The effect of aging on the microstructure and mechanical properties of AZ80 and ZK60 wrought magnesium alloys was studied with optical microscope and mechanical testers. The results demonstrate that both the tensile s...The effect of aging on the microstructure and mechanical properties of AZ80 and ZK60 wrought magnesium alloys was studied with optical microscope and mechanical testers. The results demonstrate that both the tensile strength and elongation of AZ80 alloy increase firstly and then decrease as the aging temperature rises, the peak values appear when the aging temperature is 170 ℃ The hardness of ZK60 alloy increases firstly and then decreases as the aging temperature rises, and the hardness reaches its peak value at 170 ℃. However, the toughness of the alloy is just the opposite. Moreover, ZK60 alloy has good performances in both impact toughness and other mechanical properties at the aging temperature from 140 ℃ to 200 ℃.展开更多
Flow behavior and microstructure of a homogenized ZK60 magnesium alloy were investigated during compression in the temperature range of 250-400 ℃ and the strain rate range of 0.1-50 s^-1. The results showed that dyna...Flow behavior and microstructure of a homogenized ZK60 magnesium alloy were investigated during compression in the temperature range of 250-400 ℃ and the strain rate range of 0.1-50 s^-1. The results showed that dynamic recrystallization (DRX) developed mainly at grain boundaries at lower strain rate (0.1-1 s^-1), while in the case of higher strain rate (10-50 s^-1), DRX occurred extensively both at twins and grain boundaries at all temperature range, especially at temperature lower than 350 ℃, which resulted in a more homogeneous microstructure than that under other deformation conditions. The DRX extent determines the hot workability of the workpiece, therefore, hot deformation at the strain rate of 10-50 s^-1 and in the temperature range of 250-350 ℃ was desirable for ZK60 alloy. Twin induced DRX during high strain rate compression included three steps. Firstly, twins with high dislocation subdivided the initial grain, then dislocation arrays subdivided the twins into subgrains, and after that DRX took place with a further increase of strain.展开更多
The effects of Cu addition on the microstructure and mechanical properties of the as-cast magnesium alloy ZK60 were investigated with optical microscope, SEM, TEM, XRD, EPMA and tensile tester. The mechanism by which ...The effects of Cu addition on the microstructure and mechanical properties of the as-cast magnesium alloy ZK60 were investigated with optical microscope, SEM, TEM, XRD, EPMA and tensile tester. The mechanism by which the mechanical properties are affected by Cu addition was discussed. The results show that Cu can effectively eliminate the intragranular solute segregations in the alloy, and the grain size of the alloy is decreased considerably with increasing the Cu amount. A ternary eutectic phase MgZnCu with a face-centered cubic structure is identified in the Cu-bearing alloys, which predominantly distributes at the grain boundary and acts as the nucleation sites of microcracks during the plastic deformation process. It is also found that the tensile properties of the alloy firstly increase by the trace addition of 0.5%-1%Cu and then decrease by a further addition up to 2.0%.展开更多
Fine-grained ZK60 magnesium alloy sheets of 2.0 mm in thickness were successfully joined by laser beam welding (LBW). The effects of welding parameters including laser power and welding speed on the microstructures ...Fine-grained ZK60 magnesium alloy sheets of 2.0 mm in thickness were successfully joined by laser beam welding (LBW). The effects of welding parameters including laser power and welding speed on the microstructures and mechanical properties of the joints were investigated. A sound bead, with the ultimate tensile strength (UTS) of 300 MPa and elongation of 12.0%, up to 92.5% and 65% of those of the base metal, respectively, is obtained with the optimized welding parameters. No liquation cracking is visible in the partially melted zone (PMZ) owing to the inhibitory action of the fine dispersed precipitates and the fine-grained microstructure in the as-rolled magnesium alloy sheets. The fusion zone (FZ) is featured with the equiaxed dendritic grains of the average grain size about 8 μm, which are similar to those in the heat affected zone (HAZ), and this contributes to the relatively high joint efficiency.展开更多
The oxidation behaviour of molten ZK60 and ME20 magnesium alloys in 1% 1,1,1,2-tetrafluoroethane/air atmospheres at 720 °C was compared with that of molten magnesium. The oxidation kinetics of these three melts w...The oxidation behaviour of molten ZK60 and ME20 magnesium alloys in 1% 1,1,1,2-tetrafluoroethane/air atmospheres at 720 °C was compared with that of molten magnesium. The oxidation kinetics of these three melts was determined by thermograyimetric measuring instrument, and the surface films of the oxidized samples were examined by scanning electron microscope (SEM), X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). The results show that the oxidation rate of molten ZK60 or ME20 alloys is much lower than that of molten magnesium in 1% 1,1,1,2-tetrafluoroethane/air atmospheres. The surface film formed on the molten magnesium is composed of MgF2, MgO and C, while the film formed on ZK60 melt mainly consists of MgF2, MgO, C and some ZrF4, and the film on ME20 mainly consists of MgF2, MgO, C and a small amount of CeF4. The good oxidation resistances of ZK60 and ME20 alloy melts may be caused by their major alloying elements Zr and Ce, respectively.展开更多
Microstructures and phase compositions of as-cast and extruded ZK60-xGd (x=0-4) alloys were investigated. Meanwhile, the tensile mechanical property was tested. With increasing the Gd content, as-cast microstructure...Microstructures and phase compositions of as-cast and extruded ZK60-xGd (x=0-4) alloys were investigated. Meanwhile, the tensile mechanical property was tested. With increasing the Gd content, as-cast microstructure is refined gradually. Mg-Zn-Gd new phase increases gradually, while MgZn2 phase decreases gradually to disappear. The second phase tends to distribute along grain boundary by continuous network. As-cast tensile mechanical property is reduced slightly at ambient temperature when the Gd content does not exceed 2.98%. After extrusion by extrusion ratio of 40 and extrusion temperature of 593 K, microstructure is refined further with decreasing the average grain size to 2 μm for ZK60-2.98Gd alloy. Broken second phase distributes along the extrusion direction by zonal shape. Extruded tensile mechanical property is enhanced significantly. Tensile strength values at 298 and 473 K increase gradually from 355 and 120 MPa for ZK60 alloy to 380 and 164 MPa for ZK60-2.98Gd alloy, respectively. Extruded tensile fractures exhibit a typical character of ductile fracture.展开更多
The microstructure evolution and mechanical properties of a ZK60 magnesium alloy produced by the semi-solid thermal transformation (SSTT) route and the recrystallization and partial melting (RAP) route were studie...The microstructure evolution and mechanical properties of a ZK60 magnesium alloy produced by the semi-solid thermal transformation (SSTT) route and the recrystallization and partial melting (RAP) route were studied, respectively. The microstructure evolution during partial remelting was studied at different temperatures for different time. The tensile mechanical properties of thixoformed components by the two routes at room temperature were examined. The results show that coalescence is dominant in the SSTT alloy and Ostwald ripening is dominant in the RAP alloy. Compared with the SSTT route, the RAP route can produce finer semi-solid microstructure under the similar isothermal holding condition. The microstructure of the RAP alloy is much more spheroidized compared with the SSTT alloy. Thixoforming for the ZK60 magnesium alloy produced by the SSTT and RAP route results in successful filling of the die, and the thixoforming process improves the mechanical properties of ZK60 magnesium alloy. The RAP alloy shows significantly advantageous mechanical properties over that of the SSTT alloy.展开更多
The hot deformation behavior and workability of pre-extruded ZK60A magnesium alloy were investigated by compression tests in the temperature range of 250-450 ℃and the strain rate range of 0.001-10 s 1. The constituti...The hot deformation behavior and workability of pre-extruded ZK60A magnesium alloy were investigated by compression tests in the temperature range of 250-450 ℃and the strain rate range of 0.001-10 s 1. The constitutive equation for the pre-extruded ZK60A alloy can be described by hyperbolic sine function. Processing maps were constructed from true strains of -0.2 to -0.8. The alloy experienced complete dynamic recrystallization (DRX) and showed good workability in the temperature range of 300-400 ℃ and the strain rate range of 0.01-0.001 s-Z, where hot working in pre-extruded ZK60A, such as forging, can be carried out. For large deformation to true strain of over -0.5, strain rates above 0.1 s-1 are not recommended at all temperatures, where flow instability such as local strain concentration, twinning deformation, abnormal grain growth, micro-cracks, and shear fracture were observed. Climb-controlled dislocation creep dominates both the plastic deformation and nucleation of DRX of the pre-extruded ZK60A magnesium alloy.展开更多
The ZK60 magnesium alloy plates were welded by laser beam welding (LBW) and the microstructures in the partially melted zone (PMZ) of welded joints were investigated. For the as-cast alloy, the eutectoid mixtures alon...The ZK60 magnesium alloy plates were welded by laser beam welding (LBW) and the microstructures in the partially melted zone (PMZ) of welded joints were investigated. For the as-cast alloy, the eutectoid mixtures along grain boundaries (GBs) in the PMZ are liquefied during welding, and their re-solidified materials present hypoeutectic characters, which lead to more severe segregation of the Zn element along GBs, and thus enhance the cracking tendency of the PMZ. The main reasons for liquation cracking of PMZ are described as that the absence of liquid at the terminal stage of solidification leads to the occurrence of shrinkage cavities in PMZ, from which liquation cracking initiates, and propagates along the weakened GBs under the tensile stress originating from solidification shrinkage and thermal contraction. Lower heat input can reduce the cracking tendency, and the plastic processing such as rolling also contributes to the mitigation of PMZ liquation cracking by reducing the size of eutectoid phases and changing their distribution in the base metal.展开更多
Micro-arc oxidation (MAO) process was cartied out in an optimized dual electrolyte system to fabricate a compact, smooth, and corrosion resistant coating on ZK60 Mg alloy. The microstructural characteristics of coat...Micro-arc oxidation (MAO) process was cartied out in an optimized dual electrolyte system to fabricate a compact, smooth, and corrosion resistant coating on ZK60 Mg alloy. The microstructural characteristics of coating were investigated by scanning electron microscopy (SEM) coupled with an energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). Test of mass loss was conducted at a 3.5 % NaCl solution to assess the resistance to corrosion. The bonding strength between the coating and ZK60 substrate was evaluated using scratch experiment. The results reveal that MgA1204 and MgO are the main phases of ceramic coating obtained in the dual electrolyte system. The corrosion rate of coating prepared in the optimized dual electrolyte is only 0.0061 g.m-2.h-1, which demonstrates excellent corrosion resistance. This is mainly due to the compact, uniform coating with high bonding strength.展开更多
The influence of impurity content on the microstructure and mechanical properties of ZK60 magnesium alloys was investigated by optical microscopy,scanning electron microscopy and tensile test.ZK60 alloys were prepared...The influence of impurity content on the microstructure and mechanical properties of ZK60 magnesium alloys was investigated by optical microscopy,scanning electron microscopy and tensile test.ZK60 alloys were prepared by changing holding time of alloy melt during semi-continuous casting in order to control the content of impurity elements.The alloy with lower purity content is found to have less second precipitates and larger grain size in the as-cast state.However,in the as-extruded state,reducing impurities brings about a decrease in grain size and an increase in yield strength from 244 MPa to 268 MPa,while the elongations in the as-extruded alloys with different contents of impurities are almost the same.After T5 treatment,impurity content is found to have more obvious effect on the yield strength of ZK60 alloy.The yield strength of ZK60-45 alloys with low impurity content is increased up to 295 MPa after T5 treatment.展开更多
Hot deformation of cast-homogenized and extruded(in both the extrusion and transverse directions)ZK60 magnesium alloy was conducted using the Gleeble®3500 thermal-mechanical simulation testing system.A new approa...Hot deformation of cast-homogenized and extruded(in both the extrusion and transverse directions)ZK60 magnesium alloy was conducted using the Gleeble®3500 thermal-mechanical simulation testing system.A new approach to model the high temperature constitutive behavior of the alloy was done using two well-known equations(i.e.hyperbolic sine and Ludwig equations).For this approach,the deformation conditions were divided into regimes of low and high temperature and strain rate(four regimes).Constitutive model development was conducted in each regime and the material parameters(P)were evaluated as strain,strain rate and temperature-dependent variables;P(ε,ε,T).Using this approach,the flow curves were predicted with high accuracy relative to the experimental measurements.Moreover,detailed information on the evolution of hot deformation activation energy was obtained using the modified hyperbolic sine model.Using the modified Ludwig equation,details of strain hardening and strain rate sensitivity of the ZK60 material during hot deformation were obtained.展开更多
The composition of magnesium alloys is greatly associated with initial deposition behavior of electroless Ni-P coatings.Thus,the initial deposition behavior of electroless Ni–P coatings on ZK60 and ME20 alloys was in...The composition of magnesium alloys is greatly associated with initial deposition behavior of electroless Ni-P coatings.Thus,the initial deposition behavior of electroless Ni–P coatings on ZK60 and ME20 alloys was investigated.The results indicated that differences in the alloy compositions significantly influenced the initial deposition process and the adhesive strength,corrosion resistance,and crystal structure.The initial deposition of coatings on ZK60 and ME20 alloys preferentially occurred on the precipitates.The precipitates in ZK60 alloy had higher chemical activity after HF activation and controlled the initial deposition rate of the coating.The initial deposition rate of the coating on ME20 alloy mainly depended on the density of the Mg F2 film formed by HF activation rather than on the precipitates.Owing to differences in the initial deposition process,the coating on ZK60 alloy had higher adhesive strength and better corrosion resistance than that on ME20 alloy.The coatings on ZK60 and ME20 alloys mainly had crystalline structures,and the coating on ME20 alloy had also a slight microcrystalline structure.展开更多
The low frequency electromagnetic field was applied during direct chill(DC) semi-continuous casting of the ZK60 magnesium alloy billets. Effects of low frequency electromagnetic field on surface quality, microstructur...The low frequency electromagnetic field was applied during direct chill(DC) semi-continuous casting of the ZK60 magnesium alloy billets. Effects of low frequency electromagnetic field on surface quality, microstructure and hot-tearing tendency of Φ500 mm ZK60 magnesium alloy billets were investigated. The results showed that with the application of the low frequency electromagnetic field, the surface quality of the ZK60 magnesium alloy billets is markedly improved and the depth of cold fold is decreased. The microstructure of the billets is also significantly refined. Besides, the distribution of the grain size is relatively uniform from the billet surface towards its center, where the average grain size is 42 μm at surface and 50 μm at center. It also shows that the hot-tearing tendency of DC semi-continuous casting ZK60 magnesium alloy billets is significantly reduced under low frequency electromagnetic field.展开更多
The influence of impurities on damping capacities of ZK60 magnesium alloys in the as-cast,as-extruded and T4-treated states was investigated by dynamically mechanical analyzer at room temperature.Granato and Lucke dis...The influence of impurities on damping capacities of ZK60 magnesium alloys in the as-cast,as-extruded and T4-treated states was investigated by dynamically mechanical analyzer at room temperature.Granato and Lucke dislocation pinning model was employed to explain damping properties of the alloys.It is found that reducing impurity content can decrease the amount of second-phase particles,increase grain size and improve damping capacity of the as-cast alloy slightly.The as-extruded alloy with lower impurity content is found to possess obviously higher damping capacity in the relatively high strain region than that with higher impurity concentration,which appears to originate mainly from different dislocation characteristics.The variation tendency of damping property with change of impurity content after solution-treatment is also similar to that in the as-extruded and as-cast states. Meanwhile,the purification of the alloy results in an evident improvement in tensile yield strength in the as-extruded state.展开更多
The phase structure of ZK60-1Er magnesium alloy thermally compressed at the temperature of 450℃ and the strain rate of 1×10 -4 s -1 was determined by transmission electron microscopy(TEM)and high-resolution elec...The phase structure of ZK60-1Er magnesium alloy thermally compressed at the temperature of 450℃ and the strain rate of 1×10 -4 s -1 was determined by transmission electron microscopy(TEM)and high-resolution electron microscopy(HREM).The results show that this magnesium alloy contains many new W phases(Mg3Zn3Er2,FCC structure)in the matrix.Those new W phases have two morphologies,either irregularly rectangular or rod morphology·Lattice constants of the two new W phases are slightly higher than those of W Phase(Mg3Zn3Y2)containing rare earth element of yttrium.展开更多
Twin roll cast ZK60 alloy strip/sheet with final thickness of 0.5 mm was prepared, and effect of rolling temperature on microstructure and texture development was investigated using OM and XRD technique, microstructur...Twin roll cast ZK60 alloy strip/sheet with final thickness of 0.5 mm was prepared, and effect of rolling temperature on microstructure and texture development was investigated using OM and XRD technique, microstructure and texture were measured on specimens subjected to rolling experiment at different rolling temperature, and macrotexture was also evaluated by X-ray diffraction method. In addition, the (1010)and (0002) pole figures were measured, and the tensile test was performed to reveal the influence of rolling temperature on mechanical properties. The results show that the microstrucmre of ZK60 alloy sheet consisted of fibrous structure with elongated grains, and shear bands along the rolling direction after warm rolling. Dynamic recrystallization could be found during the warm rolling process at rolling temperature 350℃ and above. And many fine recrystallized grain could be observed in the shear bands area. It is a little difficult to see the recrystallized grain in the sheet warm rolled at 300℃ because of higher density of shear bands. The warm rolled ZK60 alloy sheet exhibited strong (0002) pole texture, the intensity of (0002) pole figure decreases with the increasing of rolling temperature and the basal pole tilted slightly to the transverse direction after warm rolling.展开更多
The influence of pre-deformation and heat treatment on mechanicalproperties of as-extruded ZK60 alloy was investigated.The experimentalresults indicated that the solid solution,pre-cold rolling and artificialaging tre...The influence of pre-deformation and heat treatment on mechanicalproperties of as-extruded ZK60 alloy was investigated.The experimentalresults indicated that the solid solution,pre-cold rolling and artificialaging treatments remarkably improved the mechanicalstrength of alloys compared with the asextruded condition.Especially,pre-cold rolling in 5% reduction combined with artificialaging at 150 ℃ for 20 h was determined as the optimum heat treatment condition,which resulted in a yield strength of 333 MPa with an increment of 87 MPa and ultimate tensile strength of 373 MPa.High density of nanoscale precipitates in α-Mg matrix observed in this sample was beneficialto enhancing the strength.The as-extruded sample showed a typicalbrittle fracture while the solution treated sample exhibited ductile-fragile failure characterized by cleavage fractures,river patterns,and tear ridges.And the sample after pre-cold rolling combined with aging presented more equiaxialdimples with a great amount of cracked particles in them.The above-mentioned observations were analyzed in terms of microstructure and possible strengthening mechanism in the extruded ZK60 alloy.展开更多
The hot deformation simulation of a ZK60 magnesiuln alloy at different temperatures from 373 to 673 K and different strain rates of 0.1, 0.01 and 0.002 s^-1 was studied by using the Gleebe-1500 simulator. The plastic ...The hot deformation simulation of a ZK60 magnesiuln alloy at different temperatures from 373 to 673 K and different strain rates of 0.1, 0.01 and 0.002 s^-1 was studied by using the Gleebe-1500 simulator. The plastic deformation behavior was measured and the deformation activation energy was calculated. The microstructures of ZK60 magnesium alloy with an addition of neodymium during the deformation process were observed by using Polyvar-MET optical microscope and Tecnai G^2 20 TEM. The results show that the working hardening, the dynamic recovery and the dynamic recrystallization occur during the plastic deformation process at different temperatures and strain rates. The dynamic recrystallization starts when the temperature is over 473 K and the DRX grain size after hot deformation is only 5-10 μm. So the refined grains improve both the tensile strength and the elongation of alloys at room temperature. Neodymium is added into the alloy and a precipitate phase Mg12Nd that impedes the movement of dislocations is formed, which benefits to increasing mechanical properties of ZK60 magnesium alloy.展开更多
Finite element method was used to study the strain distribution in ZK60 Mg alloy during multi-pass cyclic extrusion and compression (CEC). In order to optimize the CEC processing, the effects of friction condition a...Finite element method was used to study the strain distribution in ZK60 Mg alloy during multi-pass cyclic extrusion and compression (CEC). In order to optimize the CEC processing, the effects of friction condition and die geometry on the distribution of total equivalent plastic strain were investigated. The results show that the strain distributions in the workpieces are inhomogeneous after CEC deformation. The strains of the both ends of the workpieces are lower than that of the center region. The process parameters have significant effects on the strain distribution. The friction between die and workpiece is detrimental to strain homogeneity, thus the friction should be decreased. In order to improve the strain homogeneity, a large corner radius and a low extrusion angle should be used.展开更多
基金Projects(50735005,50605059)supported by the National Natural Foundation of ChinaProject(2007021026)supported by the Shanxi Provincial Science Foundation for Youths, ChinaProject(20081027)supported by the Development for Science and Technology in Higher Educational Institutes, China
文摘The effect of aging on the microstructure and mechanical properties of AZ80 and ZK60 wrought magnesium alloys was studied with optical microscope and mechanical testers. The results demonstrate that both the tensile strength and elongation of AZ80 alloy increase firstly and then decrease as the aging temperature rises, the peak values appear when the aging temperature is 170 ℃ The hardness of ZK60 alloy increases firstly and then decreases as the aging temperature rises, and the hardness reaches its peak value at 170 ℃. However, the toughness of the alloy is just the opposite. Moreover, ZK60 alloy has good performances in both impact toughness and other mechanical properties at the aging temperature from 140 ℃ to 200 ℃.
基金Project (14JJ6047) supported by the Natural Science Foundation of Hunan Province,ChinaProject (51274092) supported by the National Natural Science Foundation of ChinaProject (20120161110040) supported by the Doctoral Program of Higher Education ofChina
文摘Flow behavior and microstructure of a homogenized ZK60 magnesium alloy were investigated during compression in the temperature range of 250-400 ℃ and the strain rate range of 0.1-50 s^-1. The results showed that dynamic recrystallization (DRX) developed mainly at grain boundaries at lower strain rate (0.1-1 s^-1), while in the case of higher strain rate (10-50 s^-1), DRX occurred extensively both at twins and grain boundaries at all temperature range, especially at temperature lower than 350 ℃, which resulted in a more homogeneous microstructure than that under other deformation conditions. The DRX extent determines the hot workability of the workpiece, therefore, hot deformation at the strain rate of 10-50 s^-1 and in the temperature range of 250-350 ℃ was desirable for ZK60 alloy. Twin induced DRX during high strain rate compression included three steps. Firstly, twins with high dislocation subdivided the initial grain, then dislocation arrays subdivided the twins into subgrains, and after that DRX took place with a further increase of strain.
基金Project(51201088)supported by the National Natural Science Foundation of ChinaProject(12C0324)supported by the Research Foundation of Education Bureau of Hunan Province,China+1 种基金Project(2011XQD26)supported by Doctoral Scientific Research Foundation of University of South,ChinaProject([2011]76)supported by the Construct Program of the Key Discipline in Hunan Province,China
文摘The effects of Cu addition on the microstructure and mechanical properties of the as-cast magnesium alloy ZK60 were investigated with optical microscope, SEM, TEM, XRD, EPMA and tensile tester. The mechanism by which the mechanical properties are affected by Cu addition was discussed. The results show that Cu can effectively eliminate the intragranular solute segregations in the alloy, and the grain size of the alloy is decreased considerably with increasing the Cu amount. A ternary eutectic phase MgZnCu with a face-centered cubic structure is identified in the Cu-bearing alloys, which predominantly distributes at the grain boundary and acts as the nucleation sites of microcracks during the plastic deformation process. It is also found that the tensile properties of the alloy firstly increase by the trace addition of 0.5%-1%Cu and then decrease by a further addition up to 2.0%.
基金Project(51274092)supported by the National Natural Science Foundation of ChinaProject(20120161110040)supported by the Doctoral Program of Higher Education of China
文摘Fine-grained ZK60 magnesium alloy sheets of 2.0 mm in thickness were successfully joined by laser beam welding (LBW). The effects of welding parameters including laser power and welding speed on the microstructures and mechanical properties of the joints were investigated. A sound bead, with the ultimate tensile strength (UTS) of 300 MPa and elongation of 12.0%, up to 92.5% and 65% of those of the base metal, respectively, is obtained with the optimized welding parameters. No liquation cracking is visible in the partially melted zone (PMZ) owing to the inhibitory action of the fine dispersed precipitates and the fine-grained microstructure in the as-rolled magnesium alloy sheets. The fusion zone (FZ) is featured with the equiaxed dendritic grains of the average grain size about 8 μm, which are similar to those in the heat affected zone (HAZ), and this contributes to the relatively high joint efficiency.
基金Project (SJ08-ZT05) supported by the Natural Science Basic Research Plan in Shaanxi Province of ChinaProject (ZK1050) supported by the Key Scientific Research Plan of Baoji University of Arts and Science, China
文摘The oxidation behaviour of molten ZK60 and ME20 magnesium alloys in 1% 1,1,1,2-tetrafluoroethane/air atmospheres at 720 °C was compared with that of molten magnesium. The oxidation kinetics of these three melts was determined by thermograyimetric measuring instrument, and the surface films of the oxidized samples were examined by scanning electron microscope (SEM), X-ray diffractometry (XRD) and X-ray photoelectron spectroscopy (XPS). The results show that the oxidation rate of molten ZK60 or ME20 alloys is much lower than that of molten magnesium in 1% 1,1,1,2-tetrafluoroethane/air atmospheres. The surface film formed on the molten magnesium is composed of MgF2, MgO and C, while the film formed on ZK60 melt mainly consists of MgF2, MgO, C and some ZrF4, and the film on ME20 mainly consists of MgF2, MgO, C and a small amount of CeF4. The good oxidation resistances of ZK60 and ME20 alloy melts may be caused by their major alloying elements Zr and Ce, respectively.
基金Projects(2010A090200078,2011A080403008)supported by the Major Science and Technology Project of Guangdong Province,China
文摘Microstructures and phase compositions of as-cast and extruded ZK60-xGd (x=0-4) alloys were investigated. Meanwhile, the tensile mechanical property was tested. With increasing the Gd content, as-cast microstructure is refined gradually. Mg-Zn-Gd new phase increases gradually, while MgZn2 phase decreases gradually to disappear. The second phase tends to distribute along grain boundary by continuous network. As-cast tensile mechanical property is reduced slightly at ambient temperature when the Gd content does not exceed 2.98%. After extrusion by extrusion ratio of 40 and extrusion temperature of 593 K, microstructure is refined further with decreasing the average grain size to 2 μm for ZK60-2.98Gd alloy. Broken second phase distributes along the extrusion direction by zonal shape. Extruded tensile mechanical property is enhanced significantly. Tensile strength values at 298 and 473 K increase gradually from 355 and 120 MPa for ZK60 alloy to 380 and 164 MPa for ZK60-2.98Gd alloy, respectively. Extruded tensile fractures exhibit a typical character of ductile fracture.
文摘The microstructure evolution and mechanical properties of a ZK60 magnesium alloy produced by the semi-solid thermal transformation (SSTT) route and the recrystallization and partial melting (RAP) route were studied, respectively. The microstructure evolution during partial remelting was studied at different temperatures for different time. The tensile mechanical properties of thixoformed components by the two routes at room temperature were examined. The results show that coalescence is dominant in the SSTT alloy and Ostwald ripening is dominant in the RAP alloy. Compared with the SSTT route, the RAP route can produce finer semi-solid microstructure under the similar isothermal holding condition. The microstructure of the RAP alloy is much more spheroidized compared with the SSTT alloy. Thixoforming for the ZK60 magnesium alloy produced by the SSTT and RAP route results in successful filling of the die, and the thixoforming process improves the mechanical properties of ZK60 magnesium alloy. The RAP alloy shows significantly advantageous mechanical properties over that of the SSTT alloy.
基金Projects(51171113,51301107) supported by the National Natural Science Foundation of China
文摘The hot deformation behavior and workability of pre-extruded ZK60A magnesium alloy were investigated by compression tests in the temperature range of 250-450 ℃and the strain rate range of 0.001-10 s 1. The constitutive equation for the pre-extruded ZK60A alloy can be described by hyperbolic sine function. Processing maps were constructed from true strains of -0.2 to -0.8. The alloy experienced complete dynamic recrystallization (DRX) and showed good workability in the temperature range of 300-400 ℃ and the strain rate range of 0.01-0.001 s-Z, where hot working in pre-extruded ZK60A, such as forging, can be carried out. For large deformation to true strain of over -0.5, strain rates above 0.1 s-1 are not recommended at all temperatures, where flow instability such as local strain concentration, twinning deformation, abnormal grain growth, micro-cracks, and shear fracture were observed. Climb-controlled dislocation creep dominates both the plastic deformation and nucleation of DRX of the pre-extruded ZK60A magnesium alloy.
基金Project (2011ZX06001-003) supported by the National Science and Technology Major Project, ChinaProject (51274092) supported by the National Natural Science Foundation of China
文摘The ZK60 magnesium alloy plates were welded by laser beam welding (LBW) and the microstructures in the partially melted zone (PMZ) of welded joints were investigated. For the as-cast alloy, the eutectoid mixtures along grain boundaries (GBs) in the PMZ are liquefied during welding, and their re-solidified materials present hypoeutectic characters, which lead to more severe segregation of the Zn element along GBs, and thus enhance the cracking tendency of the PMZ. The main reasons for liquation cracking of PMZ are described as that the absence of liquid at the terminal stage of solidification leads to the occurrence of shrinkage cavities in PMZ, from which liquation cracking initiates, and propagates along the weakened GBs under the tensile stress originating from solidification shrinkage and thermal contraction. Lower heat input can reduce the cracking tendency, and the plastic processing such as rolling also contributes to the mitigation of PMZ liquation cracking by reducing the size of eutectoid phases and changing their distribution in the base metal.
基金supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions and the Key Laboratory of Advanced Welding Technology of Jiangsu Province, China (No. JSAWT-11)
文摘Micro-arc oxidation (MAO) process was cartied out in an optimized dual electrolyte system to fabricate a compact, smooth, and corrosion resistant coating on ZK60 Mg alloy. The microstructural characteristics of coating were investigated by scanning electron microscopy (SEM) coupled with an energy dispersive spectrometer (EDS) and X-ray diffraction (XRD). Test of mass loss was conducted at a 3.5 % NaCl solution to assess the resistance to corrosion. The bonding strength between the coating and ZK60 substrate was evaluated using scratch experiment. The results reveal that MgA1204 and MgO are the main phases of ceramic coating obtained in the dual electrolyte system. The corrosion rate of coating prepared in the optimized dual electrolyte is only 0.0061 g.m-2.h-1, which demonstrates excellent corrosion resistance. This is mainly due to the compact, uniform coating with high bonding strength.
基金Project(50725413)supported by the National Natural Science Foundation of ChinaProject(2007CB613704)supported by the National Basic Research Program of ChinaProject(20090191120013)supported by the PhD Program Foundation of Ministry of Education of China
文摘The influence of impurity content on the microstructure and mechanical properties of ZK60 magnesium alloys was investigated by optical microscopy,scanning electron microscopy and tensile test.ZK60 alloys were prepared by changing holding time of alloy melt during semi-continuous casting in order to control the content of impurity elements.The alloy with lower purity content is found to have less second precipitates and larger grain size in the as-cast state.However,in the as-extruded state,reducing impurities brings about a decrease in grain size and an increase in yield strength from 244 MPa to 268 MPa,while the elongations in the as-extruded alloys with different contents of impurities are almost the same.After T5 treatment,impurity content is found to have more obvious effect on the yield strength of ZK60 alloy.The yield strength of ZK60-45 alloys with low impurity content is increased up to 295 MPa after T5 treatment.
基金support of the Natural Sciences and Engineering Research Council of Canada(NSERC),Automotive Partnership Canada(APC)program under APCPJ 459269-13 grant with contributions from CanmetMATERIALS,Multimatic Technical Centre,Ford Motor Company,and Centerline Windsor.
文摘Hot deformation of cast-homogenized and extruded(in both the extrusion and transverse directions)ZK60 magnesium alloy was conducted using the Gleeble®3500 thermal-mechanical simulation testing system.A new approach to model the high temperature constitutive behavior of the alloy was done using two well-known equations(i.e.hyperbolic sine and Ludwig equations).For this approach,the deformation conditions were divided into regimes of low and high temperature and strain rate(four regimes).Constitutive model development was conducted in each regime and the material parameters(P)were evaluated as strain,strain rate and temperature-dependent variables;P(ε,ε,T).Using this approach,the flow curves were predicted with high accuracy relative to the experimental measurements.Moreover,detailed information on the evolution of hot deformation activation energy was obtained using the modified hyperbolic sine model.Using the modified Ludwig equation,details of strain hardening and strain rate sensitivity of the ZK60 material during hot deformation were obtained.
文摘The composition of magnesium alloys is greatly associated with initial deposition behavior of electroless Ni-P coatings.Thus,the initial deposition behavior of electroless Ni–P coatings on ZK60 and ME20 alloys was investigated.The results indicated that differences in the alloy compositions significantly influenced the initial deposition process and the adhesive strength,corrosion resistance,and crystal structure.The initial deposition of coatings on ZK60 and ME20 alloys preferentially occurred on the precipitates.The precipitates in ZK60 alloy had higher chemical activity after HF activation and controlled the initial deposition rate of the coating.The initial deposition rate of the coating on ME20 alloy mainly depended on the density of the Mg F2 film formed by HF activation rather than on the precipitates.Owing to differences in the initial deposition process,the coating on ZK60 alloy had higher adhesive strength and better corrosion resistance than that on ME20 alloy.The coatings on ZK60 and ME20 alloys mainly had crystalline structures,and the coating on ME20 alloy had also a slight microcrystalline structure.
基金financially supported by the Major State Basic Research Development Program of China(Grant No.2013CB632203)the Liaoning Provincial Natural Science Foundation of China(Grant No.201202072)+1 种基金the Program for Liaoning Excellent Talents in University(Grant No.LJQ2012023)the Fundamental Research Foundation of Central Universities(Grant Nos.N120509002 and N120309003)
文摘The low frequency electromagnetic field was applied during direct chill(DC) semi-continuous casting of the ZK60 magnesium alloy billets. Effects of low frequency electromagnetic field on surface quality, microstructure and hot-tearing tendency of Φ500 mm ZK60 magnesium alloy billets were investigated. The results showed that with the application of the low frequency electromagnetic field, the surface quality of the ZK60 magnesium alloy billets is markedly improved and the depth of cold fold is decreased. The microstructure of the billets is also significantly refined. Besides, the distribution of the grain size is relatively uniform from the billet surface towards its center, where the average grain size is 42 μm at surface and 50 μm at center. It also shows that the hot-tearing tendency of DC semi-continuous casting ZK60 magnesium alloy billets is significantly reduced under low frequency electromagnetic field.
基金Project(50725413)supported by the National Natural Science Foundation of ChinaProject(2009BB4215)supported by the Natural Science Foundation of Chongqing Science and Technology Commission,ChinaProject(2008AB4114)supported by the Major Program of Chongqing Science and Technology Commission,China
文摘The influence of impurities on damping capacities of ZK60 magnesium alloys in the as-cast,as-extruded and T4-treated states was investigated by dynamically mechanical analyzer at room temperature.Granato and Lucke dislocation pinning model was employed to explain damping properties of the alloys.It is found that reducing impurity content can decrease the amount of second-phase particles,increase grain size and improve damping capacity of the as-cast alloy slightly.The as-extruded alloy with lower impurity content is found to possess obviously higher damping capacity in the relatively high strain region than that with higher impurity concentration,which appears to originate mainly from different dislocation characteristics.The variation tendency of damping property with change of impurity content after solution-treatment is also similar to that in the as-extruded and as-cast states. Meanwhile,the purification of the alloy results in an evident improvement in tensile yield strength in the as-extruded state.
基金Project(2008329)supported by Liaoning Provincial Education Ministry,ChinaProject supported by the Key Laboratory of Material Processing & Control of Liaoning Province,China
文摘The phase structure of ZK60-1Er magnesium alloy thermally compressed at the temperature of 450℃ and the strain rate of 1×10 -4 s -1 was determined by transmission electron microscopy(TEM)and high-resolution electron microscopy(HREM).The results show that this magnesium alloy contains many new W phases(Mg3Zn3Er2,FCC structure)in the matrix.Those new W phases have two morphologies,either irregularly rectangular or rod morphology·Lattice constants of the two new W phases are slightly higher than those of W Phase(Mg3Zn3Y2)containing rare earth element of yttrium.
基金Project(35060905) supported by the Doctoral Foundation of Jiangsu University of Science and Technology
文摘Twin roll cast ZK60 alloy strip/sheet with final thickness of 0.5 mm was prepared, and effect of rolling temperature on microstructure and texture development was investigated using OM and XRD technique, microstructure and texture were measured on specimens subjected to rolling experiment at different rolling temperature, and macrotexture was also evaluated by X-ray diffraction method. In addition, the (1010)and (0002) pole figures were measured, and the tensile test was performed to reveal the influence of rolling temperature on mechanical properties. The results show that the microstrucmre of ZK60 alloy sheet consisted of fibrous structure with elongated grains, and shear bands along the rolling direction after warm rolling. Dynamic recrystallization could be found during the warm rolling process at rolling temperature 350℃ and above. And many fine recrystallized grain could be observed in the shear bands area. It is a little difficult to see the recrystallized grain in the sheet warm rolled at 300℃ because of higher density of shear bands. The warm rolled ZK60 alloy sheet exhibited strong (0002) pole texture, the intensity of (0002) pole figure decreases with the increasing of rolling temperature and the basal pole tilted slightly to the transverse direction after warm rolling.
基金Funded by the National Natural Science Foundation of China(No.51571043)the International Science&Technology Cooperation Program of China(No.2014DFG52810)the Fundamental Research Funds for the Central Universities(No.106112015CDJZR135515)
文摘The influence of pre-deformation and heat treatment on mechanicalproperties of as-extruded ZK60 alloy was investigated.The experimentalresults indicated that the solid solution,pre-cold rolling and artificialaging treatments remarkably improved the mechanicalstrength of alloys compared with the asextruded condition.Especially,pre-cold rolling in 5% reduction combined with artificialaging at 150 ℃ for 20 h was determined as the optimum heat treatment condition,which resulted in a yield strength of 333 MPa with an increment of 87 MPa and ultimate tensile strength of 373 MPa.High density of nanoscale precipitates in α-Mg matrix observed in this sample was beneficialto enhancing the strength.The as-extruded sample showed a typicalbrittle fracture while the solution treated sample exhibited ductile-fragile failure characterized by cleavage fractures,river patterns,and tear ridges.And the sample after pre-cold rolling combined with aging presented more equiaxialdimples with a great amount of cracked particles in them.The above-mentioned observations were analyzed in terms of microstructure and possible strengthening mechanism in the extruded ZK60 alloy.
基金Project(2006BAE04B02-3)supported by the National Key Program of 11th Five-Year Plan of China
文摘The hot deformation simulation of a ZK60 magnesiuln alloy at different temperatures from 373 to 673 K and different strain rates of 0.1, 0.01 and 0.002 s^-1 was studied by using the Gleebe-1500 simulator. The plastic deformation behavior was measured and the deformation activation energy was calculated. The microstructures of ZK60 magnesium alloy with an addition of neodymium during the deformation process were observed by using Polyvar-MET optical microscope and Tecnai G^2 20 TEM. The results show that the working hardening, the dynamic recovery and the dynamic recrystallization occur during the plastic deformation process at different temperatures and strain rates. The dynamic recrystallization starts when the temperature is over 473 K and the DRX grain size after hot deformation is only 5-10 μm. So the refined grains improve both the tensile strength and the elongation of alloys at room temperature. Neodymium is added into the alloy and a precipitate phase Mg12Nd that impedes the movement of dislocations is formed, which benefits to increasing mechanical properties of ZK60 magnesium alloy.
基金Projects (51074106, 50674067) supported by the National Natural Science Foundation of ChinaProject (09JC1408200) supported by the Science and Technology Commission of Shanghai Municipality, China+1 种基金Project (2011-079) supported by the Shaanxi Scholarship Council,ChinaProject (20102015) supported by the Doctoral Startup Fund of TUST, China
文摘Finite element method was used to study the strain distribution in ZK60 Mg alloy during multi-pass cyclic extrusion and compression (CEC). In order to optimize the CEC processing, the effects of friction condition and die geometry on the distribution of total equivalent plastic strain were investigated. The results show that the strain distributions in the workpieces are inhomogeneous after CEC deformation. The strains of the both ends of the workpieces are lower than that of the center region. The process parameters have significant effects on the strain distribution. The friction between die and workpiece is detrimental to strain homogeneity, thus the friction should be decreased. In order to improve the strain homogeneity, a large corner radius and a low extrusion angle should be used.