细晶有色金属低周疲劳性能研究

发布时间：2018-03-31 15:34

本文选题：变形镁合金　切入点：多面轧制　出处：《南京理工大学》2015年硕士论文

【摘要】：镁合金作为最轻的金属结构的材料,已大量运用于交通运输、航空航天领域。特别是变形镁合金,由于其极高的比强度和优良的韧性,必然成为下一代新型镁合金的发展重点。通过对变形镁合金的挤压、轧制、ECAP (Equal Channel Angular Pressing)、锻造等加工工艺,可以细化晶粒,明显提高材料的综合力学性能。但镁合金作为结构件,必然会受到循环应力/应变的作用,为了保证材料的安全,必须对镁合金的低周疲劳性能进行研究,从而揭示其疲劳损伤规律和失效机理。本文对常规挤压AZ31镁合金进行多面循环轧制、ECAP及深冷处理,对粗晶铜进行ECAP及深冷处理,通过静态力学实验、低周疲劳实验、断口形貌观察等手段,研究材料内部微观组织结构(晶粒度、织构、第二相等)、变形机制对镁合金低周疲劳性能的影响。并对AZ80镁合金合金进行固溶与时效等处理,比较两种材料在低周疲劳下不同的棘轮行为,得出以下结果：(1)AZ31挤压态镁合金经过多面循环轧制后,力学强度明显提高,并产生一定数量的孪晶,使基面织构弱化。疲劳过程中,相较挤压态,有更多滑移系开动,孪生-退孪生现象有所减弱,但仍表现出循环硬化行为,疲劳寿命明显高于初始挤压态AZ31镁合金。(2) ECAP与ECAP深冷处理的镁合金样品,相较初始挤压态,晶粒明显细化,深冷处理后晶粒更加细小,硬度值更高,材料均显现明显的循环硬化行为。在应力幅小于0.6%的低周疲劳实验中,深冷处理的疲劳寿命明显提高。在应力幅高于0.8%的低周疲劳实验中,深冷处理样品的拉伸平均应力明显提高,疲劳寿命降低。(3) ECAP与ECAP深冷处理的粗晶铜,晶粒细化显著,低周疲劳均呈现循环软化显现。深冷处理的疲劳断口的疲劳条带间距更小,展现更高的疲劳寿命。(4)AZ80时效后析出第二相能提高时效样品的力学强度,同时降低了材料的拉压不对称性。在棘轮实验中,固溶样品展现出循环硬化的棘轮行为,而时效样品展现出一定循环软化的棘轮行为,棘轮应力门槛值明显提高。
[Abstract]:Magnesium alloy, as the lightest metal structure material, has been widely used in the fields of transportation, aerospace, especially wrought magnesium alloy, because of its high specific strength and excellent toughness. By extruding wrought magnesium alloy, rolling equal Channel Angular pressing, forging and so on, the grain can be refined and the comprehensive mechanical properties of the material can be improved obviously. In order to ensure the safety of materials, the low cycle fatigue properties of magnesium alloys must be studied. In order to reveal the fatigue damage law and failure mechanism, the conventional extruded AZ31 magnesium alloy was subjected to multi-surface cyclic rolling and cryogenic treatment, ECAP and cryogenic treatment to coarse copper, and low cycle fatigue test through static mechanical experiment. The effect of microstructure (grain size, texture, second equality, deformation mechanism) on low cycle fatigue properties of AZ80 magnesium alloy was studied by means of fracture morphology observation. The different ratchet behavior of the two materials under low cycle fatigue is compared. The following results are obtained: the mechanical strength of the extruded magnesium alloy is obviously increased after multi-plane cyclic rolling, and a certain number of twins are produced, which weakens the basic surface texture. Compared with extruded state, there are more slip systems moving, twinning and twinning are weakened, but still show cyclic hardening behavior, and the fatigue life is obviously higher than that of the initial extruded AZ31 magnesium alloy. 2) ECAP and ECAP cryogenic treatment magnesium alloy samples, the fatigue life is obviously higher than that of the initial extruded AZ31 magnesium alloy. Compared with the initial extrusion state, the grain size was finer, the hardness value was higher, and the cyclic hardening behavior was obvious. In the low cycle fatigue test, the stress amplitude was less than 0.6%. The fatigue life of cryogenic treatment was obviously increased. In the low cycle fatigue experiment with stress amplitude higher than 0.8%, the average tensile stress of cryogenic samples was obviously increased, and the fatigue life was decreased. 3) the coarse grain size of copper treated by ECAP and ECAP cryogenic treatment was obviously refined. The fatigue band spacing of fatigue fracture treated by cryogenic treatment is smaller, showing that the second phase precipitated after aging can improve the mechanical strength of the aging sample, and the fatigue life is higher than that of AZ80, and the fatigue band spacing of the fatigue fracture surface after cryogenic treatment is smaller, showing that the second phase precipitated after aging can improve the mechanical strength of the aging sample. At the same time, the asymmetry of tension and compression was reduced. In ratcheting experiment, the ratchet behavior of the solution sample showed cyclic hardening, while the ratcheting behavior of the aging sample showed a certain cyclic softening behavior, and the ratchet stress threshold increased obviously.
【学位授予单位】：南京理工大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG146.22;TG146.11

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