Papers by Author: Zhan Yi Cao

Abstract: Mg-1Mn-0.6Ce-3Y alloy was prepared by metal mould casting method. The as-cast ingot was homogenized and then hot-extruded by an extrusion ratio of 16:1 at 380 °C. Microstructure and mechanical properties of the as-cast and hot-extruded samples were investigated. The results showed that the as-cast sample mainly consisted of α-Mg, Mg₁₂Ce, and Mg₂₄Y₅ phases. The average grain size of the sample homogenized at 380 °C was about 100μm, and it was greatly refined to about 6μm by dynamic recrystallization for the hot-extruded sample. The ultimate tensile strength, 0.2% yield strength and elongation of the hot-extruded sample were 244 MPa, 178 MPa and 37.5%, respectively. They were enhanced by 82%, 197% and 400%, correspondingly compared with those of the as-cast sample. The improvement of the strengths was attributed to the grain refinement, breakup of the precipitates and increase of the dislocation density.

Abstract: Cast Magnesium alloys often exhibit large variability in fracture related properties such as ductility. In this study, the characteristics of micro-voids in high-pressure die-cast (HPDC) AM50 alloy were investigated by microstructural detecting. Specimen-to-specimen fractographic analysis of tensile fractured surface was executed to summarize the relation between microporosity and tensile properties. The results indicated that the variability in tensile properties is quantitatively correlated to the areal fraction of porosity in the corresponding fracture surface, which could be expressed by a power law equation. All the results proved that the most highly localized cluster of micro-voids is most preferentially to be the origin of fracture, and then, fracture crack will preferentially propagate through the adjacent regions that with large porosity.

Abstract: The microstructure and mechanical properties of the as-cast Mg-Zn-Ca-Zr alloys were investigated in this study. The results showed that the alloy was mainly composed of α-Mg solid solution and the secondary phases of Ca₂Mg₆Zn₃. The grain size of alloys decreased from 82 μm to 38 μm with Zr content from 0.1% to 0.5%. The addition of Zr greatly improved the ultimate tensile strength (σb) and elongation (ε), while slightly improved the tensile yield strength (σ0.2). The σb, ε and σ0.2 of the Mg-4Zn-0.5Ca-0.5Zr were 196MPa, 85MPa and 16.1% compared with the other two alloys. The reason was that grain size refinement strengthening enhanced the mechanical properties.

Abstract: The microstructure and corrosion properties of the as-cast Mg-Zn-Ca-Nd alloys were investigated in this study. The results showed that the alloy was mainly composed of α-Mg solid solution and the secondary phases of MgZn, Ca₂Mg₆Zn₃ and Mg₄₁Nd₅. The portion of precipitated phases was increased with the increment of Nd content. Both electrochemical measurement and immersion test indicated that Mg-4Zn-0.5Ca-1.0Nd had the best corrosion resistance in SBF solution. This is attributed to a combination of (1) the uniform microstructure of alloy, and (2) a lower volume fraction of second phases. In the Mg-4Zn-0.5Ca-3.0Nd alloy, the corrosion rate might slow down due to finer dendrite cells were surrounded by the more continuous second phases along the grain boundary and in the α-Mg matrix.

Abstract: In this paper, the high temperature tensile properties of the LAZ532-2RE alloy prepared by hot extruded processing after vacuum casting was investaged. The tensile properties of the extruded LAZ532-2RE alloy specimens were tested at different temperature with different strain rate. The microstructures near the fractured surfaces were observed using microscope in order to investigate the dominant deformation mechanism. The activation energy was calculated to explain the high temperature deformation mechanism. The result indicated that the strength of LAZ532-2RE alloy was high at the temperature range from 398K to 423K. Meanwhile, the fracture elongation of the alloy reaches 121% at 523K under strain rate 1×10^-3s^-1.

Abstract: Cu-based materials reinforced by titanium carbide used in the field of brake or conducting friction were fabricated by mechanical alloying (MA) and spark plasma sintering (SPS). X-ray diffraction results indicate that the composites consist of copper matrix, Cu3Ti2, titanium carbide and graphite. Transmission electron microscope (TEM) image shows that a smooth interface exists between the titanium carbide particles and copper matrix. Moreover, the laminate structure of graphite (800°C) transfers to not consistent with the laminate structure at high temperature (1000°C).

Abstract: In present work, the Mg−Gd−Nd−Y−Zn alloy sheets were prepared by hot extrusion technique. The microstructure, texture and mechanical properties of the extruded alloy were investigated. After hot extrusion, the alloy reveals a greatly refined microstructure due to the dynamic recrystallization. The coarse eutectic phases were crushed into small particles during extrusion process, which brings a promotion of grain refinement. A weak basal fiber texture was obtained in the as-extruded alloy owing to the influence of RE (rare earth) alloying elements. The as-extruded alloy exhibits mechanical anisotropy that the strengths and elongations in the extrusion direction are both higher than those in the transverse direction. And this behavior does not change with increasing temperature. It results from the weak texture and the distribution of eutectic phase particles in the alloy sheet. Through isothermal aging treatment, significant strengthening is achieved in the peak-aged alloy, and the mechanical anisotropy still exists.

Abstract: Mg-6Al-0.3Mn-0.9Y (mass fraction, %) magnesium alloy was prepared by metal mould casting. The as-cast ingot was homogenized, and then hot-rolled with total thickness reduction of 70%. Further annealing treatment was carried out on the hot-rolled sample. Microstructure and mechanical properties of the studied alloy in as-cast, hot rolled and annealed states were investigated. Results showed that main phases of the as-cast sample were composed of α-Mg, Mg₁₇Al₁₂ and Al₂Y. Average grain size was about 9 μm after hot rolling and annealing treatment. It is found that the optimal annealed parameters for the Mg-6Al-0.3Mn-0.9Y alloy should be 350 °C for annealed temperature and 30 min for the corresponding time, respectively. Tensile test results showed that ultimate tensile strength, yield strength and elongation of the annealed sample were 261 MPa, 149 MPa and 32%, respectively. Compared with those of the as-cast sample, they were enhanced by 36%, 140% and 146% correspondingly.

Abstract: Mg_-1Mn_-0.6Ce_-xY (x=0, 1, 2 and 3, mass fraction, %) magnesium alloys were prepared by casting method. And the influences of yttrium on microstructure and mechanical properties of the Mg_-1Mn_-0.6Ce magnesium alloy were investigated. The results reveal that the addition of yttrium to the Mg_-1Mn_-0.6Ce alloy could reduce the grain size of the as-cast alloys and improve mechanical properties during the investigated temperature range. The Mg_-1Mn_-0.6Ce_-1Y alloy exhibits maximum ultimate strength, yield strength, elongation and the values are 152 MPa, 72 MPa and 13.4% and enhanced about 23.1%, 63.6% and 38.1% compared with those of Mg_-1Mn_-0.6Ce alloy at room temperature, respectively. The improvement of mechanical properties are attributed to the grain refinement and the precipitation strengthening generated by the Mg₁₂Ce phase particles and the fine Mg₂₄Y₅ precipitations.

Abstract: The paper researched the isothermal time and temperature affects the solid particles evolution of semisolid AZ91D alloy, and the alloy be produced by the method of strain induced melt activation (SIMA) ,and the method of compression ratio is 40%. The research demonstrated that the solid fraction decreased and solid particle size increased when increasing temperature and time. The isothermal coarsening of solid particles obeys Ostwald ripening which depending on the curvature of the interface, thus creating concentration gradients and the diffusion transport of material. As a result, the large particles coarsening and little particles minish even dissolved and creating change of distribution frequency of solid particle size. The fitting curves reveal that the experimental data are well fitted to the coarsening equation. The quantitative analysis of solid fraction and solid particle size demonstrated that the appropriate isothermal holding temperature is 550°C -590°C and corresponding isothermal holding time is (45-60min)-(6-20min) in 40% compression ratio.