晶界相调控多晶MgAl2O4陶瓷晶界断裂韧性的内在作用机制研究

Low fracture toughness of transparent polycrystalline MgAl2O4 and its variation across bulk sample limit its bulk performance as an important structural and optical ceramic material. It has been hypothesized that grain boundary complexion transition has a significant effect on grain boundary fracture toughness, and therefore affects the bulk mechanical response of MgAl2O4. Three trusts make up the proposal, which include the exploitation of grain boundary complexion in MgAl2O4, the quantification of grain boundary fracture toughness, and the establishment of correlation between grain boundary complexion transition and grain boundary fracture toughness. A highly integrated experimental program is proposed to meet these goals. Firstly, grain boundary structure and chemistry will be characterized via atomic-scale aberration corrected electron microscopy with electron lost spectroscopy, and Auger electron microscopy, and the grain boundary complexions in MgAl2O4 will be classified based on Dillon-Harmer complexion model. The results will be linked with processing condition variations resulted from preferential vaporization of MgO during sintering, reducing sintering atmosphere, and the amount and distribution of residual LiF sintering aiding. Secondly, single grain boundary fracture toughness of MgAl2O4 will be studied via in situ TEM mechanical testing, and the effect of V shape notch geometry and micro-cantilever beam size on fracture toughness value will be quantified, too. Last but not the least, the effort will develop a correlation between grain boundary complexion transitions and grain boundary fracture toughness. The proposed work seeks to develop the database of grain boundary parameters, develop a fundamental understanding of the relationships between grain boundary structure-property-processing-performance, and harness this knowledge for mechanism-informed materials design through grain boundary engineering strategy.

多晶透明MgAl2O4陶瓷是重要的光学和结构材料，脆性大且力学性能离散度高的特点，限制了其作为关键部件的服役安全性和可靠性。通过晶界结构调控晶界力学性能是改善多晶MgAl2O4力学性能的一种有效途径。本课题拟探索制备过程中组分配比、气氛、烧结剂等因素对晶界化学组分和晶界2D原子结构的综合影响，系统研究具有复杂晶界结构的多晶MgAl2O4二元陶瓷体系中存在的新晶界相类型，提出适用于多晶MgAl2O4二元陶瓷体系的新晶界相模型。利用基于透射电镜的原位微米悬臂梁断裂法研究多晶MgAl2O4中单个晶界的断裂韧性，优化悬臂梁测试法中的切口和样品尺寸等影响因素，建立MgAl2O4晶界相类型与晶界断裂韧性的对应关系。最终，通过上述研究，阐明晶界相类型对MgAl2O4晶界断裂韧性的影响规律，揭示晶界相调控晶界断裂韧性的原子键合作用机制，为利用晶界工程策略改善MgAl2O4力学性能提供理论依据和技术参考。

纳米多晶陶瓷块体具有脆性大且力学性能离散度高的特点，限制了其作为关键部件的服役安全性和可靠性。利用原位透射电镜平台研究陶瓷材料的微观力学性能并阐明背后的演变机制，对于进一步优化材料材料在极端环境的服役性能至关重要。完成了以下研究目标：（I）通过纳米柱压缩实验，发现高压放电等离子体烧结法制备的纳米氧化钇稳定氧化锆陶瓷的屈服强度先随着晶粒尺寸的减小而增加，到达临界尺寸后屈服强度随着晶粒惠存的减小而增大，这被用来系统地评估氧化钇稳定氧化锆纳米陶瓷材料在室温下的霍尔-佩奇效应和反霍尔-佩奇效应。结合纳米柱压缩实验与原位电子衍射结果，证明材料的软化的开始伴随着每单位应变的晶体旋转量的增加，表明材料变形机制的改变。（II）进一步的，我们开发了一种结合数字图像关联（DIC）和粒子跟踪（PT）的实验方法，用于表征二氧化硅在TEM中的微尺度变形。为了验证在TEM中使用DIC和PT的可行性，使用聚焦离子束（FIB）加工了二氧化硅微米梁样品，应用DIC和PT从TEM图像中测量微米梁在加载和蠕变过程中的原位位移。这两种测量方法的结果互相吻合且与位移测量结果一致。