The effects of yttrium addition on microstructure and mechanical properties of as-extruded AZ31 magnesium alloys were investigated by OM, XRD and SEM. The results show that the addition of yttrium results in the formation of a new phase, Al₂Y. When the addition of yttrium is higher than 1.48%-2.91% (mass fraction), another new phase, Al₃Y₅Mn₇, forms, and the amount of β-Mg₁₇Al₁₂ phase in the AZ31 alloy decreases sharply. The tensile test at room temperature indicates that the yield strength of as-extruded AZ31 alloys improves with the addition of yttrium, but the elongation decreases, which is possibly related to the formation of coarse blocky compounds containing yttrium and the grain coarsening in the alloys.

The effects of yttrium addition on microstructure and mechanical properties of as-extruded AZ31 magnesium alloys were investigated by OM, XRD and SEM. The results show that the addition of yttrium results in the formation of a new phase, Al2Y. When the addition of yttrium is higher than 1.48%−2.91% (mass fraction), another new phase, Al3Y5Mn7, forms, and the amount of β-Mg17Al12 phase in the AZ31 alloy decreases sharply. The tensile test at room temperature indicates that the yield strength of as-extruded AZ31 alloys improves with the addition of yttrium, but the elongation decreases, which is possibly related to the formation of coarse blocky compounds containing yttrium and the grain coarsening in the alloys.

The development status of twin-roll strip casting for magnesium alloys in China was summarized as well as the new progress when several kinds of twin-roll strip casting technologies were developed and used. Horizontal twin-roll casting (HTRC) of magnesium alloys has attracted much attention and has been industrialized in China. Vertical twin roll casting (VTRC) of the magnesium alloys can reach a speed of higher than 30 m/min and its research and development are just beginning and exhibit exciting potential. By comparing the process characteristics of the two technologies, the process stability of HTRC for the magnesium alloys is better, and the casting speed and the cooling rate of VTRC for the magnesium alloys are higher. The quality of the products by the two technologies needs to be improved and further investigated.

The development status of twin-roll strip casting for magnesium alloys in China was summarized as well as the new progress when several kinds of twin-roll strip casting technologies were developed and used. Horizontal twin-roll casting (HTRC) of magnesium alloys has attracted much attention and has been industrialized in China. Vertical twin roll casting (VTRC) of the magnesium alloys can reach a speed of higher than 30 m/min and its research and development are just beginning and exhibit exciting potential. By comparing the process characteristics of the two technologies, the process stability of HTRC for the magnesium alloys is better, and the casting speed and the cooling rate of VTRC for the magnesium alloys are higher. The quality of the products by the two technologies needs to be improved and further investigated.

Four kinds of Mg-Y-RE-Zr alloys with different Gd contents were prepared, and the effect of Gd content on microstructure and mechanical properties of the alloys was researched. Based on the experimental investigation, the compounds at the grain boundaries are mainly Mg₂₄Y₅, Mg₄₁Nd₅, and Mg₅Gd phases. The average grain size of as-cast alloys is 50-60 μm. After T4 (535 ℃, 24 h) treatment, Mg₅Gd phases mostly decompose and dissolve into the matrix, and the disperse spotted phases are mainly Mg₂₄Y₅ and Mg₄₁Nd₅ phases. After extruding and ageing (250 ℃, 5 h), the grain size is refined and some grains abnormally grow up to about 40 μm. With Gd content increasing, the ultimate tensile strength, yield strength of as-cast alloys and the extruded bars after ageing are improved, but the elongation is decreased.

Four kinds of Mg-Y-RE-Zr alloys with different Gd contents were prepared, and the effect of Gd content on microstructure and mechanical properties of the alloys was researched. Based on the experimental investigation, the compounds at the grain boundaries are mainly Mg24Y5, Mg41Nd5, and Mg5Gd phases. The average grain size of as-cast alloys is 50−60 μm. After T4 (535 ℃, 24 h) treatment, Mg5Gd phases mostly decompose and dissolve into the matrix, and the disperse spotted phases are mainly Mg24Y5 and Mg41Nd5 phases. After extruding and ageing (250 ℃, 5 h), the grain size is refined and some grains abnormally grow up to about 40 μm. With Gd content increasing, the ultimate tensile strength, yield strength of as-cast alloys and the extruded bars after ageing are improved, but the elongation is decreased.

The effects of hot extrusion treatment on the microstructure and mechanical properties of AZ31-0.25%Sb Mg alloy were investigated by means of mechanical properties measurement and microstructure observation. The results show that the microstructure of AZ31-0.25%Sb Mg alloys consists of a-Mg matrix, Mg₁₇Al₁₂ and Mg₃Sb₂ phases. The ultimate tensile strength (UTS) and yield tensile strength(YTS) of the alloy are obviously enhanced by hot extrusion treatment, and the enhanced extent of UTS and YTS increases with the decrease of hot extrusion temperature, moreover, the YTS value of the alloy at RT, after extruded at 220 ℃, increases up to 131.4%, which attributes to the finer grains resulted from the dynamic recrystallization occurred during hot extrusion. As hot extrusion goes on, the slipping and concentration of dislocations continue to occur within the finer grains, which promotes the formation of the subgrains in the alloy. The deformation features of the extruded alloy during tensile deformation at RT are the twinning deformation and dislocation slipping in the twinning regions. Moreover, the deformation mechanisms of the alloy are a dislocation activation on the basal plane and a+c dislocation activation on the pyramidal planes.

The effects of hot extrusion treatment on the microstructure and mechanical properties of AZ31-0.25%Sb Mg alloy were investigated by means of mechanical properties measurement and microstructure observation. The results show that the microstructure of AZ31-0.25%Sb Mg alloys consists of -Mg matrix, Mg17Al12 and Mg3Sb2 phases. The ultimate tensile strength (UTS) and yield tensile strength(YTS) of the alloy are obviously enhanced by hot extrusion treatment, and the enhanced extent of UTS and YTS increases with the decrease of hot extrusion temperature, moreover, the YTS value of the alloy at RT, after extruded at 220 ℃, increases up to 131.4%, which attributes to the finer grains resulted from the dynamic recrystallization occurred during hot extrusion. As hot extrusion goes on, the slipping and concentration of dislocations continue to occur within the finer grains, which promotes the formation of the subgrains in the alloy. The deformation features of the extruded alloy during tensile deformation at RT are the twinning deformation and dislocation slipping in the twinning regions. Moreover, the deformation mechanisms of the alloy are a dislocation activation on the basal plane and a+c dislocation activation on the pyramidal planes.

Mg-7Zn-3Al-xEr (x=0.1, 0.4, 0.7) magnesium alloys were prepared by permanent mould casting. The effects of rare earth element of erbium on the microstructure and mechanical properties of as-cast Mg-7Zn-3Al alloy at both room temperature and elevated temperatures were investigated with optical microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy, differential scanning calorimetry, and tensile testing. The results show that the quasi-continuous grain boundary networked τ(Mg₃₂(Al,Zn)₄₉) phases are changed into discontinuous globular particles due to the addition of Er. Spherical Al-Er compounds are also identified in the matrix. The mechanical testing reveals that, with Er addition, the elevated temperature tensile properties can be remarkably improved. The microstructure evolution during tensile deformation under both temperature and stress and its effects on the mechanical properties were further discussed.

Mg-7Zn-3Al-xEr (x=0.1, 0.4, 0.7) magnesium alloys were prepared by permanent mould casting. The effects of rare earth element of erbium on the microstructure and mechanical properties of as-cast Mg-7Zn-3Al alloy at both room temperature and elevated temperatures were investigated with optical microscopy, scanning electron microscopy/energy dispersive X-ray spectroscopy, differential scanning calorimetry, and tensile testing. The results show that the quasi-continuous grain boundary networked τ(Mg32(Al,Zn)49) phases are changed into discontinuous globular particles due to the addition of Er. Spherical Al-Er compounds are also identified in the matrix. The mechanical testing reveals that, with Er addition, the elevated temperature tensile properties can be remarkably improved. The microstructure evolution during tensile deformation under both temperature and stress and its effects on the mechanical properties were further discussed.

Mg-3.4Nd-0.1Zn-0.40Zr alloy samples with and without containing gadolinium (0.6%, mass fraction) were prepared by sand casting. The aged hardening behavior, solidification microstructures and mechanical properties of the alloys were investigated by using the analysis methods of OM, XRD, TEM, hardness tests and mechanical property tests. The main research results are as follows. 1) Compared with the alloy without the addition of gadolinium, the alloys with the addition of gadolinium shows the more remarkable age-hardening response. 2) The as-cast microstructure of the alloy with and without containing gadolinium consists of α-Mg grains with Mg₁₂Nd phase on the grain boundary. After solution heat-treatment, Mg₁₂Nd phase of the alloy without containing gadolinium is dissolved in the matrix, however, there is still discontinued Mg₁₂Nd phase at grain boundary of the alloy with containing gadolinium. The more finely dispersed precipitates in Mg matrix are formed in the alloy with containing gadolinium during age-treatment. 3) The room temperature and high temperature mechanical properties of the alloy are satisfactory, with σ_b=280 MPa, σ_0.2=165 MPa at RT and σ_b==215 MPa, σ_0.2=155 MPa at 250 ℃. The high temperature mechanical properties decrease slightly with the increase of temperature.

Mg-3.4Nd-0.1Zn-0.40Zr alloy samples with and without containing gadolinium (0.6%, mass fraction) were prepared by sand casting. The aged hardening behavior, solidification microstructures and mechanical properties of the alloys were investigated by using the analysis methods of OM, XRD, TEM, hardness tests and mechanical property tests. The main research results are as follows. 1) Compared with the alloy without the addition of gadolinium, the alloys with the addition of gadolinium shows the more remarkable age-hardening response. 2) The as-cast microstructure of the alloy with and without containing gadolinium consists of α-Mg grains with Mg12Nd phase on the grain boundary. After solution heat-treatment, Mg12Nd phase of the alloy without containing gadolinium is dissolved in the matrix, however, there is still discontinued Mg12Nd phase at grain boundary of the alloy with containing gadolinium. The more finely dispersed precipitates in Mg matrix are formed in the alloy with containing gadolinium during age-treatment. 3) The room temperature and high temperature mechanical properties of the alloy are satisfactory, with σb=280 MPa, σ0.2=165 MPa at RT and σb=215 MPa, σ0.2=155 MPa at 250 ℃. The high temperature mechanical properties decrease slightly with the increase of temperature.

Equal channel angular pressing(ECAP) was conducted at 250 ℃ for 4 passes to the as-extruded Mg-3%Cu-1%Mn alloy with high strength and high damping capacity. After ECAP processing, the grain of as-extruded alloy is significantly refined to about 4 μm, both yield strength and tensile strength of the as-extruded Mg-Cu-Mn alloy are decreased, but the ductility is improved. After the ECAP processing, the damping capacity of Mg-Cu-Mn alloy is decreased at room temperature, while is substantially increased at elevated temperatures. After annealing at 300 ℃ for 1 h, both yield strength and tensile strength of the ECAPed alloy are further decreased, but the ductility is significantly improved. The damping capacity of the as-annealed alloy is increased at room temperature, while is decreased at elevated temperatures. The mechanisms for damping capacity of the alloy at ambient and elevated temperatures were discussed.

Equal channel angular pressing(ECAP) was conducted at 250 ℃ for 4 passes to the as-extruded Mg-3%Cu-1%Mn alloy with high strength and high damping capacity. After ECAP processing, the grain of as-extruded alloy is significantly refined to about 4 μm, both yield strength and tensile strength of the as-extruded Mg-Cu-Mn alloy are decreased, but the ductility is improved. After the ECAP processing, the damping capacity of Mg-Cu-Mn alloy is decreased at room temperature, while is substantially increased at elevated temperatures. After annealing at 300 ℃ for 1 h, both yield strength and tensile strength of the ECAPed alloy are further decreased, but the ductility is significantly improved. The damping capacity of the as-annealed alloy is increased at room temperature, while is decreased at elevated temperatures. The mechanisms for damping capacity of the alloy at ambient and elevated temperatures were discussed.

Cerium was added in AZ31 alloy with the contents of 0.4%, 0.8% and 1.2% respectively to produce experimental alloys. The grain refinement of Ce in the as-cast and rolled AZ31 alloy were studied by using Polyvar-MET optical microscope with a VSM2000 quantitative analysis system, KYKY2000 SEM and Tecnai G² 20 TEM. And the mechanical properties of AZ31+Ce alloy were tested on a CSS-44100 testing system with computerized data acquisition. The results show that the cerium has a good grain refinement effect on the as-cast AZ31 alloy because cerium can build up a solute enriched zone rapidly during the solidification process. The dynamic recrystallization (DRX) grains less than 10 μm can be obtained in hot rolled AZ31+Ce alloy. A cold rolling deformation degree over than 20% and a following annealing at 400 ℃ for 1 h will lead to refine and uniform grains with the sizes of about 25 μm. The cerium can form dispersed and thermally stable Al₄Ce phase that can prohibit the coarsening of grains in AZ31+Ce alloy during the hot rolling and annealing process.

Cerium was added in AZ31 alloy with the contents of 0.4%, 0.8% and 1.2% respectively to produce experimental alloys. The grain refinement of Ce in the as-cast and rolled AZ31 alloy were studied by using Polyvar-MET optical microscope with a VSM2000 quantitative analysis system, KYKY2000 SEM and Tecnai G2 20 TEM. And the mechanical properties of AZ31+Ce alloy were tested on a CSS−44100 testing system with computerized data acquisition. The results show that the cerium has a good grain refinement effect on the as-cast AZ31 alloy because cerium can build up a solute enriched zone rapidly during the solidification process. The dynamic recrystallization (DRX) grains less than 10 μm can be obtained in hot rolled AZ31+Ce alloy. A cold rolling deformation degree over than 20% and a following annealing at 400 ℃ for 1 h will lead to refine and uniform grains with the sizes of about 25 μm. The cerium can form dispersed and thermally stable Al4Ce phase that can prohibit the coarsening of grains in AZ31+Ce alloy during the hot rolling and annealing process.

The effect of cerium-rich misch metal addition on the microstructure and properties of squeeze cast magnesium alloys AZ81 was empirically investigated. The results indicate that the addition of cerium-rich misch metal modifies the microstructure gradually. With the increase of the RE addition, the amount of Mg₁₇Al₁₂ decreases while that of Al₁₁(RE)₃ increases, accompanied by grain refinement. When the addition reaches 1.5%, the grain refinement becomes obvious. However, when the addition exceeds 2.0%, Al₁₁(RE)₃ phase coarsens into rod shape and the grain size increases. The tensile properties of the AZ81 at both room temperature and 150℃ increase with the addition, and reach their optimal values with the addition of 1.5%. Further increase of the addition to above 2.0% decreases the tensile properties considerably. The tensile fracture of the alloy is characterized by the cleavage of the brittle second phases and ductile dimples of the matrix.

The effect of cerium-rich misch metal addition on the microstructure and properties of squeeze cast magnesium alloys AZ81 was empirically investigated. The results indicate that the addition of cerium-rich misch metal modifies the microstructure gradually. With the increase of the RE addition, the amount of Mg17Al12 decreases while that of Al11(RE)3 increases, accompanied by grain refinement. When the addition reaches 1.5%, the grain refinement becomes obvious. However, when the addition exceeds 2.0%, Al11(RE)3 phase coarsens into rod shape and the grain size increases. The tensile properties of the AZ81 at both room temperature and 150℃ increase with the addition, and reach their optimal values with the addition of 1.5%. Further increase of the addition to above 2.0% decreases the tensile properties considerably. The tensile fracture of the alloy is characterized by the cleavage of the brittle second phases and ductile dimples of the matrix.

The effects of various Mg-Sr master alloys (conventional as-cast, rapidly-solidified, rolled and solutionized) on microstructural refinement of ZK60 magnesium alloy were investigated. The results indicate that the refinement efficiency of various Mg-Sr master alloys in ZK60 alloy is different. The rolled Mg-Sr master alloy is found to have relatively higher refinement efficiency than the conventional as-cast, solutionized and rapidly-solidified Mg-Sr master alloys. After being treated with the rolled Mg-Sr master alloy, the ZK60 alloy obtains the minimum average grain size of 33 μm. The difference of various Mg-Sr master alloys in refinement efficiency might be related to the initial microstructure change of various Mg-Sr master alloys.

The effects of various Mg-Sr master alloys (conventional as-cast, rapidly-solidified, rolled and solutionized) on microstructural refinement of ZK60 magnesium alloy were investigated. The results indicate that the refinement efficiency of various Mg-Sr master alloys in ZK60 alloy is different. The rolled Mg-Sr master alloy is found to have relatively higher refinement efficiency than the conventional as-cast, solutionized and rapidly-solidified Mg-Sr master alloys. After being treated with the rolled Mg-Sr master alloy, the ZK60 alloy obtains the minimum average grain size of 33 μm. The difference of various Mg-Sr master alloys in refinement efficiency might be related to the initial microstructure change of various Mg-Sr master alloys.

The effects of Cu and Mn additions on mechanical properties and damping capacity of Mg-Cu-Mn alloy were investigated. The tensile properties and damping capacity at room temperature of as-cast Mg-Cu-Mn alloy were tested. The microstructure was studied using optical microscope, X-ray diffraction and scanning electron microscope. The Hall-Petch relation and Granato-Lücke model were used to explain the influences of Cu and Mn additions on the tensile properties and damping capacity of Mg-Cu-Mn alloy. The results show that Cu and Mn additions remarkably reduce the grain size of Mg-Cu-Mn alloy, but have little influence on phase composition and solute atoms concentration, the tensile properties increase obviously and the internal friction of Mg-Cu-Mn alloy decreases with grain refining.

The effects of Cu and Mn additions on mechanical properties and damping capacity of Mg-Cu-Mn alloy were investigated. The tensile properties and damping capacity at room temperature of as-cast Mg-Cu-Mn alloy were tested. The microstructure was studied using optical microscope, X-ray diffraction and scanning electron microscope. The Hall-Petch relation and Granato-Lücke model were used to explain the influences of Cu and Mn additions on the tensile properties and damping capacity of Mg-Cu-Mn alloy. The results show that Cu and Mn additions remarkably reduce the grain size of Mg-Cu-Mn alloy, but have little influence on phase composition and solute atoms concentration, the tensile properties increase obviously and the internal friction of Mg-Cu-Mn alloy decreases with grain refining.

The microstructures and mechanical properties of a new Mg-6%Zn-1%Mn (mass fraction) wrought magnesium alloy were studied, which could be extruded smoothly at 310-330 ℃ with a complete dynamic recrystallization. After solution treatment one and two-step aging techniques were used. All as-aged microstructures contained two types of dispersed phases: β' phases and pure α-Mn particles. The two-step aging had a better strengthening effect than the traditional one-step aging, and the strength value achieved by the two-step aging could reach that of the ZK60 wrought magnesium alloy. The outstanding precipitation strengthening effect of the alloy should be attribute to the GP zones, diffusive solute-rich zones and some metastable phases formed during the first step aging that provide more effective nuclei for Mg-Zn strengthening phases during the second step aging.

The microstructures and mechanical properties of a new Mg-6%Zn-1%Mn (mass fraction) wrought magnesium alloy were studied, which could be extruded smoothly at 310−330 ℃ with a complete dynamic recrystallization. After solution treatment one and two-step aging techniques were used. All as-aged microstructures contained two types of dispersed phases: β' phases and pure α-Mn particles. The two-step aging had a better strengthening effect than the traditional one-step aging, and the strength value achieved by the two-step aging could reach that of the ZK60 wrought magnesium alloy. The outstanding precipitation strengthening effect of the alloy should be attribute to the GP zones, diffusive solute-rich zones and some metastable phases formed during the first step aging that provide more effective nuclei for Mg-Zn strengthening phases during the second step aging.

Creep of squeeze-cast Mg-3Y-2Nd-1Zn-1Mn alloy was investigated at the constant load in the stress range of 30-80 MPa. Tensile creep tests were performed at 300 ℃ up to the final fracture. Several tests at 50 MPa were interrupted after reaching the steady state creep; and another set of creep tests was interrupted after the onset of ternary creep. Fraction of cavitated dendritic boundaries was evaluated using optical microscopy. Measurement of grain boundary sliding by observation of the offset of marker lines was carried out on the surface of the crept specimens after the test interruption by scanning electron microscopy and by confocal laser scanning microscopy. The results show that the dominant creep mechanism in this alloy is dislocation creep with minor contribution of the grain boundary sliding. Creep failure took place by the nucleation, growth and coalescence of creep cavities on the boundaries predominantly oriented perpendicular to the applied stress. Increasing amount of cavitated boundaries with time of creep exposure supports the mechanism of continuous cavity nucleation and growth.

Creep of squeeze-cast Mg-3Y-2Nd-1Zn-1Mn alloy was investigated at the constant load in the stress range of 30−80 MPa. Tensile creep tests were performed at 300 ℃ up to the final fracture. Several tests at 50 MPa were interrupted after reaching the steady state creep; and another set of creep tests was interrupted after the onset of ternary creep. Fraction of cavitated dendritic boundaries was evaluated using optical microscopy. Measurement of grain boundary sliding by observation of the offset of marker lines was carried out on the surface of the crept specimens after the test interruption by scanning electron microscopy and by confocal laser scanning microscopy. The results show that the dominant creep mechanism in this alloy is dislocation creep with minor contribution of the grain boundary sliding. Creep failure took place by the nucleation, growth and coalescence of creep cavities on the boundaries predominantly oriented perpendicular to the applied stress. Increasing amount of cavitated boundaries with time of creep exposure supports the mechanism of continuous cavity nucleation and growth.

Mg₉₇Zn₁Y₂ alloy has been studied as an elevated temperature creep resistant Mg-based alloy for nearly ten years. While, the strength of the cast Mg₉₇Zn₁Y₂ alloy with long-period stacking(LPS) structure is lower than that of the commercial AZ91 alloy at room temperature. The microstructure evolutions in Mg₉₇Zn₁2 (molar fraction, %) alloys with LPS phase, processed by rolling and annealing the as-cast alloy and rapidly solidifying/melt-spinning and age treating at different temperatures respectively, were investigated by differential thermal analysis(DTA), X-ray diffraction(XRD), and laser optical microscopy(LOM), scanning electron microscopy (SEM), and transmission electron microscopy(TEM). The evolutionary direction of microstructure prescribed by thermodynamics in the Mg₉₇Zn₁₂ alloy is reflected from experimental data of the as-cast alloy; and the actual evolution paths selected by kinetics are depicted in detail in the as-spun alloy and rolled alloy. The strong influences of thermodynamic non- equilibrium mechanism, which entails the factual complexity of microstructures typically during rapid solidification and deformation processing for strengthening the creep resistant magnesium alloy, are presented.

Mg97Zn1Y2 alloy has been studied as an elevated temperature creep resistant Mg-based alloy for nearly ten years. While, the strength of the cast Mg97Zn1Y2 alloy with long-period stacking(LPS) structure is lower than that of the commercial AZ91 alloy at room temperature. The microstructure evolutions in Mg97Zn1Y2 (molar fraction, %) alloys with LPS phase, processed by rolling and annealing the as-cast alloy and rapidly solidifying/melt-spinning and age treating at different temperatures respectively, were investigated by differential thermal analysis(DTA), X-ray diffraction(XRD), and laser optical microscopy(LOM), scanning electron microscopy (SEM), and transmission electron microscopy(TEM). The evolutionary direction of microstructure prescribed by thermodynamics in the Mg97Zn1Y2 alloy is reflected from experimental data of the as-cast alloy; and the actual evolution paths selected by kinetics are depicted in detail in the as-spun alloy and rolled alloy. The strong influences of thermodynamic non- equilibrium mechanism, which entails the factual complexity of microstructures typically during rapid solidification and deformation processing for strengthening the creep resistant magnesium alloy, are presented.

Friction stir welding (FSW) technique was utilized to weld cast AM50 magnesium alloy plates. The microstructures in the base metal (BM) and the weld joint were observed by optical microscopy. The mechanical properties were investigated by using hardness measurement and tensile test, and the fractographs were observed by scanning electron microscopy. The results show that the microstructure of the base material was characterized by bulk primary α phase, α-matrix and intermetallic compound β (or Mg₁₇Al₁₂), and the weld nugget exhibiting recrystallized microstructure consists of α-matrix and β phase. The grain size in the weld is smaller than that in the base metal. The hardness of the weld joint is improved but the tensile strength and yield strength, as well as the elongation to failure of the base material decline. The fracture of BM has a rougher surface with more dimples, which is a characteristic of the ductile fracture, whereas the fracture on the nugget reveals a quasi-cleavage feature. The ultimate tensile strength and yield strength of the FSWed AM50 are 86.2% and 94.0% of those of the base metal, respectively.

Friction stir welding (FSW) technique was utilized to weld cast AM50 magnesium alloy plates. The microstructures in the base metal (BM) and the weld joint were observed by optical microscopy. The mechanical properties were investigated by using hardness measurement and tensile test, and the fractographs were observed by scanning electron microscopy. The results show that the microstructure of the base material was characterized by bulk primary α phase, α-matrix and intermetallic compound β (or Mg17Al12), and the weld nugget exhibiting recrystallized microstructure consists of α-matrix and β phase. The grain size in the weld is smaller than that in the base metal. The hardness of the weld joint is improved but the tensile strength and yield strength, as well as the elongation to failure of the base material decline. The fracture of BM has a rougher surface with more dimples, which is a characteristic of the ductile fracture, whereas the fracture on the nugget reveals a quasi-cleavage feature. The ultimate tensile strength and yield strength of the FSWed AM50 are 86.2% and 94.0% of those of the base metal, respectively.

The microstructure and properties of Mg ZK21 laser beam weld without filler were researched using optical microscopy (OM), electron microscopy and mechanical test. The results show that the fracture strain of the joints after laser beam welding reduces by about 10.7% at room temperature. By means of laser beam welding, the fusion zones contain tensile RS, while the base material far away from the fusion line is under balancing compressive RS. The microstructures of the weld were characterized by a narrow heat affected zone and twins. Significant {1012} tension twins occur in the weld HAZ during laser welding processing. Due to the influence of temperature field and stress on morphologies, most of twins form twinning bands, which are nearly parallel to the welding direction.

The microstructure and properties of Mg ZK21 laser beam weld without filler were researched using optical microscopy (OM), electron microscopy and mechanical test. The results show that the fracture strain of the joints after laser beam welding reduces by about 10.7% at room temperature. By means of laser beam welding, the fusion zones contain tensile RS, while the base material far away from the fusion line is under balancing compressive RS. The microstructures of the weld were characterized by a narrow heat affected zone and twins. Significant { } tension twins occur in the weld HAZ during laser welding processing. Due to the influence of temperature field and stress on morphologies, most of twins form twinning bands, which are nearly parallel to the welding direction.

The microstructure of sub-rapid solidification processed AZ61A magnesium alloy was presented and discussed. The results show that the grain size of the foil is significantly refined, and the grain morphology is cellular or globular. The eutectic transformation L→α-Mg+β-Mg₁₇Al₁₂ and microsegregation in conventionally solidified AZ61A alloy are suppressed to a great extent. The β-Mg₁₇Al₁₂ phases located in the α-Mg grain boundaries are largely decreased due to high solidification cooling rate. As a consequence, the alloying elements Al, Zn, Mn show much higher solid solubility and the sub-rapid solidification microstructure dominantly consists of supersaturated α-Mg solid solution. The mechanical properties and fractographic analysis reveal that the fracture mechanism and corresponding morphology of the rapture surface of tensile bars are linked to the microstructure obtained and depend on the sub-solidification processes.

The microstructure of sub-rapid solidification processed AZ61A magnesium alloy was presented and discussed. The results show that the grain size of the foil is significantly refined, and the grain morphology is cellular or globular. The eutectic transformation L→α-Mg+β-Mg17Al12 and microsegregation in conventionally solidified AZ61A alloy are suppressed to a great extent. The β-Mg17Al12 phases located in the α-Mg grain boundaries are largely decreased due to high solidification cooling rate. As a consequence, the alloying elements Al, Zn, Mn show much higher solid solubility and the sub-rapid solidification microstructure dominantly consists of supersaturated α-Mg solid solution. The mechanical properties and fractographic analysis reveal that the fracture mechanism and corresponding morphology of the rapture surface of tensile bars are linked to the microstructure obtained and depend on the sub-solidification processes.

A design of experiment technique was used to optimize the microstructure of the AZ91D alloy produced by rheo-casting. The experimental design consists of four parameters (pouring temperature, shearing temperature, shearing time and shearing rate) with three levels. The grain size and shape factor measurements of primary α-Mg particles were conducted to determine the microstructure. The contribution of each parameter shows that pouring temperature is the most significant parameter affecting the grain size, and the shape factor highly depends on the shearing temperature. The optimized rheo-casting processing parameters are 650 ℃ for pouring temperature, 585 ℃ for shearing temperature, 40 s for shearing time, and 600 r/min for shearing rate. Under the optimized processing parameters, the average grain size is 28.53 μm, and the shape factor is 0.591.

A design of experiment technique was used to optimize the microstructure of the AZ91D alloy produced by rheo-casting. The experimental design consists of four parameters (pouring temperature, shearing temperature, shearing time and shearing rate) with three levels. The grain size and shape factor measurements of primary α-Mg particles were conducted to determine the microstructure. The contribution of each parameter shows that pouring temperature is the most significant parameter affecting the grain size, and the shape factor highly depends on the shearing temperature. The optimized rheo-casting processing parameters are 650 ℃ for pouring temperature, 585 ℃ for shearing temperature, 40 s for shearing time, and 600 r/min for shearing rate. Under the optimized processing parameters, the average grain size is 28.53 μm, and the shape factor is 0.591.

The effects of external fields such as electromagnetic field on the structure and heat treatment behavior of Mg-Li-Al alloys were studied. Mg-8Li-3Al alloys cast with and without electromagnetic stirring were used for solution treatment and aging treatment. Experimental results show that the dendritic arms are broken and a large quantity of equiaxed grains appear in the microstructure of specimens with electromagnetic stirring (EMS). With the increase of the quenching temperature(150-350 ℃), the solution of Mg and Al in solid β phase increases, and the Brinell hardness of the alloy increases as well. Aging peak and over aging happen because θ(MgLi₂Al) which precipitates in the β matrix and strengthens the alloy is unstable and transforms to stable AlLi phase. Aging curves of EMS specimens change in a smaller amplitude.

The effects of external fields such as electromagnetic field on the structure and heat treatment behavior of Mg-Li-Al alloys were studied. Mg-8Li-3Al alloys cast with and without electromagnetic stirring were used for solution treatment and aging treatment. Experimental results show that the dendritic arms are broken and a large quantity of equiaxed grains appear in the microstructure of specimens with electromagnetic stirring (EMS). With the increase of the quenching temperature(150−350 ℃), the solution of Mg and Al in solid β phase increases, and the Brinell hardness of the alloy increases as well. Aging peak and over aging happen because θ(MgLi2Al) which precipitates in the β matrix and strengthens the alloy is unstable and transforms to stable AlLi phase. Aging curves of EMS specimens change in a smaller amplitude.

Ternary alloys based on the Mg-Zn-Ca system were produced by twin-roll rapid solidification. The alloys were characterized by OM, SEM, HRTEM, XRD, EDS and Micro-hardness. The results show that the rapidly solidified flakes are of fine dendritic cell structures with the cell size ranging from 1 to 5 μm. The Mg-6Zn-5Ca alloy in RS and annealing (200 ℃ for 1 h) states are mainly composed of α-Mg, Mg₂Ca, Ca₂Mg₆Zn₃ and a small quantity of Mg₅₁Zn₂₀, MgZn₂ and Mg₂Zn ₃. Micro-hardness increases with the increment of Ca content and age hardening occurs after aging at 200 ℃ in the flakes probably due to the precipitation strengthening of the fine precipitates Mg₂Ca and Ca₂Mg₆Zn₃. Some phases at the grain boundary in Mg-6Zn-5Ca alloy are identified by means of HRTEM, which may be beneficial to the improvement in thermal stability of the alloy.

Ternary alloys based on the Mg-Zn-Ca system were produced by twin-roll rapid solidification. The alloys were characterized by OM, SEM, HRTEM, XRD, EDS and Micro-hardness. The results show that the rapidly solidified flakes are of fine dendritic cell structures with the cell size ranging from 1 to 5 μm. The Mg-6Zn-5Ca alloy in RS and annealing (200 ℃ for 1 h) states are mainly composed of α-Mg, Mg2Ca, Ca2Mg6Zn3 and a small quantity of Mg51Zn20, MgZn2 and Mg2Zn3. Micro-hardness increases with the increment of Ca content and age hardening occurs after aging at 200 ℃ in the flakes probably due to the precipitation strengthening of the fine precipitates Mg2Ca and Ca2Mg6Zn3. Some phases at the grain boundary in Mg-6Zn-5Ca alloy are identified by means of HRTEM, which may be beneficial to the improvement in thermal stability of the alloy.