Zhang Dingfei

Neodymium-based Conversion Coating on AZ31 Magnesium Alloy

Abstract A neodymium-based (Nd-based) conversion coating was obtained on the surface of AZ31 magnesium alloy. The formation process of the Nd-based conversion coating was examined by a weighing experiment and an open circuit potential (OCP) test. The coating morphology, microstructure and composition were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), respectively. The variation of morphology for the bare sample and the coated sample immersed in 3.5 wt% NaCl aqueous solution was investigated. The results show that the Nd-based conversion coating can effectively reduce the corrosion speed in 3.5 wt% NaCl aqueous solution. The composition and corrosion resistance of Nd-based conversion coating after post-treatment were measured by XRD and electrochemical experiments. The post-treatment can perfectly improve the corrosion resistance of the Nd-based conversion coating.

Precipitate Evolution in Mg-6 wt%Zn-1 wt%Mn Alloy

Abstract The evolution rules of transitional phases β 1 ′ and β 2 ′ during single and double aging and the heterogeneous nucleation of β 1 ′ rods on α-Mn particles in a Mg-6 wt%Zn-1wt%Mn (ZM61) alloy were studied by TEM technique. Age-hardening curves (130, 160, 180, 200 and 230 °C) show that double aging can more greatly enhance hardening level and rate than single aging. The rapid nucleation and the elongation of β 1 ′ rods occupies the stage before peak aging; the volume fraction increase of β 1 ′ rods results in hardness rise and also a huge accumulation of strain energy; in the over-aging stage the growth of β 2 ′ discs at the cost of β 1 ′ rods can effectively relieve such a strain energy accumulation. G. P. zones of about 10 nm in diameter are densely distributed in matrix after the pre-aging (90 °C/24 h), which can act as the heterogeneous nuclei for β 1 ′ rods and thus considerably raise their number density. Moreover, β 1 ′ rods frequently heterogeneously nucleate on α-Mn particles, because such heterogeneous nucleation can result in a coherent β 1 ′/α-Mn interface and thus reduce the interfacial energy.

Luminescence Properties of Eu/Tb Activated Y 2 O 3 Phosphors Synthesized by Solid State Process

Abstract Rare-earth doped phosphors with tunable optical properties have potential applications in solid state lighting, such as display panels and fluorescent lamps. Herein, trivalent Eu 3+ , Tb 3+ activated Y 2 O 3 phosphors were synthesized by a solid state process. The phase structure was analyzed by X-ray diffraction. The results show the pure phase of Y 2 O 3 is obtained and RE ions do not exhibit any effect on the structural properties. Single RE ions activated Y 2 O 3 phosphors display their characteristic emission. With Tb 3+ doped in Y 2 O 3 , a bright green emission is obtained; when Eu 3+ is introduced into the material Y 2 O 3 : Tb 3+ , the emission color can be adjusted to white light. The calculated CIE chromaticity coordinates of RE ions activated Y 2 O 3 phosphors confirm the green and white emissions

Diffusion models for solid-state homogenization of dendrite segregation

New mathematical diffusion models for solid state homogenization of dendrite segregation were presented to calculate the concentration evolution of alloying elements during solid-state homogenization of dendrite segregation in any cross section of a three-dimensional dendrite system. With mathematical accumulation, all the individual concentration profiles of every dendrite can be accumulated to form a concentration equation, which may be required by sine model or not. Then diffusion homogenization processes for any cross sections of dendrites in solid state can be calculated from the equation. The only assumption of the new models was that any individual concentration profile in a dendrite system could be simulated by Gauss equation of Fick-2 law.

Effect of Aging Treatment before Extrusion on Microstructure and Mechanical Properties of AZ80 Magnesium Alloy

Abstract This paper investigated the effect of aging treatment before extrusion on microstructure and mechanical properties of AZ80 magnesium alloy. Fractural microstructures of the test specimens were also analyzed by scanning electron microscopy. Results show that aging treatment before extrusion can remarkably promote grain refinement; the dispersed Mg17Al12 particles precipitated from the matrix during aging treatment before extrusion distribute in the grain boundaries; during the subsequent extrusion process these particles can pin grain boundaries migration and lead to fine microstructure; however, with prolonging the aging time, the grain refinement effect is weakened. After aging treatment and subsequent extrusion, the yield strength, tensile strength and elongation are increased. Based on fracture surface analysis, the premature cracks occur around big particles in grain boundaries lead to loss of elongation. The strengthening effect of aging before extrusion in AZ80 magnesium alloy proven in this paper provides a new method for the design of the relatively high-performance magnesium alloys.

Ultrasonic Effects on Microstructure Evolution and Mechanical Properties of AZ80 Magnesium Alloy

Abstract Ultrasonic effects on the microstructure evolution and the mechanical properties of AZ80 alloy were investigated. The results indicate that in the case of the ultrasonic melt treatment, the primary α -Mg phases is transformed from coarse dendrites to nearly fine equiaxed grains, and the β -Mg 17 Al 12 phase at α -Mg boundaries is refined and becomes discontinuous. Besides, the tensile properties of AZ80 alloy are apparently improved after ultrasonic melt treatment, in which 0.2% tensile yield strength, ultimate tensile strength and elongation are improved from 87 MPa, 118 MPa and 2.1% to 107 MPa, 170 MPa and 5.4%, respectively.

Microstructures and Elevated Temperature Mechanical Properties of As-extruded ZM61-Sn Alloys

Abstract Microstructures, elevated temperature mechanical properties and fracture mechanisms of the as-extruded ZM61- x Sn ( x =2, 4, 8, wt%) alloys were investigated by optical microscope (OM), X-ray diffraction (XRD), scanning electron microscope (SEM) and high temperature tensile tests. The results reveal that the addition of Sn can refine microstructures and the refinement effect increases with the increase of Sn content. The average grain sizes of the as-extruded ZM61- x Sn ( x =2, 4, 8) alloys are 11, 8 and 4 μm, respectively. With the increasing of Sn amount, the strength of experimental alloys increases at first and decreases afterward. ZM61-4Sn alloy shows the highest strength, whose ultimate tensile strength and yield strength are 216 and 173 MPa tested at 180 °C, respectively. The ductility increases with the content of Sn increasing, and the elongation of as-extruded ZM61- x Sn ( x =2, 4, 8) alloys are 183.8%, 235.8% and 258.6%, respectively, when tensile temperature reaches 300 °C. The ZM61-4Sn alloy has the optimal coalescence of strength and ductility. The localized necking leads to the final fracture of the specimens and the micro-void coalescence is the main fracture mechanism. Incomplete dynamic recrystallization occurs when tensile tests are carried out at 260 and 300 °C.

Effect of Extrusion Temperatures on Microstructures and Mechanical Properties of Mg-6Zn-1Mn-4Sn-0.5Y Alloy

Abstract The microstructural evolution and the mechanical properties of Mg-6Zn-1Mn-4Sn-0.5Y wrought alloy extruded at 360 and 420 °C were investigated by optical microscopy (OM), X-ray diffraction (XRD), scanning electron microscopy (SEM), electron back scattered diffraction (EBSD) and tensile test. The results show that the phase compositions of as-cast and extruded alloys consist of α-Mg, Mn, Mg7Zn3, Mg2Sn, and MgSnY phases. As the extrusion temperature increases from 360 °C to 420 °C, dynamic recrystallization completes and grain growth occurs. The yield strength, the ultimate tensile strength and the elongation decrease from 259 MPa, 350 MPa and 18.3% to 239 MPa, 332 MPa and 12.5%, respectively. The theoretical calculations combined with the experimental results reveal that fine grain strengthening and solid solution strengthening play the dominating role in the enhancement of yield strength.