| Mg-Gd serial alloys with different Gd mass fraction were fabricated bysqueeze casting method. The optimal composition of the alloy was obtainedaccording to the results of the microstructure observations and tensile tests. Andthen the alloy with the optimal component was subject to hot extrusion andageing treatment. This alloy was also used as the matrix alloy to fabricate SiCparticles reinforced Mg matrix composites. The average size of SiC particles isabout10μm were selected as the reinforcements, and the SiC volume fraction ofthe composites were10%and15%. The as-cast composites were extruded atdifferent temperatures. Optical Microscope (OM), Scanning ElectronMicroscope (SEM) and Transmission Electron Microscope (TEM) wereemployed to observe the microstructures of alloys and composites. Themechanical properties of the alloys and composites under room temperature andelevated temperatures were investigated. The microstructure-propertiesrelationship was analyzed.It was found that the grain size of the as-cast alloy was very large. Withthe increase of the Gd content, the grain sizes of the Mg-Gd as-cast alloydecreased, and the amount of the second phase increased. The second phase wasdefined as Mg5Gd through the selected area electron diffraction. The secondphase was mainly along the grain boundaries when the Gd content was relativelyhigh. With the increase of the Gd content, the yield strength (YS) and ultimatetensile strength (UTS) of the alloy was firstly increased and then decreased, thestrength reached the highest when the Gd content was10mass.%. After hotextrusion, the grain size of the Mg-Gd alloy was significantly refined because ofthe dynamic recrystallization(DRX). With the increase of the extrusiontemperature, the grain sizes of the extruded alloys increased. The tensileproperties of the alloy was improved by hot extrusion. The YS of the extrudedalloy firstly decreased and then increased with the increase of the extrusiontemperature. The alloy extruded at500℃exhibited the highest strength, becauseof the ellipsoidal precipitates formed in the extrusion process or the coolingprocess after extrusion. The yield strength hardly decreased as the tensiletemperatures were elevated to300℃, because of the formation of Mg5Gd whichwas stable at high temperature. When the alloy was subject to aging treatment at200℃, it took22h to achieve the maximum hardness. The peak-aged alloycontained a high-density ultra-fine ellipsoidal precipitates (β’) with a definite orientation relationship to the matrix. The strength of the alloy was significantlyimproved after peak aging.The addition of SiC particles significantly refined the grain size of thematrix alloy.With the increase of SiCp contents, the grain size decreased, andthe amount of the second phase also decreased. According to the SEMobservation, a quantity of secondary phase formed on the surface of SiCp, andeven some particles was wholly coated. TEM observation shew that theinterfacial reaction did not take place between the matrix and the particles.Compared with alloy, the YS and elastic modulus of the composites increased,but their elongation decreased. With the increase of the SiC volume fraction, theYS and elastic modulus increased.In the extruded composites, the distribution of SiCp and the interfacebetween the second phase and SiCp was well modified, and DRX took place.There also existed a quantity of ultra-fine ellipsoidal precipitates in the extrudedcomposites. In the extruded10vol.%composites, the grain sizes of the matrixincreased with the increase of the extrusion temperature, the distribution of theSiC particles was improved. In addition, the ellipsoidal precipitates were alsoformed in the extrusion process and the cooling process. All this had a beneficialeffect on the mechanical properties of the composites. However, themodification of the interface between the SiCp and the second phase reducedwhich has a bad influence on the mechanical properties of the composites. Giventhe above microstructure changes of the extruded composites,500℃was theoptimal extrusion temperature. In the extruded15vol.%SiCp/Mg-10wt.%Gdcomposites, the amount of second phase was fewer. Compared with the15vol.%composites extruded at500℃, the composite extruded at550℃had a moreuniform SiCp distribution and more amount of ellipsoidal precipitates. Therefore,the tensile properties of the composite extruded at550℃was superior to thatextruded at500℃. |
PDF Full Text Request