The effects of low frequency electromagnetic (LFEC) field and ultrasonic (US) field on the microstructures, macrosegregation of alloying elements and the mechanical properties of DC cast AZ80 alloy were studied. The results show that both LFEC and US fields can refine the grains of the billets, which results in the increase in mechanical properties and uniformity of alloying element distribution. The effective refinement takes place on the edge of ingots when LFEC field is applied, while in the center of billets when field US is adopted. Combined the characteristics of LFEC and US fields, a new process for direct-chilling (DC) casting of Mg-electromagnetic-ultrasonic (ECUS) casting is developed, by which the grains are refined significantly and are more uniform in the whole ingots, and the mechanical properties of the ingots are improved.

The effects of minor Sr, Sn and Sc addition on the as-cast microstructure and mechanical properties of the ZA84 magnesium alloy were compared. The results indicate that addition of 0.1%Sr, 0.5%Sn or 0.3%Sc (mass fraction) to the ZA84 alloy can refine the grains of the alloy. Furthermore, addition of 0.1%Sr to the ZA84 alloy does not obviously change the morphology and distribution of Mg₃₂(Al,Zn)₄₉ phase. However, addition of 0.5%Sn or 0.3%Sc not only refines and modifies the Mg₃₂(Al,Zn)₄₉ phase but also suppresses the formation of Mg₃₂(Al,Zn)₄₉ phase, especially with the addition of 0.3%Sc. Furthermore, addition of 0.1%Sr, 0.5%Sn or 0.3%Sc to the ZA84 alloy improves the tensile properties at room temperature and 150 ˚C, especially with the addition of 0.1%Sr and 0.3%Sc. However, addition of 0.1%Sr is not beneficial to the creep properties, and addition of 0.5%Sn has no obvious influence on the creep properties. Oppositely, addition of 0.3%Sc to the ZA84 alloy greatly improves the creep properties.

A series of die casting heat-resistant magnesium alloys based on Mg-Al system were developed for automotive application by adding Y and various amounts of Ca. The mechanical properties and microstructures of die casting AZ91 alloy with combined addition of Y and Ca were investigated by optical microscopy, scanning electronic microscopy, X-ray diffractometry and mechanical property test. The results show that the combined addition of Y and Ca can refine the as-die-cast microstructure, result in the formation of Al₂Ca phase and Al₂Y phase, and inhibit the precipitation of Mg₁₇Al₁₂ phase. The combined addition of Y and small amount of Ca has little influence on the ambient temperature tensile properties, but increasing the content of Ca can improve significantly the tensile strength at both ambient and elevated temperatures. It is found that for AZ91-1Y-xCa alloy, the hardness and the elevated temperature tensile strength increase, while the elongation decreases with increasing the addition of Ca. The mechanism of mechanical properties improvement caused by the combined addition of Y and Ca was also discussed.

A new magnesium alloy plate added elements Zn, Sn and In was manufactured by twin-roll continuous casting method to improve the precipitation of AZ91 alloy. The effects of elements addition and casting method on microstructure and mechanical properties of the Mg-Zn-In-Sn alloys were investigated by optical microscopy (OM), transmission electron microscopy (TEM), electron probe micro-analysis (EPMA), X-ray diffractometry (XRD) and energy dispersion spectrograph (EDS). The results show that the Mg-Zn-In-Sn alloy has higher tensile strength and better corrosion protection than the AZ91 alloy. The outstanding precipitation strengthening effect of the alloy is attributed to the small grain size and the hard precipitates between the grain boundaries.

Mg-1.5Zn-0.2Zr-xCe (x=0, 0.1, 0.3, 0.5, mass fraction, %) alloys were prepared by conventional semi-continuous casting. The effect of rare earth Ce on the microstructure of Mg-1.5Zn-0.2Zr-xCe alloys was studied and the distribution of Ce was analyzed by optical microscopy (OM), X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results indicate that Ce element exists in the form of Mg₁₂Ce phase and has an obvious refining effect on the microstructure of test alloys. As the Ce content increases, the grain size reduces, the grain boundaries turn thinner, and the distribution of Mg₁₂Ce precipitates becomes more and more dispersed. The Mg-1.5Zn-0.2Zr alloy with 0.3%Ce has the best refinement effect. From center to periphery of the ingot, the amount of granular precipitates in the grain reduces. In longitudinal section of the ingot, some relative long columnar grains appear.

To develop AZ91D alloys with fine microstructure, effects of the addition of rare earth (RE), Sr and RE + Sr on the dendrite growth and phase precipitation in AZ91D magnesium alloy were studied, respectively. The results show that the microstructure is refined and the morphology of β-Mg₁₇A1₁₂ phase is modified with RE or Sr addition, especially with the RE+Sr composite addition which can reduce the average grain size of AZ91D alloy obviously to 141 μm. The needle-like or block-like new phases adhering to β-Mg₁₇A1₁₂ phase form at interdendrites during solidification. The enrichment of RE or/and Sr elements in front of the solidification interface, especially at the tips of α-Mg dendrite, which restricts the growth of α-Mg dendrite, changes the preferential growth of α-Mg and finally results in the grain refinement and the blunting of α-Mg dendrite.

In order to meet the demands of high temperature components in automobile, the microstructure and mechanical properties of several new die-casting AZ91-rare earth (RE) magnesium alloys were studied. The alloys were characterized by optical microscopy (OM), scan electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX), tensile and creep tests. The results show that Ce addition has little effect on the mechanical properties of AZ91 alloy at high temperature, while Y and Nd addition play important role in the improvement of creep resistance. New alloys containing Y or Nd with excellent high temperature performance are selected to produce cylinder head cover of high power diesel engine of Red Flag car and oil pan of Besturn car. The new magnesium alloys with RE addition for die-casting have potential to produce power-train parts, and can greatly decrease weight.

The Mg-5Sn-1Ca-xGd (x=0, 1) alloys were chosen to investigate the change in solidification paths, phase formation and mechanical properties. The microstructure of as-cast Mg-5Sn-1Ca alloy is composed of α-Mg, Mg₂Sn and CaMgSn phases. With the addition of Gd, the formation of the Mg₂Sn phase is impeded and the CaMgSn phase is refined, whereas the ultimate tensile strength and elongation decrease. The possible reasons for the variation in microstructure and mechanical properties were discussed.

Mg-10Ho-0.6Zr-xNd (x=0, 1, 3 and 5, mass fraction, %) alloys were prepared by metal mould casting, and the microstructures and mechanical properties were investigated. The results show that the grain size of as-cast alloys reduces and the hardness and strength increase with the increase of Nd content. The alloys are aged followed by solid solution treatment. Mg-10Ho-0.6Zr-3Nd and Mg-10Ho-0.6Zr-5Nd alloys exhibit obvious age hardening response. The hardness value of Mg-10Ho-0.6Zr-5Nd alloy increases from HV104 at as-cast state to HV136 at peak-aged state. The maximum ultimate tensile strength and yield strength of the Mg-10Ho-0.6Zr-5Nd alloy are obtained in at peak-aged state, and the values are 323 MPa, 212 MPa at room temperature, and 258 MPa, 176 MPa at 250 ℃, respectively. The improvement of the tensile strength is mainly attributed to the fine and dispersively distributed plate-shaped β′ metastable phase.

The solidification microstructures and hardness of Mg-2%Zn (mass fraction) based alloys with addition of 0.4%Ce, 0.4%Gd, 0.4%Y or 0.4%Nd (mass fraction) were investigated, and the effects of the rare earth elements on the microstructures and mechanical properties of these alloys extruded at 310 ˚C were also compared. The results indicate that the trace rare earth Ce, Gd, Y or Nd in the Mg-2%Zn alloy has obviously different grain refinement effects on its solidification microstructures, and the as-cast and hot-extruded alloy with 0.4%Ce has the smallest average grain size and the highest strength. However, the extruded alloys containing 0.4%Nd or 0.4%Y with the elongation of 26.6% and 30%, respectively, show higher plasticity in spite of lower strength as compared with the alloy containing 0.4%Ce.

The effects of yttrium addition on microstructure and mechanical properties of as-cast Mg-6Zn-3Cu-0.6Zr-xY (x=0, 0.5, 1.0, 1.5 and 2.0, mass fraction, %) (ZCK630+xY for short in this study) alloys were investigated by means of OM, XRD and SEM. The results show that the average grain size of Mg-Zn-Cu-Zr magnesium alloy is effectively reduced (from 57 μm to 39 μm) by Y addition. The analysis of XRD indicates the existence of I-phase (Mg₃Zn₆Y) and W-phase (Mg₃Zn₃Y₂) in ZCK630 alloys with Y addition. The ultimate tensile strength of ZCK630 alloys is significantly deteriorated with increasing Y addition, which is possibly related to the continuous networks of intergranular phases and the increase of W-phase.

Mg-Al-Zn alloy thin strips were prepared by vertical twin-roll casting. Effects of Al and Ca on the microstructure and the surface defects of the thin strips were investigated. The results show that with the increase of Al content, the quantity of surface cracks of the thin strips increases rapidly and their locations are moved to the center of the thin strips. The alloy melts with more than 0.10% Ca (mass fraction) stick on the roller and a poor surface quality of the thin strips appears. With the increase of Al content, the microstructure of the thin strips evolves from single α-Mg phase, characterized by the uniform and equiaxed recrystallization grains, to the combination of dendritic α-Mg with Al supersaturated α-Mg. The minor addition of Ca can obviously refine the grains of Mg-3Al-1Zn thin strip because the formation of Al₂Ca and the finest grains are obtained by adding 0.08% Ca.

The microstructure, mechanical properties and damping capacity of ZK60-xY (x=0, 1.5%, 2.5%, 4.0%, mass fraction) magnesium alloys were investigated by using the optical microscope (OM), X-ray diffractometer (XRD), universal tensile testing machine and dynamic mechanical analyzer (DMA). The mechanisms for damping capacity of referred alloys were discussed by Granato-Lücke theory. The results show that Y additions remarkably reduce grain size (the average grain size is 21.6, 13.0, 8.6 and 4.0 μm, respectively), and the tensile properties are enhanced with grain refining (the yield tensile strength increases to 292 MPa from 210 MPa and ultimate tensile strength increases to 330 MPa from 315 MPa). For the ZK60-xY (x=0, 1.5%, 4.0%) alloys, the damping capacity decreases with the increase of Y content. However, for the ZK60-xY (x=2.5%) alloy, the damping capacity improves abnormally, which is possibly related to the formation of Mg₃Y₂Zn₃ (W) FCC phase in this alloy.

The response and failure of magnesium alloy AZ31 specimens subjected to different pre-loaded-stress levels and heating rates were investigated with a Gleeble−1500 thermo-mechanical material testing system. It is found that the increases of either pre-loaded stresses or heating-rates decrease the failure temperatures of the specimens. The metallographs of the tested specimens were also observed. It is shown that the high heating-rate may cause stronger local thermal inconsistency, which remarkably increases the microdefects and reduces the macroscopic mechanical properties of the material.

Under the high-intensity ultrasonic field, AZ80 magnesium alloy was semi-continuously cast. The effects of ultrasonic intensity on the as-cast microstructures and mechanical properties were investigated. The results show that the microstructures of the alloy cast under high-intensity ultrasonic field are fine and uniform, and the grains are equiaxed, rose-shaped or globular with an average size of 257 μm. High-intensity field significantly decreases the grain size, changes the morphologies of the β-Mg17Al12 phases and reduces their area fraction. It is also shown that a proper increase in ultrasonic intensity is helpful to obtain fine, uniform and equiaxed as-cast microstructures. The optimum ultrasonic parameters are that frequency is 20 kHz and ultrasonic intensity is 1 368 W. The mechanical tests show that the mechanical properties of the as-cast AZ80 magnesium alloy billets cast under ultrasonic field are greatly improved, and with increasing the ultrasonic intensity, the mechanical properties of the entire alloy billets are much higher and more uniform than those of the alloy without ultrasonic field.

It was attempted to enhance and accelerate the separation of oxidation inclusions from magnesium alloy melt by virtue of ultrasonic agglomeration technology. In order to investigate the feasibility and effectiveness of standing waves for ultrasonic purification of magnesium alloy melt, numerical simulation and relevant experiment were carried out. The numerical simulation was broken into two main aspects. On one hand, the ultrasonic field propagations within the cells with various shapes were characterized by numerical solutions of the wave equation and with a careful choice of geometry a nearly idealized standing wave field was finally obtained. On the other hand, within such a standing wave field the agglomeration behavior of oxidation inclusions in magnesium alloy melt was analyzed and discussed. The agglomeration time and agglomeration position of oxidation inclusions were predicted with numerical simulation method. The results show that the oxidation inclusions whose apparent densities are close to the density of the melt can agglomerate at wave nodes in a short time which to a great extent enhances and accelerates the separation of oxidation inclusions from magnesium alloy melt.

Effects of electromagnetic stirring on the microstructure and mechanical properties of the magnesium-lithium-aluminum alloy were studied. The results reveal that, the morphology of the α phase changes from the long block to globular structure and β phase distributes more widely in the periphery of α phase when the electromagnetic stirring voltage is higher than 110 V. The mechanical properties are increased significantly with the increasing electromagnetic stirring. The tensile strength is improved from 172 to 195 MPa, and the elongation is increased from 10.65% to 25.75%.

The effects of cooling rates corresponding to different diameters of the steel mould and laser surface melting (LSM) on the as-cast microstructures of Mg-9Al-xSi (x=1, 3) (mass fraction, %) alloys were investigated by XRD and OM. The results show that obvious refinement of the alloy microstructure is obtained with increasing cooling rate by conventional ingot metallurgy. However, no evident modified morphologies of both dendritic primary Mg₂Si and Chinese script eutectic Mg₂Si in the Mg-Al-Si alloy occurs. Surprisingly, the morphologies of Mg₂Si phases within the laser-melted Mg-Al-Si alloy transform drastically from both coarse Chinese script shape for the eutectic Mg₂Si and dendrite for the primary Mg₂Si to fine spherical particles with an average size of about 3 μm due to the rapid cooling of the melted layer, and the Mg₂Si particulates distribute more uniformly in the α-Mg matrix.

AZ61 alloys with different levels of Al5Ti1B master alloy additions were prepared by conventional casting method. The effects of Al5Ti1B contents and holding time on microstructures and microhardness of AZ61 alloys were studied by XRD, OM and microhardness testing techniques. The results show that when the addition level of Al5Ti1B master alloy is less than 0.5% (mass fraction), the average grain size of the alloys decreases with the increase of Al5Ti1B content at the same holding time. But the grain size increases somewhat with further addition of Al5Ti1B. The average grain size of the alloys decreases with the increase of the holding time as it is less than 30 min at the same addition level of Al5Ti1B. It is considered that TiB₂ particles can serve as the heterogeneous nucleation sites of α-Mg during solidification, and heterogeneous nucleation is the main reason for the grain refinement of AZ61 alloys. The microhardness of the refined AZ61 alloys with 1.0% Al5Ti1B addition is increased by about 8%.

The effects of semi-solid isothermal process parameters on the microstructure evolution of Mg-Gd rare earth alloy produced by strain-induced melt activation (SIMA) were investigated. The formation mechanism of the particles in the process of the isothermal treatment was also discussed. The results show that the microstructure of the as-cast alloy consists of α-Mg solid solution, Mg₅RE and Mg₂₄RE₅ (Gd, Y, Nd) phase. After being extruded with an extrusion ratio of 14:1 at 380 ˚C, the microstructure of Mg-Gd alloy changes from developed dendrites to near-equiaxed grains. The liquid volume fraction of the semisolid slurry gradually increases with elevating isothermal temperature or prolonging isothermal time during the partial remelting. To obtain an ideal semisolid slurry, the optimal process parameters for the Mg-Gd alloy should be 630 ℃ for isothermal temperature and 30 min for the corresponding time, respectively, where the volume fraction of the liquid phase is 52%.

The effects of trace element Fe on the corrosion behavior of AZ80 magnesium alloy were investigated by salt spray test and electrochemical measurements. The results show that the corrosion rate decreases with decreasing the trace element Fe content in an approximately linear relation even though the amount of trace element Fe reduces to 0.000 2% (mass fraction). The electrochemical measurements show that the corrosion potential (φ_corr) of the alloy with lower trace element Fe content shifts to less negative value. It is suggested that the control trace element by purification is an effective way to enhance the corrosion resistance of AZ80 magnesium alloy.

The refining effect and mechanism of Sb on Mg₂Si and the microstructure of the matrix were investigated. The results indicate that there are Mg₃Sb₂ particles in the composites with the addition of Sb, and Mg₃Sb₂ can promote the formation of fine polygonal type Mg₂Si by providing nucleation site. Meanwhile, the grain size of Sb modified alloy is finer than that of the matrix. The improved microstructure results in the improvement of mechanical properties. The ultimate tensile strength is increased by 12.2% with the addition of 0.8% Sb.

The effects of solution heat treatment on the microstructure and mechanical properties of AZ61-0.7Si magnesium alloy were investigated. The results indicate that the solution heat treatment can modify the Chinese script shaped Mg₂Si phases in the AZ61-0.7Si magnesium alloy. After being solutionized at 420 ˚C for 16−48 h, the morphology of the Mg₂Si phases in the AZ61-0.7Si alloy changes from the Chinese script shape to the short pole and block shapes. Accordingly, the tensile and creep properties of the AZ61-0.7Si alloy are improved. After being solutionized at 420 ˚C for 24 h and followed by aging treatment at 200 ˚C for 12 h, the heat-treated alloy exhibits relatively high tensile and creep properties than those of the as-cast alloy.

The effects of second phases on the fracture behavior of Mg-10Gd-3Y-0.6Zr alloy were investigated. The results show that the fracture mode can be generally described as ductile transgranular fracture in as-extruded condition and intergranular fracture in peak-aged condition. In as-extruded condition, the ductile transgranular fracture occurs by the formation and transgranular propagation of the microcrack from the broken primary phases. However, as the collaboration effects of precipitates inside grains and on the grain boundaries have the tendency to reduce the cohesive strength of the grain boundary, and make the grain boundaries the favorable path for crack propagation, the intergranular fracture occurs in peak-aged condition.

Effects of temperature and heating rate on the mechanical properties of the tensile specimens of magnesium alloy AZ31 were experimentally investigated using a Gleeble-1500 thermo-mechanical material testing system. The metallurgraphs of the fracture section of the specimens were also experimentally observed and analyzed for exploring their failure mechanism under different temperatures and heating rates. The results show that the higher the temperature, the lower the ultimate strength of the specimens. And the higher the heating rate, the higher the ultimate strength of the specimens. The high temperatures and high heating rates will induce microvoids in the specimens which make the specimens failure under relatively low loads.

The effects of isothermal holding time on the semisolid microstructure of Mg-9Al-1Si (mass fraction,%) alloy were investigated. The research results indicate that the Mg-9Al-1Si alloy with non-dendritic microstructure can be produced by the semi-solid isothermal heat treatment. With holding time varying from 5 to 30 min, the volume fraction of liquid is gradually enlarged from 29.3% to 38.6%, the morphology of α-Mg grains changes from initial dendritic shape to spherical types and their average sizes increases from 41.1 to 56.1 μm. In addition, during the isothermal heat treatment, the eutectic Mg₂Si phase changes from the initial Chinese script shape to granule and/or polygon shape in Mg-9Al-1Si alloy. The modification of Mg₂Si phase is possibly attributed to a shift of the eutectic composition of the liquid in semi-solid slurries, towards lower silicon contents with increasing Al content due to a redistribution of Al during isothermal heat treatment.

Mg-9Al-xPr (x=0.4, 0.8 and 1.2, mass fraction, %) magnesium alloys were prepared by high-pressure die-casting technique. The effects of Pr on the microstructures of die-cast Mg-9Al based alloy were investigated by XRD and SEM. Needle-like Al₁₁Pr₃ phase and polygon Al₆Mn₆Pr phase are found in the microstructure. With 0.4% Pr addition, fine needle-like Al₁₁Pr₃ phase and a small amount of polygon Al₆Mn₆Pr phase near the grain boundary are found in the microstructure. Increasing Pr addition to 0.8%, lots of coarse needle-like Al₁₁Pr₃ phase within grain and polygon Al₆Mn₆Pr phase on grain boundary are observed. Further increasing Pr addition, the size of needle-like Al₁₁Pr₃ phase decreases, while the size of polygon Al₆Mn₆Pr relatively increases. The mass fraction of Pr at around 0.8% is considered to be suitable to obtain the optimal mechanical properties. The optimal mechanical properties are mainly resulted from grain boundary strengthening obtained by precipitates and solid solution.

Forward extrusion experiments of as-cast AZ31 magnesium alloy were conducted at different temperatures and different extrusion ratios using the as-cast billets with and without homogenizing treatment. The mechanical properties of pre- and post-extrusion of the two kinds of billets were investigated. Experimental results show that the mechanical properties of post-extrusion of the two kinds of billets all are obviously improved compared with those of pre-extrusion. The elongation of post-extrusion using the billet with homogenizing is higher than that without homogenizing, but the tensile strength is lower than that without homogenizing. When the extrusion ratio increases, the elongation and tensile strength of post-extrusion of two kinds of billets all will increase obviously. When the extrusion temperature of billet without homogenizing increases, the tensile strength of post-extrusion will decrease obviously and the elongation of post-extrusion will change to a small extent. For the billet with homogenizing, the tensile strength of post-extrusion will decrease in some sort when extrusion temperature increases.

Tensile tests with small deformation amounts of 0.5%, 1%, 3% and 5% were performed at room temperature on as cast Mg-1%Al alloy. Microstructures of the Mg-1%Al alloys before and after deformation were observed by optical microscopy (OM) and transmission electron microscopy (TEM). The strain amplitude dependent and temperature dependent damping capacities of the as-cast and deformed Mg-1%Al alloys were investigated by dynamic mechanical analysis (DMA). The mechanism of deformation on damping capacity of Mg-1%Al alloy was discussed. The results show that the as-cast Mg-1%Al alloy has high damping value at high strain. When the tensile elongation is higher than 3%, the damping values of this alloy in high strain region are significantly decreased at room temperature. But the large amount of dislocations produced by tensile deformation are activated by heat, and then increase the damping value at high temperature.

The dynamic mechanical analyzer (DMA) was applied to investigate the damping properties of Mg-Cu based alloys. The results show that the as-cast hypoeutectic Mg-Cu binary alloys exhibit ultra-high damping capacities, while the eutectic Mg-Cu alloy exhibits low damping capacity. The strain amplitude dependent damping performance reveals that the dislocation damping mainly dominates in Mg-Cu alloys. Furthermore, the influence of eutectic phase on damping mechanisms of Mg-Cu binary alloys was discussed in detail and the effect of Si addition on the damping of Mg-1%Cu based alloy was also reported. Two damping peaks are observed on the temperature dependent spectrum of Mg-Cu based alloys. One is located at room temperature, which is dislocation related peak; and the other is located at moderate temperature, which is caused by the grain boundary sliding.

The effect of precipitation on the internal friction (IF) of AZ91 magnesium alloy was investigated by using X-ray diffraction (XRD) analysis, scanning electron microscope (SEM) observation, and dynamic mechanical analysis (DMA). Six different states of alloy were prepared by applying different heat treatment processes: as-cast, in-complete solid solution, complete solid solution, micro-precipitation, continuous precipitation and continuous−discontinuous precipitation. It was found that the internal friction of in-completely solid-solutionized, completely solid-solutionized and micro-precipitated specimens showed a similar characteristic, and the grain boundary relaxation is completed depressed due to the Al atoms supersaturated in the α-Mg solution. However, a thermal relaxation internal friction peak was observed for continuously precipitated and continuously− discontinuously precipitated specimens at around 438 K and frequency of about 1 Hz, which was attributed to the grain boundaries relaxation. Furthermore, it was found that the relaxation of the β-Mg₁₇Al₁₂/α-Mg phase interfaces should give its contribution to the background internal friction in the as-cast, continuously precipitated and continuously−discontinuously precipitated specimens.

Novel AZ91D Mg alloy/fly-ash cenospheres (AZ91D/FACs) composites were fabricated by melt stir technique. Fly-ash cenosphere particles with 4%, 6%, 8%, 10% in mass fraction and 100 µm in size were used. Hardness and compressive strength of the composites were measured. The effects of mass fraction of cenospheres on the microstructure and compressive properties were characterized. The results show that the cenospheres are uniformly distributed in the matrix and there is no sign of cenosphere  cluster or residual pore. The densities of the composites are 1.85−1.92 g/cm³. By comparing with matrix, the compressive yield strength of the composites is improved, and the cenospheres is filled with Mg matrix alloy. SEM, XRD and EDX results of the composites show clear evidence of reaction product at cenosphere/matrix interface. On the basis of XRD and EDX, composition, structure and thermodynamic analysis, the main interfacial phase between the cenosphere and AZ91D Mg alloy was identified to be MgAl₂O₄.

The magnesium matrix double interpenetrating composites reinforced by nickel foam were fabricated by pressureless infiltration technology. Then the morphology of the nickel reinforcement and the microstructures of composites were characterized by SEM. The results show that not only is the nickel foam reinforcement reticular in three dimensions, but also the struts of foam keep the network structure, which ensures that the Ni foam/Mg composites are double interpenetrating. The interface bonding of composites between magnesium matrix and nickel foam reinforcement is good, without reaction around the interface, which is the indispensable condition that advanced composites should possess. Magnesium matrix distributes in the windows of nickel foam, the triangle center holes and microhole of nickel struts, and the composites have double interpenetrating structure, which makes the composites have unique properties.

In order to investigate the effect of microvoids on the mechanical behavior of casting magnesium alloy, a spherical void−cell model of the material was presented. The velocity and strain fields of the model were obtained from the assumption that the material matrix is homogeneous and incompressible. The hardening and softening functions, which respectively reflect the deformation-hardening and void-softening behaviors of the material, were presented and introduced to an endochronic constitutive equation for describing the mechanical behavior of the material including microvoids. The corresponding numerical algorithm and finite element procedure were developed and applied to the analyses of the elastoplastic response and the porosity of casting magnesium alloy ZL102. The computed results show satisfactory agreement with experimental data.

The magnesium matrix composites reinforced by graphite particles and Al₂O₃ short fibers were fabricated by squeeze-infiltration technique. The additions dispersed uniformly and no agglomeration and casting defect were observed. The microstructures and wear properties of the composites with different Ce contents of 0, 0.4%, 0.8% and 1.0%, respectively, were investigated. Especially, the effect of Ce on the properties was discussed. The results reveal that Ce enriches around the boundaries of graphite particles and forms Al₃Ce phase with Al. The addition of Ce refines the microstructures of the composites. With the increase of Ce content, the grain size becomes smaller and the wear resistance of the composite is improved. At low load, the composites have similar worn surface. At high load, the composite with 1.0% Ce has the best wear resistance due to the existence of Al₃Ce phase. The Al₃Ce phase improves the thermal stability of the matrix so the graphite particles can keep intact, which can still work as lubricant. At low load, the wear mechanism is abrasive wear and oxidation wear. At high load, the wear mechanism changes to delamination wear for all the composites.

The semi-solid antiburning AZ61-1.0%Y magnesium alloy slurry with fine circular solid phase was fabricated by a novel type continuous mechanical stirring in this work. The microstructure of the semisolid slurry was characterized by a metallography microscope. The results show that the fine circular solid phase distributes uniformly in the slurry when the stirring temperature ranges from 600 to 605 ˚C. With the increase of the stirring velocity, the size of the solid phase becomes smaller and smaller. With the increase of the stirring time, the size of solid phase gets finer, but if the stirring time is longer than the critical time, it will be coarsened abnormally. The mechanical properties of semi-solid AZ61-1.0%Y alloy are superior to those of the normally casting magnesium alloy.