Measuring the Impact of Dopant on Grain Boundary Fracture Toughness

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.

Characterization of Grain Boundaries

An extensive characterization of grain boundary interfaces, both chemical and mechanical, facilitates researchers to improve materials for a given application. This article presents a mechanical characterization method that involved a Hysitron PI 85L SEM PicoIndenter to provide quantitative measurement of fracture toughness through deflection and fracture of bi- crystal cantilever specimens (Figure 1). Chemical characterization of the grain boundaries was also performed by atomic-resolution high-angle annular dark- field (HAADF) scanning transmission electron microscopy (STEM).

Figure 1. SEM micrographs of a bi-crystal spinel cantilever before and after in-situ deflection testing.

Magnesium Aluminate Spinel

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

This analysis used controlled bi-crystal spinel samples made with and without Ytterbium (Yb) dopant at the interface. The interfaces were produced by diffusion bonding of two single crystal substrates with {100} and {111} orientation. Specimens were hot pressed at 1200°C for 1 hour and then subjected to thermal annealing at 1400°C for 4hr. The doped samples used were Ytterbium dopant, which was introduced in solution before hot pressing.

Further annealing for 4 hours at temperatures of 1400, 1600, and 1800°C produced three variations of doped samples. Defect-free regions of the interfaces determined and characterized through HAADF STEM utilizing a JEOL 2200FS electron microscope. After confirming the presence of defect-free regions, Focused Ion Beam (FIB) milling was used to extract nearby specimens in cantilever form (Figure 2).

Figure 2. Low magnification SEM micrograph showing fractured cantilevers milled via FIB.

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.

Deflection Testing and Analysis

The Hysitron PI 85L SEM PicoIndenter was employed for deflecting the cantilevers to the point of failure at a constant loading rate of 5µN/sec. The load/displacement curves show changes in the event of the failure of each ligament (Figure 3). This also acts as a crack initiation point for the entire beam. The slope of the load/displacement line was determined to relate to the notch depth for a given specimen, followed by the estimation of the fracture toughness (K_IC) of each beam as a function of width, and notch depth.

Figure 3. The load/displacement curve from representative cantilever deflection tests on doped/undoped bi-crystal specimens.

The single crystal samples showed an average K_IC of 1.62MPa m½, which is in line with other published results. The doped samples demonstrated similar K_IC values that remained constant to annealing temperature. HAADF measurements also verified similar grain boundary structure for the three sets of doped samples, supporting the insignificant difference in mechanical strength. Average K_IC for 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.

Conclusion

The results clearly demonstrate the ability of the Hysitron PI 85L SEM PicoIndenter to provide quantitative measurements of grain boundary fracture toughness of bi-crystal spinel cantilevers. The strength of the grain boundary interface is considerably increased by the presence of Ytterbium dopant. The Hysitron PI 85L SEM PicoIndenter also supports a new method for microscale fracture toughness characterization to optimize a variety of material systems.

This information has been sourced, reviewed and adapted from materials provided by Bruker Nano Surfaces.

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