The intention of this paper is to give an overview of selected RD high resolution microscopy and electron backscatter diffraction. Amongst emerging trends, coupling between experiments and modelling at different scales is growing, as well as three dimensional modelling of microstructure evolution. Trends are discussed and an outlook for future development in the respective fields is given.

Trends in the P/M hard metal industry

In order to achieve improved properties and performance with WC-based cemented carbides, research efforts have been directed towards the development of nanostructured cemented carbides. With the recent development of ‘spray conversion process’ for synthesizing nanosized powders and the advent of spark plasma sintering technique, it has been possible to successfully develop bulk nanostructured cemented carbides, possessing improved hardness and wear resistance. On a different note, realisation of the fact that the presence of metallic binder phase is deleterious towards certain applications of WC-based cermets has led to a recent surge of interest towards the development of novel ‘binderless’ WC-based ceramics by replacing the metallic binder phase with ceramic sinter-additives. More recently, it has been possible to develop dense WC-based ceramic composites without considerable deterioration of fracture toughness in the absence of the metallic binder phase. In the above perspective, the present review focuses mainly on the recent research results concerned with the processing and characterisation of nanostructured WC-based cermets and binderless WC-based ceramic composites.

Recent developments on WC-based bulk composites

The deformation of the cutting edge of inserts made from WC–Co and Ti(C, N)–19%WC–Co upon radial turning was measured as a function of the cutting speed. Ab initio calculations of some grain boundary geometries in WC–Co indicated that Co segregates to WC/WC grain boundaries in submonolayer proportions and increases their strength and resistance to Co infiltration. This was also confirmed with TEM-EDX. SEM studies showed that during plastic deformation the hard phase skeletons in both materials were partly broken up and infiltrated by binder phase. TEM observations showed a considerable deformation of the WC grains and some infiltrated grain boundaries were facetted along low energy planes. In the cermet, no deformation of the hard phase grains could be seen. The plastic deformation at high temperatures observed by three-point bending corresponds to a number of relaxation processes, detected by internal friction, related to grain boundary sliding. Both materials deform by grain boundary sliding, accommodated in the cermet by Co diffusion and in the WC–Co by deformation of the hard phase and diffusion of Co.

Mechanisms of plastic deformation of WC–Co and Ti(C, N)–WC–Co

https://iopscience.iop.org/article/10.1016/S1468-6996(00)00006-1/pdf

High resolution transmission electron microscopy study in VC-doped WC–Co compound

Effects of cobalt intergranular segregation on interface energetics in WC-Co

The high temperature deformation of tungsten carbide-cobalt composites has been performed in the 1050 to 1350° C temperature range either in compression or in three point bending. The creep behaviour of these materials exhibits a sigmoidal log
 
$$\mathop \varepsilon \limits^ \circ - log$$

 
σ plot with three domains of steady state creep rate possessing a different value of the stress exponent. From recent microscopical and analytical results a microstructural model of the composite is proposed. This model assumes the existence of carbide chains formed of crystals linked with coincidence grain boundaries and takes the number of coincidence grain boundaries into account. It has been used to interpret the creep results. The controlling component of the composite is the tungsten carbide and two deformation processes, grain boundary sliding and intragranular deformation, are the main mechanisms which are involved in the explanation of the creep behaviour. The results of the creep tests are discussed in correlation with transmission electron microscope (TEM) investigations on annealed then deformed samples. TEM microstructural studies of the carbide phase after deformation show an extensive intragranular deformation which has been carefully analysed. Attention has also been paid to the defect structure of grain boundaries.

High temperature creep of WC-Co alloys

Interfaces between carbide grains in the tungsten carbide-cobalt composite have been considered. Different techniques, such as transmission electron microscopy and energy dispersive X-ray analysis have been used to characterize the orientation relationship, the nature of the planes and the chemical composition of the grain boundaries.The cobalt concentration at WC-WC grain boundaries was determined by X-ray energy selective analysis in the TEM. Cobalt profiles were performed across low-angle grain boundaries, coincidence and general grain boundaries. Cobalt segregation was found whenever dislocations were imaged in the grain-boundary plane of a low-energy grain boundary. The segregation value was compared with the segregation ratio measured in special grain boundaries characterized by a coincidence site lattice.

Cobalt intergranular segregation in WC-Co composites

Grain boundaries in tungsten carbide crystals (hexagonal structure c/a = 0.976) show many coincidence orientations. The most frequent orientation has A coincidence index £= 2. H.R.E.M. images of such grain boundaries have been realized. These grain boundaries show a defect structure, only due to the mismatch of the a and c parameters. These defects consist of dissociated 1/6<1120>. or 1/2 <0001>„ dislocations regularly spaced. An atomic step is associated with each dislocation. This particular grain boundary can be described as a E= 2 (c/a =1) grain boundary with a periodic structure of dislocations or as a Z= 39 (c/a = 0.975) grain boundary with a 1*5 deviation to the exact coincidence orientation.

Structural analysis and properties of grain boundaries in hexagonal carbides

Des observations en M.E.T. sur des echantillons de carbure pur deforme a 1450° montrent une deformation intergranulaire importante. L'analyse des defauts a ete realisee. Proposition d'un modele pour la deformation des composites WC-Co

TEM INVESTIGATIONS OF WC-Co ALLOYS AFTER CREEP EXPERIMENTS

Cobalt concentration at WC-WC grain boundaries (G.B.) in WC-Co composites was determined via X-ray energy dispersive analysis in S.T.E.M. It was shown that only general G.B. were characterized by a high increase in cobalt. From these results, it was possible to infer a continuous carbide skeleton through coincidence G.B.

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J. Vicens

Papers

High temperature creep of WC-Co alloys

Cobalt intergranular segregation in WC-Co composites

Structural analysis and properties of grain boundaries in hexagonal carbides

TEM INVESTIGATIONS OF WC-Co ALLOYS AFTER CREEP EXPERIMENTS

Microanalyse des joints carbure-carbure dans les composites carbure de tungstène-cobalt