There is a correlation between bulk mechanical properties of polycrystalline materials and the strength of individual grain boundaries at the micro- or nano-scale. The strength of the grain boundary interface can be strongly influenced by the addition of dopants. This, in turn, substantially changes fracture path and fracture toughness, ultimately affecting the strength and durability of engineered materials at the macroscale.
晶界的表征
化学和机械的晶界界面的广泛表征促进了研究人员改善给定应用的材料。亚博网站下载本文提出了一种机械表征方法，涉及Hysitron Pi 85L SEM Picoindenter通过偏转和裂缝的双晶悬臂标本来定量测量断裂韧性（图1）。晶界的化学表征还通过原子分辨率的高角度环形暗场（HAADF）扫描透射电子显微镜（STEM）进行。
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图1。SEM micrographs of a bi-crystal spinel cantilever before and after in-situ deflection testing.
镁氧化镁尖晶石
Magnesium aluminate spinel (MgAl2O4), a ceramic system, exhibits great strength and high transmissibility of visible and near IR light, with several benefits over conventional glass. The material is increasingly gaining interest for a myriad of military applications, especially as transparent armor and windshields for combat aircraft, helicopters, ground vehicles, and spacecraft. Other applications include lenses for night vision goggles, lasers, and nonmilitary applications for biomedical sensors and LEDs.
Bi-Crystal Fabrication
该分析使用了在界面处有和没有Ytterbium（YB）掺杂剂的受控双晶型尖晶石样品。接口是通过{100}和{111}方向的两个单晶体底物扩散键合产生的。将样品在1200°C下热按1小时，然后在1400°C下进行热退火4小时。所使用的掺杂样品是ytterbium掺杂剂，在热压之前，该样品是在溶液中引入的。
在1400、1600和1800°C的温度下进一步退火4小时可产生三种掺杂样品的变化。通过使用JEOL 2200FS电子显微镜确定和表征界面的无缺陷区域。在确认了无缺陷区域的存在之后，使用聚焦的离子束（FIB）铣削以悬臂形式提取附近的标本（图2）。
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Figure 2.低放大的SEM显微照片显示通过FIB铣削的悬臂断裂。
Cantilever dimensions were roughly 12µm long by 3µm thick by 3µm wide. Electron backscatter diffraction (EBSD) was used to determine the position of the interface, while the FIB was used to notch the top of the interface to form an initiation point for failure. Notch depth varied from 20 to 50% of total thickness. At the edges of each notch, roughly 100nm ligaments were left intact to prevent edge rounding caused by FIB milling. Single crystal (SC) {100} cantilevers were also prepared and notched in the same way as a control.
挠度测试和分析
息肌PI 85L SEM Picoindenter用于将悬臂偏转到失败点，以5µn/sec的恒定加载速率。负载/位移曲线显示在每种韧带发生故障的情况下变化（图3）。这也是整个光束的裂纹启动点。确定了负载/位移线的斜率与给定样品的缺口深度相关，然后估算断裂韧性（KIC）每个光束作为宽度的函数和缺口深度。
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Figure 3.来自代表性的悬臂偏转测试的负载/位移曲线在掺杂/未掺杂的双晶体标本上。
单晶样品显示平均KICof 1.62MPa m½, which is in line with other published results. The doped samples demonstrated similar KIC在退火温度上保持恒定的值。HAADF测量还验证了三组掺杂样品的类似晶界结构，从而支持机械强度的微不足道差异。平均kICfor the undoped specimens was estimated to be 1.23MPa m½, roughly 30% compared to the SC and doped specimens. Strong bonding between Oxygen and Ytterbium, seen in STEM mode, presents itself as a potential source of the improved strength of the doped interface.
结论
结果清楚地表明了息护物PI 85L SEM Picoindenter提供对双晶型尖晶石悬臂的晶界断裂韧性的定量测量。晶界掺杂剂的存在大大提高了晶界界面的强度。Hysitron PI 85L SEM Picoindenter还支持一种新的微观裂缝韧性表征的方法，以优化各种材料系统。
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