【作者】 盛绍顶；

【导师】 陈振华；

【作者基本信息】 湖南大学 ， 材料加工工程， 2008， 博士

【摘要】 镁基复合材料具有低密度、高比强度、高比模量、良好的尺寸稳定性等优异的性能,在航空航天、交通、电子、武器装备等领域具有广阔的应用前景。快速凝固/粉末冶金为制备颗粒增强金属基复合材料的有效方法之一,本文介绍了一种自行发明的新型快速凝固粉末的制备方法,该方法将普通气体雾化液滴喷射到旋转的双辊辊缝中进一步模冷,形成了雾化-双辊急冷的快速凝固粉末制备工艺。利用该方法安全有效地制备了快速凝固AZ91镁合金粉末,并与SiCp、Al₂O₃、ZrO₂粉末混合,通过热挤压研制了颗粒增强AZ91镁基复合材料,研究了复合材料组织性能的演变规律。本文还研究了快速凝固/粉末冶金AZ91镁合金热压缩变形流变应力行为,快速凝固/粉末冶金法原位生成Mg₂Si增强AZ91镁基复合材料,经过系统的研究,获得如下结论:1、雾化-双辊急冷法在下列工艺参数组合下:双辊线速度25m/s;喷嘴直径1mm;熔体压射压力0.1MPa;雾化气体压力0.3MPa.,可获得宏观尺寸细小、微观组织均匀细小、综合性能优良的的快速凝固AZ91镁合金细碎箔带,快速凝固箔带在挤压温度为673K、挤压速度为0.1mm/min,挤压比为25:1时,可获得的外表光洁、尺寸均匀、组织性能优异的镁合金棒材。2、雾化-双辊急冷法制备的AZ91镁合金粉末态为细小等轴晶组织,晶粒尺寸1-3μm,组织为α-Mg过饱和固溶体和微量的细小β-Mg₁₇Al₁₂相组成;粉末挤压棒材为等轴晶组织,晶粒尺寸5-7μm,组织中含有大量细小的β-Al₁₂Mg₁₇以及AlMg₂Zn析出相,室温力学性能,抗拉强度383MPa,屈服强度275MPa,断后伸长率7.5%。室温下表现为韧性断裂特征。3、快速凝固/粉末冶金AZ91镁合金在加热过程中发生明显的沉淀相析出和静态再结晶现象。加热温度为118-200℃时,晶粒尺寸无明显变化,而在晶内和晶界处析出了大量的β-Mg₁₇Al₁₂相粒子。加热温度在200℃时发生再结晶,当加热温度在350℃以下时,合金的晶粒尺寸仍保持稳定,但第二相粒子开始重溶。当加热温度达到400℃时,β-Mg₁₇Al₁₂相粒子大量重溶,在短时间内发生了晶粒异常长大现象。退火温度以及挤压比对快速凝固/粉末冶金AZ91镁合金的硬度及组织有显著影响。4、在应变速率为0.001-1s^-1和变形温度为250-400℃条件下,快速凝固粉末冶金AZ91镁合金热压缩变形流变应力行为与普通铸造镁合金不同,在微应变阶段,应力上升很快,该阶段加工硬化占主导,动态回复或动态再结晶现象不明显,尤其在较低的温度变形时,硬化效果突出。材料的热压缩变形流变应力行为强烈地受到变形温度的影响,流变应力主要呈现幂指数关系。在本实验条件下,RS/PM AZ91镁合金热变形应力指数n为8.7,其热变形激活能Q为132.6kJ/mol。在热变形温度较低,材料处于较高应力水平时,其拟合方程为:在热变形温度较高,材料处于较低应力水平时,其拟合方程为:5、首次采用快速凝固/粉末冶金法制备了Si颗粒增强AZ91镁基复合材料,增强相分布均匀,在挤压过程中增强相与基体反应形成反应层,反应层中存在高温相Mg₂Si。在热处理过程中,随热暴露温度提高和时间的延长,增强相与基体的反应层拓展,Mg₂Si明显增多。反应区显微硬度明显高于其它区域,同时热处理过程中晶粒有所长大,增强相有明显抑制基体晶粒长大的作用,增强相附近基体晶粒长大速率小于其它区域。Si/AZ91复合材料在不同的时效制度下,材料的性能与固溶时间存在一定的规律性。450℃退火处理时,随固溶时间的延长,显微硬度没有明显变化。随后200℃/60h时效时,随退火时间延长,材料的显微硬度呈上升趋势;200℃/120h时效时,固溶时间为16h时,材料的显微硬度达到峰值,而后迅速降低。Si颗粒增强AZ91镁基复合材料的力学性能室温δ、σ_b、σ_0.2分别为3.50%、322MPa、241MPa。而且随Si含量的提高,材料的性能明显下降。同时,复合材料表现出较好的高温性能。6、采用雾化.双辊急冷法制备出了原位生成Mg₂Si快速凝固2%Si/AZ91镁合金碎箔带,并用热挤压方法制备出合金板材。板材室温σ_b、σ_0.2、δ_分别为429 MPa、322 MPa、6.4%,弹性模量71 GPa。断口表现为韧性断裂特征,有明显的增强相拔断现象。在423K时合金的屈服强度大于240MPa,在473K时合金的屈服强度和铸态AZ91镁合金相当。7、采用雾化-双辊急冷法制备的合金粉末与增强颗粒均匀混合后热挤压成形制备了SiC（Al₂O₃、ZrO₂）_p/AZ91复合材料。增强颗粒分布较均匀。SiCp/AZ91复合材料的屈服强度、抗拉强度、延伸率随着SiC颗粒含量的增加有所降低,在热挤压过程中,基体合金中的Mg与SiC颗粒表面的SiO₂发生了界面反应,生成了Mg₂Si相,在拉伸变形过程中存在SiC颗粒断裂、SiC颗粒与基体脱粘现象。Al₂O_3p/AZ91复合材料的高温力学性能优异。由于ZrO₂增强颗粒在基体中的分布不均匀,同时与基体发反应,影响了ZrO_2p/AZ91复合材料的力学性能。更多还原

【Abstract】 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.更多还原