Strength, deformation and equation of state of tungsten carbide to 66 GPa

Carbides such as tungsten carbide are found in reducing environments on Earth including the mantle and core and are distinguished by high incompressibility and strength. These properties at ambient and extreme conditions have led to widespread application of WC in research and industry in protective coatings, cutting tools, and other durable components. Despite the utility of WC, its quasi-static yield strength and deformation have not been studied at high pressure. Its equation of state has also been debated, with reported bulk modulus ranging from 329 to 452 GPa with a pressure derivative from 1.25 to 5.45. The stiffening and strengthening effects of carbon on the WC lattice can be revealed through detailed study of its compression and deformation mechanisms.

We studied the EOS, strength, and plasticity of WC to 66 GPa with synchrotron X-ray diffraction in the diamond anvil cell. Experiments were conducted at the Advanced Photon Source beamline 16-BM-D. EOS parameters were determined by compressing WC in a Ne pressure medium in the axial diffraction geometry. Measured unit cell volumes were fit to a 3^rd order Birch-Murnaghan EOS yielding V₀ = 20.80 ± 0.04 ų, K₀ = 371 ± 10 GPa, and K₀' = 4.2 ± 0.4. Strength and plasticity were determined by complementary analysis of lattice strain and texture via full-profile Rietveld refinement of radial diffraction patterns. Elasto-ViscoPlastic Self-Consistent simulations of strain and texture were used to determine deformation mechanisms. Large scale yielding and development of preferred crystallite orientation begins at ~30 GPa, at which pressure WC sustains a differential stress of ~13-15 GPa, depending on whether shear modulus is assumed based on ultrasonic or theoretical constraints. Above yielding, the strength of WC is similar to other hard materials TiB₂ and B₆O, with a maximum flow stress of 23-28 GPa at 66 GPa. During deformation a texture maximum develops near 2-1-10 in inverse pole figures of the compression direction. EVPSC simulations indicate that plastic deformation is accommodated by prismatic slip on {10-10}<-12-10> and {10-10}<0001>, with pyramidal slip on {10-11}<-2113> becoming activated at ~40 GPa. These mechanisms differ from basal slip observed in W and hcp metals due to blocking effects of carbon on dislocations in the lattice.