AZ31镁合金宽应变率下各向异性力学行为及形变机制研究

发布时间：2018-10-23 18:05

【摘要】：镁合金作为一种轻质结构材料,在汽车和航空等领域的应用前景广阔。然而,室温环境中较强的各向异性限制了镁合金的广泛应用。为克服这一缺点,许多学者开展了大量宏观力学行为和微观结构演化方面的研究,但微观结构演化如何影响镁合金的宏观力学行为仍不清楚,缺乏联系宏观与微观变形的细观尺度的测量。有关镁合金各向异性变形的原因众说纷纭,尚未达成共识。本文利用原位实时同步辐射X射线相衬成像和衍射多尺度测量方法研究不同应变率加载下镁合金的各向异性变形。实验设计两种加载方式,加载方向垂直或平行于变形镁合金的c轴,分别记为LA⊥c和LA‖c。在镁合金变形过程中,实现宏观应力—应变曲线、细观应变场和微观衍射图谱的同时采集。电子背散射衍射技术用于回收样品中的变形孪晶的表征。1、室温环境中准静态压缩加载,应变率5XI0-4 s-1,LA⊥c和LA‖c样品的应力—应变曲线、应变场和衍射图谱演化均呈现明显的差异。对于LA⊥c样品,{1012}拉伸孪生主导塑性变形,应力梯度通过孪生被迅速释放并使应变场变得均匀化,应变局域化程度的降低有效激发了应变硬化率的增加。然而LA‖c样品中的塑性变形主要依赖位错运动,位错在缺陷处形核并缠结引起应变集中,细观非均匀变形导致应变硬化率的降低。2、高温环境中准静态压缩加载,应变率10-3 s-1,镁合金的宏观力学性能、细观应变场和微观晶格变形呈现明显的各向异性。由于初始织构的差异,在室温和高温下,{1012}拉伸孪晶主导LA⊥c样品的塑性变形,而位错运动则在LA‖c样品中较为盛行。随着温度的升高,LA⊥c样品中激发的{1012}拉伸孪晶的数量逐渐减少,使得应变场的均匀化程度降低;高温下,LA‖c样品中{1122}c + a锥面滑移较易启动,使得LA‖c样品在高温下呈现更加均匀的变形。{1012}拉伸孪晶和锥面c + aa滑移均能通过协调垂直和平行于加载方向的变形使塑性变形变得均匀化。应变场不均匀度的差异导致LA⊥c和LA‖c样品的应变硬化率呈现明显的差异。3、分离式霍普金森压杆加载,应变率约为5.5×103 s-1,镁合金的动态响应同准静态加载类似呈现较强的各向异性。{1012}拉伸孪晶使得应变场的不均匀度减小,应变场局域化程度的减小有效激发了应变硬化率的增加;位错运动引起非均匀变形,应变场不均匀度的升高导致应变硬化率的减小。然而,在塑性变形的初期,动态加载过程中孪生主导塑性变形,而准静态加载下主导塑性变形的则是位错运动,表明加载应变率在一定程度上影响着镁合金的变形模式。4、平板撞击加载,应变率0.92～1.35×105s-1 LA⊥c和LA‖c样品的弹塑性转变呈现明显的各向异性,但二者的雨贡纽弹性极限基本相同,约为0.32 GPa。低速撞击下LA⊥c样品的层裂强度略高于LA‖c样品,随着撞击速度的增加,二者的层裂强度均随撞击速度的增加而增加,且差异减小;当撞击速度增加至400 m/s时,LA⊥c和LA‖c样品的层裂强度基本一致。LA⊥c样品中形成大量的{1012}拉伸孪晶,LA‖c样品中孪晶数量较少。
[Abstract]:As a lightweight structural material, magnesium alloy has a wide application prospect in automobile and aviation. However, the strong anisotropy in the room temperature environment limits the wide application of magnesium alloys. In order to overcome this shortcoming, many scholars have carried out a lot of research on macroscopic mechanical behavior and microstructure evolution, but how microstructure evolution affects the macroscopic mechanical behavior of magnesium alloy is still unclear, and lack of micro-scale measurement of macroscopic and microscopic deformation. There is no consensus on the reasons for the anisotropy deformation of magnesium alloys. In this paper, the anisotropic deformation of magnesium alloy under different strain rate is studied by using in-situ real-time synchronous radiation X-ray phase contrast imaging and diffraction multi-scale measurement method. In the process of deformation of magnesium alloy, the macroscopic stress relaxation strain curve, micro-strain field and micro-diffraction pattern are simultaneously acquired. The results show that the stress relaxation strain curve, strain field and diffraction pattern evolution of the specimen under quasi-static compression loading, strain rate 5X0-4s-1, LA-c and LA-c in room temperature environment show significant difference. For LA-c samples, {1012} tensile twinning dominant plastic deformation, the stress gradient is rapidly released by twinning and the strain field becomes uniformized, and the decrease in strain localization degree effectively stimulates the increase of strain hardening rate. However, the plastic deformation in LA-c samples depends mainly on dislocation movement, dislocation is at the defect and entanglement causes strain concentration, and meso-non-uniform deformation results in a decrease of strain hardening rate. The quasi-static compressive loading and strain rate of 10-3s-1 and the macroscopic mechanical properties of magnesium alloy in high-temperature environment are studied. The micro-strain field and the micro-lattice deformation show obvious anisotropy. Due to the difference in the initial texture, the {1012} tensile uniaxial crystals dominate the plastic deformation of the LA-c samples at room temperature and high temperature, while dislocation movements are more prevalent in the LA-c samples. As the temperature increases, the number of {1012} tensile microcrystals excited in the LA-c sample is gradually decreased so that the degree of homogenization of the strain field is reduced; at high temperatures, the {1122} c + a cone slip in the LA-c sample is easier to start, so that the LA-c samples exhibit more uniform deformation at high temperatures. The {1012} tensile stressor and the cone c + aa slip can uniformize the plastic deformation by coordinating the vertical and the deformation parallel to the loading direction. The difference of non-uniformity of strain field results in a significant difference in strain hardening rate of LA-c and LA-c samples. 3. Split Hopkinson pressure rod is loaded with a strain rate of about 5. 5-103 s-1. The dynamic response of magnesium alloy is similar to quasi-static loading. The increase of strain hardening rate is effectively excited by the decrease of the degree of localization of the strain field and the decrease of strain hardening rate due to the increase of the non-uniform deformation of the strain field and the increase of the non-uniformity of the strain field. However, during the initial stage of plastic deformation, the twin dominant plastic deformation during dynamic loading, while the dominant plastic deformation under quasi-static loading is dislocation movement, indicating that the loading strain rate affects the deformation mode of magnesium alloy to some extent. The elastic-plastic transition of the strain rate of 0. 92 ~ 1. 35-0105s-1LA-c and LA-c-c samples showed obvious anisotropy, but the elastic limit was about 0.32GPa. When the impact velocity increases, the crack intensity increases with the increase of the impact velocity, and the difference is reduced; when the impact velocity increases to 400 m/ s, The fracture strength of LA-c and LA-c samples was consistent. A large number of {1012} tensile microcrystals were formed in the LA-c samples, and the number of crystals in LA-c samples was small.
【学位授予单位】：中国科学技术大学
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TG146.22

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