Research On Rapidly Solidified AZ91 Magnesium Alloy And AZ91 Composite Reinforced By Ceramic Paticles

Magnesium alloy matrix composites have great potential for applications in the fields of aerospace, automotive, electron and weapon equipment due to its low density, high specific strength and specific module, good dimension stability, etc. Rapid solidification/Powder metallurgy(RS/PM) is an effective method to prepare particle reinforced metal matrix composites (MMCs). In the present dissertation, a novel preparation method of rapidly solidified (RS) powders was developed, in which alloy melt is atmized into fine droplets and subsequent splat-quenched on the water-cooled copper twin-rollers, this process was termed as atomization-twin rolls quenching technology. RS AZ91 magnesium alloy powders were prepared safely and effectively, and mixed with SiC, Al₂O₃, ZrO₂ particles respective to develop particle reinforced AZ91 composites by hot extrusion and the evolution of microstructures and properties of the composites were investigated. Moreover, the hot compressive deformation behavior of the RS/PM AZ91 magnesium alloy and AZ91 alloy matrix composite were also investigated, the conclusions are drawn as follows:1. The processing parameters of the atomization-twin rolls quenching technology were optimized as follows: the diameter of the nozzle 1mm, the wheel velocity 25m/s, the pressure of atomization gas 0.3MPa and Ar gas for melt injection 0.1MPa. The RS AZ91 alloy flakes exhibited fine and uniform microstructures. When the flakes were extruded at 673K, extruded velocity of 0.1mm/min and extrusion ratio of 25:1,the magnesium alloy rods with clean surface, uniform dimension and excellent mechanical properties were obtained.2. The RS AZ91 magnesium alloy powders prepared by atomization-twin rolls quenched technology exhibited fine equiaxed grains with the grain size of 1-3μm, the phase constituent included supersaturate solid solution phaseα-Mg and miner fineβ-Mg₁₇Al₁₂ phase. The as-extruded materials also exhibited equiaxed grains with the size of 5-7μm and a large number of fineβ-Al₁₂Mg₁₇ and fewer AlMg₂Zn phases were detected in the alloy. The yield strength, tensile strength and elongation of the alloy bars at room temperature were 383 MPa, 275 MPa and 7.5%, respectively. The fracture characterization exhibited ductile rupture.3. The precipitation of phases and static recrystallization phenomenon occurred in the RS/PM AZ91 magnesium alloy during thermal exposure. When the thermal exposure temperature was in the range of 118-200℃,the grain size was not changed remarkably, however, a large number ofβ-Mg₁₇Al₁₂ precipitated both within the grains and on the grain boundaries. The recrystallization occurred at 200℃.As the thermal exposure temperature was blew 350℃,the grain size of the alloy still maintained invariant, but the precipitates began to remelting. When the heating temperature up to 400℃,manyβ-Mg₁₇Al₁₂ phases remelted, and the grains grew up abnormally in a short time. In addition, the aging temperature and extrusion ratio had an obvious influence on the hardness and microstructures of the RS/PM AZ91 magnesium alloy.4. When the strain rate and deformation temperature were in the range of 0.001-1s^-1 and 250-400℃respectively, the flow stress behavior of the RS/PM AZ91 alloy during hot compressive deformation was different from that of the cast magnesium alloys. At the stage of micro-strain, the stress raised rapidly, in which work hardening was dominant but the dynamic recovery or recrystallization phenomenon was not apparent, especially when the deformation temperature was relative lower, the hardening effect was visible. The flow stress behavior during hot compressive deformation for the RS/PM AZ91 magnesium alloy was intensively influenced by deformation temperature, and flow stress mainly exhibited power exponent relationship. In the present dissertation , the values of stress exponent n and the activation energy Q of the RS/PM AZ91 alloy during hot deformation were 8.7 and 132.6kJ/mol respectively.At relative low deformation temperature and high stress level, the fitting equation was:With relative high deformation temperature and low stress level, the fitting equation was:5. AZ91 magnesium alloy matrix composites reinforced by Si particles were prepared by RS/PM method. The Si particles were uniformly distributed in the composites, and during hot extrusion the reaction between silicon particles and alloy matrix occured and formed a reaction layer, in which high temperature phase Mg₂Si was present. During heat treatment process, the reaction layer between the reinforced particles and matrix was extended, and Mg₂Si obviously increased. The microhardness in the reaction zone was obviously higher than that in other zones. In addition, though the grains coarsed,the growing rate of grains in the matrix nearby reinforced particles was lower than that in other zones due to the effective suppression effect of reinforced particles on the grain growth in the matrix. For Si/AZ91 composites, there were some regularities between the properties and solution treatment time. When annealed at 450℃,the microhardness of the composite material was not visible changed with increasing the solution treatment time. Subsequently aging at 200℃for 60h, the microhardness of the material increased with aging time. As aging at 200℃for 120h with solution treatment time of 16h, the value of hardness was maximum, then decreased rapidly. Theδ,σ_b,σ_0.2 of AZ91 magnesium alloy matrix composites reinforced by Si were 3.50%, 322MPa, 241MPa respectively at room temperature, and the mechanical properties of composites obviously decreased with increasing the content of Si particles. But the composites exhibited good mechanical properties at elevated temperatures.6. 2% Si/AZ91 magnesium alloy flakes reinforced by in situ Mg₂Si were prepared by atomization- twin rolls quenched technology, and the flakes were hot extruded into sheet. Theσ_b,σ_0.2,δand G of the composite material at room temperature were 429 MPa, 322 MPa, 6.4% and 71 GPa respectively. The fracture characterization exhibited ductile rupture. The yield strength of the 2%Si/AZ91 magnesium alloy at 423K was above 240 MPa, and was equivalent with that of cast AZ91 magnesium alloy at 473K.7. The powders prepared by atomization-twin rolls quenching technology were mixed with the reinforced particles of SiC, Al₂O₃,ZrO₂ respectively to prepare composites by hot extrusion. The distribution of reinforced particles SiC, Al₂O₃ was relatively uniform. The yield strength, tensile strength and elongation of the SiC_p/AZ91 composites decreased with increasing the content of SiC. During hot extrusion, the interface reaction between Mg and the surface of SiC particles occurred and formed Mg₂Si. During the tensile failure, SiC particles cracked and sticky point phenomenon occurred between the SiC particles and the matrix. Al₂O_3p/AZ91 composites exhibited good mechanical properties at elevated temperatures. The mechanical properties of the ZrO_2p/AZ91 composites were influenced due to the non-uniform distribution of ZrO₂ particles in the matrix and the reaction with matrix.