed in the process of immersion firstly, and it was dissolved into Mg(OH)₂ over time; (3) With the extension of immersion time, Mg(OH)₂ increased continuously and played a major role in corrosion prevention. The deposited Mg(OH)₂ was divided into two layers. In the initial deposition stage, it was mainly evenly dispersed on the surface of the alloy to form a tightly arranged inner layer. Afterwards, the crystals of Mg(OH)₂ agglomerated and formed a sphere, becoming the outer layers.

Abstract: The effect of Ca and Zr Additions and Aging Treatments on Microstructure and Mechanical Properties of Mg-Sn alloy was investigated. It was found that the grain size of as-cast Mg-4Sn-xCa and Mg-4Sn-xZr alloys was refined with the increase of alloying elements addition. The alloys were solution-treated at 480 °C and aged at 160 °C, and the aging peak appeared after 4-5 h. The difference was that the maximum tensile strength and Brinell hardness of Mg-4Sn-0.3Ca were 140.7 MPa and 44.5 HB, respectively, while in Mg-4Sn-xZr alloy, Mg-4Sn-0.5Zr was optimal. The maximum tensile strength and Brinell hardness of Mg-4Sn-0.5Zr were 137.4 MPa and 41.5 HB, respectively. This difference was mainly due to the formation of the brittle phase CaMgSn in the Mg-4Sn-xCa alloy. The excessive brittle phase was not conducive to the strength of the alloy, but could increase the hardness of the alloy. However, Zr existed as a simple substance in the alloy, which can be used as a nucleation particle to inhibit grain growth and play a role of fine grain strengthening. But the addition of Zr did not form many hard phases, so the hardness did not change much.

Abstract: The effects of rare earth Y addition on microstructure and properties of pure copper were investigated. Mechanical test, electrical test, oxidation resistance test, metalloscope, scanning electronic microscope (SEM) and X-ray difffraction (XRD) were performed to study the properties, microstructure and constitution. The results showed that both the hardness and antioxidant properties obviously increased with the increase of Y, confirmed the successful refinement role of Y. A small amount of Y (less than 0.5 wt.%) could improve the electrical conductivity of pure copper. When the Y content reached 0.2 wt.%, pure coppers obtained optimum electrical conductivity which is 96.8% IACS. However, over-added Y (>0.5 wt.%) resulted in second phase of Cu₇Y coarsening and non-homogeneous microstructures forming, which reduces the conductivity of copper. In addition, Y can effectively purify the organization of molten copper.

Abstract: A356.2 aluminum alloy (Al–7Si–0.35Mg) has been widely used in automotive and aircraft industries. Previous studies found that the metamorphism effect of rare earth is better than other type of elements because of long modification time and good stability. The influence of Yb addition (0%, 0.2%, 0.4% and 0.6%) and T6 heat treatment on A356.2 alloy has been investigated in this work. The microstructures and mechanical properties of the specimen after T6 treatment were examined by optical microscope, scanning electronic microscope and tensile tests. Experimental results showed that Yb could reduce the size of α-Al and change the Si morphology from needle-like to fine spheroidal particles. With the increase of Yb content, the ultimate tensile strength increased gradually. When adding 0.4%Yb, the alloy achieved the highest ultimate tensile strength (252 MPa) and hardness (97.3HB), 10.12% and 37.66% higher than the alloy with no Yb addition. Tensile fracture analysis showed that the fracture mechanism for A356.2 aluminum alloy after T6 treatment is transgranular/intergranular mixed mode of fracture.

Abstract: The influence of cooling rates on the solidification and microstructure of rapidly solidified quasicrystal alloy Mg_70.8Zn₂₈Nd_1.2(at.%) was investigated. The microstructure, phase constitution, phase transition and phase structure of the alloys were examined by means of scanning electron microscopy, x-ray diffraction, energy dispersive spectrometer, differential scanning calorimetry. The experimental results showed that the phase composition of as-cast Mg_70.8Zn₂₈Nd_1.2 alloy includes quasicrystal I-phase and Mg₇Zn₃ phase. For the rapidly solidified alloy ribbons, when the speed is not higher than 400 r/min, the microstructure includes I-phase, Mg₇Zn₃ phase and α-Mg phase. When the speed is at the range of 400-2000r/min, the Mg₇Zn₃ phase disappears and only quasicrystal with α-Mg phase exist. With the increase of cooling rate, the grain size decreases and there are a large number of microcrystals in the microstructure. When the speed reaches higher than 2500 r/min, amorphous phase appeared. Differential thermal analysis showed that quasicrystal exist at about 340°C.

Abstract: In this paper, the microstructure and mechanical properties of the as-cast and heat treatment of Mg-Zn-Nd alloy was investigated. The alloy was manufactured by a conventional casting method, and then subjected to a heat treatment. The results showed that the microstructure of as-cast alloy was comprised of α-Mg matrix and Mg₁₂Nd phase. With increase of Nd content, the grain size gradually decreased from 25.38 μm to 9.82 μm. The ultimate tensile strength and elongation at room temperature of the Mg₉₄Zn₂Nd₄ alloy can be reached to 219.63 MPa and 5.31%. After heat treatment, part of the second phase dissolved into the magnesium matrix and the grain size became a little larger than that of the as-cast. The ultimate tensile strength was declined by about 2.5%, and the elongation was increased to 5.47%.

Abstract: The microstructure of as-cast Mg_97-xY₂Zn₁Nd_x (x = 0, 0.5, 1.0 and 2.0 at. %) alloys was investigated using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical properties were measured by universal testing machine. The corrosion behavior was evaluated by Tafel curves and hydrogen evolution reaction. The results showed that the α-Mg, MgZn, Mg₁₂Nd and Mg₁₂YZn phases exist in the Mg-Zn-Y-Nd alloy. Mg₁₂YZn is a long period stacked ordered (LPSO) phase. When x is, the addition of less than 1at. % Nd can promote the formation of LPSO phase. The addition of more than 1at. %Nd could result in the formation of Mg₁₂Nd phase, and the volume fraction of the Mg₁₂Nd phase in Mg₉₅Zn₁Y₂Nd₂ alloy is larger than that of others. The ultimate tensile strength (UTS) of Mg₉₆ZnY₂Nd₁ alloy reaches 197.8MPa because of small grain size and uniform distribution of the LPSO and MgZn phases. The corrosion rate of Mg_96.5Zn₁Y₂Nd_0.5 alloy is lower than that of other alloys due to its lower self-corrosion current density and hydrogen evolution rate.

Abstract: As-cast Mg-3Er-xZn (x = 0, 0.5, 1 and 2 at.%) alloys were prepared by a cast ingot metallurgy process. The effect of Zn contents on the microstructure and phase constitution of the alloys was investigated by optical microscope (OM), scanning electron microscope (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and transmission electronic microscope (TEM). The results revealed that the as-cast Mg-3Er alloy mainly consisted of α-Mg phase and MgEr eutectic phase. Typically, a small amounts of LPSO phase was precipitated at grain boundaries by adding 0.5 at.% Zn into Mg-3Er alloy. With the addition of 1 at.%Zn, the volume fraction of LPSO phase increased obviously. When the addition of Zn increased to 2 at.%, the volume fraction of LPSO phase decreases and Mg₈ZnEr phase forms at grain boundaries. Tensile test indicated that Mg-3Er-1Zn alloy exhibits the excellent mechanical properties. The tensile strength, yield strength and elongation of Mg-3Er-1Zn alloy are 213 MPa, 187.6 MPa and 5.52%, increased by 38.8%, 60% and 3.19%, respectively, corresponding to the alloy without Zn addition. The excellent mechanical properties could be attributed to the introduction of Zn element in alloys, which leads to the strengthening of LPSO phase and grain refinement of α-Mg.

Abstract: Metal matrix composites reinforced by graphene particles exhibit physical and mechanical property and are developed and qualified for use in aerospace structure, bioengineering, energy storage material and photoelectric device. In the present paper, graphene was fabricated by modify Hummers method, and then was surface modified by chemical plating copper. The graphene/Al composites were fabricated by powder metallurgic method. Morphology characterization of graphene and composites were detected by XRD and SEM,the fabrication parameters of composites were optimized by testing harness and density. The volume fraction of graphene particles was 3%, the density of composites was maximum of 96.5%. The hardness had a maximum of HB 42.6, and the hardness of graphene/Al composites increased by 33.5%.

Abstract: The TiO₂ reduced graphene oxide (TiO₂/RGO) nanocomposites were synthetized by hydrothermal method. The microstructure and morphologies of them were characterized by XRD and SEM. The photocatalytic activity was investigated by the methyl orange photogradation under UV illumination. The results showed that GO sheets had wrinkles and folds, and anatase-structured TiO₂ covered on graphene surface after hydrothermal reaction. Compared to the TiO₂ nanoparticles, the TiO₂/RGO nanocomposites display the higher photogradation efficiency. In 90 minutes, the gradation percentage of TiO₂/RGO nanocomposites to methyl orange is 80%, higher than TiO₂ nanoparticle (40%). This is attributed to the large surface area of TiO₂/RGO nanocomposites and their improved separation efficiency of electron-hole pair.