【作者】 文丽华；

【导师】 吉泽升；

【作者基本信息】 哈尔滨理工大学 ， 材料学， 2009， 博士

【摘要】 镁合金是目前工程应用的最轻的金属结构材料,它具有密度低,比强度和比刚度高,且尺寸稳定,易于机械加工,阻尼性能好等优点。目前广泛应用于航空、航天、汽车、计算机、通信和家电等行业。但由于镁合金高温强度和高温抗蠕变性能差,限制了镁合金在高温条件下的应用。人们用加稀土等合金元素的方法来提高其使用温度,制备耐热镁合金。本文以固相再生的方法再生ZM6耐热镁合金料。利用透射电子显微镜、扫描电镜、金相显微镜、等离子耦合电感光谱仪、电子万能拉伸机和蠕变试验机等分析和测试手段,研究了合金成分、组织的演变规律、合金拉伸性能、蠕变性能和断裂行为。讨论了各种状态下合金的强化机制和断裂机制。研究了固相再生工艺参数对ZM6镁合金组织和性能的影响,分析了固相再生过程中ZM6镁合金屑变形的基本特征。ZM6镁合金屑在冷压成坯过程中,压力为350MPa,坯料密度为1.71g/cm³,形成少量新的结合面。ZM6镁合金随着挤压比和挤压温度的提高,晶粒细化,强度和延伸率同时提高。挤压温度为500℃和挤压比为25:1时,再生试样具有较好的综合力学性能,这时抗拉强度为280MPa,延伸率为29.7%,在250℃时,高温强度为171.9MPa。研究了固相再生ZM6镁合金固溶时效过程中组织演变、拉伸性能和断裂方式。时效初期,晶界产生不连续的沉淀相。随着时效时间的延长,沉淀相增多而不均匀。时效16h后,合金的强度达到峰值,主要的强化相为β′相和β相。欠时效时合金以穿晶剪切断裂为主;峰时效时合金以穿晶韧窝和沿晶断裂为主;过时效时粗大的第二相对断裂过程有着较大的影响。研究了ZM6耐热镁合金不同加工状态下的力学性能。铸态下,合金的抗拉强度和延伸率都很低,分别为141.3MPa和3.3%。经挤压后,合金的致密性增加,晶界组织状态被破坏,合金的强度和塑性大幅度提高。屑挤压合金的抗拉强度和铸锭挤压态合金的抗拉强度相当,塑性稍差一些。固相再生ZM6镁合金经T5处理后,抗拉强度略有增加,延伸率下降;经T4处理后,延伸率增加,抗拉强度明显下降。研究了Ce对固相再生ZM6镁合金组织和性能的影响。一次挤压后,Mg-Ce中间合金屑没有被打碎,合金的力学性能较差。经五次挤压后,Mg-Ce中间合金屑的均匀分布使合金的力学性能有较大的改善。随着挤压次数的增加,合金的抗拉强度和延伸率增大,增大的幅度随着挤压次数的增加而变小。五次挤压后,合金主要由α-Mg相、Mg₁₂Ce相、Mg₁₂Nd相和氧化相组成,这时合金的抗拉强度为300MPa,延伸率为14.8%。研究了固相再生ZM6耐热镁合金的抗蠕变性能。ZM6镁合金在200℃蠕变温度下,随着蠕变应力的增加,合金的抗蠕变性能下降,蠕变第一阶段的变形量增大;ZM6镁合金在110MPa下,随着蠕变温度的增加,合金的抗蠕变性能下降,蠕变第一阶段的时间缩短。ZM6镁合金的应变硬化指数为4.4,蠕变激活能为104kJ/mol。ZM6镁合金在蠕变过程中受位错攀移机制控制。更多还原

【Abstract】 At present, magnesium alloy is the lightest metal structural materials. It has many advantages including low density, good specific strength and stiffness, steady dimension, good machinability and excellent damping capacity. Magnesium alloy is being used in aviation, spaceflight, automotive, computer, communications and electronic industries. Because mechanical and creep properties at elevated temperature of magnesium alloy are inferior, its applications at elevated temperature are limited. Mechanical and creep properties of magnesium alloy at elevated temperature are improved by adding lanthanon and heat-resistant magnesium alloy is made.In this paper, ZM6 heat-resistant magnesium alloy chips and scraps were prepared by solid state recycling. Microanalysis and test of mechanical properties were carried out by transmission electron microscope, scanning eletron microscope, optical microscope, inductively coupled plasma-atomic emission Spectrometry, electron universal strength testing machine and creep testing machine. Composition of the alloy, microstructural evolution, tensile properties, creep properties and fracture behavior were investigated. Strengthening mechanism and fracture mechanism of the alloy at different conditions were discussed.Effect of technologic parameters on micrcostructures and mechanical properties of ZM6 magnesium alloy prepared by solid state recycling were studied and plastic characteristics in solid state recycling were analyzed. At first, ZM6 magnesium alloy chips were cold-pressed to form a compact billet with the pressure of 350MPa. The density of the billet was 1.71g/cm³ and the surface of the chips exhibited some breaking and bonding. With extrusion ratios and temperatures increasing, grains of ZM6 magnesium alloy became refiner and tensile strength and elongation of the alloy improved. Recycled specimen that was prepared with extrusion temperature of 500℃and extrusion ratio of 25:1 showed higher mechanical property. Its ultimate tensile strength and elongation to failure were 280MPa and 29.7%, respectively. Ultimate tensile strength of ZM6 magnesium alloy prepared by solid state recycling was 171.9MPa at 250℃.Microstructural evolution, tensile properties and fracture mode of ZM6 magnesium alloy prepared by solid state recycling during ageing treatment were investigated. At the beginning of ageing, The discontinuous precipitation occurred near the grain boundaries. Precipitates became more and uneven distribution with aging time increasing. When aging time reached 16 hour, tensile strength of the alloy was maximal and main strengthening phases of the alloy wereβ’ phase andβphase. Fracture mode of the alloy was mainly transgranular tearing fracture at the initial stage of ageing. Fracture mode of the alloy was a mix mechanism with transgranular dimple fracture and intergranular fracture in the peak-ageing. Bigger second phases had largely effected on fracture in the further-ageing.Mechanical properties of ZM6 heat-resistant magnesium alloy at different conditions were studied. Ultimate tensile strength and elongation of the cast alloy were 141.3MPa and 3.3%, respectively. Strength and ductility of the alloy were largely improved because microstructural state at the grain boundaries was destroyed through extrusion. Ultimate tensile strength of the alloy prepared by chip extrusion was equal to ultimate tensile strength of the alloy prepared by cast extrusion. Ductility of the alloy prepared by chip extrusion was lower than that of the alloy prepared by cast extrusion. Ultimate tensile strength of the alloy prepared by solid state recycling lightly increased and elongation to failure decreased after T5. Ultimate tensile strength of the alloy prepared by solid state recycling obviously decreased and elongation to failure increased after T4.Effect of Ce on microstructures and mechanical properties of ZM6 magnesium alloy prepared by solid state recycling was investigated. After the first extrusion, tensile strength and elongation to failure of the alloy were very low because Mg-Ce intermediate alloy chips have not been broken. Tensile strength and elongation to failure of the alloy were obviously improved because of well-proportioned distribution of Mg-Ce intermediate alloy chips after the fifth extrusion. Ultimate tensile strength and elongation to failure of the alloy increased with the extrusion times increasing. Increasing range decreased with the extrusion times increasing. The alloy was made ofα-Mg phase, Mg₁₂Ce phse, Mg₁₂Nd phase and MgO phase after the fifth extrusion. Ultimate tensile strength and elongation to failure of the alloy were 300MPa and 14.8%, respectively.Creep properties of ZM6 heat resistant magnesium alloy prepared by solid state recycling were studied. At 200℃temperature, creep properties of the alloy decreased and creep strain of Class I creep increased with the creep stress increasing. At 110MPa stress, creep properties of the alloy decreased and time of Class I creep reduced with the creep temperature increasing. The stress exponent and the activation energy of ZM6 magnesium alloy were 4.4 and 104kJ/mol. ZM6 magnesium alloy was controlled by the dislocations climbing mechanism.更多还原