UCLA

The differential strain behavior of Ta x W 1-x B solid solutions has been studied as a function of composition using high-pressure radial X-ray diffraction in a diamond-anvil cell under non-hydrostatic pressure (up to ∼65 GPa) to understand the hardening mechanisms in this family of materials. The hardness of tungsten monoboride (WB) can be increased by adding tantalum and reaches a maximum at a doping level of 50 at. % with a value of 42.8 ± 2.6 GPa under an applied load of 0.49 N. Plateaus were observed in the differential strain data for both the (020) and (002) directions, suggesting that this is the primary slip system in this material. These plateaus were modified by the addition of Ta, indicating that strengthening of the (002) and (020) planes by solid solution hardening was primarily responsible for the hardness enhancements in Ta x W 1-x B solid solutions. In contrast, the differential strain supported by the (200) plane linearly increases with pressure up to the highest pressures reached in this work (>60 GPa) and shows almost no change with metal composition. Because of the very different compression behavior in the (200) and (020) planes, change in the b/a ratio with pressure provides a unique way to visualize the onset of plastic behavior. This onset varies from ∼15 GPa for samples with 5% Ta to more than 30 GPa for the sample with 50% Ta. In addition, the ambient bulk modulus of each solid-solution sample was determined using the second-order Birch-Murnaghan equation-of-state and found to be ∼340 GPa for all phases.