新型弛豫铁电单晶裂纹扩展及裂尖微纳尺度变形场的原位实验研究

Due to the excellent electromechanical properties, the new relaxor ferroelectric single crystals, such as Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN–PT) and Pb(In1/2Nb1/2)O3–Pb(Mg1/3Nb2/3)O3–PbTiO3 (PIN–PMN–PT), have become the important materials to fabricate the new generation electromechanical devices, including ultrasound transducers, actuators, sensors and other electromechanical devices, with greatly improved performance. However, because of their inherent brittleness, the applied loading may lead to crack and fracture of the ferroelectric single crystal devices. So studying the crack propagation and micro- and nano-scale deformation fields evolution around crack tip in the relaxor ferroelectric single crystals is the key to improving reliability and assessing service life of ferroelectric single crystal devices. In this project, the micro-crack nucleation and propagation in PMN–PT and PIN–PMN–PT single crystals will be studied experimentally by in situ polarized optical microscopy and scanning electron microscopy. The effect of domain switching on crack propagation will be analyzed, and the mechanisms of crack propagation and fracture for ferroelectric single crystals will be explored. Geometric phase analysis and digital image correlation method will be used for quantitative measurement of the micro- and nano-scale deformation fields around crack tip. The effect of switching strain on deformation fields around crack tip will be shown and analyzed, and the application scales of the available theories of deformation fields around crack tip in ferroelectrics will be verified. These studies will provide the reliable experimental basis for establishing and developing the fracture theoretical model of ferroelectric single crystals.

以铌镁酸铅-钛酸铅(PMN-PT)和铌铟酸铅-铌镁酸铅-钛酸铅(PIN-PMN-PT)为代表的新型弛豫铁电单晶，由于其优异的机电性能，已成为制造新一代高性能超声波换能器、致动器及传感器等机电器件的重要材料。然而，铁电单晶具有固有脆性，在外加载荷作用下，常常会导致铁电单晶器件发生断裂。因此，研究弛豫铁电单晶的裂纹扩展和裂纹尖端微纳尺度变形场演化，对提高铁电单晶器件的可靠性及寿命评估具有关键意义。本项目拟采用原位偏光显微镜及扫描电子显微镜实验方法研究PMN-PT及PIN-PMN-PT铁电单晶中微裂纹的萌生及扩展过程，分析畴变对裂纹扩展的影响，探索铁电单晶的裂纹扩展及断裂机理，采用几何相位分析及数字图像相关方法对裂纹尖端微纳尺度变形场进行定量测定，展示并分析畴变应变对铁电单晶裂纹尖端变形场的影响，检验现有铁电体裂纹尖端变形场理论的适用尺度，为铁电单晶断裂理论模型的建立和发展提供可靠的实验依据。

近十多年来，三元弛豫铁电单晶铌铟酸铅–铌镁酸铅–钛酸铅（Pb(In1/2Nb1/2O3)– Pb(Mg1/3Nb2/3)O3–PbTiO3, PIN–PMN–PT）及二元弛豫铁电单晶铌镁酸铅–钛酸铅（Pb(Mg1/3Nb2/3)O3–PbTiO3, PMN–PT）由于具有优异的机电性能，已成为功能材料领域的热门研究对象。然而由于其固有脆性，弛豫铁电单晶在材料加工、器件制作及服役过程中，会不可避免地产生微裂纹、孔洞等缺陷，从而会导致器件性能下降甚至是失效。因此，开展弛豫铁电单晶的微裂纹扩展和裂纹尖端变形场的实验研究，对提高铁电单晶器件的性能、可靠性和使用寿命具有重要意义。.本研究对不同晶向的PIN–PMN–PT及PMN–PT铁电单晶进行了纳米压痕实验，研究了铁电单晶的微纳米尺度的力学性质。研究结果表明：压入硬度、弹性模量及断裂韧度具有各向异性；随着压入深度的增加，PIN–PMN–PT及PMN–PT铁电单晶的压入硬度随着压入深度的增加而减小，压入硬度具有明显的尺寸效应，但弹性模量与压入深度无关。以2000目透射电子显微镜样品用方孔铜载网为模板，采用离子溅射沉积技术在PIN–PMN–PT及PMN–PT铁电单晶单边缺口试样的表面成功制作了微米尺度的周期性网格；对PIN–PMN–PT及PMN–PT铁电单晶试样进行了原位扫描电子显微镜单轴拉伸实验，分析了铁电单晶中微裂纹的萌生及扩展过程；采用几何相位分析方法测定了裂纹尖端附近的微米尺度的应变场。研究结果表明：微裂纹萌生于缺口根部，并随载荷增加沿着垂直于拉伸的方向扩展，属于Ⅰ型裂纹；裂尖应变场主要由正应变εyy控制。.此外，本研究还将微米尺度周期性网格制作工艺和几何相位分析方法成功应用于5A05铝镁合金及多晶钼的裂纹尖端变形场分析中，证实了其在铁电单晶变形场分析中的可行性。本研究还设计了一种扫描电子显微镜原位拉伸压缩实验装置和一种扫描电子显微镜原位弯曲断裂实验装置。