Residual stresses in cemented carbides — An overview

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Thermal residual stresses in cemented carbide composites are large, interact with applied stresses, and affect deformation and toughness. Their magnitudes are high (e.g., +2GPa for Co and −0.4GPa for WC in WC–10wt.% Co) and their distributions are complex. Magnitudes depend on expansion coefficients, binder content and particle size; distribution depends on particle angularity — wide ranges of local values occur in both phases. The response of WC-based cemented carbides to applied uniaxial compression and tension is profoundly influenced by thermal residual stresses. They account for long-observed non-linearity at very low strains, plasticity-induced relaxation, unusual Poisson's ratio behavior, and changes in density with loading. Mechanical response is also asymmetric and a function of load direction and history. The response to cyclic loading gives insight into the role of thermal residual stresses in the toughness of these materials, known for their high toughness considering their high hardness.Results for WC–Co and WC–Ni systems are presented. These studies substantially depend on the application of neutron diffraction: neutrons facilitate good bulk sampling of the volumetric thermal residual stresses in the presence of heavy elements and the use of in situ measurements; diffraction enables each phase to be monitored independently.Results show a complex plasticity behavior that comprises a primary source of toughening characteristic in cemented carbides. However, it is now clear that assumptions implicit in the application of fracture mechanics to cemented carbides, including far-field linear elasticity and isotropy are not valid. The emergent view of cemented carbide mechanics seems to require a new, non-linear-elastic model of toughness behavior in these materials, both in terms of bulk “continuum” response and response in the presence of defects. Moreover, such models, if they are to provide accuracy, must also take into account the documented anisotropic relaxation and plasticity effects and their sensitivity to load directionality and history.