Evolution of WC-Co cemented carbide microstructure during creep testing

Licentiate thesis, 2014

The aim of this work is to understand the mechanisms behind the plastic deformation of WC-­Co based cemented carbides. The microstructures of two materials have been investigated before and after creep deformation using quantitative microscopy, atom probe tomography, and transmission electron microscopy. One material was a pure WC‐Co material with 10 vol % binder phase. The other material had a higher fraction of binder phase, 16 vol %, and contained a small addition of Cr. High temperature compressive creep tests have been performed under a load of 900 MPa at 900, 1000 and 1100 °C and the test bars were deformed to different strains. Grain size measurements by the linear intercept method showed that WC grain growth took place during compressive creep testing of the Cr doped material, and that this growth took place preferentially in the plane perpendicular to the load axis. Transmission electron microscopy showed that the WC grains in the crept materials had a significantly increased dislocation density, with a large number of dislocation lines merging at the grain surfaces. It is suggested that merging matrix dislocations having a screw component may act as nucleation points to grow new layers of WC. The binder phase grains became, on the other hand, smaller during creep deformation, and lost their dendritic morphology. This may be explained by binder phase grain rotation caused by dislocation glide. The crept microstructures had an increase number of binder phase lamellae separating adjacent WC grains. This suggests that a redistribution of the WC grains occured, and that the associated grain boundary sliding was accommodated by the lamella formation. Intergranular cavities were also formed during creep deformation. This cavity formation indicates that also unaccommodated grain boundary sliding took place during creep deformation. Atom probe tomography revealed an increased amount of C in the binder phase after creep deformation at 1000 °C. This could possibly be related to the comparatively rapid cooling from the test temperature.