Daniele Mari

TiMoCN based cermets: Part II. Microstructure and room temperature mechanical properties

Abstract Cermets obtained by sintering powders of TiCN and Mo 2 C with cobalt or nickel binder are studied in this paper. Different TiCN/Mo 2 C ratios are used in order to vary the grain size. Morphology parameters such as grain size, contiguity, and phase ratios are related with fracture toughness and hardness. The hardness can be adequately modelled considering the hardness of each phase weighed by the respective volume fraction. The toughness is mainly controlled by the properties of the binder, in particular the binder flow stress and the size of binder regions.

High temperature mechanical behaviour of Ti(C,N)-Mo-Co cermets

Abstract The high temperature behaviour and the microstructure of Ti(C,N)-Mo 2 C-Co cermets were investigated in order to describe the deformation mechanisms that can appear during their use in cutting tool applications. Different grades were considered in order to take into account the effects of the molybdenum and of the cobalt content. The results of three point bend tests, consisting in constant strain rate tests and creep tests performed between 1173 and 1553 K, suggest a description of the mechanical behaviour in three temperature domains: elastic and brittle from room temperature to about 1150 K, tough with limited plasticity between 1150 and 1350 K and creep above 1350 K. Internal friction (IF) measurements were performed at high temperature using forced oscillations. Five thermally activated relaxation peaks and one thermally activated high temperature background were observed. Each peak is related to the motion of a structural defect (dislocation, grain boundary) in one of the two cermet phases. The results obtained from the IF measurements and from the three point bend tests were confirmed by transmission and scanning electron microscope observations, as well as by in situ electron microscope observations performed at high temperature.

TiMoCN based cermets: Part I. Morphology and phase composition

Abstract Cermets obtained by sintering powders of TiCN and Mo 2 C with cobalt or nickel binder are studied in this paper. Different TiCN/Mo 2 C ratios are used in order to vary the grain size. A characteristic core–rim structure of the ceramic phase is formed by precipitation of TiMoCN cubic carbide around undissolved TiCN cores. The morphological parameters of the materials such as grain size and the respective volume of rims and cores are determined by image analysis. The ceramic phase and the binder composition are measured by analytical scanning and transmission microscopy. The chemical composition of the phases, obtained after sintering, is modelled by a second order interface reaction controlled by molybdenum precipitation.

Experimental strategy to study the mechanical behaviour of hardmetals for cutting tools

Abstract Two main families of hardmetals for cutting tool applications can be defined according to the ceramic matrix: WC and TiMoCN based hardmetals with either cobalt or nickel binder. In this paper, we illustrate an original approach to study this kind of materials combining different techniques, macroscopic and microscopic. The morphology of hardmetals can be well described by a structure made of two interpenetrated skeletons: the ceramic and the metal ones. This morphology makes it easy to remove the metal by chemical etching. The comparison between the properties of the ceramic skeleton and those of the whole material allow to locate the mechanisms responsible for a given mechanical behaviour. The macroscopic mechanical behaviour is studied by bending tests made as a function of temperature and composition. Generally, three distinguished domains can be defined while increasing the temperature: brittle, limited plasticity and plasticity. A transition from bulk deformation in the limited plasticity region to grain boundary sliding in the plasticity region is observed in cobalt base hardmetals. Only bulk deformation of TiMoCN and nickel is observed in nickel base hardmetals. The kinetics of the high temperature deformation is studied by temperature jumps leading to better stability of the microstructure compared to traditional creep tests. However, apparent activation energies are often obtained due to changes in the internal composition of the binder. A finer tool to study the dynamics of defects responsible for deformation and their kinetics is the mechanical spectroscopy. The analysis of the thermal activation of damping peaks can be made isothermally as a function of frequency which means without structural changes: this method provides true activation energies with respect to deformation tests. Mechanical tests are supported by electron microscopy imaging.

TiMoCN-based cermets: high-temperature deformation

Abstract The high-temperature behaviour of Ti(C,N)–Mo 2 C–Co cermets is investigated by three point bend tests in order to describe the deformation mechanisms that appear during their use in cutting tool applications. The results of constant strain rate tests and activation energy measurements by creep tests suggest a description of the mechanical behaviour in three temperature domains: elastic and brittle from room temperature to about 1150 K, limited plasticity between 1150 and 1350 K, and creep above 1350 K. Two mechanisms control the cermet deformation at high temperature: grain deformation (in the second domain) and grain boundary sliding (in the third domain).

Study of the mechanical properties of TiCN-WC-CO hardmetals by the interpretation of internal friction spectra

Abstract TiCN–WC–Mo–Co mixed carbide hardmetals have an interesting application potential for cutting tool fabrication combining the high toughness of WC–Co with the resistance to plastic deformation of TiCN–Co cermets. Mechanical spectroscopy (MS) is used in order to separate the effects of the constituents on the mechanical properties. Internal friction (IF) spectra are measured in a torsion pendulum on WC–TiCN–Mo–Co samples where TiCN/WC ratio is varied as well as the Co content. Six components of the characteristic IF spectrum of WC–TiCN–Mo–Co have been identified and interpreted. Two peaks are located in the cobalt, two peaks in the TiCN phase and two peaks in the ceramic grain boundaries. Four temperature domains are defined depending on the mechanical behaviour: brittle (I), anelastic (IIa), limited plasticity (IIb) and extended plasticity (III). The anelastic domain is characterized by the bulk deformation of cobalt. In the limited plasticity domain, both cobalt and TiCN are deformed by dislocation movement. The high temperature extended plasticity should be attributed to grain boundary sliding in the ceramic phase (mainly WC) enhanced by cobalt diffusion in the grain boundaries.

Internal friction in a martensitic high-carbon steel

Abstract This paper presents a study on defect mobility in a high-carbon martensitic tool steel. Mechanical spectroscopy investigations are performed in the temperature range 80–700 K. Torsion pendulum (about 1 Hz) and flexural vibrating-reed (about 3 kHz) techniques are used and compared in order to obtain thermal activation parameters. Differential scanning calorimetry (DSC) measurements are also performed in the same temperature range in order to detect structure transformations. The internal-friction spectra obtained as a function of temperature show four peaks P1, P2, P3 and P4. The physical mechanisms responsible for the peak formation are analysed on the basis of the peak activation energies, the influence of cold work and tempering on the internal-friction spectra and the comparison with DSC. The conclusions are supported by present information available on similar materials. It is found that P1, P2 and P4 are all relaxation peaks and are related to dislocation movement in the martensite and to their interaction with solute atoms. P3 should be considered as a maximum in the internal-friction spectrum related to a transformation of the material, probably leading to carbide-forming reactions. This transformation coincides with a hardness decrease, which marks the first step of steel tempering.

Investigation into the potential of a composite combining toughness and plastic deformation resistance

AbstractThe hard materials considered in this work cover the range from the Ti(C,N)–Mo 2 C–Ni model cermet to the WC–Co modelhardmetal. Special emphasis is given to the intermediate systems Ti(C,N)–Mo 2 C–Co and the hybrid Ti(C,N)–WC–Mo 2 C–Co withdi•erent Ti(C,N)/WC ratios. This paper illustrates how the sintered microstructure and the mechanical behavior of these grades arerelated to their initial composition. Image analysis based on SEM observations is used to measure morphologic and structuralcharacteristics. Some of the hybrid materials exhibit a very fine structure and a high contiguity, giving rise to high hardness andexcellent high temperature mechanical behavior. O 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Hardmetal; Cermet; Microstructure; Mechanical properties; Image analysis 1. IntroductionThis paper presents data from an ongoing project,which is a collaboration between Stellram S.A. (a unit ofAllegheny-Teledyne Company) and the Swiss FederalInstitute of Technology of Lausanne (EPFL, Switzer-land). The aim of this research is to correlate the sin-tered microstructure and the high temperaturemechanical behavior with the initial composition ofseveral cemented carbides or carbonitrides intended forcutting tools applications. A complete description of theexperimental strategy is presented by Mari et al. in thisissue [1].The WC–Co material is known to be tough, but itdeforms plastically at high temperature. This constitutesa limitation to the use of this material as a cutting tool.In high speed machining, the temperature of the edgegets high enough to induce plastic deformation of thetool nose, and the cutting quality is rapidly lost. TheTi(C,N)–Mo

The Mechanical Behaviour of Cemented Carbides at High Temperatures

Abstract Specimens of WC-11.wt%Co were tested under three-point bending conditions at temperatures ranging between 20 and 1000°C to investigate the effect of chromium and ruthenium additions on the mechanical behaviour of cemented carbides. Below 800 °C, a linear elastic brittle behaviour is observed. Above 800 °C, creep exponents of about n = 2 and activation energies between 3 and 5 eV were measured. Pore formation and growth were observed during creep. These results agree well with models based on creep by grain boundary sliding and pore formation. Displacement-controlled bend tests of chevron-notched specimens were performed to introduce stable crack growth. Above 800 °C, creep crack growth occurs owing to pore formation and growth. The pore distribution around the crack tip has been observed using a scanning acoustic microscope. It is shown that the equivalent strain distribution around a creep crack according to the models of Riedel agrees well qualitatively with the experimentally determined pore density. The transition between the linear elastic and viscoplastic behaviour (i.e. creep) is accompanied by a Portevin-Le Chatelier type of effect. If creep occurs in a tool made from this material, the edge blunts and is no longer useful for cutting. A characteristic time t ft is introduced describing the transition between the brittle and creep behaviour. It is shown that cemented carbides with a ruthenium addition have longer transition times than those without. Therefore, cemented carbides with ruthenium are advantageous as cutting tool materials since they can be used for longer times at a high temperature.

Fracture toughness of coated TiCN–WC–Co cermets with graded composition

Abstract Mixed TiCN–WC–Co cermets are developed to improve at the same time toughness and resistance to deformation of materials for cutting tool applications. Moreover, graded materials joining optimum properties according to the functional part of the tool are elaborated. To this end, TiCN–WC–Co cermets are interesting because they develop a WC–Co layer at the surface during the sintering. This tough layer at the surface limits the crack propagation that can lead to the rupture of the tool. Such materials show a good resistance to the deformation in the bulk and a good toughness at the surface, where the cracks are initiated upon machining. Cutting tools are often coated by CVD to improve the wear resistance. This paper proposes a method to measure the toughness KIC at high temperature by using this CVD coating for initial crack formation. The coating thickness is the precrack length of traditional KIC measurements. Samples are fractured by three point bend tests. The rupture stress is measured by Weibull statistics. This method is particularly interesting for graded structure materials where the influence of surface layers on toughness must be estimated. The comparison between cermets with and without WC–Co layer shows an improvement of 28% of the toughness when the layer is present. The possible bias of internal stresses on the results is discussed.