Search

According to the principles of diffusional creep, the normal and tangent components of the velocity jumps between adjacent grains arise from, respectively, the climbing and sliding of disconnections along grain boundaries. Stationary deformation thus implies a balance between nucleation and recovery of moving disconnections. The model considers a periodic lattice of hexagonal grains with nucleation of disconnection multipoles at triple junctions. The strain energy coupled to the population of climbing disconnections is calculated by inferring that the internal strain field associated to disconnection pile-ups brings a distribution of tractions along GBs that is consistent with the field of diffusion potential gradient that drives disconnection climb. It follows that the distribution of the density of climbing disconnections is parabolic and that the dissipation due to the nucleation and recovery of climbing disconnections is equal to 50% of the dissipation arising from diffusion fluxes. These results hold for both Nabarro-Herring creep and Coble creep. The analysis of the disconnection nucleation process highlights the sources of non-Newtonian behaviour and the existence of a threshold stress as an intrinsic feature of diffusional creep.

The model of diffusional deformation is revisited by accounting for the dependence of the diffusion potential on grain boundary curvature. The issue is developed through the analysis of two case-studies: the deformation of a lattice of columnar grains in conditions of Coble creep, and the rotation of a grain embedded in a polycrystal in conditions of either Nabarro-Herring creep or Coble creep. The analysis reveals that, unless grain boundary mobility is infinite, grain boundary curvature is dynamically induced by strain rate. A link is established between the curvature distribution and the transfer of diffusion fluxes across grain boundaries. For the two case-studies, the equation expressing the balance of grain boundary motions at steady-state is solved for calculating, within a range of grain boundary mobilities, the grain boundary profiles, the diffusion fluxes, and the contributions to power dissipation arising from curvature. The latter contributions are found to scale closely as the square of grain size. It follows that the dissipation contribution due to curvature is larger in conditions of Nabarro-Herring creep. In conditions of Coble creep, the dissipation contribution due to curvature translates into a lower bound for the apparent boundary viscosity parameter to be used in numerical simulations. This lower bound is consistent with previous identifications of the parameter in the literature. The classical model assuming flat grain boundaries with transfer of fluxes via triple junctions emerges as a particular case involving the implicit assumption of an infinite grain boundary mobility.

Modeling of bulk sintering viscosity usually neglects the contribution of pore surface diffusion with respect to grain‐boundary diffusion. This approximation is questionable at the high densification rates used today in advanced fast sintering techniques. A two‐dimensional analysis of the problem shows that the influence of surface diffusion on bulk viscosity at high strain rate can be decomposed as the sum of two terms: a term linked to the change in pore surface curvature and a term linked to the change in grain‐boundary size. The computational procedure relies on the partition of pore profile evolution into a transient component accounting for non‐densifying phenomena and an asymptotic component accounting for strain‐rate‐controlled phenomena. The largest impact of surface diffusion is found to arise from the change in grain‐boundary size. It follows a transition from Newtonian viscosity at low strain rate to non‐Newtonian viscosity which, during densification, increases nearly linearly with strain rate. In some conditions, viscosity can then reach more than twice the value estimated when neglecting pore surface diffusion. Reversely, expansion is accompanied by a decrease in grain‐boundary size which causes a decrease in viscosity and can lead to grain separation at high strain rate.

The effect of strain-induced grain shape anisotropy on diffusional creep viscosity is analysed in two dimensions via a model representing grains by cylinders with elliptical cross section. Both cases of dominance of grain boundary diffusion and lattice diffusion are considered. Anisotropic creep viscosity is described by two coefficients calculated by considering different loading configurations with respect to the ellipse axes. Upper and lower bounds on these coefficients are obtained using kinematic and statical variational principles and assuming affine velocity, or uniform stress trial boundary fields, respectively. The analysis emphasises the dependence of the viscosity coefficients on aspect ratio and grain boundary viscosity. The difference between the bounds increases with grain elongation. A method is proposed for deriving estimates for the effective viscosity coefficients by coupling the two bounds. The strain hardening effect is analysed. Lattice diffusion contributes less to viscosity anisotropy than diffusion and sliding at grain boundaries.

The diffusion processes involved in the accommodation of grain boundary sliding in diffusional deformation are revisited by considering the role of grain boundary curvature and of the associated grain boundary migration. A method is developed for calculating grain boundary profiles, diffusional fluxes, and the dissipation power arising from the coupling of diffusion fluxes and grain boundary migration during the quasi-steady-state stage of deformation. In conditions of dominance of Coble creep, the controlling length scale is the square root of the ratio of grain boundary diffusivity to grain boundary mobility. Zero curvature is the particular solution valid when mobility is infinite. The analysis brings the definition of a lower bound for the grain boundary viscosity parameter to be used in numerical simulations of diffusional deformation. This lower bound is consistent with previous identifications of the parameter in literature.

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.

The evolution of an aggregate of particles embedded in a fluid phase, no matter whether a liquid, a vapor, or a mixture of both, is determined by the dependence of the equilibrium interface area on porosity volume fraction. In system with open porosity, this equilibrium can be analyzed using a model representing the particles as a collection of cones of revolution, the number of which is the average particle coordination number. The accuracy of the model has been assessed using in situ X-ray microtomography. The model makes possible the computation of the driving force for sintering, commonly called sintering stress. It allows the mapping of the domains of relative density, coordination number, and dihedral angle that bring about aggregate densification or expansion. The contribution of liquid/vapor interfaces is enlightened, as well as the dependence of the equilibrium fluid phase distribution on particle size. Applied to foams and emulsions, the model provides insight into the relationship between osmotic pressure and coordination. Interface-governed transport mechanisms are considered dominant in the macroscopic viscosity. Both sintering stress and viscosity parameters strongly depend on particle size. The capacity of modeling the simultaneous particle growth is thus essential. The analysis highlights the microstructural parameters and material properties needed for kinetics simulation.

An original procedure is developed for simulating pore surface evolution during sintering at high strain rate while distinguishing two types of diffusion fluxes: transient surface fluxes governed by short-range curvature gradients and coupled fluxes at surface and grain boundary governed by strain rate. The latter fluxes become dominant asymptotically, i.e. after damping-out of transient fluxes. The procedure aims at allowing the prediction of the strain rate dependence of macroscopic viscosity, a concept which is meaningful only during the asymptotic stage. The problem is addressed in two-dimension. It is shown that the asymptotic solution of the general partial differential equation of the problem can be obtained as the solution of an ordinary differential equation, of which the resolution lends itself to a semi-analytical procedure. An estimate is also proposed for the rate of convergence of the general solution towards the asymptotic solution. The accuracy of the mathematical procedure is validated by a comparison of the evolution of asymptotic profiles and exact profiles calculated fully numerically during densification or expansion of the system. A method is proposed for mapping the conditions of existence of an asymptotic stage. The method can account for the dependence of average grain coordination on relative density.

The basic science underneath the phenomenon of thermal expansion in composite materials is presented on the basis of a comprehensive review of literature. Owing to the high yield strength of the matrix, the effective thermal expansion of metal matrix composites depends on an intricate manner on the highly anisotropic internal stresses that are induced by the mismatch between the thermo-elastic constants of the phases. Three cases are discussed successively: (1) pure elastic behavior of the phases; (2) plastic yielding in the matrix; (3) role of porosity and damage.

Friction Stir Processing (FSP) is an attractive manufacturing technique to produce Mg matrix composites since it avoids the problem of excessive reactivity between reinforcement and matrix encountered in liquid-phase processing routes. However, the strength of the interface in C-reinforced Mg matrix composites produced by FSP remains to be assessed. A short fibre composite has been fabricated by FSP a stack of a C-fabric between two Mg-AZ91D alloy sheets. In order to elucidate the interplay between matrix hardness and interface bonding strength, the work investigates the influence of heat treatment on the mechanical properties of the composites. An incremental Mori-Tanaka model is developed to analyse the relative roles of heat treatment and C-fibre reinforcement on the flow strength and ductility of the composites in tension and compression. The mean-field model provides an estimate of the stress at the matrix/fibre interface, from which a simple debonding criterion can be derived. Comparison between model predictions and experimental data indicates that damage in the FSP composites is triggered by early interfacial debonding. Based on Finite Element simulations of a tensile test carried out in-situ in a scanning electron microscope, the critical interfacial stress for debonding was identified to be 435 MPa in simple traction but only 250 MPa when damage is governed by shear. This explains the limited strengthening by C fibres observed in heat treated composites.

Composite plates in which the low CTE phase has the shape of a honeycomb are anticipated to present optimum anisotropy of thermal expansion and thermal conductivity for baseplates in electronic packaging. This design is explored by choosing an invar alloy for the low CTE phase. In order to allow the formation of a passivation layer protecting from reaction with liquid Al during squeeze casting, the honeycomb is made of the Cr-rich alloy commonly called “stainless-invar”. Composite plates containing 20 vol.% and 38 vol.% stainless invar were processed using honeycombs with the same thickness over cell side ratio. Experimental CTE values are significantly lower than the predictions of three different thermo-elastic models. The very limited amplitude of the strain hysteresis precludes the occurrence of global plastic yielding in the matrix. It appears that, owing to the high contiguity of the low CTE phase, the low value of the experimental CTE results from void closing and opening by localised plastic flow. A honeycomb volume fraction of 38% is necessary for bringing the average CTE down to the level suitable for packaging applications. The ratio of transverse thermal conductivity to density then amounts to about half of the performance of the best Al/SiC composites. Keywords: Metal matrix composites (MMCs), Interface, Thermomechanical properties, Anisotropy, Squeeze casting

The influence of dihedral angle and particle coordination on the evolution of densification rate is investigated throughout the intermediate and final stages of sintering. From a review of literature data, heuristic expressions are proposed for representing different trajectories of evolution of coordination from the onset of skeleton formation up to full density. The influence of dihedral angle and coordination on the threshold density for pore closure is established by revisiting the Plateau–Rayleigh instability criterion. The intermediate and final stages are modeled using representative volume elements that do not restrict coordination to integer values. Computational results enlighten the interplay between coordination and dihedral angle on the evolution of sintering stress, bulk viscosity, and sintering rate. The results allow quantifying the amplitude of the drop of densification rate at pore closure transition. The monotonous increase of coordination with relative density brings about a monotonous increase of sintering rate at constant particle size. Simulations emphasize the beneficial effect of higher initial density and coordination in the green part.

The evolution of sintering stress, bulk viscosity, and densification rate during the intermediate stage and the final stage of sintering of random aggregates of particles is simulated using a new model able to account for the monotonous increase in the coordination number during sintering. For the intermediate stage, the representative volume element is a cone of revolution whereas, for the final stage, it is volume centered on a closed pore located at quadruple point. Care is taken to warrant the continuity between the two models at the transition between the two stages. Computations allow tracing the variation in sintering stress and bulk viscosity with relative density, coordination, and dihedral angle during the two stages, as well as at the transition between them. They show how, as a result, densification rate is enhanced by the increase in coordination. The model is assessed via comparison of the computed curves with the predictions of previous models based on periodic 3D arrangements, in which grain coordination number is constrained to jump between integer values. The agreement is generally very good in spite of the different approximations involved.

International audience; The role of liquid phase and grain growth during the intermediate stage of sintering is investigated by comparing the isothermal densification rate of the tungsten skeleton in two systems: solid-state sintered W and liquid-phase sintered W-lOwt%Cu. Densification curves at 1380 degrees C, i.e. well after completion of capillary rearrangement in W-Cu, are confronted to the predictions of a model that considers a single representative grain size with quasi-equilibrium interfaces and a monotonic increase in grain size and coordination with density. In both systems, the densification mechanism is taken to be governed by W diffusion along grain boundaries. For system W-Cu, a phenomenological law is proposed to allow accounting for the decrease in liquid surface curvature during funicular filling of the pores by the liquid phase. The kinetics of grain growth is described via current literature model. Good agreement is demonstrated between experimental measurements and model predictions when relying on material parameter values given in literature. In system W-Cu, the detrimental effect of the decrease in dihedral angle is, at the beginning of the funicular stage, compensated by the contribution to sintering stress brought about by the concave curvature of liquid/vapour interface. The sensitivity of computational results to model hypotheses and material parameters is assessed. The most influential parameters are diffusivities along W grain boundaries and along W/vapour and W/Cu interfaces. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Spark Plasma Sintering (SPS) has attracted a lot of interest in recent years owing to its ability to enable the densification of a broad range of materials in a very short processing time. It is well documented in the literature that the very high heating rates that can be applied with this technology can lead to the apparition of large thermal gradients in the tool and thus affect the homogeneity of the compact. In the present study, the influence of the compact thermal and electrical properties on the thermal gradients was studied. Al 2 O 3 , AlN and TiC powders were used to produce series of Al 2 O 3 -TiC and AlN-TiC composites (0, 25, 50, 75, 100 vol%TiC) showing different electrical and thermal conductivities. Two pyrometers were used in order to observe and measure the thermal gradients and the percolation of the current during sintering at a high heating rate and without insulation. Electrical conductivity measurements were carried out on samples presenting different relative densities. This samples were obtained through interrupted sintering cycles at temperatures below and above the identified percolation threshold temperature. It was shown that high thermal gradients can appear during SPS depending on the processing parameters (dimensions of the die and heating rate) but also on the composition of the compact (proportion of conductive phase) and on its density.

In this study, the densification of alumina by spark plasma sintering (SPS) was investigated and compared to conventional hot pressing. It was shown that SPS is very effective in the sintering of alumina leading to higher densities and allows to work at lower temperatures and with shorter sintering cycles. The effect of the heating rate is dependent on the heating mode (SPS or HP). The identification of active sintering mechanisms was attempted by an isothermal and an anisothermal methods, showing that other mechanisms probably related to electrical effects enhance the densification. We suggest the higher contribution of surface diffusion mainly during the initial stage of sintering and an influence of the presence of impurities segregated at the grain boundaries. They could create conductive layers and also introduce ions with a lower valence than Al 3+; defects are created in the surface layers and the diffusion of the species is increased. © 2011 Elsevier Ltd.

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.

Mg matrix composites appear as suitable competitors to Al alloys, as they allow for a significant weight reduction while exhibiting potentially comparable mechanical properties. A large variety of processing routes have already been considered for their production, including 'liquid state' processes such as squeeze casting. However these techniques necessitate a very careful control, simultaneously, of the solidification process, the wetting behaviour and the possible interfacial reactions between the reinforcements and the molten metal. And this is more particularly true in the case of Mg matrix composites with carbon fibers reinforcements, as Mg alloys are known to exhibit a poor wetting behaviour on carbon substrates. As a consequence, 'solid state' processing routes such as friction stir processing, that has been developed more recently, provide promising alternatives for the production of C-Mg composites. In the present work, Mg-Al-Zn alloys have been reinforced with carbon fibers using squeeze casting and friction stir processing. The microstructures and mechanical behaviour of composites produced by both processes have then been characterised and compared to those of non-reinforced samples, with a particular attention to the damage mechanisms prevailing in the composite materials. As a result, both processes have been compared in terms of the properties of their final products and hence their ability to produce sound composites.

Ion scattering spectrometry (ISS), a method which allows to measure the atomic composition of the uppermost surface monolayer, has been used to study the atomic arrangement of cobalt and molybdenum deposited on the surface of CoMo/y-AL2O3, hydrodesulfurization catalysts in their oxidic precursory form. The results prove that the oxidic molybdenum layer is on top of the oxidic cobalt layer. The depth profiling measurements are in excellent agreement with calculations from a straightforward model of the process.

Unsupported CoMo sulfide catalysts prepared by the homogeneous sulfide precipitation method were submitted to 3 MPa 4% H2S-H2 at 573K, and 0.1 MPa H2 at 573 and 723K. The samples were characterized by MES, XPS, XRD and temperature-programmed sulfur extraction. Their activity was measured in the HDS of thiophene and hydrogenation of cyclohexene. MES reveals the presence of Co9S8, CoS1+x, Co-Mo-S and two unidentified species. Treatment under H2 at 573K caused an increase of activity which was correlated with an increase of the Co9S8 concentration at the expense of all other species. Treatment under H2 at 723K caused a further increase of activity which was correlated with the sintering of the Co9S8 phase. Part of the sulfur extracted by treatment under H2 up to 733K cannot be recovered by subsequent treatment under H2S/H2. These results do not provide straightforward evidence for deciding between the current models used for explaining the synergetic effect, namely the existence of a Co-Mo-S phase or the remote control hypothesis. The sulfur deficiency of the catalyst appears to play a major role in the activity.

Two chromium-doped Raney nickel catalysts have been prepared from Al-Ni-Cr alloys where Cr was introduced in such a way as to substitute for nickel in the Ni2Al3 intermetallic phase. In comparison with the undoped catalysts, the BET and metallic surface areas remain little affected but more aluminium is retained after etching. X.P.S. shows that aluminium and nickel are in the metallic state whereas chromium is completely oxidized. Very large amounts of chromium are found on the surface, which suggests migration of chromium to the surface during leaching. Analytical electron microscopy indicates that the local chromium concentration is not perfectly uniform. Electron probe microanalysis of the precursor alloy reveals the segregation of part of chromium in a separate phase. These results are evaluated in the framework of the available literature data on the microstructure of Raney nickel catalysts.

A commercial cobalt-molybdenum catalyst supported on γ-alumina was activated by reduction-sulfidation under 15% H2S in H2 in the temperature range 573 to 1073K. Optimum hydrodesulfurization activity was achieved after activation between 673 and 803K. At lower temperatures lower activity was observed and X-ray photoelectron spectroscopy indicated that this was due to incomplete sulfidation of cobalt and molybdenum. Sintering of the supported phase, which occured after activation above 800K, was measured by high resolution electron microscopy, ion scattering spectroscopy and dynamic oxygen chemisorption. The hydrodesulfurization activity varied in the same proportion as the surface area of the MoS2-like crystallites. It was concluded that the number of active sites per unit surface area of supported phase and the intrinsic activity of these sites for hydrodesulfurization did not change during activation above 800K.

On the basis of a collection of experimental data, this paper analyses the interplay between dihedral angle, coordination and particle volume fraction in aggregates governed by capillary forces. Novel data are derived via a model describing the dependence of capillary energy on aggregate topological parameters. An increase in the dihedral angle above 60 degrees favours the formation of a skeleton with lower coordination. It is shown that dihedral angle also controls the conditions for the closure of pores. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The mechanical behaviour of transformation-induced plasticity (TRIP)-assisted multiphase steels is addressed based on three different microstructures generated from the same steel grade. The mechanisms responsible for the work-hardening capacity and the resulting balance between strength and resistance to plastic localization are investigated at different length scales. The macroscopic mechanical response is determined by simple shear, uniaxial tension, Marciniak and equibiaxial tension supplemented by earlier tensile tests on notched and cracked specimens. It is shown that the transformation rate reaches a maximum for stress states intermediate between uniaxial tension and equibiaxial tension. At an intermediate length scale, the true in situ flow properties of the individual ferrite–bainite and retained austenite phases are determined by combining neutron diffraction and digital image correlation. This combined analysis elucidates the partitioning of stress and strain between the different constitutive phases. Based on these results, supplemented by transmission electron microscopy and electron backscattered diffraction observations, a general overview of the hardening behaviour of TRIP-assisted multiphase steels is depicted.

The stress and strain partitioning between the different phases of transformation-induced plasticity (TRIP)-aided multiphase steels is evaluated using a mean field homogenization approach. The change of the austenite volume fraction under straining is predicted using a micromechanics-based criterion for the martensitic transformation adapted to the case of small, isolated, transforming austenite grains. The parameters of the model are identified from the mechanical response and transformation kinetics measured under uniaxial tension for two steels differing essentially by the austenite stability. The model is validated by comparing the predictions with tests performed under different loading conditions: pure shear, intermediate biaxial and equibiaxial. An analysis of the effect of the austenite stability on strength and ductility provides guidelines for optimizing properties according to the stress state. (c) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Short C fibres-Mg matrix composites have been produced by friction stir processing sandwiches made of a layer of C fabric stacked between two sheets of Mg alloy AZ31B or AZ91D. This novel processing technique can allow the easy production of large-scale metal matrix composites. The paper investigates the microstructure of FSPed C fibre-Mg composites in relation with the fragmentation of the C fibres during FSP and their influence on the tensile properties. 3D X-ray tomography reveals that the fibres orient like onion rings and are more or less fragmented depending on the local shear stress during the process. The fibre volume fraction can be increased from 2.3% to 7.1% by reducing the nugget volume, i.e. by using a higher advancing speed in AZ31B alloy or a stronger matrix alloy, like AZ91D alloy. A higher fibre volume fraction leads to a smaller grain size which brings about an increase of the composite yield strength by 15 to 25%. However, a higher fibre volume fraction also leads to a lower fracture strain. Fracture surface observations reveal that damage occurs by fibre/matrix decohesion along fibres oriented perpendicularly to the loading direction.

This work addresses the plastic flow properties of a composite material in which the reinforcing phase is continuous and cannot be suitably represented by isolated ellipsoidal inclusions. The dual-phase metal under consideration is composed of a network of Inconel-601 fibres infiltrated by pure aluminium. Hence, both phases exhibit elastic-plastic behaviour and are continuous in the three dimensions of space. The fibre network presents a large morphological anisotropy that is reflected in the mechanical response of the composite. The modelling is based on Eshelby's equivalent inclusion theory. Strain partitioning between the phases is computed incrementally based on tangent operators derived from the isotropic response of individual phases. Assessment of the model relies on extensive experimental data. Uniaxial tensile tests, involving measurement of the Lankford coefficient, have been performed at various temperatures on samples containing different volume fractions of fibres. Measurement of the phase stresses by neutron diffraction supplements the information provided by the macroscopic stress-strain curves. It is demonstrated that predictions are valid only when the micro-macro averaging scheme accounts for the co-continuous character of the constitutive phases. (c) 2006 Elsevier Ltd. All rights reserved.

Twinning-Induced Plasticity steels (TWIP steels) are extensively studied due to their ultra-high strain-hardening rate, that brings about a remarkable combination of ductility and strength. Twinning can be observed in high manganese-carbon steels. This paper considers hardening by combination of mechanical twinning with carbide precipitation. The kinetics of precipitation and the morphological evolution of carbides with annealing time were studied for two different TWIP steels with high manganese and carbon contents. The steels are first cold-rolled and then annealed at 800°C for recrystallization and carbide precipitation. Depending on the steel composition, the kinetics of precipitation and the morphology of the carbides are quite different. The influence of the annealing cycle on the mechanical properties has also been assessed. The results are used to discuss the influence of composition, stacking fault energy (SFE) and carbide precipitation on twinning. We show that the usual criteria based on the SFE only are not sufficient to characterize the twinning ability of a steel.

Constraint effects in adhesive joint fracture are investigated by modelling the adherents as well as a finite thickness adhesive layer in which a single row of cohesive zone elements representing the fracture process is embedded. Both the adhesive and the adherents are elastic-plastic with strain hardening. The bond toughness Gamma(work per unit area) is equal to Gamma(0) + Gamma(p), where Gamma(0) is the intrinsic work of fracture associated with the embedded cohesive zone response and FP is the extra contribution to the bond toughness arising from plastic dissipation and stored elastic energy within the adhesive layer. The parameters of the model are identified from experiments on two different adhesives exhibiting very different fracture properties. Most of the tests were performed using the wedge-peel test method for a variety of adhesives, adherents and wedge thicknesses. The model captures the constraint effects resulting from the change in Gamma(p): (i) the plastic dissipation increases with increasing bond line thickness in the fully plastic regime and then decreases to reach a constant value for very thick adhesive layers; (ii) the plastic dissipation in the fully plastic regime increases drastically as the thickness of the adherent decreases. Finally, this model is used to assess a simpler approach which consists of simulating the full adhesive layer as a single row of cohesive elements. (c) 2005 Elsevier Ltd. All rights reserved.

Cold compaction (200-1900 MPa) and sintering (1250 degrees-1350 degrees C) of cermets based on WC-Co were experimentally studied using die compaction, cold isostatic pressing, sintering, and creep tests. Two different-sized WC powders were used. The cobalt content varied over a range of 10-30 wt%. Cold-compaction behavior has been described by using a Cam-Clay model. Die-wall friction was measured by using green powder compacts that had different aspect ratios. Friction coefficients were 0.28-0.85, depending on the WC particle size and cobalt content. Simple constitutive equations have been used to model the high-temperature behavior (sintering and creep). The constitutive equations were implemented in a finite-element program to model the compaction, ejection, and sintering of bilayer structures that had different cobalt contents. The model can represent the effect of die-wall friction on the average density, as well as deformation inside the green compact. Density gradients were generated; they were revealed during sintering, because the compact does not deform homogeneously. Simulation also can be used to evaluate deformations that are induced by sintering.

The tranformation-induced plasticity (TRIP) effect, i.e. the mechanically activated martensitic transformation of metastable austenite, has been proven for some years to contribute very effectively to the deformation process in a large variety of iron-based alloys. In order to enlighten the influence of the stress triaxiality on the kinetics of the mechanically-induced martensitic transformation in TRIP-assisted multiphase steels. several specimens presenting austenite with different mechanical stabilities were strained under different stress states (pure uniaxial tension, uniaxial tension of notched and DENT specimens and stretching). It is shown that the stress triaxiality has a large effect on the mechanical stability of austenite dispersed in a multiphase microstructure and consequently on the mechanical properties of the investigated steels.

The phenomenon of variant selection in a bainitic steel containing retained austenite was investigated. The steel was hot-rolled below the Tnr to true strains of epsilon = 0.2 and epsilon = 0.8 in the austenite temperature range. The orientation relationships between bainite and austenite are expressed in Rodrigues space. A strain of 0.2 is shown to be insufficient to induce variant selection. A hot deformation of 0.8 leads to a much smaller number of variants being created, which are shown to be related to the slip systems active during austenite deformation.

Homogenisation is an essential step in the preparation of aluminium billets for extrusion. It is designed to produce an homogeneous solid solution and to transform the beta-Al5FeSi particles to the more equiaxed and thus more acceptable cubic alpha-Al-12(Fe-x,Mn1-x)(3)Si variety. The effect of the homogenisation conditions on high temperature surface damage and fracture is investigated through full scale industrial extrusion experiments on AlMgSi alloy. The transformation of AlFeSi intermetallic particles from the beta-Al5FeSi thin plate like shape to the more rounded alpha-Al-12(Fe,Mn)(3)Si form, quantified by EDS measurements, is shown to postpone the formation of surface defects. Transmission Electron Microscopy (TEM) analysis of extruded samples close to the location of extrusion defects, reveals that damage initiates by microcracks within the brittle beta-Al5FeSi particles or due to incipient melting caused by the eutectic transformation Al + beta-Al5FeSi + Si --> Liquid at 578degreesC. These microcracks open by plastic deformation around the cracked brittle phase and, after sufficient growth, coalesce into a large crack.

The influence of intercritical rolling on the microstructural development during the processing of hot rolled multiphase steels is characterised. Intercritical rolling leads to dynamic recovery of ferrite and to strain-induced transformation of austenite into ferrite. The prior deformation of austenite considerably influences the bainite transformation. It is shown that a smaller amount of austenite transforms into bainite when bainite forms from deformed austenite. This is explained by the physics of the bainite transformation.

This study deals with the influence of the austenite grain size and deformation state on the mechanism and kinetics of bainite transformation in several low-alloy steels differing by their C content. Nucleation and growth conditions are thoroughly modified when the austenite grain size becomes of the same order as the length of the individual bainitic ferrite platelets. As a consequence, the kinetics show some peculiarities. The austenite prior deformation also influences in a large way the morphology of the bainite - retained austenitc, microstructure.

An extensive numerical study of the mechanics of the "wedge-peel test" is performed in order to analyze the mode I steady state debonding of A sandwich structure made of two thin plastically deforming metallic plates bonded with an adhesive. The constitutive response of the metallic plates is modeled by J(2) flow theory, and the behavior of the adhesive layer is represented with a cohesive zone model characterized by a maximum separation stress and the fracture energy. A steady-state finite element code accounting for finite rotation has been-developed for the analysis of this problem. Calculations performed with the steady-state formulation are shown to be much faster than simulations involving both crack initiation and propagation within a standard, non-steady-state code. The goal of this study is to relate the measurable parameters of the test to the corresponding fracture process zone characteristics for a representative range of adherent properties and-test conditions. An improved beam bending model for the energy release rate is assessed by comparison with the numerical results. Two procedures are proposed for identifying the cohesive zone parameters from experimental measurements. (C) 2003 Elsevier Science Ltd. All rights reserved.

Squeeze cast Al matrix composites reinforced with continuous fibers of Inconel 601 were submitted to different annealing treatments aiming at tuning the amount of reaction at the fiber/matrix interface. The reaction develops in the form of intermetallic nodules growing onto the fibers. The tensile flow stress of the composites increases with increasing nodule volume fraction at the expense of a progressive loss of ductility. This loss of ductility is due both to the low cohesion of the oxide layer separating the matrix from the nodules and to brittle cracking at the root of attachment of the nodules onto the fibers. Damage development is evaluated from the evolution of strain hardening. The nodules grow underneath the oxide barrier layer that protects the fibers from reacting with Al during squeeze casting. Their mechanism of growth involves the partial reduction of the oxide layer by Al, followed by diffusion of Al and Ni through the Cr-rich oxide layer. (C) 2002 Elsevier Science B.V. All rights reserved.

TRIP-assisted multiphase steels have been thoroughly studied in the cold-rolled and annealed state. The effects of hot-rolling conditions on these steels are much less studied even though these are of major importance for industrial practice. This study was carried out in order to understand the effect of the hot deformation of austenite on the tensile properties of TRIP-assisted multiphase steels. Two different compositions and microstructures are investigated. The first one is a low-carbon steel (mass content of 0.15 %) with a microstructure consisting of an intercritical ferritic matrix, bainite and retained austenite. The second one is a medium-carbon steel (mass content of 0.4 %) that consists of bainite and retained austenite. Both steels were deformed to various strain levels below the non-recrystallisation temperature of austenite. The medium carbon steel was deformed in the fully austenitic temperature range whereas the low-carbon steel was deformed in the intercritical temperature range. In both cases, the prior hot deformation of austenite brings about a large enhancement of the work-hardening capabilities. In the case of the medium-carbon steel, this effect can be attributed to a much larger TRIP effect taking place during straining. In the case of the low-carbon steel, the improvement of the work-hardening behaviour was attributed to an interaction between the martensitic transformation and the dislocations already present within the surrounding ferrite matrix.

The nanohardness of the phases present in the microstructure of two TRIP (for TRansformation Induced Plasticity)-assisted multiphase steels differing by their silicon content was measured by nanoindentation in an atomic force microscope. It is observed that the softest phase in both steels is the ferritic matrix, followed by bainite, austenite and martensite. It is also shown that the silicon content of the steel grades is responsible for an increase of the hardness of the ferritic matrix due to solid solution strengthening. Finally, the influence of the preparation mode of the surface prior to the nanoindentation measurements has been investigated. An electropolishing stage after mechanical polishing is acceptable to allow valuable nanohardness measurements. (C) 2002 Elsevier Science B.V. All rights reserved.

The essential work of fracture (EWF) and the J-integral methods were applied in a study of the effect of the thickness on the cracking resistance of thin plates. The paper discusses two themes: (1) the relationships between the two methods or concepts is elucidated, and (2) a new, thickness independent plane stress toughness parameter is proposed. For that purpose, cracked aluminium 60820 thin plates of 1-6 mm thickness were tested in tension until final separation. The EWF, w(e), and the J-integral at cracking initiation, J(i), increase identically with thickness except at larger thickness for which the increase of J(i) levels off. J(i) reaches a maximum for 5-6 mm thickness whereas w(e) keeps increasing linearly with thickness. This difference is related to the more progressive development of the necking zone in front of the crack tip when thickness increases: at large thickness, cracking initiates well before the neck has developed to its stationary value during propagation. A linear regression on the fracture toughness/thickness curve allows partitioning the two contributions of the work of fracture: the plastic work per unit area for crack tip necking and a plane stress work per unit area for material separation. The pertinence of this new measure of the pure plane stress cracking resistance is critically discussed based on a micromechanical model for ductile fracture. The micromechanical void growth model incorporates void shape effects, which is essential in the low stress triaxiality regime. (C) 2002 Elsevier Science Ltd. All rights reserved.

An original approach for the synthesis of iron aluminides of the type FeAl1-x is presented, based on the use of a squeeze-casting equipment for inducing the reactive infiltration of Al into a porous preform made by sintering continuous Fe-based fibres. The casting is subsequently homogenised by heat treatment in the solid state. Preforms with fibre volume fractions ranging between 40 and 80% (fractions that correspond to stoichiometries close to FeAl and Fe3Al. respectively) are prepared using steel-wool fibres with various diameters. Small fibre diameters and high processing temperatures are found to be beneficial for obtaining, after squeeze casting a low-porosity material with a large amount of uniformly distributed reaction compounds. However, the preform thickness that can be properly infiltrated is limited to about 6 mm. The homogenisation of the intermetallic phase is complete after only 5 h at 1000 degreesC. A major problem to be solved is the heterogeneity of the porosity distribution in the sample after homogenisation. This heterogeneity results from an insufficiently uniform distribution of the fibre volume fraction. (C) 2002 Elsevier Science B.V. All rights reserved.

The paper aims at contributing to a better appraisal of the wedge-opened double cantilever beam method for testing the debonding toughness of adhesively bonded plates. The test specimens consisted of two plates of either hot-dip galvanized low carbon steel or alloy Al-6082-T6 bonded with an epoxy-based adhesive. The classical method of crack length measurement by visual observation of the crack tip along the side of the specimen is compared with a method making use of displacement sensors for continuously monitoring the deflection of the plates. The variation of toughness with debonding rate was derived from the evolution of the debond length either with respect to a static wedge or with respect to a wedge advancing at a constant rate. The paper enlightens the uncertainty arising from the anticlastic effect and compares the experimental reproducibilities provided by the different test procedures. (C) 2002 Elsevier Science Ltd. All rights reserved.

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.

TRIP-assisted multiphase steels show enhanced mechanical properties thanks to the strain-induced transformation of retained austenite to martensite (TRIP effect). Stabilization of austenite is made possible by the combination of appropriate chemical composition and heat-treatment. It has been shown recently that aluminium could be substituted to silicon, whose effect has been mainly studied in the literature so far, for this austenite retention. In this work, the influence of aluminium content and heat-treating conditions on the retention of carbon-enriched austenite is investigated in two 0.12 wt. % C - 1.5 wt. % Mn steels with 0.51 wt, % Al and 1.16 wt. % At respectively, Special attention is given to the effect of aluminium on the phenomena developing during bainitic holding. The bainitic transformation kinetics is followed by dilatometry, Coupled with a characterization of the microstructures by X-ray diffraction, scanning electron microscopy and image analysis. these dilatometry experiments enabled us to draw transformation maps giving the volume fractions of the different phases. The retarding effect of aluminium on carbide precipitation during the bainitic transformation is highlighted, although Al appears less efficient than Si.

Ni-based heat resistant fibres were used for reinforcing Al-based matrices. The goal was to improve the high-temperature mechanical properties of the matrix while keeping a good ductility of the material, thanks to the ductile nature of the fibres. In order to prevent reaction when liquid Al comes into contact with the Ni-based fibres during the processing of the composites by squeeze casting, it was found beneficial to passivate the fibres by oxidation before infiltration. In order to optimise the composite, as far as the oxidation conditions of the fibres are concerned, tensile tests have been performed on composites processed using fibres oxidised during 30 minutes at 600°C, 750°C and 900°C. The best properties were obtained for the intermediate temperature of 750°C. Thus, a compromise was reached between a sufficient roughness and/or an appropriate nature of the interface for good fibre/matrix adhesion and a limited oxidation to prevent degradation of the fibres. Indeed, TEM characterisation of the interfaces revealed the presence of cavities below the surface of the fibres, due to oxidation mechanisms.

The influence of heat-treating conditions on the retention of carbon-enriched austenite of TRIP-assisted multiphase steel grades containing different amounts of silicon and/or aluminium is investigated. The ensuing mechanical properties resulting from the TRIP effect are also scrutinised. The bainite transformation kinetics was followed by dilatometry whereas a detailed characterisation of the microstructures led to the construction of transformation maps giving the volume fractions of the different phases and the carbon content of austenite. The role of silicon and aluminium additions (i) on the retention of austenite by partial bainite transformation and (ii) on the mechanical properties is enlightened. A strong influence of the solid-solution strengthening effect of silicon is highlighted. Aluminium seems to be an effective alloying element for the retention of austenite in TRIP-aided steels even if lower strength levels can be attained. A mixed Al-Si TRIP-aided steel seems to be a very good compromise between the processing needs, the required mechanical properties and the industrial constraints.

The damage resistance, fracture toughness and austenite transformation rate in transformation-induced plasticity (TRIP)-assisted multiphase steel sheets were comparatively characterised on two steel grades differing by the volume fractions of the phases (i.e. ferrite, bainite, retained austenite) and by the mechanical stability of retained austenite. The influence of stress triaxiality on austenite transformation kinetics and the coupling between martensitic transformation and damage were investigated using double edge notched (or cracked) plate specimens tested in tension. The map of the distribution of transformation rates measured locally around the notch (or the crack) was compared with the map of the effective plastic strains and stress triaxialities computed by finite element simulations of the tests. The mechanically-activated martensitic transformation was found to progress continuously with plastic straining and to be strongly influenced by stress triaxiality. Fracture resistance was characterised by means of J(R) curves and crack tip opening displacement (CTOD) measurements using DENT specimens. The fracture toughness at cracking initiation was found to be lower for the steel with higher tensile strength and ductility. The contrasted influence of the TRIP effect, which improves formability by delaying plastic localisation but reduces fracture toughness at cracking initiation, is shown to result from parameters such as the volume fraction of non-intercritical ferrite phases or the mechanical properties of martensite. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.

A new peel test method has been developed for allowing debonding of adhesively bonded, thin metallic substrates. The rolling of the substrates on rollers allows control of the plastic dissipation during substrate bending and prevents dissipation associated with substrate unbending. The measured debonding toughness is shown to be independent of substrate thickness when proper account is taken of the work required for straining the adhesive layer. Mechanical analysis shows that control of plastic dissipation in substrates remains effective, as long as the ratio of the plate thickness to the roller radius is large enough to prevent wound-up. Mode mixity during debonding can be conveniently modified by changing the radii of the rollers. This possibility of altering mode mixity allows a better control of the occurrence of the cohesive, or near-interfacial debonding mechanisms. Evidence is given for the fact that the debonding mechanism is not governed by the minimum in the debonding toughness. A significant increase of debonding toughness with increasing mode mixity is observed only when the substrate roughness is high.

A transition from an adhesive mode of debonding at low debonding rate to a cohesive mode at high debonding rate was observed when peeling assemblies consisting of thin metallic plates bonded with an epoxide adhesive. Stable cohesive debonding was found to be characterised by a river pattern aspect of the debond surfaces and by an increase of the apparent debonding resistance with increasing debond extension. Mechanisms are proposed for the initiation and extension of cohesive debonding. Second phase nodules in the adhesive are shown to be the sites of nucleation of cavities. Initiation of cohesive debonding is ascribed to the onset of this cavitation, which is promoted by the increase of the adhesive yield strength with straining rate. Cohesive debond extension is analysed in terms of a meniscus instability mechanism whereas the R-curve behaviour is related to the finger-like extension of the debond front. (C) 2001 Elsevier Science Ltd. All rights reserved.