The characteristics of microstructures and mechanical properties of the multi-layer spray deposited SiC_P/Al-6.5Fe-0.6V-1.3Si, SiC_P/Al-8.5Fe-1.3V-1.7Si and SiC_P/Al-10Fe-1.3V-2Si composite sheets obtained by rolling after extruding were investigated. The evolution of the grain and phases of these composites during processing were examined, and the influence of the microstructures on the mechanical properties was analyzed. The experimental results show that the ultimate tensile strengths σ_b of the three kinds of composite sheets are 420, 535, 470 MPa respectively at room temperature, and 232, 285, 300 MPa at 315 ℃ and 148, 180, 200 MPa at 400 ℃. The excellent mechanical properties can be attributed to the high solid solubility, fine grain size, Al₁₂(Fe,V)₃Si precipitation particles and the SiC particles. And the composition of the matrix alloy has an obvious effect on the mechanical properties of the as-rolled sheets.

Two kinds of Al-6.0Zn-2.0Mg-0.12Zr and Al-6.0Zn-2.0Mg-0.2Sc-0.12Zr alloy plates were prepared by ingot-metallurgy. The alloy plates with 3 mm thickness were welded by argon shield welding method, and the mechanical properties and microstructures of the two welded joints filled with Al-Mg-Sc welding wire were studied comparatively. The results show that firstly, minor Sc can raise the mechanical properties of the Al-Zn-Mg-Zr base alloy greatly. The reason for the increment is the fine grain strengthening, precipitation strengthening and the substructure strengthening caused by Al₃(Sc, Zr). Secondly, η′ phase (MgZn₂) and grain size in the heat-affected zone of the alloy without Sc become coarse obviously, the η′ phase (MgZn₂) in the heat-affected zone of the alloy with Sc becomes coarse also, but the grain size has no visible change. Al₃(Sc, Zr) particles are rather stable and can inhibit the movement of dislocation and sub-grain boundaries, overaging softening is not serious. Thirdly, adding minor Sc can raise the strength of welded joint remarkably, the tensile strength of alloy with Sc increases from 395 MPa to 447 MPa and the welding coefficient increases from 0.7 to 0.8 as well. The reason for the high strength of welded joint with Sc addition is the fine grain strengthening, precipitation strengthening and the increasing of resistance to thermal cycling softening caused by Al₃(Sc, Zr).

The d 120 mm ingots of 7050 aluminum alloy were made by low frequency electromagnetic casting (LFEC) and conventional DC casting process, respectively. After homogenization treatment the ingots were extruded to rods and the solution and aging treatment were carried out for the rods. Constituents evolution during processing and effects of LFEC on constituents and remnant constituents were studied. The results show that 7050 aluminum alloy mainly contains Al-Zn-Mg-Cu type and Al-Cu-Fe type constituents. Al-Zn-Mg-Cu type constituents dissolve during homogenization, while Al-Cu-Fe type constituents could not dissolve. After homogenization treatment, the main remnant constituent is Al₇Cu₂Fe which crushes and orients along the extrusion direction after extrusion. Compared with DC process, by the process of LFEC, the constituents or remnant constituents are smaller in size and less in content. The LFEC process shows significant improvement in elongation by LFEC in both as-cast state and final state.

The crystal plasticity finite element method(CPFEM), which incorporates the crystal plasticity constitutive law into the finite element method, was developed to investigate the rolling processes of the cubic oriented and Goss oriented Al single crystal. The simulation results show that after rolling the crystal predominantly rotates around the transverse direction(TD) for both orientations. The rotations around the rolling direction(RD) and the normal direction(ND) are negligible. The reduction plays a significant role in the texture evolution. The TD rotation angle increases with increasing reduction. The deformation bands exist in the rolled specimens with the cubic initial orientation. Compared with the cubic oriented specimens, the TD rotation angles in the Goss oriented specimens are very small.

Material flow phenomena during friction stir welding (FSW) and the forming mechanism of “onion rings” are complicated and currently not fully understood. In the present FSW study aluminium alloys 5083 and A356, with the latter more readily welded than the former, were used. The experiments were conducted to obtain samples of tool and workpiece “frozen” together during FSW for analysis. Two deposition modes for forming the weld nugget zone were observed, one for each alloy with the present FSW parameters. The first is the deposition of shear layers forming the “onion rings” in 5083 alloy nugget. The tool-workpiece interaction leading to the layer thickness being equal to the ratio of welding speed and tool rotation speed is suggested. The second mode is the combination of depositing the rotational shear material in the lower part on the advancing side and the drag flow of material from the retreating site forming the rest of the A356 nugget. The latter mode resulted in the absence of a clear ring structure.

The aged characteristics and microstructure of two Al-Cu-Mg-Ag alloys with and without Ce addition are investigated by TEM and mechanical properties test. The results show that trace Ce addition inhibits precipitating process and delays peak aging time. 0.25% Ce additions decrease both tensile strength and ductility. TEM observation shows that the morphology of Ω precipitate can be modified by the small addition of Ce. The trace Ce affects the microstructure by reducing the volume fraction of Ω precipitate. It can be concluded that the intensive interaction between Ce and Cu decreases the Cu atom concentration in alloy matrix and reduces the precipitated kinetics for Ω phase.

The hot deformation behavior of a new Al-Cu-Li-Mg-Zr alloy was studied, and its microstructure and true stress were characterized as function of the deformation temperature and the strain rate using Gleeble-1500 thermal mechanical simulator. The results show that, with the increase of the strain rate from 0.001 s⁻¹ to 10 s⁻¹, the peak value of true stress is elevated at the same deformation temperature, and at the same strain rate the peak value of the true stress decreases with the increase of the deformation temperature from 360 ℃ to 520 ℃. Dynamic recrystallization easily occurs in the new Al-Cu-Li-Mg-Zr alloy under the lower strain rate and the higher deformation temperature, and dynamic recovery can usually be seen in this alloy under the higher strain rate of 10 s⁻¹ and the lower deformation temperature.

SiC_p/5210 Al metal matrix composites with a high volume fraction (50%) of SiC particles were fabricated by squeeze casting method. The effect of particle size on the mechanical properties of the composites was studied. The results show that with the decreasing of particle size, the bending strength of the composites increases, while the fracture toughness of the composites decreases. The bending strength and fracture toughness of 10 μm SiC_p/5210 Al metal matrix composite are 825 MPa, 10.0 MPa∙m^1/2, and the fracture toughness of 63 μm SiC_p/5210 Al metal matrix composite reaches 12.8 MPa·m^1/2. The main fracture mechanism changes from particle crack to particle/matrix interface de-bonding with the decreasing of particle size.

The flow stress behavior of 2197 Al-Li alloy during hot compression deformation was studied in the strain rate range from 0.01 to 10 s⁻¹ and the temperature range from 360 to 510 ℃ by isothermal compression test on a Gleeble-1500 thermal-mechanical simulator. The results show that the flow stress of 2197 Al-Li alloy decreases with the increase of deformation temperature and increases with the increase of strain rate. The peak flow stress during high temperature deformation can be represented by Z parameter in a hyperbolic sine function. The analytical expression of peak flow stress was fitted with the hot deformation activation energy of 260.6 kJ/mol.

A novel ceramic rolling technique was developed to improve the formability of spray deposited porous aluminum alloy sheets, in which the sheet preform was canned and ceramic particulates were adopted as the medium for transferring pressure to make a homogenous hydrostatic stress field. The fractional thickness deformation, fractional longitudinal elongation, and fractional lateral spreading were calculated from the dimensions of samples before and after ceramic rolling. The results show that the flowing of metals in the longitudinal and transverse directions can be restrained during rolling. So ceramic rolling can effectively improve the density of the spray deposited performs. When the thickness reduction ratio is up to about 60%, the full density preforms can be obtained. The effect of the kinds of ceramic particulates, including Al₂O₃, SiO₂ and graphite on the densification behavior of the preforms during rolling was investigated. The results imply that the Al₂O₃ particles with the size of about 74 μm are most effective.

The high cycle fatigue properties and fracture behavior of 2124-T851 aluminum alloy were investigated roundly, including the fatigue crack growth rate, fracture toughness and fatigue S—N curve. Furthermore, the fatigue crack growth rate was analyzed by fitting the curves. And the microstructure of the alloy was studied using by optical microscopy, transmission electron microscopy and X-ray diffractometry, scanning electron microscopy. The results show that the fatigue strength and the fracture toughness of 2124-T851 thick plate are 243 MPa and 29.6 MPa∙m^1/2 at room temperature and R=0.1, respectively. At high cycle fatigue condition, the characteristics of fatigue facture were observed obviously. And the higher the stress amplitude, the wider the space between the fatigue striations, the faster the rate of fatigue crack developing and going into the intermittent fracture area and the greater the ratio between the intermittent fracture area and the whole fracture area.

Nickel anode was investigated as a potential anode of aluminium electrowinning for preparation of Al-Ni master alloys. The electrolysis tests were carried out in Na₃AlF₆-Al₂O₃ based melts at 940 ℃. The results show that the cell voltage during electrolysis has only minor instability, and there exists NiO phase in electrolyte after 0.5 h electrolysis. Ni content in Al-Ni master alloys increases with increasing the electrolysis time. Concentration limit of Ni in Al-Ni master alloys can be up to 33.8% (mass fraction). However, substantial corrosion of the Ni-metal substrate is observed, and the oxide scale on the nickel anode after electrolysis is porous and loose that does not prevent corrosion of the substrate.

The 5Cu40Zn55Al and 15Cu20Zn65Al alloys were prepared in the Al-Zn-Cu system. There exist the metastable phases ε and θ in the two alloys after homogenization treatment and furnace cooling, respectively. It is shown that the particles are refined from 3 mm to less than 10 μm after hammering the two alloys but there are still metastable phases. This means that the phase constituents of the two alloys have no changes by the deformation, which is different from that by balling. The phase constituents are not changed at room temperature by hammering, which is dependent on the deformation mechanism of hammering.

The effects of microalloying elements Ti, Sc, Zr and Er on grain refinement behaviors and hardness properties of wedge-shaped Al-Mg-Mn alloy castings were investigated. The results indicate that alloys containing Sc and Zr can remarkably reduce the grain sizes of Al-Mg-Mn castings. Combination of Sc, Zr and Er can completely eliminate the columnar dendritic grains and further obtain refined grains with nondendritic sub-structure; the whole wedge-shaped cross-section of the casting consequently exhibits more homogeneous cast structures instead of the typical tri-crystal region structures. Large amounts of Al₃Sc-based intermetallic compound particles, such as Al₃(Sc_1−x, Zr_x), Al₃(Sc_1−x, Ti_x), Al₃(Sc_1−x−y, Zr_x, Ti_y) and Al₃(Sc_1−x−y, Zr_x, Er_y) are present in the microalloyed alloys, resulting from their numerously forming in high-temperature melt before solidification. These phases have the same L1₂-type crystal structure to Al₃Sc phase as well as smaller misfits with the primary α(Al) grains, which leads to more efficient epitaxial growth for α(Al) grains on all crystal planes of these composite phases. The experimental alloys have been hardened in different levels and, show the low susceptibilities of hardness change with varying cooling rate. The high hardness of the castings are caused by grain-refined strengthening and solid solution strengthening.

In situ TiC-TiB₂ diphase ceramic reinforced aluminum metal matrix composites were successfully fabricated via thermal explosion (TE) reaction in the Al-Ti-B₄C system. Using DTA and XRD analyses, the combustion reaction characteristic was examined. The results show that Al serves not only as a diluent but also as a reaction participant, affecting the reaction process and its final products. Combining with the DTA and the TE temperature-time curves, the ignition temperature is estimated to be about 970 K. With increasing Al content, the adiabatic combustion temperature is lowered and the sizes of the TiC and TiB₂ particulates decrease. When the Al content in the reactants is more than 50%, Al₃Ti intermediate phase is detected in the synthesized products. SEM observations reveal that the nearly spherical TiC particles and hexagonal or rectangular TiB₂ particles distribute relatively uniformly in the Al matrix.

The effects of solid solution and aging processing on the microstructures and mechanical properties of the multi-layer spray co-deposited 7090Al/SiC_p composite were investigated. The experimented results show that fine grains and homogeneous microstructures can be obtained, the average grain size of the as-solid solution treated and as-aged composites after extrusion is under 3.0 μm. A large amount of the Cu-rich phase particles form in the as-extruded samples, and solve into the matrix after solid solution treatment. After aging, the size of the precipitate phases, mainly MgZn₂ and CuAl₂ is less than 1.0 μm, which homogeneously distribute inside the grains and at the grain boundaries. The ultimate tensile strength of the composite treated at T6 state, i.e. solid solution treated at 475 ℃ for 1 h then aged at 120 ℃ for 24 h, is up to 765 MPa.

The effect of tensile stress on thermal microstructure evolution of Ω phase in an Al-Cu-Mg-Ag alloy with high Cu/Mg ratio and higher Ag content was investigated by transmission electron microcopy (TEM) .The samples were aged at 200 ℃ for 1 h (T6 condition), then thermal exposed at 250 ℃ for 100 h with and without a tensile stress (130 MPa), respectively. The results indicate that Ω precipitates uniformly disperse in the matrix as a major precipitate after artificially aging at 200 ℃ for 1 h (T6 condition). Exposed at 250 ℃ for 100 h without stress, Ω precipitates dissolve dramatically. Whereas, during stress exposure they coarsen unexpectedly rather than dissolve into matrix. It can be deduced that the stress retards the redissolution of Ω phase.

High reinforcement content Al/Si composites were fabricated by pressure infiltration technology. The composites are free of porosity and silicon particles distribute uniformly in the composites. The properties and fracture behavior of the composites were studied. The composites fracture is aroused by silicon brittle fracture and extends to the composites inward through the metallurgical structure and fracture analysis. The thermal expansion behavior of the composites was investigated by a high-precision thermomechanical analyzer，and compared with the predictions of theoretical model. The mean linear coefficient of thermal expansion (CET) of Al/Si composites ranges from 8×10⁻⁶ to 10×10⁻⁶/℃ and decreases with increasing silicon volume fraction.

The effect of the second phase precipitation behavior on the mechanical properties and fracture behavior of the modified casting Al-Cu alloys was investigated. The tensile strength of the alloys increases firstly and then decreases due to the appearance of θ′ precipitation phases, which increases firstly and then become coarser with the aging time increasing from 10 h to 20 h at 155 ℃. The strength of the alloys reaches the peak, resulting from Ω and θ′ precipitation phases, and decreases due to Ω phases becoming coarser and θ′ precipitation decreasing with the aging time increasing from 10 h to 20 h at 165 ℃. Ω phase becoming coarser and θ′ precipitation decreasing result in the strength of the alloys drastically decreasing after aging at 175 ℃ for 20 h. The ductility remains high level with increasing aging time at 155 ℃. The ductility irregularly changes as aging time prolongs at 165 ℃. The ductility is very low and at the same time gradually decreases with increasing aging time at 175 ℃. The Al-Cu alloy with a promising combination of tensile strength and ductility of about 474 MPa and 12.0% after aging at 165 ℃ for 10 h is due to a dense, uniform distribution of Ω precipitation phases together with a heterogeneous distribution of θ′ precipitations.

In hot-compression process, the various factors have obvious effects on the deformation behavior of AZ31 magnesium alloy deformation behavior. To understand the hot-compression constitutive relation thoroughly, the stress-strain behavior of AZ31 magnesium alloy at various strain rates and different deformation temperatures were investigated under maximum strain of 60%. The microstructure of the experimental alloy was studied in the hot-compression procedure. The experimental results show that the relation of peak flow stress, strain rate and temperature can be described by Z parameter which contains Arrheniues item. The strain rate and the deformation temperature are the key parameters affecting deformation activation energy.

During the welding of AZ91 cast alloy, the presence of eutectic b-Mg₁₇Al₁₂ phase results in constitutional liquation in the original interdendritic regions and in the formation of a partially melted zone (PMZ). In this study, gas tungsten arc welding (GTAW) and partial melting (simulated using furnace, salt bath and Gleeble) experiments were conducted. The results show that practically there would not be a critical heating rate during the welding to prevent constitutional liquation. The gradual change of the re-solidification microstructure within PMZ from base metal side to weld metal side was characterized. A sharp transition from base metal to PMZ has been observed. It is found that the original partially divorced eutectic has become a more regular eutectic in most of the PMZ, although close to the fusion boundary the re-solidified eutectic is again a more divorced one. Proceeding the eutectic re-solidification, a-Mg re-solidified with a cellular growth resulting in a serrated interface. The morphological change affected by the peak temperature and cooling rate will be presented and explained.

The deformation behavior of AZ31 was examined by compression and tension testes over a wide strain rate and temperature range, strain rate from 10⁻³ to 10³ s⁻¹, temperature from 300 to 623 K. Analysis of flow behavior and microstructural observations indicate that in tension tests dislocation glide is the most important deformation mechanism in the test strain rate and temperature range, while in compression tests twinning deformation mechanism is important at lower temperature when the strain rate ranges from 10⁻³ to 10 s⁻¹. At 10³ s⁻¹ strain rate, dislocation glide and twinning are present at the same time. At the strain rate of   2 964 s⁻¹, adiabatic shear band can be found easily, even at the strain rate of 1 537 s⁻¹ adiabatic shear localization zone can be found. In adiabatic shear localization zone, there are fine recrystallization grains. But in adiabatic shear band, the grains cannot be identified by optical microscopy.

One kind of Mg3.5Zn0.6Gd-based alloy strengthened with quasicrystals was designed, and the effect of alloying elements on microstructure and mechanical properties of as-cast Mg-Zn-Gd alloy at room temperature and elevated temperatures were studied. The results indicate that MgZnCu Laves phase, which coexists with quasicrystal at grain boundary, emerges with the addition of copper element in Mg-Zn-Gd alloy. The strength of alloys exhibits the parabola curve with the increase of copper content. The alloy with 1.5% (mole fraction) Cu shows better mechanical properties at room temperature: tensile strength 176 MPa, yield strength 176 MPa and elongation 6.5%. The existence of MgZnCu Laves phase can effectively improve the heat resistance and elevated temperature properties of the alloy. The alloy with 1.5% Cu has better mechanical properties at 200 ℃: tensile strength 130 MPa and elongation 18.5%. The creep test of the alloys at 200 ℃ and 50 MPa for 102 h indicates that Mg3.5Zn0.6Gd alloy reinforced with quasicrystal has better creep properties than AE42, which can be further improved with the introduction of Laves phase in the alloy.

The combination of magnesium alloys with the low-pressure expendable pattern casting (LP-EPC) process would bright future for application of magnesium alloys. The researches are focused on the effect of process parameters on the internal casting quality of magnesium alloy parts. AZ91D magnesium alloy castings were produced for different combinations of the LP-EPC process parameters. Specifically, pouring temperature, vacuum, filling velocity and coupling action of these factors were manipulated to observe their effect on the casting porosity and density distribution. The results indicate that the pouring temperature with LP-EPC process is lower than it in gravity casting. The selected process parameters, such as vacuum, filling velocity and coupled modes of them, must ensure melt metal flowing front profile exhibiting smooth and convex shape. The optimal process parameters for the castings are pouring temperature 983−1 023 K, vacuum 0.02−0.03 MPa, filling velocity 60−95 mm/s, and simultaneous filling with sucking.

The experiments of heat treatment, hot extrusion and hard drawing were employed to study their effects on the structure and mechanical properties of Mg-9Al-1.2Nd-0.45Y-0.7Zn magnesium alloy. The results indicate that the mechanical properties of the ingot are unstable and exhibit typical brittle failure. After heat treatment (693 K, 24 h), most β-Mg17Al12 phases decomposed into the α-Mg matrix and the distribution of Rare earths compounds remained the state of as-cast, which have little effects of the mechanical properties of the alloy. As the casting defects disappeared and the grain was refined after hot extruded, the mechanical properties of the alloy were drastically increased. By hard drawing, the ultimate tensile strength and the yield strength of the alloy were sharply increased while the elongation decreased rapidly. The failure of as-cast samples was mainly brittle fracture. After plastic deformation, the fracture patterns all exhibited ductile rupture features.

Mg-10Gd-3Y-0.5Zr alloy was cast in a step-like mould with five different cooling rates. The as-cast microstructures of the different steps were examined with optical microscope(OM) and scanning electron microscope(SEM). The room temperature mechanical properties were examined by tensile test. The results show that the microstructures are refined and the second phase particles are distributed much uniformly with the increase of cooling rate. The increase of yield strength, ultimate strength and elongation can be ascribed mainly to the strengthening effect of fine grains. The relationship between grain size and yield strength/hardness agrees with the Hall-Petch behavior.

The high tensile ductility of Mg-Gd-Y alloy at temperatures above 623 K and average engineering strain rate of 4.6×10⁻⁴ s⁻¹ was investigated. The results show that the high ductility (elongation 77%) at 623 K is attributed to the basal, non-basal dislocations glides. The superplasticity deformation (elongation 180%) at 673 K includes three stages. The first stage is the plastic deformation induced by dislocation gliding and twining. The second stage is the grain refinement controlled by dynamic recrystallization. The third stage includes two processes of the fine grain growth and grain refinement of the elongated grain with high stored energy.

LA141-(0−1.2)Ce alloys were prepared with vacuum induction melting method. The effects of Ce addition on the microstructure and mechanical properties of LA141 alloys were studied. The microstructure and phases composition of these alloys were analyzed by optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffractometry.  The mechanical properties of these alloys were measured with tensile tester. The results show that Ce has refining effect on the alloys. In the alloys, some Al₂Ce compounds exist, which make the Al content dissolved in α and β phases decrease and the hard brittle Mg₁₇Al₁₂ phase refined. The refining effect improves the mechanical properties of alloys. When Ce content is 0.9%(mass fraction), the tensile strength reaches 206.8 MPa and the elongation is two times as high as that of LA141 alloy. Due to the generation of Al₂Ce, the content of Al solid soluted in β phase decreases resulting in the decrease of alloy hardness with the addition of Ce.

Several Mg-xLi-3Al-1Ce alloys were prepared by vacuum induction heating. These alloys are Mg-5Li-3Al-1Ce, Mg-8Li-3Al-1Ce and Mg-14Li-3Al-1Ce, respectively. The microstructure and phase composition of these alloys were analyzed by optical microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffractometry. The mechanical properties of these alloys were measured with tensile tester. The results show that, Mg-5Li-3Al-1Ce has a single phase (α) structure, Mg-8Li-3Al-1Ce has a double phases (α+β) structure, Mg-14Li-3Al-1Ce has a single phase (β) structure. And some compounds distribute in the matrix. After being rolled, the grain size of all the alloys is refined. Under the condition of the same content of other alloying elements, the mechanical properties of Mg-5Li-3Al-1Ce are relatively high. With increasing Li content, the strength of both as-cast and as-rolled alloy decrease. For the as-cast alloys, with increasing Li content, the elongation increases. While for the as-rolled alloys, with increasing Li content, the elongation decreases. Ce has refining effect on these alloys.

The samples were fabricated by 220 t thixomolded machine made by Japan Steel Works. The microstructure from the AZ91D magnesium alloy chips to the thixomolded products was investigated. Melting behavior of the chips in thixomolding process was analyzed. The evolution processing of solid phase morphology was studied, and evolution model was put forward. The results show that microstructures in outer zone of a chip and the inner zone are obviously different, and the severe distortion takes place in the brim of the chip, where the grains are observed to be bent, distorted, even broken. The severe plastic deformation region is firstly molten, then segregation area in the inner of the chip continues to melt. The liquid phase in solid phase does not formed by liquid entrapped during shearing process, but primarily induced by internal composition segregation.

Coarse and agglomerated primary Mg₂Si phase in in-situ synthesized Mg₂Si/Mg composite with 4%Si was treated in remelting process by means of high intensity ultrasonic vibration. The effects of ultrasonic vibration duration and temperature on size, morphology and distribution of the primary Mg₂Si were studied. The evolution mechanism was discussed. The microstructures of the composites were investigated by means of optical microscopy (OM) and scanning electronic microscopy (SEM). The components were inspected with energy dispersion spectrum (EDS) and X-ray diffraction (XRD). The results indicate that ultrasonic vibration does not alter two constituents of the composites, but changes the size and distribution of aggregated primary Mg₂Si particles. The size of primary Mg₂Si particles decreases with the increase of vibration duration and vibrating temperature. High intensity ultrasonic has little effects on the primary Mg₂Si morphology. The high intensity ultrasonic vibration is an effective means to prepare well-proportioned in-situ synthesized magnesium matrix composites.

As-cast d 50 mm ZK60 magnesium alloys were reciprocating extruded for different passes under (335±5)℃. The microstructures and properties of the alloys were studied under different reciprocating extrusion passes. The results show that, reciprocating extruded microstructures were refined and uniformed effectively, in compared with as-cast condition. For 2 passes, the grain size is about 7 μm. After 4 passes, the grain size is about 6 μm. During reciprocating extrusion, refined as-cast eutectic and reinforcement phases in dispersed particles are uniformly distributed on matrix, and their size is less than 50 nm. The tensile strength, yield strength and the elongation of 4 passes reciprocating extruded as-cast ZK60 magnesium alloy are 311 MPa, 214 MPa and 17.93%, respectively. The thermal expansion coefficient of RE-4-EX-ZK60-CT magnesium alloy is the lowest. More or less extrusion passes can not decrease the value of the thermal expansion coefficient.

The flow stress features of MB26 magnesium alloy were studied by isothermal compression at 300−450 ℃ and strain rate of 0.001−1 s⁻¹ with Gleeble 1500 thermal simulator. In addition, the deformation activation energy Q was calculated. The results show that the strain rate and deformation temperature have obvious effect on the true stress. The peak value of flow stress becomes larger with increasing strain rate at the same temperature, and gets smaller with the increasing deformation temperature at the same strain rate. The alloy shows partial dynamic recrystallization. The flow stress of MB26 magnesium alloy during high temperature deformation can be represented by Zener-Hollomon parameter including the Arrhemius term. The temperature range of 350−400 ℃ is suggested for hot-forming of this alloy.

Zirconium and rare earth element cerium were added in magnesium and magnesium alloys to study their different grain refinement mechanisms. The results show that zirconium has an obvious refinement effect on the cast grain of magnesium and its alloys without the alloy element Al because the crystal structure of zirconium is the same as magnesium matrix, and the lattice parameters are close to magnesium. Zirconium can decrease the grain size of magnesium from 150 to 20 μm. The rare earth cerium also has a grain refinement effect on Mg and Mg-Al alloy. The cerium atoms tend to remain in the liquid rather than solidify with the solvent atoms magnesium at the solid-liquid interface. The liquid constitutional undercooling can provide a heterogeneous crystal nucleation. The grain is refined from 200 μm to 40−80 μm. These two elements have different grain refinement mechanism on Mg alloy. The mechanism of zirconium is that it acts as the nuclei of α-Mg. But the mechanism of cerium is that it increases the liquid constitutional undercooling that can provide a heterogeneous crystal nucleation for the alloy.

The superplasticity of coarsed grain AZ31 magnesium alloy was investigated in the temperature range of 623−723 K at strain rates from 10⁻³ to 10⁻¹ s⁻¹. It is found that with increasing strain rates, the elongation increases firstly, and then reaches the maximum value of 278% nearly at the strain rate of 5×10⁻² s⁻¹. The best superplastic conditions are at 723 K and at the strain rate of 5×10⁻² s⁻¹. During superplastic deformation, it is found that the mean grain size decreases from original 30 μm to 18 μm, indicating grain refinement is the dominant process at the initial stage of the superplastic deformation, meanwhile the grains become more homogenous and most of the grains are still equiaxed. However, the size and distribution of deformation cavity vary with deformation conditions at the last stage of deformation. At best superplastic condition, the cavity exhibits homogenous and ball-like. It is indicated that the dominant superplastic deformation mechanism at high strain rate is both grain refinement and grain boundary sliding.

A hypereutectic aluminum-silicon alloy with content of 17%Si, designed as A390, was modified. The change of microstructure was identified by optical microscopy. The results show that phosphorus can change the morphology and size of the primary silicon and control the length of needle-like eutectic silicon or dendrite. After modified by phosphorus, the primary silicon of star-like and coarse platelet is changed to polygonal with less cute angles and decreasing average size. The eutectic silicon is also changed because it grows from the tip of angles on the primary silicon and is influenced by the morphology and size of primary silicon. The eutectic silicon changes from needle-like or dendrite shape to short bars and dots with less average length.

The microstructures and tensile properties of Mg-9Gd-3Y-0.3Zr alloys were investigated by OM, SEM and mechanical property tester. The results show that the microstructure of as-cast sample is a typical dendritic structure, the grain size of extruded sample is finer than that of the as-cast because of recrystallization. The aging response of extruded specimens is quick and marked when the samples are aged at 200 to 250 ℃ for different time, and the peak aging hardness is about HV120. The tensile strengths at 25, 200, 250 and 300 ℃ are 375, 364, 329 and 286 MPa, respectively. The maximum elongation is higher than 18% at 300 ℃. The fracture mode is mainly microvoid coalescence fracture combining with brittle cleavage fracture at room temperature, and microvoid coalescence fracture at 250−300 ℃.

The cylindrical billet of Mg-9Al-1Zn alloy was produced by spray forming and hot extruded at 698−723 K with an extrusion ratio of 22׃1. The microstructural evolution was investigated systematically. The mechanisms of grain refinement, solubility extension of Al and Zn elements in Mg matrix, and the precipitation of the second phase β-Mg₁₇Al₁₂ as well as their effect on the mechanical properties of the Mg-9Al-1Zn alloy were analyzed and discussed. The results show that the spray formed Mg-9Al-1Zn alloy has homogeneous equiaxed fine grains with the average grain size of 17 μm, of which the average grain size is refined further to 5 μm due to dynamic recrystallization during hot extrusion process. The evolution of size, proportion and distribution of β-Mg₁₇Al₁₂ phase was also observed and analyzed. The mechanical properties for the extruded rods are improved remarkably at ambient temperature.