挤压态镁合金高周疲劳与断裂行为研究

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	挤压态镁合金高周疲劳与断裂行为研究
	杨帆
学位类型	博士
导师	李守新 ; 张哲峰
	2011
学位授予单位	中国科学院金属研究所
学位授予地点	北京
学位专业	材料物理与化学
关键词	镁合金疲劳裂纹萌生环境效应腐蚀疲劳 Magnesium Alloys Fatigue Crack Initiation Environmental Effect Corrosion Fatigue
摘要	"（1）对传统挤压态AZ31 (Mg-3Al-1Zn, wt.%)镁合金进行高周疲劳实验。这类合金由于在变形过程中形成很强的织构，抑制了沿挤压方向加载时基面滑移的开动并使变形孪生仅在压缩受力情况下开动，因此这类合金往往具有明显的非对称拉伸－压缩屈服行为。这一特性导致在材料受到交变载荷的作用时，滑移承载塑性变形能力不足，而孪生－退孪生机制被迫开动来承担较大量的塑性变形。孪生变形本身的不均匀性使得在循环过程中，孪晶带附近应变局部化程度偏高，材料疲劳损伤程度加重，最终导致疲劳裂纹优先沿孪晶带萌生，削弱了材料的整体疲劳性能。（2）通过超声疲劳加载方法对挤压态AZ80 (Mg-8Al-0.5Zn, wt.%)镁合金进行高周疲劳实验，一方面肯定了由AZ31合金所做实验的可靠性，另一方面可以获得特征清晰的疲劳断口用以考察裂纹尖端应力强度因子大小和裂纹尖端循环塑性区尺寸的关系。在估算AZ80合金裂纹尖端循环塑性区尺寸时，把材料的拉伸－压缩非对称屈服行为引入对裂纹尖端塑性流变过程的分析，修正了传统的估算模型，并获得相对更加合理的结果。这一过程提示了在处理类似可能存在拉伸压缩非对称行为或具有其他一些特性的材料时，需要综合判断传统的分析结果是否合理，必要时要对传统分析模型进行修正。（3）挤压态GW123k (Mg-12Gd-3Y-0.5Zr, wt.%)镁合金由于存在大量弥散分布的析出相粒子和固溶原子，在经过热挤压变形时再结晶形核取向大大分散，从而获得比较随机的织构分布。这类合金基本消除了变形态镁合金中经常见的力学性能各向异性尤其是拉伸压缩屈服非对称性，并使得变形孪晶很难开动。在循环加载的过程中滑移成为材料变形的主要承载方式。材料的应变局部化现象减弱，滑移带、晶界和内部夹杂物都成为可能的裂纹萌生源。由于没有占主导地位的裂纹萌生机制，GW123k合金的疲劳裂纹萌生过程更加困难，抗疲劳破坏能力相应得到提升。这些结果提示我们可以通过促进材料的均匀化变形来获得更好的综合疲劳性能。（4）分别在低应变速率和高应变速率，真空环境和大气环境中进行挤压态GW123k合金与AZ31合金的拉伸实验。在真空或者高速加载条件下，GW123k合金的拉伸延伸率比其在空气中低速加载条件下有明显提升，同时拉伸强度也略有提升。挤压态AZ31合金在真空或者高速加载条件下则表现出与空气中准静态加载类似的拉伸性能。据此判断GW123k合金力学性能对空气环境的敏感性比AZ31合金要高，也说明如果不考虑环境因素的影响，GW123k合金的本征疲劳性能可能比AZ31合金的疲劳性能更为优越。"
其他摘要	"(1) High cycle fatigue test has been conducted on conventional extruded AZ31 (Mg-3Al-1Zn, wt.%) Mg alloy. This alloy possesses a strong basal texture through the extrusion process, which favors extensive twinning under compressive loading while suppresses both the basal slip and the twinning under tensile loading, resulting in a much higher tensile yield strength compared to the compressive yield strength. This character forces the operation of twinning-detwinning mechanism to undertake cyclic strain in order to compensate the lack of strain contribution from the slip mechanism. The heterogeneity nature of twinning deformation favors the strain localization along the twin bands, aggravates the material local damage level, and finally leads to the preferential crack initiation along the twin bands, resulting in a relatively moderate fatigue property. (2) Ultrasonic fatigue test has been conducted on extruded AZ80 (Mg-8Al-0.5Zn, wt.%) magnesium alloy and generated fatigue fractured samples. On typical fracture surfaces, the transition area of crack propagation from the near threshold regime to the Paris Law regime was identified, where the cyclic plastic zone size ahead of crack tip was estimated and compared to that of the material microstructure dimension (e.g., grain size). The unique feature of extruded magnesium alloy’s tension-compression yield asymmetry was accented and a comprehensive analysis of crack tip plastic flow behaviors was proposed. Based on the specialty of the current material, the previous models used for calculating the crack tip plastic zone size were updated and sounder results were obtained. (3) Through the addition of rare-earth elements Gd and Y, GW123k (Mg-12Gd-3Y-0.5Zr, wt.%) alloy was produced with the presence of large amount of precipitate particles and solute elements. Due to the particle stimulated nucleation (PSN) of recrystallization mechanism, this alloy possesses quite a random texture after hot extrusion, which eliminates the strong mechanical anisotropy especially the severe tension-compression yield asymmetry usually suffered by conventional wrought Mg alloys. The formation of deformation twins is thus suppressed and basal slip takes the major role of bearing material’s deformation. Therefore, under fatigue loading, the severe strain localization is alleviated. Slip bands, grain boundaries and inclusions all become possible fatigue crack initiation sites. The ruling out of a preferential crack initiation mechanism makes the alloy more difficult to crack, which suggests that promoting material’s deformation homogeneity is a crucial factor in achieving enhanced fatigue performance. (4) Tensile tests have been conducted on GW123k and AZ31 alloys under low and high strain rates and in air and vacuum environment. In vacuum or under high strain rate conditions, GW123k alloy have exhibited much longer elongation and slightly higher strength than that in vacuum or under low strain rate conditions. By contrast, AZ31 alloy exhibits similar properties in all testing conditions. It suggests that the GW123k alloy is more environmentally sensitive than AZ31 alloy. In another words, the superiority of intrinsic mechanical properties of GW123k alloy to that of AZ31 alloy may be even more significant. GW123k alloy’s environment sensitivity is arbitrarily ascribed to the presence of water vapor and its influence on hydrogen embrittlement."
文献类型	学位论文
条目标识符	http://ir.imr.ac.cn/handle/321006/64274
专题	中国科学院金属研究所
推荐引用方式 GB/T 7714	杨帆. 挤压态镁合金高周疲劳与断裂行为研究[D]. 北京. 中国科学院金属研究所,2011.