Yun Dong

Microstructure and compression deformation behavior in the quasicrystal-reinforced Mg-8Zn-1Y alloy solidified under super-high pressure

Abstract The microstructure of Mg-8Zn-1Y alloy solidified under super-high pressure was analyzed through X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). And, compression deformation behavior at room-temperature was studied. The results showed that the microstructure of Mg-8Zn-1Y alloy solidified under ambient pressure and super-high pressure was both mainly composed of α-Mg and quasicrystal I-Mg 3 Zn 6 Y. Solidification under super-high pressure contributed to refining solidified microstructure and changing morphology of the intergranular second phase. The morphology of intergranular second phase (quasicrystal I-Mg 3 Zn 6 Y) was transformed from continuous network (ambient pressure) to long island (high pressure) and finally to granular (super-high pressure) with the increase in pressure. The compressive strength, yield strength and rupture strain of the samples solidified under ambient pressure were significantly improved from 262.6 MPa, 244.4 MPa and 13.3% to 437.3 MPa, 368.9 MPa and 24.7% under the pressure of 6 GPa, respectively. Under ambient pressure, cleavage plane on compressive fracture was large and smooth. When it was solidified under the pressure ranging from 4 to 6 GPa, cleavage plane on compressive fracture was small and coarse. In addition, dimple, tear ridge and lobate patterns existed.

Investigation on the modification behavior of A356 alloy inoculated with a Sr-Y composite modifier

Abstract In the present paper, differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and X-ray diffraction (XRD) were used to examine the effects of a Sr-Y composite modifier on the microstructure of A356 alloy. After adding Y to A356, YAl3 compounds formed, and the size of the α (Al) crystal nucleus increased. The degree of supercooling caused by Sr-Y composite modifier was higher than Sr modification by 2.7 °C, leading to an increased nucleation rate. This increase in supercooling temperature was favorable to the refinement of eutectic structure of the alloy and its eutectic reaction was delayed to the maximum extent. The Si phase in the as-cast Sr-Y composite-modified A356 alloy was either granular or flaky. No large flakes of eutectic Si were found, and the modification effects were completely comparable with those obtained using a lone Sr modifier. After T6 heat treatment, most of the eutectic Si showed a grain-like shape with smaller grains. No eutectic Si with long-strip shapes, significant enhancements in the particle roundness and evenness of the Si crystals, and increased globosity were observed. Both the roundness and evenness of the grained Si crystals were enhanced, and the amount of globular eutectic Si available increased, these findings showed that excellent modification effects were achieved.

Constitutive model and deformation microstructure of fine-grain Mg-Zn-Y alloy solidified under high pressure

Abstract Fine-grain Mg95.50Zn3.71Y0.79 alloy was prepared by high pressure solidification. By comparison with the conventional casting alloy, the true stress-strain curve characteristic and deformation microstructure of Mg95.50Zn3.71Y0.79 alloy solidified under high pressure were studied via unilateralism compress tests under the strain rate of 0.001–1 s−1 and deformation temperature of 523–623 K. Constitutive equations were constructed. According to the experimental results, compared to the conventional casting alloy, the true stress-strain curve of the fine-grain alloy solidified under high pressure not only had the high strain hardening characteristic but the dynamic recrystallization softening after the peak stress was more than the working hardening and would soon reach a stable flow stress – strain state. The deformation activation energy of the alloy solidified under high pressure was 151 kJ/mol, around 49 kJ/mol lower than that of the conventional casting alloy. The fine-grain Mg-Zn-Y alloy solidified under high pressure could obtain 95 percent of dynamic recrystallization grain at 573 K during hot deformation process.

Microstructure of Mg–8Zn–4Al–1Ca aged alloy

Abstract The microstructure of Mg–8Zn–4Al–1Ca aged alloy was investigated by TEM and HRTEM. The results show that the hardening produced in the Mg–8Zn–4Al–1Ca alloy is considerably higher than that in the Mg–8Zn–4A1 alloy. A dense dispersion of disc-like Ca 2 Mg 6 Zn 3 precipitates are formed in Mg–8Zn–4Al–1Ca alloy aged at 160 °C for 16 h. In addition, the lattice distortions, honeycomb-looking Moire fringes, edge dislocations and dislocation loop also exist in the microstructure. The precipitates of alloy aged at 160 °C for 48 h are coarse disc-like and fine dispersed grainy. When the alloy is subjected to aging at 160 °C for 227 h, the microstructure consists of numerous MgZn 2 precipitates and Ca 2 Mg 6 Zn 3 precipitates. All the analyses show that Ca is a particularly effective trace addition in improving the age-hardening and postponing the formation of MgZn 2 precipitates in Mg–8Zn–4Al alloy aged at 160 °C.

Dynamic recrystallization kinetic of fine grained Mg-Zn-Y-Zr alloy solidified under high pressure

Abstract Fine grained Mg 96.17 Zn 3.15 Y 0.79 Zr 0.18 alloy with an average grain size of 20 μm was prepared by high pressure solidification. The dynamic recrystallization (DRX) behavior of the fine grained Mg alloy solidified under the pressure of 4 GPa was studied via isothermal compression experiments. The tests were performed under the strain rate of 0.001–1.0 s −1 and at a deformation temperature of 523–623 K on a Gleeble-3500D thermal-mechanical simulation machine. The DRX kinetic of the fine grained Mg alloy solidified under high pressure was established, and the microstructures of the alloy under different hot compression conditions were analyzed by electron back-scattering diffraction (EBSD). According to the experimental results, the DRX kinetic model of the fine grain Mg alloy solidified under high pressure was X DRX = 1 - exp [ - 0.75445 ( ɛ - ɛ c ɛ * ) 1.066208 ] . The Avrami exponents of n and k were 1.066208 and 0.75445 respectively, higher than those in the conventional casting alloy. The DRX volume fraction of the fine grain Mg alloy solidified under the pressure had a tendency to increase obviously with the strain rate decreasing and the deformation temperature increasing, which is different from the one in the conventional casting alloy. When compressed at 523 K, the DRX volume fraction of the fine grained Mg alloy solidified under high pressure was 85% under the strain rate of 1.0 s −1 and could be up to 95% under the strain rate of 0.001 s −1 . The DRX volume fraction of the conventional casting alloy was only 67% although under the condition of 623–0.001 s −1 . It was shown that the fine grained Mg alloy solidified under high pressure had a strong DRX capacity.

Study on Aging Strengthening of Mg-Zn-Cu Alloy Based on Component Optimization Design

In this study, we investigated the aging strengthening of Mg-Zn-Cu alloy based on component optimization design by FactSage software, optical microscope (OM), X-ray diffraction (XRD) and Vickers hardness tester. The results show that the precipitation rate of MgZn2 phase in Mg-6Zn-1Cu is significantly higher than that of the other alloys. When Mg-6Zn-1Cu alloy is subjected to aging at 160°C for different time, the phase consists of α-Mg, MgCu2 and MgZn2. The content of main strengthening phase MgZn2 is increasing with the prolonging of aging time. When Mg-6Zn-1Cu alloy aged at 160°C for 10h, the kinetics of precipitation is considerably accelerated. The results indicate that the hardening produced in the Cu-containing alloy is considerably higher than in the Mg-Zn alloy. Therefore, based on component optimization design to establish Mg-Zn-Cu alloy solidification database, and to predict the phase equilibrium and thermodynamic properties of the alloy, is an effective method for the development of new magnesium alloy.

Microstructure and Compression Deformation Behavior in the Quasicrystal-Reinforced Mg-6Zn-2Y Alloy Solidified under Super-High Pressure at Room-Temperature

The microstructures of the Mg-6Zn-2Y alloy solidified under high pressures were investigated using scanning electronic microscopy (SEM) and X-ray diffraction (XRD). The room-temperature compression behavior was analyzed through experiments, showing that the microstructures of the alloys are consisted of α-Mg and quasicrystal I-Mg3Zn6Y phases. With solidification pressure increasing, the microstructures were refined, and the morphologies of the inter-dendritic secondary phase were improved from continuous networks into long-island and granule. The compression strength, yielding strength and compressibility were increased significantly corresponding with solidification pressure, from 259.02 MPa, 230.39 MPa and 18.3% under ambient pressure to 361.43 MPa, 272.25 MPa and 33.1% under high pressure of 6 GPa. The cleavage planes are flat, and the cleavage steps are straight under ambient pressure. However, the cleavage planes are small and rough under 4-6 GPa; tearing dimples occur in the tearing area, indicating that the degree of cleavage fracture decreases under high pressure.

Wear Performance of the Plasma Sprayed Fine WC-Co Composite Powders Coatings

WC/Co; Composite coating; Plasma spraying; Friction and wear Abstract: WC- Co composite powders were synthesized by direct mechanical grinding in a rotary-vibration mill under 8h, and then analyzed by SEM and XRD. WC and WC/Co composite coatings were prepared by supersonic plasma spraying fine WC-Co composite powders. The wear and friction properties of both coatings were evaluated. The results showed that the wear resistance of the WC/Co composite coating was superior to that of the WC coating. The improvement in wear resistance of the WC/Co composite coating was attributed to its higher fracture toughness and adhesion strength as well as better thermal diffusivity. As for the WC/Co composite coating, the mechanism was mainly adhesion with micro-abrasion and fatigued-induced brittle fracture within splats, and the delamination along splat boundaries only occurred at high load. However, the failure of the WC coating was predominantly detachment of transferred film and brittle fracture within the splats and delamination along splat boundaries, which were enhanced with the increasing load.

Synthesis of Fine Co/ WC Composite and Application to Supersonic Plasma Spraying

Co/ WC composite powders (with 10 wt% content of Co) were synthesized by direct mechanical grinding in a rotary-vibration mill. The powders with different mill time were evaluated. WC and WC/Co composite coatings were prepared by supersonic plasma spraying. The results showed that the milled powders consist of composite particles that were formed in the first 2h of milling. Longer milling times improve the distribution of phases inside the composite particles. The formation of the composite particles involves sequential steps of deformation, fragmentation, cold welding, work hardening and piercing of particles of the hard phase in the soft phase. X-ray spectra of the sprayed coating are shown that only very weak W2C and Co6W6C peaks are observed. Compared with WC coating, the Co/ WC coating is denser, and less large pores within composite coating.

Phases Evolution and Y Solid Solution Microstructure in the Mg-6Zn-3Y Alloy Solidified under Super-High Pressure

The solidification microstructure of Mg-6Zn-3Y alloy under super-high pressure was investigated by using X-ray diffraction (XRD) and scanning electron microscope (SEM). The results show that the dendritic structure of Mg-6Zn-3Y alloy under super-high pressure (GPa level) can be evidently refined with the increase of solidification pressure. When the pressure increases to 2 GPa, Y element can’t solubilize in matrix of a-Mg, the primary Y solid solution is distributed in the shape of polygon block in the matrix. When the pressure is up to 4 GPa, the primary Y solid solution appears as symmetrical petaline shape. So Y solid solution exhibits the different morphology with the change of the pressure