Effect of metal and cubic carbide additions on interface chemistry, phase composition and grain growth in WC-Co based cemented carbides

Doctoral thesis, 2010

A cemented carbide is a composite material used in metal cutting operations. A hardness providing WC-based skeleton is embedded in a toughness providing binder, mainly consisting of Co. The material is produced by powder metallurgical methods. Additions to the powder mixture are often made of grain growth inhibitors such as V or Cr, in order to retain a fine grained WC, and cubic carbides such as TiC, ZrC, NbC or TaC, in order to increase the material wear resistance. Two series of materials were produced. In the first series, small amounts of V, Cr or Mn were added, and in the second series, larger amounts of TiC, ZrC, NbC or TaC were added. The microstructure of the two series was investigated with scanning electron microscopy, transmission electron microscopy and atom probe tomography. In this study, the effects the additions have on interface chemistry, phase composition and grain growth are systematically investigated. Segregation corresponding to between one half and one monolayer of close packed Co was observed to WC/WC grain boundaries as well as to WC/(M,W)C phase boundaries. For the grain boundaries, some of the Co atoms were replaced of Ti, V, Cr, Mn, Fe, Zr, or Nb but not Ni or Ta. Segregation corresponding to approximately one monolayer of close packed VC was observed to WC/binder phase boundaries. Segregation of Ti, V, Cr, Mn, Zr, Nb and Ta to the WC/binder phases corresponding to less than one monolayer close packed MC was also observed. Of the investigated additions, Ta had the highest solubility in WC, followed by Nb, Cr and V, all with atom fraction solubilities in the 10-3-range. Mn and Ti showed lower, yet detectable, solubilities in the 10-6 or 10-5-range. The solubilities of Co and Zr were too low to be measured. All additions inhibited WC grain growth. For the first series of materials, V was the most efficient grain growth inhibitor with a mean WC grain size that was 38 % smaller than for a corresponding reference material. For the second series of materials, Ti was most efiicient with a mean WC grain size that was 40 % smaller than for a corresponding reference material.