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Understanding How Bonding Controls Strength Anisotropy in Hard Materials by Comparing the High-Pressure Behavior of Orthorhombic and Tetragonal Tungsten Monoboride


In this work, we investigate the high-pressure behavior of the stabilized high-temperature (HT) orthorhombic phase of WB using radial X-ray diffraction in a diamond-anvil cell at room temperature. The experiments were performed under nonhydrostatic compression up to 52 GPa. For comparison, the low-temperature (LT) tetragonal phase of WB was also compressed nonhydrostatically to 36 GPa to explore structurally induced changes to its mechanical properties. Although our microindentation hardness tests indicate that the HT WB possesses slightly higher hardness, synchrotron-based high-pressure compression data yield significant distinct incompressibilities. The ambient pressure bulk modulus of the HT phase of WB is 341 ± 5 GPa obtained by using the second-order Birch-Murnaghan equation of state, while for the LT phase of WB the incompressibility increased to 381 ± 3 GPa. The elastically supported differential stress was measured in a lattice-specific manner and analyzed by using lattice strain theory. Greater strength anisotropy was observed in the HT WB phase, compared to the LT materials. DFT energy shift calculations indicate that W-B bonds rather than B-B bonds are responsible for the lattice-dependent mechanical properties.

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