| In order to meet the requirements of metal materials with light weight, highproperties, three series of (submicron+nano) bimodal size,(micron+nano) bimodalsize and (submicron+micron+nano) bimodal size SiCp/AZ31B composites weredesigned and fabricated, and then were subsequented to hot extrusion. Theinterparticle combination mode, content and spatial arrangement on microstructureand mechanical properties of composites before and after hot extrusion wereinvestigated, and then strengthening reason of different systematic SiCp/AZ31Bcomposites were investigated. Effect of particles parameter on the microstructure ofdifferent systematic SiCp/AZ31B composites was researched by OpticalMicroscopy, Scanning Electron Microscope and Transmission Electron Microscopeduring hot extrusion, and then mechanism of the different size SiCp on dynamicrecrystallization were obtained. Effect of different systematic SiCp on roomtemperature tensile properties were researched by the investigation ofmicrostructure of SiCp/AZ31B composites. The mechanism of room temperaturetensile and strengthening reason were researched.The optimization of process parameters for (submicron+nano) bimodal sizeSiCp/AZ31B composites was obtained while the stirring time, stirring temperature,stirring velocity and ultrasonic processing time were15min,625℃,1500rpm/minand20min, respectively. The bimodal size SiCp mainly distributed at the grainboundary, in the meantime nano SiCp distributed around the micron-SiCp, and thendistribution uniformity of nano SiCp increased with increasing the micron-SiCpcontent at cast condition. The result of investigation illustrated that the mechanicalproperties were improved by the application of hot extrusion due to grainrefinement and uniform particles distribution. During hot extrusion, on one hand,thermal misfit dislocation was formed around submicron SiCp, on the other hand,the dislocation piled up around submicron SiCp due to the hinder effect ofsubmicron SiCp on dislocation movement in (1μm or0.5μm+60nm) bimodal sizeSiCp/AZ31B composites. The these effects led to the increasing dislocation densityaround smaller sized micron SiCp, which was propitious to promote dynamic recrystallization nucleation. Meanwhile, the pinnin effect of nano SiCp(submicronSiCp) on grain boundary led to lower growth rate during hot extrusion. As a results,the addition of bimodal size SiCp was propitious to grain refinement of AZ31Bmatrix. When micron SiCp content was lower than3vol.%, distribution mode ofnano SiCp was uniformly distribution and banded distribution. However, whenmicron SiCp content was higher than3vol.%, distribution mode of nano SiCpmainly was uniformly distribution. The mechanical properties of bimodal sizecomposites were better than single size composite and AZ31B alloy, and the idealmechanical properties of composites cound be obtained at the volume ratio of1:4between nano and submicon SiCp. The addition of bimodal size SiCp led to adramatic increase in ultimate tensile strength and yield strength. The dislocationstrengthening mechanism and grain refinement effect played major role onimprovement of yield strength in (1μm or0.5μm+60nm) bimodal size SiCp/AZ31Bcomposites.Through optimizing the compositions, N-1+M10-14and N-1+M5-14composites with uniform particle distribution and high strength were designed andfabricated. The investigation of microstructure illustrated that the particledeformation zone(PDZ) with high density dislocation and high degree dislortionwas formed around micron SiCp in the (10μm or5μm+60nm) bimodal sizeSiCp/AZ31B composites, which promoted grain refinement. Meanwhile, the pinnineffect of nano SiCp on grain boundary led to lower growth rate during hot extrusion.Both of PDZ and nucleation ratio of DRX increased as increasing volume fractionof micron SiCp. Distribution uniformity of nano SiCp increased as increasingvolume fraction of micron SiCp. The bimodal size SiCp can play an effectivesynergistic strengthening role during room temperature tensile text. The density ofdislocation around micron SiCp increased with increasing the tensile strain, andmicrocracks initiated mainly at the ends of micron SiCp due to stress concentration.However, interface bonding between nano SiCp and matrix was better, and nomicrocracks could be found. Dispersed nano SiCp around micron SiCp could hindercrack propagation.The mechanical properties of (10μm+1μm+60nm) multi size SiCp/AZ31Bcomposites were better than bimodal size composite and single size composite, andthe ideal mechanical properties of multi size composites cound be obtained at the volume ratio of1:4:10between nano, submicon and micron SiCp. For(10μm+1μm+60nm) multi size composites, the multi size SiCp can give full play tothe synergy between each size SiCp. On one hand, the PDZ with high densitydislocation and high degree dislortion was formed around10μm-SiCp, meanwhile,hinder dislocation movement and deformation mismatch effect of1μm-SiCp led tothe increasing dislocation density around1μm-SiCp, which was propitious topromote DRX nucleation. On the other hand, the presence of nano SiCp can hindergrain boundary migration led to inhibit ot delar grain growth. These reasons led tothe grain refinement. The distribution of multi size SiCp were improved by theapplication of hot extrusion, and they form directional arrangement along theextrusion direction. The strengthening mechanism of multi size composites weredislocation strengthening mechanism, grain refinement, Orowan strengtheningmechanism and load transfer effect, however, dislocation strengthening mechanismand grain refinement played a major role among them. Compared with nano and1μm-SiCp, microcracks initiated predominately at the ends of micron SiCp due tostress concentration during tensile test. Dispersed nano and1μm-SiCp aroundmicron SiCp could hinder crack propagation. Microcracks propagated along theweak interfacial as increasing load. Once microcracks throughout the matrix, theywould link together quickly, leading to the fracture of SiCp/AZ31B composites. |
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