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International audience; A model is developed to compute the capillary pressure for the migration of the liquid phase out or into a uniform solid-liquid-vapor system. The capillary pressure is defined as the reduction of the overall interface energy per volume increment of the transferred fluid phase. The model takes into account the particle size of the solid particle aggregate, the packing configuration (coordination number, porosity), the volume fractions of the different phases, and the values of the interface energies in the system. The model is used for analyzing the stability of the composition profile during processing of W-Cu functionally graded materials combining a composition gradient with a particle size gradient. The migration pressure is computed with the model in two stages: (1) just after the melting of copper, i.e., when sintering and shape accommodation of the W particle aggregate can still be neglected and (2) at high temperature, when the system is close to full density with equilibrium particle shape. The model predicts well the different stages of liquid-phase migration observed experimentally.

Mg-C composites offer a suitable alternative to Al alloys while allowing for a significant weight reduction, but their production can be impaired by the poor wettability of C substrates by Mg. In this study, a new ‘‘liquid’’ processing route has been investigated. By making use of the well-known effect of a pre-treatment of the C fibers with an aqueous solution of K2ZrF6 in favoring spontaneous wetting of C with Al, C yarns have been pre-impregnated with Al and the feasibility of further using them as reinforcements in Mg matrix composites has been assessed. More particularly, it has thus been shown that the, under the thermal conditions involved in the process, C fibers did not suffer damage due to chemical reaction with Al, and also that special care should be taken to control the surface condition of the pre-infiltrated yarns.

The mechanical stability of dispersed retained austenite, i.e., the resistance of this austenite to mechanically induced martensitic transformation, was characterized at room temperature on two steels which differed by their silicon content. The steels had been heat treated in such a way that each specimen presented the same initial volume fraction of austenite and the same austenite grain size. Nevertheless, depending on the specimen, the retained austenite contained different amounts of carbon and was surrounded by different phases. Measurements of the variation of the volume fraction of untransformed austenite as a function of uniaxial plastic strain revealed that, besides the carbon content of retained austenite, the strength of the other phases surrounding austenite grains also influences the austenite resistance to martensitic transformation. The presence of thermal martensite together with the silicon solid-solution strengthening of the intercritical ferrite matrix can “shield” austenite from the externally applied load. As a consequence, the increase of the mechanical stability of retained austenite is not solely related to the decrease of the M s temperature induced by carbon enrichment.

In the first part of the paper, the use of circumferentially cracked round bars (CRB geometry) for characterizing fracture toughness of a ductile material, namely copper, is assessed experimentally through a comparison with the single edge notched bend (SENB) geometry. The J(R) curve method with multiple-specimens was applied, but, as unstable cracking appeared very early in the CRB specimen, an engineering definition of fracture toughness was not pertinent. Unloaded specimens were analyzed metallographically to determine the CTOD at physical cracking initiation. The fracture toughness measured using the CRB geometry was 50% larger than using the SENB geometry. The second part of the paper aims at justifying this difference of fracture toughness at cracking initiation. Finite element simulations revealed a slightly higher constraint in the SENB specimens. The main difference between the two specimen geometries lies in a 50% larger extension of the finite strain zone with respect to the CTOD in the case of the SENB specimens. Based on the observation that, in the studied material, the critical CTOD is one order of magnitude larger than the void spacing, we conclude that the geometry dependence of the fracture toughness is caused by the difference in the finite strain zone extension rather than by a stress triaxiality effect.

Dual-phase and transformation-induced plasticity (TRIP)-assisted multiphase steels are related families of high-strength formable steels exhibiting excellent mechanical characteristics. This study shows how a ferrite-bainite-martensite microstructure containing retained austenite can improve the mechanical properties of a cold-rolled low-carbon, low-silicon steel. Such a multiphased microstructure is obtained by a heat treatment involving intercritical annealing followed by a bainite transformation tempering. Depending on the heat-treatment parameters, the samples present a variety of microstructures. Due to the presence of retained austenite, some samples exhibit a TRIP effect not anticipated with such a low silicon content. A composite strengthening effect also results from the simultaneous presence of a ductile ferrite matrix with bainite and martensite as hard second phases. A true stress at maximum load of 800 MPa and a true uniform strain of 0.18 can be obtained by forming a ferrite-bainite-martensite microstructure containing up to 10 pct of retained austenite. These properties correspond to a favorable evolution of work hardening during plastic deformation.

This work investigates void growth in cold-drawn copper bars containing a fine dispersion of small inclusions at which voids nucleate. Using the Rice and Tracey (RT), the Gurson-Tvergaard (GT), and the Gurson-Leblond-Perrin (GLP) void growth models, a procedure is proposed for deriving the porosity distribution from density measurements on specimens sectioned from the neck of a tensile bar. This procedure allows identification of the parameters of the models. The effect of strain hardening on porosity evolution is analyzed by comparing the behavior of the material in the cold-drawn state (n approximate to 0.1) and in the recrystallized state (n approximate to 0.4). Inclusion dimensions and distributions were found to be identical in these two states. The parameter ct of the RT model is found to depend on n, whereas the parameter q of the Gurson-type models does not vary with n. Numerical modeling of porosity variations in notched, round copper bars shows that both the parameter cu and the parameter q in the GT model depend on the stress triaxiality in the recrystallized material, whereas the parameter q remains a constant in the GLP model. Accounting for the ellipsoidal void shapes and for the presence of the inclusion significantly affects the prediction of porosity variations.

A comprehensive investigation is made of the parameters affecting the extent of interface reactions during squeeze casting of composites consisting of a matrix of either pure Al or alloy AS13 reinforced with fibers of INCONEL 601. The process parameters are the preform thickness and temperature, the fiber volume fraction, the temperature and mass of the liquid metal, and the temperature of the die. Adjustment of these process parameters made possible the full control of reactions. It is found that reactions proceed mainly in the solid state after decomposition of the oxide barrier layer covering the fibers. A simple kinetic model is developed that enlightens the role of this barrier layer. No trace of reaction could be detected in composites processed using preoxidized preforms. Alloying Al with Si also induces a drastic reduction of reactivity. The high ductility of the composites attests to the processing quality. An original procedure is proposed for measuring the activation energy for initiation of reactions by differential thermal analysis.

A glass allowing the wetting of titanium has previously been developed. Two surface treatments promoting glass adhesion on titanium are compared: preoxidation and phosphatation combined with preoxidation. After preoxidation at 700 degrees C, a 1 mu m-thick TiO2 layer covers titanium. The phosphated titanium substrates are oxidized at 600 degrees C to obtain a 20 mu m-thick oxifluoride layer. After firing, glass adhesion is obtained with both surface treatments, but a "critical time of firing" appears with preoxidized titanium. WDS analysis suggests that diffusion of TiO2 into the glass is responsible for adhesion on preoxidized titanium, while a complex oxifluoride layer allows redox phenomena in the case of phosphated titanium. Pull-off tests have measured a maximum strength from 1.5 to 3 MPa whatever the surface treatment. Measurement of transverse crack densities in the vitreous coatings gives a higher value for phosphated than for preoxidized titanium. This confirms that better adhesion is obtained after phosphatation treatment. (C) 1998 Kluwer Academic Publishers.

This work investigates the formability of a composite made of a pure aluminium matrix reinforced with an Inconel fibre network. Composites were formed into wires by swaging, to a true strain ɛtrue = 2.7. Thermal expansion, Young's modulus and tensile properties of these wires were measured. Young's modulus of the wires is slightly larger than that of the as-cast composites, indicating that no significant porosity is present in the composite after deformation. Tensile strength and ductility are found to decrease with increasing reduction of diameter. Scanning electron microscope (SEM) observations show that fibres align during swaging and that fibre fragmentation occurs beyond a certain strain. Fibre fragments reach a critical aspect ratio of about 7 after a swaging true strain of 1.5. Qualitatively, the evolution of the properties is explained by a combination of fibre fragmentation and fibre alignment effects. Properties of the wires are discussed using the Clyne model and the Nardone and Prewo model (modified shear lag models).

The precision of the measurement of the essential work of fracture requires an accurate determination of the critical ligament length below which the ligament is in a mixed mode stress state and the failure mechanisms considerably change. A statistical procedure is proposed which allows to determine easily and accurately this critical ligament length. After rejection of the specimens having too small ligament lengths, we is obtained by extrapolation of the remaining data for zero ligament length. The number of data points from which we is calculated is also shown to strongly influence the precision of the measurement. The procedure is applied to the measurement of the toughness of sheets of Al and Zn alloys and of a low density polyethylene.

The Essential Work of Fracture (EWF) concept is used to characterize the fracture of thin plates of a zinc alloy. The consistency and applicability of the EWF approach are discussed. The successive stages of the fracture process were studied: the evolution of the shape of the crack tip was observed with a scanning electron microscope, the shape of the plastic zone and the evolution of necking in the ligament ahead of the crack tip were measured using a laser profilometer and the onset of cracking in the ligament was detected by means of a TV camera coupled with acoustic emission recording. The influence of both test parameters and material parameters on the Essential Work of Fracture (We) and on the CTOD (δc) are elucidated. The effects of specimen geometry, deformation rate, texture and grain size are especially investigated. The main advantages and drawbacks of the EWF method are highlighted.

The aim of this work is to explore experimentally the validity of the concepts of J-integral and crack tip opening displacement for characterizing the stress and strain state at the tip of an axisymmetrical crack in a bar undergoing large plastic strain before crack extension. The tests are made on extruded copper round bars presenting a very high ductility. Three different analytical formulations of the J-integral proposed in the literature for circumferentially cracked bars are compared at initiation of cracking. The limit between shallow crack and deep crack geometries is experimentally demonstrated. It is found that, in neither of these geometries, J and delta CTOD are dominant. However, the ratio J(c)/delta(c) is constant for deep cracks, which suggests an alternative fracture criterion consisting in postulating the dissipation of an average critical energy per unit volume until crack extension.

Double-edge notched (DENT) steel plates were pulled until complete fracture and several experimental observations were made (using profilometry and scanning electron microscopy). The essential work of fracture (EWF) model was found to be well verified. Numerical simulations - up to the maximum load only - of some experiments were performed using the finite element method (FEM), and incorporating geometric and material non-linearities (large deformation elasto-plasticity). Some experimental measurements were compared with the corresponding numerical computations and excellent agreement was found.

The steady-state creep behavior of directionally solidified eutectic alloy Ni-30Mo-6Al-1.6V-1.2Re (wt pet) was investigated at temperatures between 1223 and 1323 K using constant strain rate tension creep tests. The steady-state stress is found to depend strongly on creep rate and temperature. The apparent power law stress exponent for steady-state stress is n = 7.5 +/- 0.3, and the apparent activation energy for creep of the eutectic gamma/gamma'-alpha composite is determined to be Q = 517 +/- 11 kJ mol(-1). When the steady-state creep is analyzed in terms of the effective stress and normalized with respect to the temperature dependence of the elastic modulus, the corrected activation energy for creep Q(c) is calculated to be between 412 and 424 kJ mol(-1) and the stress exponent between 5.7 and 6.0. The kinetics of the steady-state creep deformation within the studied temperature range involves the contribution of both the fibers and the matrix which creep during steady-state. Analysis of the fracture surfaces of the composite shows ductile fracture mode. The composite fails by growth and coalescence of microvoids in the matrix and by fiber fragmentation.

The kinetics of the thermal decomposition of stoichiometric hydroxylapatite (HA) has been studied up to 1500°C for the purpose of determining the maximum admissible combinations of temperature and time for sintering HA. The influence of the sintering temperature on shrinkage, density and grain growth is then investigated in the temperature range from 1000 to 1450°C. Nearly theoretical density was achieved above 1300°C. A maximum fracture toughness is obtained for the samples sintered at 1300°C whereas hardness increases up to a sintering temperature of 1400°C. These results are discussed in terms of the roles of porosity and grain size.

In a study of the thermal expansion behaviour of a variety of metal matrix composites, it appeared to the authors that a wealth of information could be extracted from thermal expansion curves. In the present work they aim at showing how a fairly straightforward analysis of these curves can provide quantitative data on the internal stresses and on the temperature dependence of the yield stress of the matrix. The method is illustrated on the basis of the thermal expansion curves of a composite consisting of an Al-3% Mg matrix reinforced by 30 vol.% continuous SiC fibres (Nicalon).

This review aims at making a bridge between the fundamentals of the wetting of solids by liquid metals and the practice of the preparation of metal-matrix composites. One recalls first the significance of concepts such as surface tension, work of adhesion, adsorption and the relation between these concepts, the phenomenon of wetting and the process of liquid metal infiltration. Thereafter, the wetting of various types of solids is considered: metals, oxides, carbon and carbides. !n the !light of this body of science, one proposes finally a critical evaluation of the literature concerning the preparation of metal-matrix composites by liquid metal infiltration techniques. Particular emphasis is devoted to reinforcements made of graphite, alumina or silicon carbide multifilament fibres; the use of coatings and the addition of alloying elements to the metal are successively discussed.

Samples of hematite and of hematite doped with 1 at% lithium and 1 at% vanadium are prepared by the hot kerosene drying technique. The doped samples are also prepared by an impregnation method. The various samples are used to study the reduction with hydrogen to form iron. The results indicate that the rates of reduction are strongly influenced by the particle size of the initial samples and by the repartition of the additive in the bulk and surface layer of the hematite particles. It has been shown that both additives can retard the rate of reduction. The lithium exerts its maximum influence when present in solid solution and vanadium when concentrated in the surface layer. The importance of a careful preparation of samples to be used for reduction studies, is stressed.

Precursors of unsupported NiMo and FeMo sulfide hydrodesulfurization catalysts with concentration ratiosr=Ni(Fe)/(Ni(Fe) + Mo) ranging from 0.1 to 0.3 were prepared by three methods: homogeneous sulfide precipitation (HSP), inverse HSP and coprecipitation. Differential thermal analysis was used to study the decomposition under argon, and the reduction/sulfidation under 15% H2S—H2 of the precursors and the subsequent oxidation under air of the samples obtained after these reactions. The reactivity of the solids varies as a function of the preparation method, the nature of the promoter and the concentration ratio. The degree of sulfidation of the precursor and the presence of either NH4NO3 or NH2Cl formed from group VIII metal salts and (NH4)2S may affect the thermal behaviour of samples during DTA.

The results of measurements of the transport properties of high‐Tc superconductors containing different amounts of silver are reported. The Tc and Jc values were found to be similar to that of the single phase YBaCuO compound whereas the electrical and thermal conductivities above Tc were drastically increased. A very low contact resistance was observed.