AZ31镁合金塑性变形机制及再结晶行为的研究

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英文题名：Research on Plastic Deformation Mechanism and Recrystallization of AZ31Magnesium Alloy 作者：黄洪涛 论文级别：博士 学科专业名称：材料科学与工程 中文关键词：AZ31镁合金 ; 孪生 ; 滑移 ; 再结晶 ; 显微硬度点法 英文关键词：AZ31Mg alloy ; twinning ; slip ; static recrystallization ; micro-grid method 学位年度：2013 导师：Godfrey Andrew 学科代码：0805 学位授予单位：清华大学 论文提交日期：2013-04-01

摘要

镁合金具有比强度高、导热性能好及容易回收等优点具有广泛的应用前景。但是室温下由于其独立的滑移系少，塑性成型能力差，极大地阻碍了镁合金的普遍应用，因此需要深入研究低温下镁合金的孪生变形行为，探索激活非基面滑移的途径，这对工业生产中镁合金成型能力的提高具有重要的指导意义。
     本文利用电子背散射衍射技术(EBSD)结合显微硬度点法，深入研究了应变状态、应变路径对AZ31镁合金孪生行为的影响，探讨了AZ31镁合金变形机制由孪生向滑移转变的变形条件，并在此基础上深入研究了样品取向对AZ31镁合金静态再结晶行为的影响。
     取得的主要结论如下：(1)利用显微硬度点法测量了试样表面应变分布；深入研究了不均匀变形对孪生分数及显微组织的影响，结果表明，随着宏观应变增加不均匀变形对孪生分数的影响逐渐减小。统计分析了不同应变状态区域变体的斯密特因子(SF)及应变张量，发现{101|-2}拉伸孪生变体激活均满足斯密特定律，提出了考虑局部区域应变状态的SF计算模型。宏观上来说孪生变体与局部区域变形是相协调的。(2)设计了两种压缩路径，跟踪研究了两向压缩路径以及三向压缩路径下的变形行为，发现沿板材横向(TD)压缩中以{101|-2}拉伸孪生为主，中间道次压缩中以基面滑移为主，沿板材法向(ND)压缩中以退孪生为主，且完全退孪生所需应变比原始孪生所需应变要小，而滑移和孪晶的交互作用加速退孪生行为。(3)切取与板材法向成不同角度的样品进行压缩实验，结果表明，在150℃和200℃之间发生了拉伸孪生主导向柱面滑移主导变形转变行为，且80°样品是最敏感取向，文中利用迹线分析法和晶粒内取向差轴分析法确定滑移为柱面，也对此协调转变行为进行了模拟计算；基于应力应变曲线以及平均SF计算，计算得到多晶AZ31镁合金材料拉伸孪生和柱面滑移临界分切应力比值。(4)基于上述变形机制的探讨，采用沿板材ND(N样品)及TD(T样品)切取的两种试样进行形变退火试验。结果表明，由于微观变形机理的差异，与N样品相比，相同压缩应变下T样品形变储存能要小，因而相同退火参数下静态再结晶开始及结束都被推迟。
Magnesium alloys have a wide range of potential applications, on account of theirexcellent mechanical properties, good thermal performance and ease of recycling.However, the plastic formability of magnesium alloys is poor, due to the lack of asufficient number of independent slip systems at room temperature. Therefore, studiesof the twinning behavior of magnesium alloys during low temperature deformation, andof non-basal slip system activation, has an important guiding significance for industrialproduction.
     In this study the effect of strain state and strain path on twinning behavior of the Mgalloy AZ31has been investigated, making use of a micro-grid method to measure localstrains, combined with electron backscatter diffraction (EBSD) measurements tomeasure local changes in orientation. In addition, the effect of sample orientation anddeformation temperature on the transition in dominant deformation mechanism fromtwinning to slip was also investigated. Based on the the results, an attempt was made toinvestigate the effect of sample orientation on the static recrystallization (SRX)behavior of the AZ31magnesium alloy.
     The main conclusions obtained are as follows:(1)The strain distribution on thesample surface can be measured using a micro-grid method. The effect ofinhomogeneous deformation on twinning behavior was thereby investigated and theresults show that the effect on the twin area fraction decreases with increasingmacroscopic strain. A statistical analysis of twin variants at regions experiencingdifferent strain states was made using both a Schmidt factor (SF) and strain tensorapproach. The results indicate that in all regions more than80%of {101ˉ2} twin variantscan be accounted for by a Schmid factor criterion. A SF model based on a considerationof the local stress state has been proposed for explaining the twin variant selectionphenomena physically. An analysis of twin variant selection based on a strain tensorapproach shows that individual twin variants can exhibit a wide range of local straintensor values, though the average values over many grains match well locally measuredvalues.(2) The deformation behavior during two compression paths, designed toinvestigate the process of detwinning, was investigated. In one case compression wasapplied first along the transverse direction (TD) and then along normal direction (ND). In the second case an intermediate deformation along a direction midway between TDand ND was applied. It was found that {101|-2} twinning dominates the deformationprocess during initial deformation along TD, basal slip dominates the paldeformationprocess during the intermediate compression, and detwinning dominates thedeformation process during the final compression along ND. In both cases a smallerstrain is required for detwinning than for the initial twinning. Moreover the interactionbetween slip and twinning accelerates the detwinning behavior.(3) The transition from{101|-2} twinning to prismatic slip-dominated deformation has been investigatedthrough compression testing of samples cut from the sheet with the compression axisaligned at different angles to the sheet normal direction. The data show a clear transitionbetween twinning-and slip-dominated deformation in the range150℃to200℃forsamples cut with the compression axis at80°to ND. EBSD orientation maps of themicrostructure and slip trace observations have been used to verify the dominantdeformation mode for each sample and test temperature. Model calculations have alsobeen carried out to examine the sensitivity of this transition in deformation mode. Theresults have been used to estimate the critical resolved shear stress (CRSS) values forthe {101|-2} twin and prismatic slip of the AZ31magnesium alloy as a function oftemperature, based on the experimentally collected stress-strain curves and the averageSchmid factors for each sample.(4) A comparative study of the SRX behavior betweensamples compressed along ND and TD has been carried out on the basis of the aboveinvestigations of deformation mechanisms. The results indicate that due to the differentdeformation mechanisms, both the initiation and the completion of SRX aresignificantly retarded in samples compressed along TD compared to samplescompressed along ND.