Mg-Gd合金室温变形行为的原位观察及分析

发布时间：2018-03-14 08:21

本文选题：Mg-Gd合金　切入点：原位拉伸　出处：《上海交通大学》2015年硕士论文　论文类型：学位论文

【摘要】：镁合金晶粒内部的变形机制及晶粒间变形机制的交互作用对宏观的应力应变反应具有重要的影响,但是目前缺乏介观尺度针对镁合金晶粒度、晶粒间位向等因素对变形行为影响的讨论;开展镁合金介观尺度变形行为的研究,讨论晶粒间位向关系、晶界结构等对位错滑移、孪生、晶界滑动和转动等变形机制的影响具有重要的意义。本研究拟制备不同晶粒度、具有织构差异性的Mg-Gd合金,采用原位背散射电子(EBSD)及扫描电镜(SEM)的分析方法,跟踪室温变形过程中晶内变形机制(滑移、孪生)及晶间变形协调性(晶界滑动、微裂纹萌生和扩展),分析晶粒度、织构及晶界结构等对晶内及晶间变形机制的影响,探讨开发高成形性能镁稀土合金材料的理论依据及方法,取得了如下进展:对比分析了Gd含量对挤压态Mg-Gd合金棒材微观组织和力学性能的影响,随着Gd含量的升高,连续动态再结晶过程因溶质拖拽效应而受到抑制,继而引起Mg-Gd合金微观组织不均匀。Mg-0.5Gd挤压态合金的织构呈现典型的基面丝织构,而挤压态Mg-2Gd、Mg-5Gd合金织构发生明显改变,形成了2-1-11弱稀土织构。织构的差异直接导致了挤压态Mg-x Gd合金室温力学性能的不同,其中Mg-0.5Gd合金的室温屈服强度更高(达255MPa),而Mg-2Gd和Mg-5Gd合金的室温延伸率更好(35%)。研究了退火工艺对Mg-x Gd合金再结晶及晶粒长大的影响,结果表明:合金在退火过程发生了静态再结晶,晶粒随着退火时间的延长而长大,获得了平均晶粒尺寸(5-57μm)不同的退火态Mg-2Gd合金。退火过程中,由于静态再结晶过程的择优形核和晶粒的择优长大,10-10织构组分逐渐减弱,2-1-11和10-11织构组分逐渐增强。此外,退火处理能有效地改善合金的力学性能拉压不对称性,并且Mg-2Gd合金在5-57μm的晶粒尺度范围内依然满足经典的Hall-Petch关系。对Mg-2Gd挤压态(挤压温度300℃;13.6μm)和退火态(400℃/60min;38.5μm)合金进行室温原位拉伸试验,结合原位EBSD和原位SEM表征和分析了不同晶粒度Mg-2Gd合金的室温变形行为,结果表明:挤压态Mg-2Gd合金中,晶界的滑动和转动以及位错滑移成为室温拉伸变形的主要协调机制,而孪生机制对应变的贡献很小;在退火态Mg-2Gd合金中拉伸孪生和位错的滑移成为室温拉伸变形的主要协调机制,而晶界的转动和滑动对应变的贡献较小。研究发现在镁合金中,细晶组织能有效地抑制孪晶的形核。挤压态合金由于具有更为细小的晶粒(13.6μm),晶粒间的协调性更好,拉伸孪生过程受到抑制;退火态合金晶粒尺寸较大(38.5μm),孪晶容易开动,而晶粒的转动和滑动受到抑制。此外,变形过程中的孪生形核方式为滑移诱发孪晶形核(slip-induced twinning),孪生的大量开动能促进更多的滑移系开动,增强合金的变形协调性。引入了一个描述相邻晶粒基面滑移几何兼容性参数m,充分讨论晶粒度、晶粒施密特因子、晶粒间取向差等因素对微裂纹的萌生和扩展的影响,结果表明:在室温拉伸变形过程中,挤压态和退火态Mg-2Gd合金的微裂纹都在晶界处萌生,影响微裂纹萌生的主要因素有:晶界与拉伸加载方向的夹角、孪晶的形核和长大、相邻晶粒的基面滑移开动情况(m值的大小)。当晶界和拉伸加载方向的夹角越大,孪晶的形核受到的抑制越强,m值越小,越有利于微裂纹在晶界的萌生。然而微裂纹向相邻晶界的扩展行为并不依赖m值的大小,微裂纹往往更趋于扩展到与微裂纹萌生的晶界连接更平滑的晶界上。此外,孪晶的形核能有效释放局部应力集中,从而抑制微裂纹的扩展。
[Abstract]:The interaction of the deformation mechanism of magnesium alloy grains and grain between the internal deformation mechanism has an important effect on the macro stress and strain response, but the current lack of meso scale for magnesium alloy grain size, grain orientation and other factors influence on deformation behavior; to carry out the research on deformation behavior of magnesium alloy medium scale view, discuss grain orientation relationship between the structure of grain boundary, dislocation slip and twinning, has important implications for grain boundary sliding and rotation deformation mechanism. This study was to prepare Mg-Gd alloy with different grain size and texture difference, using in situ backscattered electron (EBSD) and scanning electron microscopy (SEM) analysis method, tracking room temperature deformation mechanism of the deformation process of MICROTEK (slip, twinning) and intergranular deformation (grain boundary sliding, micro crack initiation and propagation), analysis of grain size, texture and grain boundary structure of intragranular and intergranular deformation machine The influence of the theory and method of developing high performance magnesium rare earth alloy forming materials, has made progress as follows: comparative analysis of the effect of Gd content on the microstructure and mechanical properties of extruded Mg-Gd alloy bars, with the increase of Gd content, continuous dynamic recrystallization process due to the solute drag effect was inhibited, and then the microstructure of Mg-Gd alloy caused by uneven alloy.Mg-0.5Gd extrusion texture showed basal fiber texture typical, and extruded Mg-2Gd, Mg-5Gd alloy texture changed obviously, the formation of 2-1-11 rare earth weak texture. The texture difference led directly to the extrusion of room temperature mechanical properties of Gd alloy with different states of Mg-x, the yield strength of Mg-0.5Gd alloy at room temperature higher (up to 255MPa), while Mg-2Gd and Mg-5Gd alloy at room temperature elongation better (35%). The effects of annealing process on Mg-x Gd alloy recrystallization and grain growth,. The results show that the static recrystallization occurs in the alloy during annealing, the grain size grows up with the increase of annealing time, the average grain size (5-57 m) annealed Mg-2Gd alloy with different annealing process. The static recrystallization process, due to preferential nucleation and the preferential growth of grains, 10-10 texture components gradually weakened 2-1-11, and 10-11 components gradually increased. In addition, the annealing treatment can effectively improve the mechanical properties of the alloy tensile compressive asymmetry, and grain size of Mg-2Gd alloy in a range of 5-57 m still satisfy the classical Hall-Petch relation. The Mg-2Gd extruded (extrusion temperature of 300 DEG C; 13.6 m) and annealed (at 400 /60min; 38.5 m) alloy at room temperature in situ tensile test, combined with in situ EBSD and in situ SEM characterization and analysis of the different grain size of Mg-2Gd alloy at room temperature deformation behavior, the results show that the extruded Mg-2Gd alloy, grain boundary sliding Dynamic rotation and dislocation slip and become the main coordination mechanism of tensile deformation, and the twin mechanism of strain contribution is very small; in the annealed Mg-2Gd alloy tensile twinning and dislocation slip has become the main coordination mechanism of tensile deformation, while the smaller contribution of grain boundary rotation and sliding on the strain. The study found that in magnesium alloys the fine grain structure, can effectively inhibit the nucleation of twins. The extruded alloy has finer grains (13.6 m), better coordination between grains, tensile twinning process was inhibited; the annealed alloy grain size is large (38.5 m), twin easy to start, but the grain rotation and the sliding was inhibited. In addition, the twin nucleation modes during the deformation of slip induced twinning nucleation (slip-induced twinning), a large number of twin slip system start can promote more running, enhance the deformation coordination of the alloy. In a description of the adjacent grain basal slip geometry compatibility parameter m, fully discussed the grain size, grain Schmidt factor, grain orientation difference between the factors of micro crack initiation and propagation are investigated, the results showed that: in the tensile deformation process, the micro cracks of extruded and annealed Mg-2Gd alloys have sprouted in the grain boundaries. And the main factors affecting the micro crack initiation angle: grain boundary and tensile loading direction, the nucleation and growth of twin grains, adjacent basal slip condition (m value). When the angle of the grain boundary and the tensile loading direction is larger, the nucleation inhibition twinning is stronger, the smaller the value of m, more conducive to the micro crack initiation at grain boundaries. However, the micro crack propagation behavior to the adjacent grain boundaries does not depend on the magnitude of m the grain boundary cracks tend to be extended to the micro crack initiation and connection of the grain boundary more smooth. In addition, the twins The crystalline nuclear energy effectively releases the local stress concentration and thus inhibits the propagation of microcracks.

【学位授予单位】：上海交通大学
【学位级别】：硕士
【学位授予年份】：2015
【分类号】：TG146.22

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