高温氧化物陶瓷的形变和相变过程中的位错机制研究

Attributed to their high melting points, good mechanical properties, excellent thermal and chemical stabilities at elevated temperatures, oxide ceramics such as Al2O3 and Y2Si2O7 have attracted increasing research attention as potential high temperature structural materials. Dislocation slip is an important mode of plastic deformation in these oxide ceramics at elevated temperatures. However, these oxides generally possess complex structures in which dislocations slip in groups, even involving short-range diffusion along different directions. The coulombic repulsion between charged ions further complicates the dislocation core structures and the slip mechanisms. For those reasons, the physical processes of dislocation glide with such local mutual motion of atoms are still largely speculative. In this work, we are planning to resolve dislocation core structures in Al2O3 and Y2Si2O7 crystals and interface structures between alpha and gamma phases in Al2O3 by using aberration-corrected high-resolution transmission electron microscopy (HRTEM), the focal series exit-wave reconstruction technique and annular bright-field (ABF) scanning transmission electron microscopy (STEM). With the aid of quantitative high-resolution electron microscopy techniques, structural models of the dislocation cores and interphase interfaces will be refined by iterative digital image matching between experimental and simulated images. In this way, possible partial occupation of ions could be determined in the dislocation cores. First-principles calculations are utilized to reveal slip mechanisms of these dislocations. The nature of these slip mechanisms will lead to general concepts for dislocation motion and reaction in complex-structured ionic crystals. By the virtue of electron microscopy and theoretical calculation, we pursue an insight of the dislocation mechanisms of plastic deformation and phase transitions in oxide ceramics. These results will also help understand high temperature mechanical properties of the oxide ceramics as well as the brittle-to-ductile transition of oxide ceramics. In the mean time, quantitative high-resolution electron microscopy methods will be developed to obtain the defect structures in complex-structured materials at an atomic scale.

具有高熔点和优异力学性能的氧化物陶瓷是高温结构材料的一个重要发展方向。研究这些氧化物晶体中的位错运动和反应机制对于深入认识高温陶瓷材料的力学性能有着重要的意义。但是在这些复杂结构的离子晶体中，位错的运动往往需要几个甚至几十个原子的协调运动，离子之间强烈的电荷作用也使得滑移过程更加复杂。我们拟利用像差校正高分辨像(HRTEM)、环形明场像(ABF)等技术对氧化物陶瓷Al2O3、Y2Si2O7在高温形变时的位错核心结构和Al2O3中alpha/gamma相变界面的精细结构进行研究。结合定量电子显微学分析，优化样品结构模型中原子的种类、位置、数量，从而解析位错核心离子的部分占位状态。利用所得到的缺陷结构模型进行第一原理计算，揭示位错运动机制和相变机理，然后和已有的材料力学性能的实验结果比较，加深对位错机制对高温陶瓷材料作用物理本质的理解。同时，发展对复杂结构材料中缺陷组态的定量电子显微学研究。

高温结构陶瓷具有高熔点、高硬度、抗氧化性以及优异的高温力学性能，在航空航天等领域具有重要应用价值。高温结构陶瓷的力学性能与材料的微观结构以及其中的相变过程具有密切关系。研究高温结构陶瓷的微观形变和相变机制，对于进一步改善高温结构陶瓷的性能以及促进该材料的应用具有重要的理论指导意义。我们采用透射电子显微学方法，结合像差校正的成像和谱学分析，研究了高温结构陶瓷中的缺陷结构和化学组成。采用高分辨成像方法，对脉冲激光沉积的Gd:CeO2(GDC)中的缺陷结构进行了研究，发现GDC/YSZ界面存在周期性分布的失配位错，其类型为纯刃型位错，该失配位错释放了不同陶瓷材料之间的晶格失配。GDC薄膜内部，存在着两种不同类型的位错，一种为Burgers矢量为1/2 <011> 的纯刃型位错，一种为Burgers矢量为1/2<110> 的混合型位错。这些网络状位错的存在促进了GDC/YSZ系统的离子传导率。对脉冲激光沉积的LSMO:ZrO2的界面结构和离子价态进行了研究，原子分辨率的电子能量损失谱结构显示，界面两侧具有明显的元素扩散和离子价态的变化，从而影响了该复合薄膜的电子输运性能。对LSMO薄膜中穿透型位错的研究表明，该穿透位错以刃型位错为主，位错核心中存在反位缺陷和氧离子空位。利用等离子放电烧结的方法，制备了Hf-Al-C三元层状陶瓷，发现该陶瓷以Hf2Al4C5为主相，对其微观结构的研究表明其内部存在三种不同类型的层错，即两种基面层错和一种非基面层错，解释了不同层错的形成机制。同时，提出了层错诱导HfC相转变为Hf-Al-C三元相的相变机制。对高温形变后的Cr2Nb晶体研究发现，其内部存在大量的基面层错和非基面畴界，这些缺陷将晶体分割成小至2nm的纳米晶体畴。这种畴界结构具有优异的高温稳定性，从而带来了该材料优异的高温结构稳定性和高温力学性能。