The Investigation On Microstructures And Mechanical Properties Of Multi-Scale SiC_p /AZ91 Composites

In this paper,AZ91 alloy was used as matrix,and the method of semi-solid stirring casting was applied to fabricate 1vol.%submicron Si C_p reinforced magnesium matrix composite?S-1?,10vol.%5?m SiC_p reinforced magnesium matrix composite?M-10?and bimodal sized SiC_p/AZ91 composite?M-9+S-1?.The effect of extrusion temperature and extrusion speed on microstructures and mechanical properties of M-9+S-1 composite has been investigated systematically.M-10,S-1 composites and AZ91 alloy have also been extruded at 250^oC with the extrusion speed of 0.1mm/s so as to study the effect of SiC_p scale on AZ91matrix's microstructures and mechanical properties.The results indicated that the grains of as-cast AZ91 alloy are coarse,and there exist plenty of coarse and continuous Mg₁₇Al₁₂ phases along the grain boundaries.However,the incorporation of multi-scale SiC_p alters nucleation site of Mg₁₇Al₁₂ phases in AZ91 matrix.And the nucleation site of Mg₁₇Al₁₂ phases is in connection with SiC_p scale.When Si C_p is 5?m,Mg₁₇Al₁₂ phases adhere to SiC_p surface to nucleate in M-10 composite.When SiC_p scale decreases to 0.5?m,submicron SiC_p is covered by Mg₁₇Al₁₂ phases in S-1 composite.When submicron and micron SiC particles coexists AZ91 matrix,0.5?m SiC particles distribute around the 5?m SiC particles,and Mg₁₇Al₁₂2 phases mainly nucleate at5?m SiC_p.Furthermore,the addition of multi-scale SiC_p refines the grains of AZ91 marix.Particularlly in as-cast M-9+S-1 composite,synergistic effect of0.5?m and 5?m SiC_p generates obvious grain refinement.The grain refinement results in higher YS and UTS in as-cast M-9+S-1 composite.M-9+S-1 composite has been extruded at different temperature?250^oC,300^oC,350^oC?with different extrusion speeds?0.01mm/s,0.1mm/s,1mm/s?.It is found that a complete dynamic recrystallization occurred in M-9+S-1 composite.The grain size of extruded M-9+S-1 composite is much finer than that of the cast one.With the increase of extrusion temperature and extrusion speed,DRXed grain size gradually increases.During extrusion process,Mg₁₇Al₁₂ phases dynamically precipitate mainly along DRXed grain boundaries and around SiC_p.The dynamic precipitation behavior is affected by extrusion temperature and extrusion speed during slow-speed extrusion.The number and the volume fraction of Mg₁₇Al₁₂phases are decreased,however,the size of Mg₁₇Al₁₂ phases gradually increases with the enhancement of extrusion temperature.With the increase of extrusion speed,the amount and volume fraction of Mg₁₇Al₁₂ phases gradually decreases.Besides,the size of Mg₁₇Al₁₂ phases first decreases,then increases.The mechanical properties of as-extruded M-9+S-1 composite are improved obviously.With the improvement of extrusion temperature,YS and UTS of M-9+S-1composite gradually decrease.With the increase of extrusion speed,the YS and UTS don't present linear change.M-9+S-1 composite obtains high strength?YS³42.24MPa,UTS³79.64MPa?when extruded at 250^oC with 0.1mm/s.In order to investigate the effect of SiC_p scale on the microstructure and mechanical properties of AZ91 matrix.M-10,S-1 composites and AZ91 alloy were also extruded at 250^oC with 0.1mm/s.The results indicate SiC_p scale has important influence on Mg₁₇Al₁₂ precipitation.In extruded AZ91 alloy,the fine Mg₁₇Al₁₂ phases precipitate along DRXed grain boundary.In extruded M-10composite,Mg₁₇Al₁₂ phase adhere to the surface of 5?m Si C_p to precipitate.And there are more Mg₁₇Al₁₂ phases in particle dense zone than in particle sparse zone.In extruded S-1 composite,Mg₁₇Al₁₂ phase tend to cover the 0.5?m SiC_p,leading to a higher size and volume fraction of Mg₁₇Al₁₂ phase compared with AZ91 alloy,M-10 and M-9+S-1 composites.As for M-9+S-1 composite,the grain size of M-9+S-1 composite is much finer than M-10 and S-1 composites before extrusion.The grain refinement of M-9+S-1 composite before extrusion causes coordinate deformation during extrusion.And deformation resistance of M-9+S-1 composite is much smaller than that of M-10 and S-1 composites,leading to generating fewer dislocations.The fewer dislocations of M-9+S-1 composite are adverse to Mg₁₇Al₁₂ precipitation,which results in less volume fraction of Mg₁₇Al₁₂ phases.SiC_p scale does not only affect Mg₁₇Al₁₂ precipitation in AZ91 matrix,but also has important effect on mechanical properties.Based on dynamic precipitation of Mg₁₇Al₁₂ phase during extrusion,Multi-scale SiC_p/AZ91 composites?S-1,M-9+S-1,M-10?show an obvious strength advantage over other SiC_p reinforced magnesium matrix composites.The size of Mg₁₇Al₁₂ phase is smaller and the volume fraction of Mg₁₇Al₁₂ phase is higher in M-10 composite than in M-9+S-1composite,which leads to higher YS in M-10 composite.Thus it can be seen that the synergistic effect of micron SiC_p addition and Mg₁₇Al₁₂ precipitation is superior to the addition of bimodal sized SiC_p for material strengthening.