H. E. Exner

The relationship between electrochemical behaviour and in-service corrosion of WC based cemented carbides

Abstract In long-life applications, the corrosion properties of cemented carbides can have a large influence on overall performance. Cemented carbides with improved corrosion resistance have been developed and are now commercially available. The understanding of the corrosion behaviour has been mostly empirical and satisfying explanations of the relationship between the electromechanical behaviour and in-service corrosion have been lacking. In this paper, the electromechanical behaviour of WC-Co is modelled using the behaviour of pure WC and Co(W,C) alloys, according to a linear rule of mixtures. By comparing WC-Co with WC-Ni(Cr,Mo) in both normal sulphuric acid and a synthetic mine water, it is shown that the behaviour of the two grades is inherently different. WC-Co exhibits a ‘pseudo-passivity’ during electro-mechanical tests but corrodes actively in industrial applications. In contrast, WC-Ni( Cr,Mo) passivates and the rate of corrosion can be several orders of magnitude lower than that of WC-Co.

Electrochemical behaviour of tungsten-carbide hardmetals

Accurate measurements of the polarisation curves of tungsten carbide-cobalt composites as well as those of the individual components (tungsten carbide, pure cobalt and a cobalt-tungsten-carbon alloy representing the composition of the binder phase of the liquid-phase sintered composites) in normal sulphuric acid were carried out. The linear rule of mixtures first proposed by Stern for heterogeneous alloys was re-examined. The corrosion behaviour of the hardmetal composites cannot be predicted from that of tungsten carbide and pure cobalt. Two models were tested, one in which the carbide crystals are separated from the binder alloy by a thin layer of cobalt low in both carbon and tungsten, and one where this layer is not considered. The second model agrees very well with the experimental results while the first one shows significant deviations. Calculations of the relative removal rates show that the binder corrodes faster than the carbide and is leached out in spite of exhibiting a pseudopassive behaviour owing to the formation of a porous corrosion layer. The influence of the carbide grain size and of the cobalt content on the corrosion kinetics is shown to be too small as to be of any practical importance.

An extended numerical procedure for predicting microstructure and microsegregation of multicomponent alloys

A numerical model for the prediction of microstructure and microsegregation in multicomponent alloys during dendritic solidification using a finite difference scheme is presented. The main kinetic and thermodynamic effects that can influence microsegregation (solid state back diffusion, secondary dendrite arm coarsening, primary tip and eutectic undercooling and the thermodynamic correction of the interface concentrations) are accounted for. The liquid/solid phase equilibria in the thermodynamically stable and metastable range are calculated with thermodynamically formulated phase diagrams. For the calculation at high cooling rates non-equilibrium phase diagrams for multi-component alloys are assessed. The consideration of undercooling effects extends the applicability of the model to very low and very high cooling rates. The model thus covers the whole range of cooling conditions where dendritic solidification occurs. As compared with other models in the literature, the number of adjustable parameters is reduced to a minimum.

Simulation of Solutal Remelting

Starting with an interface between a solid and a melt with compositions far off equilibrium, calorimetric experiments were conducted in order to determine melting rates due to solutal supersaturation. Good agreement was found for melting rates calculated from a heat balance and from the volume loss of the solid sample. However, simulation calculations using macroscopic and microscopic models assuming local equilibrium at the interface predict melting rates that are significantly lower than those determined experimentally. After introducing a quantitative description of deviations from equilibrium, the simulations predict melting rates that agree well with the ones determined by the heat balance and volume loss methods.

Korrosionsverhalten anorganischer Materialien in nah- und überkritischen wäßrigen Lösungen

The synthesis potential of Supercritical Water Chemistry has been limited owing to corrosion problems. Until now there is no universal material for reactors or catalysts available, which is stable under the applied conditions (300 to 500°C, 25 to 50 MPa). The subject of this paper is to study the corrosion behaviour of some potential oxidation catalysts such as copper, silver, nickel, palladium ruthenium and cobaltoxide in subcritical and supercritical aqueous solutions (H 2 O/oxygen and H 2 O/acetic acid) as a function of temperature and pressure. A continuously operating high-pressure testing-plant with a 250 ml autoclave of Inconel 625 was used for the investigations. The samples were analysed before and after treatment with SCW-solutions by using different methods (gravimetric analysis, light microscopy, scanning electron microscopy, energy-dispersive of X-rays spectroscopy and X-ray diffractometry). The effluent of the test reactor was analysed using atomic absorption spectroscopy. The investigations have shown that all materials except of nickel are stable in pure subcritical and supercritical water. Treatment in solutions containing acetic acid or oxygen causes corrosion of all materials.

Prediction of the strength distribution for unidirectional fibre reinforced composites

Abstract A statistical model is proposed which predicts the strength distribution of composites. It accounts for the following parameters: fibre strength distribution, fibre spatial distribution, fibre-matrix interface properties, matrix plasticity, load transfer at broken fibres and load profile of the specimen (bending or tension). The model is solved via Monte Carlo simulation for small specimens and via an analytical approach for large parts. Both techniques predict well the strength distribution of the composite. Aluminium matrix composites reinforced with unidirectional ceramic fibres show a large scatter in three point bending strength. This scatter is caused by the probability that a cluster of broken fibres of a critical size occurs at a given stress level. When the limiting stress is reached the critical cluster grows in an unstable way leading to catastrophic failure of the specimen. The results obtained by this model agree favourably with the experimental findings and indicate that strength prediction using other concepts, like fracture mechanics, global load sharing, a simple rule of mixtures or Weibull statistics, is unreliable for predicting the strength of large composites.

Cooling schedule effects on the microsegregation in AlMgSi alloys

Abstract By means of a numerical simulation technique the extent of microsegregation during dendritic solidification for given cooling conditions can be predicted. A model including dendrite arm coarsening and solid state back diffusion has been developed in which a thermodynamic formulation of the ternary phase diagram is used. Following up the results for binary alloys, the influence of the shape of the cooling curve on the extent of microsegregation and the solidification path is investigated for aluminium alloys containing 0.8 to 1.2 wt.% Mg and 2.8 to 3.2 wt.% Si. The varying amounts of binary and ternary eutectic phases, dendrite arm spacings and the exact position of the solidification path on the liquidus surface are calculated for solidification times ranging from 0.1 to 10 000 seconds with three types of cooling curves (slow/fast, linear, fast/slow). Extreme changes of the cooling curve shape for a given solidification time affect the microstructural parameters as much as changes in the solidification time by several orders of magnitude for a given cooling curve shape. Therefore, predictions of microsegregation and microstructure from solidification time are valid only for a specified shape of the cooling curve.

A critical assessment of porosity coarsening during solid state sintering

Understanding porosity coarsening is essential for controlling the properties of sintered materials and for producing high density or near net shape parts. As porosity is interconnected up to relative densities in excess of 80% or even 95%, measuring, monitoring and modelling of the growth of individual pores is relevant only at very high relative densities. This paper tries to convey two essentials of porosity coarsening: (i) characterization of the geometry of the pore space in the lower range of densities is possible by using stereological parameters, and (ii) coarsening of porosity takes place during all sintering stages by various mechanisms. Porosity coarsening has been reported in metals, ceramics and glasses. Recent and classical approaches, experimental results with two dimensional model arrangements as well as computer simulations of formation and opening of particle contacts are discussed. The effect of porosity dimensions on the properties of sintered materials and measures for preventing and provoking porosity coarsening are briefly addressed.

Fractography of critical and subcritical cracks in hard materials

Abstract In testing hard and brittle materials like hard metals and ceramics for fracture resistance, stable and unstable crack propagation can be observed. The question arises if critical and subcritical cracks proceed in the same way, and, as a corollary, if results from subcritical fracture can be used to describe catastrophic failure. Results from earlier work are re-evaluated to demonstrate the equivalence or difference of stable and unstable crack growth in hard materials. It is shown and statistically assured by χ 2 tests for the length distributions of the fracture facets that fracture at room temperature produces identical surface geometry in tungsten carbide–10 wt% cobalt alloys as well as in an alumina–3 wt% SiO 2 ceramic while significantly different fracture surfaces are obtained for the ceramic at high-temperature testing. Some general features of fracture mechanisms are discussed. Assessment of fracture surfaces (measurement of distributions of the lengths and the angles of inclination of profiles along the fracture surface) was carried out using an instrumented stereometer. Recent progress in quantitative analysis of rough surfaces is reviewed which appears to be very useful for a large number of applications in the field of hard materials.

Effects of Microstructure on Wear of Aluminium Bearing Alloys with Systematic Variation of Hard and Soft Phases

By systematic variation of the amount and size of silicon and tin inclusions, two types of aluminium alloys are modified to display the effects of hard and soft phases on wear at low and high rates, using pin-on-disc testing devices. The predominant wear mechanism when sliding against steel is delamination. Three types of wear curves were found for the different alloys: constant rate, continuously dropping rate, and a transition from significant to zero wear. Enrichment of silicon in the pin surface and smoothening of the roughness of the disc are the major factors for decreasing wear rates. A sufficiently large size of the silicon particles is essential for preventing their break away. If the particle size in the cast alloy is below the critical size, this can be adjusted by heat treatment. The amount and size of the soft particles is found to affect seizure but to be less critical in mild wear. Therefore, synergistic effects become relevant when the wear mechanism is changing.