On the fracture toughness of cemented carbides

Tungsten-carbide cobalt alloys in the range of technically relevant compositions an microstructures were fractured under controlled conditions. Stable and unstable crack propagation are shown to be equivalent with respect to crack geometry and energy release rates. Based on microscopic observations and on measurements using SEM micrographs, the nature of the process zone, the maximum extension of plastic deformation, the types of crack paths, and the area fractions of these crack paths are determined. The results, which are in partial disagreement with earlier work, provide the basis for the qualitative understanding and the quantitative description of fracture processes in composites combining a ductile phase embedded in a brittle matrix.

Experimental study of the mechanics of fracture in WC-Co alloys

Critical stress intensity factor, and related parameters have been measured in three-point bending for 18 different combinations of different volume fractions of cobalt (5 to 37%) and grain size of tungsten carbide (0.7, 1.1 and 2.2 μm). In particular, a study was made of the correlations between the strength and mechanical and microstructural parameters, such as ¯LCo,CWC, ¯LCo/¯DWC, ¯LCo2/¯DWC,HV and wear resistance. A hypothesis for the mechanism of fracture has been proposed following an analysis of these results and a study of the mode of fracture.

Fracture toughness and fracture of WC-Co composites

Simple expressions were derived to predict the elastic properties of two-phase systems containing discontinuous reinforcements, on the basis of approximation of composite microstructure to a unit cell incorporating isostrain and isostress type elements, arranged in two different ways. The bounds on elastic modulus obtained in this manner have been shown to accurately describe the variation of elastic modulus as a function of volume fraction of one of the phases, for a wide variety of two-phase composites. The present expressions offer predictions of elastic modulus that are much closer to experimental data than the commonly used Hashin and Shtrikman bounds, particularly for composites with constituents having large differences in elastic moduli. Similarly, it has been shown that the shear moduli and the Poisson's ratios of composites as a function of second-phase volume fraction can also be predicted. the present unit-cell-based method of calculation presents a promising approach for the prediction of properties of multiphase materials. especially for those consisting of more than two dissimilar phases.

Elastic Properties of Two‐Phase Composites

Microstructure-based simulation of thermomechanical behavior of composite materials by object-oriented finite element analysis

Measurements of Young’s modulus, shear modulus, compression modulus, and Poisson’s ratio on WC-(1 ~ 30) wt pct Co alloys were carried out by dynamic resonance method. The effects of volume fraction of the WC phase, the carbon content, and the WC particle-size on the elastic moduli were investigated in some detail. The result shows that the various elastic moduli of the alloys depend solely on volume fractions of the constituent phases and they invariably fall within Hashin and Shtrikman’s bounds. Moreover, the moduli are found to be approximated by Paul’s “strength of materials” type formulas. It is concluded that elastic behavior of the alloys can be essentially predicted on the basis of a simplified geometrical model of WC particles embedded in continuous cobalt matrix.

A systematic investigation of elastic moduli of Wc-Co alloys

Strengthening phenomena of hot-isostatic alloy pressed WC-10%Co alloy (H.I.P. alloy) were investigated mainly by means of direct observation of the defect structures (appearing in the so-called white spot on fracture-surface), which are thought to have acted as the origin of fracture, and by comparing with the results of normally vacuum-sintered alloy (N.S. alloy). Results obtained were as follows:(1) Slight changes in the mean grain size of WC, the composition and distribution of the binder phase, etc., were observed in H.I.P. alloy. But these changes were considered to have no relation to the strength-increase in H.I.P. alloy.(2) It was made clear that the strengthening mechanism of H.I.P. alloy was directly related to the disappearance of a small number of large pores (which usually exist in N.S. alloy). However, it was shown here that a sharp increase in strength was not always visualized due to H.I.P., because it was usual that some large WC grains were contained in the structure.(3) The strength level which would be obtained in H.I.P. alloy was suggested to be comparable to the level obtainable in N.S. alloy.

Strengthening Phenomena of WC-Co Cemented Carbide Prepared by Hot Isostatic Pressing

As previously reported by the present authors, studies on the strengthening mechanism of hot-isostatically pressed cemented carbides (H.I.P. alloy) had been carried out. In this paper, some properties such as specific gravity, lattice parameter of the binder phase (γ phase), micropores developed, and those Properties affected by vacuumannealing or resintering were investigated in H.I.P. alloy, the results being compared with those of normally-sintered cemented carbides (N.S. alloy).Results obtained were as follows:1) Specific gravity and lattice parameter in H.I.P. alloy were generally observed to be a little lower and higher, respectively, than those of N.S. alloy, and these properties easily changed towards the values of N. S. alloy by annealing or resintering. 2) Micropores having the diameter less than about 1.5 μφ were observed in H.I.P. alloy, and the numbers were increased by annealing. The transverse-rupture strength, however, was not affected by this increasing numbers. 3)The above mentioned properties of H.I.P. alloy could qualitatively be explained as being caused by the argon gas dissolved in the γ phase of H.I.P. alloy.

Some Properties of WC-Co Cemented Carbides Prepared by Hot-Isostatic Pressing

Plastic Deformation and Fracture in Tension of WC-Co Alloys

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Kazuo Miyake

Papers

A systematic investigation of elastic moduli of Wc-Co alloys

Strengthening Phenomena of WC-Co Cemented Carbide Prepared by Hot Isostatic Pressing

Some Properties of WC-Co Cemented Carbides Prepared by Hot-Isostatic Pressing

Plastic Deformation and Fracture in Tension of WC-Co Alloys

Erratum to: A systematic investigation of elastic moduli of WC-Co alloys