Nanocrystalline WC–Co materials have been the subject of interests and focus of research programs around the world for the past two decades owing to the expectations that the mechanical behavior of the material may improve significantly when grain sizes reduce to nanometer scale. However, although numerous technologies are available for making nanosized tungsten carbide powders, obtaining true nanocrystalline WC–Co (average WC grain size

Synthesis, sintering, and mechanical properties of nanocrystalline cemented tungsten carbide - A review

Cemented carbides cover a wide range of applications in many relevant industries, i.e. as cutting tools (turning, milling, drilling) for machining of metal components in the automotive and/or aerospace industry, as components of drill bits or road headers in the rock tools and mining area or as wear parts in wire drawing dies or punch tools. In this review selected cemented carbide and cermet microstructures are presented. The focus is on microstructures, both those that are already established in the cemented carbide industry and those which have drawn scientific attention due to new potential applications. Cemented carbides are here divided in four groups based on microstructure and chemistry: WC morphology and chemistry, cubic carbide containing cemented carbide and cermets, functionally graded cemented carbides, and binder design of cemented carbides. Furthermore, this review covers some historical background that motivated the microstructure design as well as the status of each class of materials nowadays. The paper aims at categorising cemented carbides in a structured way and to serve as an introduction to cemented carbide microstructures for engineers, researchers and scientists.

Cemented carbide microstructures: a review

Extensive calculations based on density functional theory have been carried out to understand the origin of magnetism in undoped ZnO thin films as found in recent experiments. The observed magnetism is confirmed to be due to Zn, instead of O, vacancy. The main source of the magnetic moment, however, arises from the unpaired 2p electrons at O sites surrounding the Zn vacancy with each nearest-neighbor O atom carrying a magnetic moment ranging from 0.490 to 0.740 B. Moreover, the study of vacancy-vacancy interactions indicates that in the ground state, the magnetic moments induced by Zn vacancies prefer to ferromagnetically couple with the antiferromagnetic state lying 44 meV higher in energy. Since this is larger than the thermal energy at room temperature, the ferromagnetic state can be stable against thermal fluctuations. Calculations and discussions are also extended to ZnO nanowires that have larger surface to volume ratio. Here, the Zn vacancies are found to lead to the ferromagnetic state too. The present theoretical study not only demonstrates that ZnO samples can be magnetic even without transition-metal doping but also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZnO nanostructures. In addition, vacancy mediated magnetic ZnO nanostructures may have certain advantages over transition-metal doped systems in biomedical applications.

https://absimage.aps.org/image/MAR08/MWS_MAR08-2007-004197.pdf

Zinc Vacancy induced magnetism in ZnO thin films and nanowires

Metal‐Based Nanomaterials in Biomedical Applications: Antimicrobial Activity and Cytotoxicity Aspects

Computer simulation has been used to study the interaction of a perfect, basal dislocation with a {1012} twin boundary in a hcp metal for the situation where the 1/3{1120} Burgers vector is inclined at 60° to the interface. It is found that slip is not transferred from one crystal to the other with a residual dislocation left at the interface. Instead, the matrix dislocation decomposes into interfacial defects. We show that as a result of this decomposition the matrix dislocation becomes a new source of twinning dislocations that produce twin growth when the appropriate stress is applied to the crystal. The mechanism described does not require twinning dislocations to multiply by a pole process.

A new model for {1012} twin growth in hcp metals

The microstructure of sintered WC–Co composites doped with VC and Cr3C2 is investigated by several techniques (TEM, HREM, EDXS, EELS) in order to accurately locate the VC phase. Small VC precipitates are frequently found at the junction between two WC grains in the corner of cobalt pockets. Due to the good match between the WC and VC parameters, the VC grains adopt a preferential orientation with regards to WC: (1 1 1) VC //(0 0 0 1) WC with 〈 1 1 0〉 VC //〈 1 2 1 0〉 WC . Moreover steps along the 〈1 1 2 0〉 WC directions are observed at the interface between cobalt and WC. Small VC precipitates lying on the (0 0 0 1) WC and {1 0 1 0} WC facets of the steps are identified. A thin VC layer less than 1 nm thick covering all (0 0 0 1) WC surfaces of WC grains is also pointed out. Some WC grain boundaries contain a thin film of VC phase whereas some high coincidence grain boundaries seem to be free from second phase.

Location of VC in VC, Cr3C2 codoped WC–Co cermets by HREM and EELS

Cr-rich layer at the WC/Co interface in Cr-doped WC–Co cermets: segregation or metastable carbide?

In two alloys WC–(24 wt.%) Co containing a C and W excess respectively, sintered at 1450 °C (10 h) the shape of the larger WC grains that is a prism with a truncated triangle base is studied by transmission electron microscopy. The truncation that is the ratio of the short/long triangle sides and the elongation that is the ratio of the prism/triangle heights are quantified. The grains are less truncated and flatter in the C rich alloy. In equilibrium condition the ratio of the interface energies of the prismatic facets determine the truncation. The ratio of the energies of the basal and prismatic planes determines the elongation. The measured truncation confirms the ratio of the interface energies predicted from atomistic calculation for the prismatic facets. The experimental elongation is of the same range as the ratio between the calculated energies in the W rich alloy but much smaller in the C rich alloy. The possible origins of the discrepancy – departure from the equilibrium WC grain shape, model of the WC–Co interfaces used to calculate the interface energies – are discussed.

Morphology of WC grains in WC–Co alloys

Morphology of WC grains in WC-Co alloys: Theoretical determination of grain shape

The morphology and the interface features of WC grains are examined by SEM and TEM in cemented carbides WC-8 wt% Co. Three compositions - the mixture {WC + Co binder) and the mixtures with graphite or η - are studied after treatments at 1200 and 1450 °C. The effect of the C content is observed both on the WC crystal features and on the microstructure evolution. In all the alloys, large faceted grains that are defect free single crystals are formed at 1450 °C. On the C rich side, the increase in number and size of the large grains is more regular, the WC grains are more faceted and grow already during heating. On the W rich side, the WC grains have slightly rounded corners and interface steps. The C content influence on the interface energy anisotropy and the growth process of the WC grains is discussed from the microstructure results.

Sabine Lay

Papers

Location of VC in VC, Cr3C2 codoped WC–Co cermets by HREM and EELS

Cr-rich layer at the WC/Co interface in Cr-doped WC–Co cermets: segregation or metastable carbide?

Morphology of WC grains in WC–Co alloys

Morphology of WC grains in WC-Co alloys: Theoretical determination of grain shape

Microstructure Evolution in the Cemented Carbides WC–Co I. Effect of the C/W Ratio on the Morphology and Defects of the WC Grains