【作者】 乐国敏；

【导师】 Godfrey A；

【作者基本信息】 清华大学 ， 材料科学与工程， 2013， 博士

【摘要】 细晶材料由于可达到高强度而受到广泛关注，但一般情况下能达到的塑性有限，为了得到良好的性能结合，十分有必要进行细晶材料的形变机理研究，此研究不仅可在工业领域为其应用提供基础理论指导，同时在科学领域可作为连接粗晶和纳米晶的桥梁，对目前颇具争议的纳米晶形变机理研究起指导作用。本文主要选用细晶铝来进行形变机理研究，关注形变微观组织演变，及组织与力学性能之间的关系。一个简单的形变前组织是研究的关键，本文利用放电等离子体烧结(SPS)技术制备了平均晶粒尺寸为5.2~0.8m的铝样品，这些样品具有等轴的完全再结晶晶粒和随机织构。在此基础上，利用ECC，EBSD和TEM等技术对压缩变形中的微观组织演变进行了详细的研究；利用弱束暗场技术对位错界面及位错柏氏矢量进行表征，结合Frank公式，Schmid因子和Bishop-Hill模型对开动的滑移系进行了深入的分析和讨论；最后，基于系统的微观组织研究结果和拉伸/压缩测试，对微观组织与力学性能的关系进行了定性与定量的分析。研究主要结果如下：(1)晶粒尺寸影响形变微观组织演变。随着晶粒尺寸的减小，晶粒内部形成的微观组织类型发生转变，位错界面的排列更加不规则。而当晶粒尺寸减小至1m以下后，晶粒内部形成的位错界面越来越少。(2)晶粒尺寸影响组织与取向的对应关系。随着晶粒尺寸的减小，III型组织对应的取向范围增加，从反极图三角形的[111]附近扩张到[100]-[110]线附近。(3)以上晶粒尺寸的影响是由于细晶中开动滑移系与粗晶不同，通过标定和分析细晶中位错界面上位错网络的柏氏矢量，发现细晶开动的滑移系比粗晶多，且Schmid因子最大的滑移系的主导地位减弱。对于细晶的变形，利用Bishop-Hill模型比Schmid因子能更好地预测滑移系的开动。(4)形变微观组织演变影响着力学性能，具体表现为平均晶粒尺寸大于1.3m的样品在形变中形成位错界面产生加工硬化，而平均晶粒尺寸降低到0.8m的样品在形变中不形成或很少形成位错界面而缺乏加工硬化。(5)对于产生了加工硬化的细晶铝样品，利用氧化物颗粒强化、林位错强化和晶界强化建立模型对流变应力进行预测，发现当利用2~3o区分林位错强化和晶界强化时，能得到预测值和实验值的良好匹配。更多还原

【Abstract】 Fine-grained metals have attracted much interest due to the possibility to obtainhigh mechanical strengths, although in general such metals only have a limited ductility.Studies of the deformation mechanisms in fine-grained metals are therefore important,and can also help fill a gap in knowledge between nano-grained metals andconventional coarse-grained metals, which is an area of both scientific and industrialinterest.In the present work, the deformation mechanism of fine-grained metals has beenstudied, primarily using Al as an example system. Efforts have been focused oninvestigations of deformation microstructure evolution, and on the relationship betweenmicrostructure and mechanical properties. For such an investigation it is very importantto use a starting material with a simple starting microstructure. For this purpose thespark plasma sintering (SPS) technique has been used. By careful selection of powdercharacteristics and processing conditions, samples of fully dense, fine-grained Al withaverage grain sizes ranging from5.2m to0.8m, in a fully recrystallized condition,with equiaxed grains and a random texture, have been produced. The microstructureevolution during compression of the fine-grained Al SPS-samples up to a strain of0.3has been systematically investigated using a range of techniques, including ECC, EBSDand TEM. Detailed investigations of the dislocation content of dislocation boundarieshave also been carried out using a weak-beam technique, and the observations havebeen interpreted via a slip system analysis including Schmid factor calculations, use ofthe Frank formula and the Bishop-Hill crystal plasticity model. Based on the abovestudies, relationships between the microstructure and mechanical properties have beenestablished.The main conclusions arising from these studies are as follows:(1) The formation ofdeformation microstructure depends on grain size. It is found that as the grain sizedecreases, the deformation microstructure changes from one containing well definedfeatures to one with less-well defined features. Additionally as the grain size decreasesbelow1m, only few dislocation boundaries are formed.(2) The relationship betweenmicrostructure and grain orientation changes with decreasing grain size, showing an expansion of grain orientations with type3microstructure from [111] corner toward the[100]-[110] line.(3) The above grain size effect is attributed to different slip systemactivity in fine-grained Al compared to coarse-grained Al. By investigating the Burgersvector of dislocations in the dislocation boundaries, it is found that more slip systemsare activated in fine-grained Al, and that slip systems with the highest Schmid factor areless dominant in the dislocation boundaries formed during deformation. The use of aBishop-Hill model is found to give better predictions compared to a Schmid factoranalysis.(4) The mechanical properties of fine-grained Al show a transition behavior asthe average grain size decreases, and this transition can be related to the differentmicrostructure evolution behavior. For samples with grain size larger than1.3m,dislocation boundaries are stored in each grain during deformation, causingwork-hardening after yielding. However, for samples with grain size below0.8m, fewdislocation boundaries are stored in each grain during deformation, causing a lack ofwork-hardening after yielding.(5) For samples showing work-hardening after yielding,the relationship between microstructure and flow stress has been established. By a linearcombination of oxide particle hardening, forest dislocation hardening and grainboundary hardening, the flow stress can be well predicted by choosing a criticalmisorientation angle of2~3oto distinguish forest dislocation hardening and grainboundary hardening.更多还原