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2020 Research Highlight: Characterization of the Mechanical Properties of Single Crystal Boron Carbide

Characterization of the Mechanical Properties of Single Crystal Boron Carbide

Mr. Michael Straker
Morgan State University
Dr. Arezoo Zare
Johns Hopkins University
Prof. Mo-Rigen He
Johns Hopkins University
Prof. MVS Chandrashekhar
University of South Carolina
Dr. Christopher Marvel
Lehigh University
Prof. Kevin Hemker
Johns Hopkins University
Prof. K.T. Ramesh
Johns Hopkins University
Prof. Michael Spencer
Morgan State University
Dr. Jerry LaSalvia
CCDC Army Research Laboratory

Despite the need for engineering composites based on boron carbide, and the numerous studies characterizing mechanical failure in these composites there are few studies on the mechanical properties of single crystal boron carbide. Boron carbide is highly anisotropic, with measured elastic moduli varying by as much as 10x between different crystallographic orientations. It is critical to obtain high purity single crystals to completely characterize the elastic moduli, hardness, and dynamic failure mechanisms along various crystal directions. This information can be used in constitutive computer models of boron carbide mechanical failure. For these models to have predictive power for rational material design the mechanical properties as a function of crystal orientation should be determined with a high degree of confidence.

Well-oriented boron carbide single crystals can be grown using a Laser-Diode Floating Zone (LD-FZ) method (Fig. 6a). In LD-FZ growth, lasers melt the polycrystalline feed material small single crystals spontaneously nucleate and grow into long boules as the crystal cools. Due to the anisotropy of the crystal properties, one growth domain dominates and large area single crystal regions are achieved. The phase diagram of boron carbide shows that at the eutectic point, solid phases with carbon content in the range of ~17-20% exist. The large phase region allows for the potential creation of a wide range of boron carbide compositions. Single crystals of boron carbide (Fig. 6b) ~6 cm long with a diameter of 8mm could be reproducibly obtained. There were no apparent graphitic inclusions in the center of the crystal. The crystal orientation was measured with a white beam X-ray Laue camera (Fig. 6c) and electron backscattered diffraction (EBSD). Using these two techniques the crystal growth direction was found to be [003] ± 15 degrees and uniform single crystal regions were found to extend distances of greater than 100 microns. Using a calibrated X-ray energy dispersive spectrometry (XEDS) technique the boron to carbon ratio was determined to be 4.9 (17%). Using Glow discharge mass spectroscopy (GDM) it was found that Si (7.7ppm), Al (.93ppm) and Mg (.60ppm) were the predominate impurities.

Transmission electron microscopy (TEM) was used to characterize the as grown defects. It was determined that the principal defects were twins and stacking faults. In order to determine the mechanical properties of the material quasi-static nanoindentation experiments were performed using a Berkovich indenter under an applied strain rate of 0.05 s-1 to a maximum indentation depth of ~1 μm. As shown in Fig. 7a, to examine the dependence of elastic modulus and hardness on in-plane orientation, nanoindentation experiments were performed by rotating the single crystal about the indenter axis from 0° to 60° and then to 120°, with each of these directions approximate to the <1120> axis.

For each rotation angle, Fig. 7b shows the average values (plus/minus one standard deviation) of elastic modulus and hardness obtained from 10 separate indentations. For a rotation angle of 0°, an elastic modulus of 541.2 GPa and a hardness of 49.7 GPa were obtained. The elastic modulus and hardness values at a rotation angle of 120 were comparable to those measured at 0°. However, a rotation angle of 60 resulted in an increase in the elastic modulus, while decreasing the hardness. Such variations in elastic modulus and hardness by rotation angle suggest in-plane anisotropy in both elastic and quasi-plastic response of the crystal. Post-indentation SEM image of an indent, shown in Fig. 7c, reveals formation of surface cracks at a rotation angle of 60°. Similar observations were made for rotation angles of 0° and 120°and the orientation of surface cracks at each rotation angle was found to be highly repeatable, suggesting preferred crystallographic orientations may exist for indentation crack propagation.

The quasi-plastic behavior under the indent in Fig. 7c was further examined in a cross-sectional specimen lifted out and polished to electron transparency by using focused ion beam (FIB). In the bright field TEM image as shown in Fig. 7d, un-deformed region of the specimen was tilted to the [1210] zone axis, whereas the region around the indent was highlighted with brighter contrast, indicating severe lattice rotation and distortion.

Detailed characterization of the deformed region revealed various microstructural defects. For instance, the high-resolution TEM images in Figs. 7e and 7f shows formation of amorphous bands and fragmentation, respectively. These features observed at different locations may be driven by the inhomogeneous and anisotropic stress fields under the indent or reflect the different stages of deformation and failure. The surface crack (as seen in Fig. 7c) was also found to initiate from an amorphous band formed in the deformed region (details not shown here). Importantly, the mediators of quasi-plasticity were always found along specific low-index crystallographic planes, as marked in Figs. 7e and 7f, indicating the role of crystal anisotropy that governs the mechanical behaviors at the atomic scale.