Microstructure Development of WC-Co Based Cemented Carbides During Creep Testing

Doktorsavhandling, 2016

The aim of this work is to understand the mechanisms behind the plastic deformation of WC‐Co based cemented carbides. The microstructure of two series with different WC grain size was investigated before and after creep deformation using quantitative microscopy, atom probe tomography, and transmission electron microscopy. The first series consisted of two WC-Co based cemented carbides with fine WC grain size used for machining applications, an un-doped WC-Co material with 10 vol % binder phase and a Cr-doped WC-Co material with a higher fraction of binder phase, 16 vol %. The second series consisted of two WC-Co based cemented carbides with coarser WC grain size used for mining applications, an un-doped and a Cr-doped WC-Co based materials. In both materials, the binder phase represented 10 vol% of the total volume. High temperature compressive creep tests were performed under a stress of 900 MPa at 900, 1000 and 1100 °C and 300 MPa at 1100 °C, and the test bars were deformed to different strains. A heat treatment was also performed at 1000 °C in order to see the effect of an applied load on WC grain growth. Preferential WC grain growth perpendicular to the applied load was found to occur during creep deformation at 1000 and 1100 °C. WC grains in the crept materials had a significantly increased dislocation density, with a large number of dislocation lines merging on the grain surfaces. It is suggested that merging matrix dislocations having a screw component may act as nucleation points to grow new layers of W and C. The binder phase grains became, on the other hand, smaller during creep deformation at 1000 °C, and lost their finger-like morphology. This may be explained by binder phase grain rotation caused by dislocation glide. The crept microstructures had an increased number of density of WC/WC grain boundaries infiltrated by the binder phase. This suggests that grain boundary sliding occurred, accommodated by binder phase infiltration. Some WC grain surfaces facing the infiltrated grain boundaries had a step surface of an angle of 90 and 120 °. The atoms at the surface were rearranged to reduce the surface energy. Intergranular cavities were also formed during creep deformation. This cavity formation indicates that also unaccommodated grain boundary sliding took place during creep deformation. APT revealed a variation in the concentration of W and C in the binder phase, and that the solubility of W and C increased with increase of temperature. The rapid cooling from the creep test temperature ensures that the composition of the binder phase in crept materials represents the composition during the test. WC/binder phase boundaries of the as-sintered fine grained Cr-doped material showed an enrichment in Cr at the phase boundary and formation of thin (Cr, W) carbide layer on the surface of WC grains at the phase boundary. This (Cr, W) carbide layer seems to disappear when the material is heat treated at 1000 °C. The thermodynamic calculations found that this cubic layer is stable below 940 °C when no M7C3 carbides are formed.