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Ce chapitre s’intéresse aux aciers à haute tenue mécanique résistants à la corrosion humide, à l’oxydation et au fluage. Les effets liés à la composition, à la microstructure, aux défauts et aux procédés mis en œuvre sont discutés. En filigrane, il apparaît qu’il existe un compromis entre les propriétés mécaniques et le comportement qui découle des interactions avec l’environnement.

An explanation is proposed to martensite inhibition beyond characteristic concentration thresholds in titanium binary alloys. The method combines the phenomenological theory of martensite crystallography (PTMC) and thermodynamic calculations (TCs) to describe the conditions under which martensite formation is favourable. It is shown that martensite can be crystallographically prevented while being thermodynamically favourable. The PTMC is implemented by taking into account the influence of composition. After a comprehensive comparison to experiments, two twinning systems and two glide systems are inferred to be able to produce the lattice invariant shear. The critical concentrations above which martensite cannot form are computed and compared to experimental results on binary and ternary systems, showing good agreement. The proposed method may be used as a guide to design titanium alloys for controlled martensitic behaviour.

Nitrogen is a well-known gamma-stabiliser in austenitic steels, also responsible for significant solid solution hardening of these materials. Yet, only few papers have studied its impact on austenitic high-entropy alloy (HEA) matrixes. This study focuses on a cobalt-free, non equimolar CrFeMnNi HEA doped with nitrogen. A series of alloys was cast under a nitriding atmosphere to promote nitrogen absorption into the liquid alloy. Study of as-cast alloys has shown nitrogen presence in solid solution up to 0.3 wt % (1.2 at. %). Over the whole range of compositions, a linear increase of hardness (134 HV/wt. % of N) was measured as well as an expansion of the lattice parameter of Δa/a = 1.01/wt. % N due to nitrogen addition in the interstitial sites of the lattice. Tests on forged and annealed samples showed that the increase of hardness with nitrogen addition is higher than in as-cast state (210 HV/wt. % of N) surely due to presence of other strengthening mechanisms. Tensile tests confirmed that the presence of dissolved nitrogen increases yield strength and ultimate strength and enhances strain-hardening, without any modification of ductility.

International audience; The sensitivity to ductility dip cracking was measured in two model high chromium nickel alloys, with the same composition, apart from different sulphur contents. The newly developed Refusion Cracking Test was used, that consists in repetitive refusion lines conducted at the specimen surface. Grain boundary cracks develop in the heat affected zone near the refusion line. Cracking is much more pronounced in the sulphur-enriched alloy, which demonstrates a strong deleterious effect of sulphur. However post-mortem analyses using WDS and STEM-EDS revealed no segregation of sulphur at grain boundaries. In contrast, grain boundary fracture surfaces are covered with sulphur. This suggests a dynamic type of grain boundary embrittlement where sulphur acts as a surfactant, facilitating crack opening. Sulphur is efficiently provided to the crack tip as it propagates, due to accelerated diffusion by plastic deformation. This allows crack growth rates higher than 10 µm/s.

This paper deals with Ductility Dip Cracking (DDC) during multipass welding of 690 filler metals. In addition to industrial alloys (152 and 52 M), model alloys of controlled purity were also used to unambiguously demonstrate the effect of elemental sulphur and carbon. The sensitivity of each alloy to DDC was measured with the recently developed Refusion Cracking Test (RCT). It is confirmed that the cracks observed in this study result from DDC, not from liquation nor solidification cracking. DDC cracks are in most cases observed after several welding passes. The detrimental effect of sulphur and beneficial effect of carbon on the resistance to DDC are clearly evidenced. The good resistance to DDC of the industrial alloys, compared to the model alloys, is related to their niobium content.

A Gaussian process regression model for estimating the creep rupture stress of single crystal nickel-based superalloys is constructed. It is built and validated on data disclosed in patents as well as scientific and technical reports. This model is coupled with computational thermodynamics for the prediction of microstructural features, and a model for the estimation of density. Using this combination, the key characteristics of a large number of potential alloys are systematically computed as a function of their composition. Materials specifications targeted towards applications as turbine blades, which none of the alloys from the creep database follow, narrow down the search from 300 000 000 to 180 000 alloys. A large number of these candidates are predicted as featuring greater specific properties than existing single crystal nickel-based superalloys. The selection criteria used to isolate alloys for experimental validation are discussed, and lead to several alloys displaying good agreement between their predicted microstructure and estimated properties.

Fe–35Ni–25Cr–0.4C alloys with different compositions are aged between 750 and 1150°C up to ∼10,000 h. As-cast microstructure contains interdendritic carbides of type M7C3 (‘Cr7C3’) and MC (‘NbC’). ...

Relations between microstructure, phase transformations and creep resistance of austenitic Fe–Ni–Cr alloys are investigated. As-cast alloys with different silicon contents and an ex-service tube ar...

International audience; New theory is presented to describe the occurrence of plasticity-induced transitions in titanium alloys. The approach is able to predict the composition dependence of transformation induced plasticity (TRIP), superelasticity, as well as martensite formation upon quenching. Martensite formation in the absence of stress is considered as the result of a competition between elastic strain energy and chemical driving force. Assuming that the formation of martensite is the result of a thermally activated nucleation process followed by athermal growth, a nucle-ation parameter is postulated to describe the conditions under which martensite is formed upon quenching; the parameter accounts for the ratio between the available thermal energy and an energy barrier for nucleation, suggesting that phase is not the main factor controlling martensite inhibition. This nucleation parameter is able to describe, for the first time, martensite occurrence in 130 alloys from the literature, quantifying the martensite start temperature (M s) reported for 49 alloys with great precision. An empirical parameter ([Fe] eq) is proposed and, when combined with the M s prediction, it allows to define regions within which TRIP and superelasticity occur. By defining threshold values for the M s , the [Fe] eq and the nucleation parameter, candidate alloys likely to display TRIP, superelasticity or martensitic transformation upon quenching can be identified. As a result, this method can be adopted to design alloys with tailored plasticity behaviour.

International audience; The design of new metallic alloys is faced with the challenge of an increasing complexity of the alloys composition, processing and resulting microstructures necessary to answer to multiple property targets, together with a requirement that the design stage be faster and less expensive. This paper shows that combinatorial methods, combining numerical and experimental approaches, can be applied to the specific requirements of alloy design and lead to improved understanding of fundamental processes of physical metallurgy, such as precipitation, together with improved alloy compositions and processing.; La conception de nouveaux alliages métalliques est confrontée au défi d'une complexité croissante de leur composition, du traitement et des microstructures résultantes nécessaires pour répondre à de multiples objectifs quant à leurs propriétés, ainsi qu'à l'exigence d'une étape de conception plus rapide et moins coûteuse. Cet article montre que les méthodes combinatoires, associant des approches numériques et expérimentales, peuvent être appliquées aux exigences spécifiques de la conception des alliages et conduire à une meilleure compréhension des processus fondamentaux de la métallurgie physique, tels que la précipitation, ainsi qu'à des compositions et des traitements d'alliages améliorés.

A new integrated computational HEA design strategy is proposed. It combines a multi-objective genetic algorithm with (i) statistical criteria to guide the formation of a single phase, supplemented by computational thermodynamics (Thermo-Calc) and (ii) models for the estimation of alloy yield stress via solid solution hardening, to be maximised, and alloy density, to be minimised. This strategy is applied to the design of face-centered-cubic (FCC) HEAs and yields several thousands of new alloys. An alloy featuring an interesting combination of predicted stability, strength and density, AlCoFeMoNi (at%), is chosen among them, fabricated by vacuum arc melting and experimentally tested. The microstructure of this new HEA consists in a single FCC solid solution, as evidenced by X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray energy dispersive spectroscopy (EDS) mapping. With a density of 7.95 g⋅cm, a Vickers hardness of 1.78 GPa, a yield stress of 215 MPa and an ultimate tensile strength of 665 MPa in the annealed state, its properties surpass those of existing FCC HEAs of comparable density., The work of Franck Tancret at the University of Cambridge was funded by the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie Actions grant agreement No 656 249: “Computer-Aided Design Of Revolutionary Superalloys” (CADORS). I. Toda-Caraballo wishes to acknowledge Professor Mark Blamire for provision of facilities at the University of Cambridge and the financial support of the fellowship 2016-T2/IND-1693, from the programme Atracción de talento investigador, (Consejeria de Educación, Juventud y Deporte, Comunidad de Madrid) at CENIM-CSIC.

A multi-objective optimisation genetic algorithm combining solid solution hardening (SSH) and thermodynamic modelling (CALPHAD) with data mining is used to design high entropy alloys (HEAs). The approach searches for the best compromise between single-phase stability, SSH and density. Thousands of Pareto-optimal base-centred cubic (BCC) HEAs are designed. AlCrMnMoTi (at.%) is chosen for experimental validation. The alloy was cast and characterised. Its microstructure consists of large grains of a single disordered solid solution displaying a Vickers hardness of 6.45 GPa (658 HV) and a density below 5.5 g/cm; uniquely combining exceptional hardness with medium density., Philippe Castany acknowledges the THEMIS platform of the University of Rennes for providing access to TEM facilities. The work of Franck Tancret at the University of Cambridge was funded by the European Union's Horizon 2020 Research and Innovation Programme under the Marie Sklodowska-Curie grant agreement No. 656249: “Computer-Aided Design Of Revolutionary Superalloys” (CADORS). The present document reflects only the author's view; the European Union's Agency is not responsible for any use that may be made of the information it contains.info:eu-repo/grantAgreement/EC/H2020/656249

International audience; This work presents a critical review of the application of CALPHAD techniques in the development of high entropy alloys (HEAs) and complex concentrated alloys (CCAs). This assessment covers three major themes: thermodynamics of mixtures and stability, retrospective thermodynamics, and predictive thermodynamics. Based on statistical and thermodynamic analysis, we assess the concept of entropic stabilization. A brief description of the major accomplishments of the CALPHAD technique applied to explain the stability and microstructure of HEAs and CCAs is presented. We describe the role of CALPHAD and its integration with other design tools, such as physicochemical criteria, data mining, and optimization techniques, to accelerate the discovery of new materials. Finally, we recommend future efforts in the development of the next generation of HEAs and CCAs in connection with the design of their microstructures, with an emphasis on precipitation strengthening and twinning-induced or transformation-induced plasticity (TWIP, TRIP).

High entropy alloys (HEAs), a category of highly concentrated multicomponent alloys, have become a subject of interest in the past years due to their combination of properties. The development of these single phase solid solution alloys, containing between 5% and 35% of at least five different elements, has mainly relied on trial-and-error experiments, and more recently on modelling. The latter has notably focused on criteria to guide the formation of a single solid solution: (1) Hume-Rothery rules or their modification based on elemental variations in atomic radius, electronegativity, valence or number of itinerant electrons; (2) the use of thermodynamic concepts relying on estimates of enthalpy or entropy of mixing, and/or on melting or spinodal decomposition temperatures; (3) criteria based on lattice distortion; and (4) computational thermodynamics using the CALculation of PHAse Diagrams (CALPHAD) method. However, none of these criteria or methods, taken alone, can reliably predict the formation of a single solid solution. Instead, based on a critical assessment and a Gaussian process statistical analysis, a robust strategy to predict the formation of a single solid solution is proposed, taking into account most of the previously proposed criteria simultaneously. The method can be used as a guide to design new HEAs. Keywords: HEA, Neural network, Thermo-Calc, Data mining

Integrated Computational Materials Engineering (ICME), and Integrated ComputationalMaterials Science (ICMS) are developing fields with an aim of alloy design, by combining physicalmodels describing materials behavior through lengthscales and processing steps. It has beensuspected, however, that uncertainties in input parameters may cumulate in a hereditary way andyield to a high variability in the final output, independently of the quality of models themselves.Such a variability is however rarely quantified. In this aim, an illustrative example is here given,using a set of “cascade models”, each model being voluntarily very simple (grain growth,precipitation, hardening…) whereas assumed to be exact, so that only the effect of parameteruncertainties on the variability of the output (yield stress of a Ni-base superalloy) can be studied. Itis demonstrated that, with usual uncertainty levels in input parameters, the final dispersion (error)can become very high. Additionally, considering that models are not exact themselves would renderthe situation even worse. Besides, global and implicit models, like neural networks or Gaussianprocesses, have been shown to be able to perform reliable predictions and to be used for alloydesign, with acceptable levels of error, the latter being estimated by statistical methods. In addition,unlike ICME or ICMS, predictions are very fast so that automatic alloy composition optimisation ispossible using, for instance, genetic algorithms. Other fast predictive tools, like computationalthermodynamics (Thermo-Calc), can then be used as constraints during alloy optimisation.

International audience; Silica monoliths featuring either mesopores or flow-through macropores and mesopores in their skeleton are prepared by combining spinodal phase separation and sol-gel condensation.The macroporous network is first generated by phase separation in acidic medium in the presence ofpolyethyleneoxides while mesoporosity is engineered in a second step in alkaline medium, possibly in the presence of alkylammonium cations as surfactants. The mesoporous monoliths, also referred as aerogels, are obtained in the presence of alkylpolyethylene oxides in acidic medium without the use of supercritical drying. The impact of the experimental conditions on pore architecture of the monoliths regarding the shape, the ordering, the size and the connectivity of the mesopores is comprehensively discussed based on a critical appraisal of the different models used for textural analysis.

International audience; A new computational framework for systematic and optimal alloy design is introduced. It is based on a multi-objective genetic algorithm which allows (i) the screening of vast compositional ranges and (ii) the optimisation of the performance of novel alloys. Alloys performance is evaluated on the basis of their predicted constitutional and thermomechanical properties. To this end, the CALPHAD method is used for assessing equilibrium characteristics (such as constitution, stability or processability) while Gaussian processes provide an estimate of thermomechanical properties (such as tensile strength or creep resistance), based on a multi-variable non-linear regression of existing data. These three independently well-assessed tools were unified within a single C++ routine. The method was applied to the design of affordable nickel-base superalloys for service in power plants, providing numerous candidates with superior expected microstructural stability and strength. An overview of the metallurgy of optimised alloys, as well as two detailed examples of optimal alloys, suggest that improvements over current commercial alloys are achievable at lower costs.

LTA zeolites are used in many applications as diverse as chemistry, physics, medicine, agriculture. By exploiting the concept of pseudomorphic transformation, i.e. using silica monoliths featuring macro- and mesopores as silica source, we manage to shape LTA zeolites as centimetric monoliths (6 mm diameter, 5 cm length) with trimodal hierarchical porosity (micro-, meso- and macropores). The skeleton of the monolith is formed by an aggregation of LTA nanocrystals of ca. 100 nm leading to mesopores due to voids between the nanocrystals. Diverse LTA nanocrystal shapes and sizes were realized by controlling aging time and crystallization temperature. These unique self-standing monoliths of LTA nanozeolites feature flow-trough macropores of 3–4 microns highly suitable for high mass transport of liquids, homogeneous distributions of mesopores for high diffusion of molecules and ions and therefore a high accessibility to zeolites active sites. Together with an easy handling, these materials offer the possibility to intensify many processes using classical LTA zeolite crystals.

LTA zeolite monoliths (6 mm diameter, 3 cm length) featuring a hierarchical trimodal network of micro-/meso- and macropores (obtained by either controlling the nucleation step of LTA crystallization into nanocrystals or by creating mesopores into micronic crystals by using organosilane surfactant) were used for strontium capture in aqueous medium. LTA monoliths were compared to other LTA zeolite architectures: LTA microcrystals, commercial LTA beads and bimodal micro-/macroporous LTA zeolite monoliths. In batch mode, the presence of mesopores allowed to increase remarkably by a factor 15 the diffusion of ions, whereas macropores had no influence on ions transport. In flow mode, only LTA monoliths featuring flow-through macropores proved suitable as microreactors. The trimodal LTA monoliths were 1000 times more efficient than packed-beds of LTA beads, and 4 times than bimodal (micro-/macroporous) LTA monoliths due to higher rates of diffusion. Trimodal LTA monoliths were able to treat efficiently 4 L of Sr 2+ solution (10 mg L −1 ) with 1 mL of material at a flow rate of 0.5 mL min −1 (or 1 m h −1 ); ie. 4200 bed volumes (BV) were efficiently treated at a flow rate of 34 BV h −1 , with no Sr 2+ detectable by ionic-chromatography in the effluent. This result highlights the fact that the multiscale pore architecture engineering of an adsorbent is crucial for process intensification: macropores allow uniform mass transport of solutions with low pressure drop while the generation of mesopores in zeolites leads to faster ionic transport and more efficient crystal use in cation-exchange processes, both in batch and flow modes.

Deforming nickel base superalloys under the γ′ solvus retards dynamic recrystallisation (DRX) by hampering grain boundary motion; this is an efficient way of refining grain size during forging and other hot working processes. A previously proposed model for describing the kinetics of DRX in nickel alloys is here further developed to account for the influence of γ′ intermetallics, allowing predicting microstructural evolution during subsolvus deformation. The model incorporates alloy composition and initial grain size, as well as temperature and strain rate. It is calibrated with data for both supersolvus and subsolvus deformation. Plotting the calculated steady-state grain size as a function of the strain necessary to complete DRX allows visualising both processing and compositional trends, which are relevant to materials engineering and can be used for design. It is shown that efficient grain refinement cannot be achieved with a limited amount of strain if process or alloy optimisation alone is performed. Instead, it appears that this may only be reached by joint design of the forming process (strain rate and temperature) and composition. Keywords: Dynamic recrystallization, Ni base, Alloy design, Hot forming, Wrought, Modelling

In this study, we propose a simple and effective strategy to prepare injectable macroporous calcium phosphate cements (CPCs) by syringe-foaming via hydrophilic viscous polymeric solution, such as using silanized-hydroxypropyl methylcellulose (Si-HPMC) as a foaming agent. The Si-HPMC foamed CPCs demonstrate excellent handling properties such as injectability and cohesion. After hardening the foamed CPCs possess hierarchical macropores and their mechanical properties (Young's modulus and compressive strength) are comparable to those of cancellous bone. Moreover, a preliminary in vivo study in the distal femoral sites of rabbits was conducted to evaluate the biofunctionality of this injectable macroporous CPC. The evidence of newly formed bone in the central zone of implantation site indicates the feasibility and effectiveness of this foaming strategy that will have to be optimized by further extensive animal experiments.A major challenge in the design of biomaterial-based injectable bone substitutes is the development of cohesive, macroporous and self-setting calcium phosphate cement (CPC) that enables rapid cell invasion with adequate initial mechanical properties without the use of complex processing and additives. Thus, we propose a simple and effective strategy to prepare injectable macroporous CPCs through syringe-foaming using a hydrophilic viscous polymeric solution (silanized-hydroxypropyl methylcellulose, Si-HPMC) as a foaming agent, that simultaneously meets all the aforementioned aims. Evidence from our in vivo studies shows the existence of newly formed bone within the implantation site, indicating the feasibility and effectiveness of this foaming strategy, which could be used in various CPC systems using other hydrophilic viscous polymeric solutions.

Calcium phosphate cements have been widely used in medical and dental applications for decades. However, their intrinsic high brittleness and low strength prohibit their use in many stress-bearing locations, which would require an improvement in mechanical properties. The influence of microstructural parameters on the latter have nevertheless barely been investigated in a systematic manner. Furthermore, due to their inferior reproducibility which is sensitive to the variations introduced during the preparation and the way they are measured, mechanical properties of CPC cannot simply be characterized using mean values, but request a more reliable method. In this aim, apatite cements have been fabricated by mixing liquid and powders based on α-TCP (α-tricalcium phosphate), and their mechanical properties have been measured (Young’s modulus, fracture toughness, compressive strength and flexural strength) in wet environment as a function of various parameters (liquid-to-powder ratio; amount and morphology of porosity, including macropores created by porogens). The reliability of compressive strength of CPC is analysed using Weibull statistics. The above results indicate that fabrication and microstructural features of CPC significantly influence their mechanical properties.

A finite element approach is used to simulate the precipitation of Ni3(Al,Ti) intermetallics in nickel-based superalloys containing a low volume fraction of spherical g’ precipitates, in which precipitation occurs following nucleation and growth mechanisms. Classical differential equations of nucleation and growth are implemented in the software Comsol (formerly Femlab), to compute the number of precipitates per unit volume and their mean size. Another originality of the model is the use of thermodynamic quantities coming from phase diagram computations (Thermo-Calc), like the temperature variation of the equilibrium g’ volume fraction, and the evolution of the concentration of g’ forming elements (Al, Ti) in the matrix with the volume fraction of precipitates. Once adjusted to experiment in the case of isothermal ageings, the model can be used to simulate precipitation during complex thermal histories. Finally, automatic heat treatment optimisation procedures are proposed and tested, which can reduce heat treatment times by a factor of more than five.

Macroporous biphasic calcium phosphate (BCP) bioceramics, for bone substitution applications, have been synthesized, cold isostatically pressed and pressureless sintered, using naphthalene particles as a porogen to produce macropores. The resulting materials are mixtures of P-tricalcium phosphate and hydroxyapatite with various microporosities and macroporosities. Mechanical properties (Young's modulus, compressive strength and fracture toughness) were measured on specimens over the widest attainable ranges of porosities, and compared to previously proposed analytical models and hypotheses. These models describe the evolution of the mechanical properties as functions of macroporosity and microporosity separately, the strength model considering macropores as critical flaws in the ceramic. Results show that the presence of macropores strongly influences the critical flaw size, but the latter appears to increase with macroporosity. This phenomenon can be explained by the presence of clusters of macropores, acting as critical flaws, becoming larger as macroporosity increases. (C) 2009 Elsevier Ltd. All rights reserved.

Segregation of trace elements on a surface, at grain boundaries or more generally in any interface can have important consequences: adhesion of thin films, catalytic activity, embrittlement of steels by P or of nickel alloys by S, reinforcement of nickel alloys by B, etc. Segregation kinetics can be simulated by a finite element (FE) approach, by implementing the Darken–Du Plessis equation at the interface and Fick’s diffusion laws in the bulk. It is then possible to simulate segregation kinetics in non-isothermal conditions, and to couple segregation and macroscopic heat transfer calculations. A previously developed model is here adapted to the case of complex interfacial segregation phenomena: (i) segregation of a single species with a solute–solute or solute–solvent interaction, (ii) co-segregation of two species with a site competition in the interface, and (iii) segregation of a single species at an interface between two phases. Results are compared with available experimental data.

The microstructure of centrifugally cast heat resistant alloys of the HP-series (Fe–35Ni–25Cr–0.4C, wt.%) contains several minor phases, like carbides (M 7 C 3 , M 23 C 6 and MC). Several alloys are characterized by means of scanning electron microscopy in back-scattered electron imaging and energy dispersive spectroscopy, of electron probe micro-analysis by X-ray wavelength dispersive spectroscopy, and of electron back-scattered diffraction. On the one hand, scanning electron microscopy imaging in the back-scattered mode, associated with energy dispersive spectroscopy analysis and electron back-scattered diffraction, can lead to the identification of all phases, but with a lack of accuracy in certain cases. On the other hand, electron probe micro-analysis by X-ray wavelength dispersive spectroscopy alone can always provide a strict identification of all phases, with additional precise information on phase composition, including for light elements, which is important in the case of carbides.

Concrete blocks, made of a mixture of lime and hemp shives (also called “hemp hurds”), have been manufactured by a recently developed projection process. Lime carbonatation kinetics is determined by X-ray diffraction. Density measurements are made within blocks, and thermal and mechanical properties are measured (flexural strength, compression strength and hardness). The main observations are moderate density variations within a given block, and an influence of the projection distance on density. Both thermal conductivity and mechanical properties increase with the mortar density, which is well described by existing theoretical models. Compression tests induce a compaction, or densification, of the material.

The softwares Thermo-Calc and Dictra, based on the one hand on the calculation of phase diagrams (CALPHAD), and on the other hand on the finite difference simulation of diffusion in multicomponent alloys, are used to simulate the out-of-equilibrium formation of a liquid film during the rapid heating or welding of nickel-based superalloys. The softwares rely on the thermodynamic and mobility databases TTNI7 and MOB2, respectively. When temperature increases, γ′ particles of Ni3(Al,Ti,Nb) dissolve progressively. If temperature reaches the own melting point of the γ′ phase before it has completely dissolved, the particle melts (liquation); if the particle dissolves completely before temperature reaches its melting point, no γ′ liquation is possible. However, γ′ dissolution leads to a local enrichment of the γ matrix in Al, Ti and/or Nb, which decreases locally the melting point. If temperature reaches this melting point before the alloy has been completely homogenised, a sort of “γ liquation” can take place, the material melting locally at a temperature inferior to the equilibrium solidus of the alloy. As the kinetics of precipitate dissolution is a major aspect of the phenomenon, the latter depends strongly on precipitate size. Results are in good agreement with published experimental observations.

Macroporous biphasic calcium phosphate bioceramics, for use as bone substitutes, have been fabricated by cold isostatic pressing and conventional sintering, using naphthalene particles as a porogen to produce macropores. The resulting ceramics, composite materials made of hydroxyapatite and β-tricalcium phosphate containing various macroporosities and microporosities, have been submitted to compression and three-point bending tests. The mechanical tests performed on the sintered ceramics tend to validate the modelling approach and its hypothesis, i.e. the material can be considered as a microporous matrix containing isolated macropores, and the critical flaw is a macropore.

The finite element software Comsol is used to simulate surface or grain boundary segregation in dilute alloys. The model computes simultaneously the evolution of interfacial concentration and diffusion in the bulk. The solute exchange between bulk and interface is governed by Darken’s equation. The model is able to reproduce thermodynamic and kinetic aspects of the phenomenon, in particular the saturation segregation level and the short-time segregation kinetics expressed by the McLean approximation. It is also able to reproduce experimental trends in the case of surface segregation of sulphur in a Ni superalloy. In the case of the grain boundary segregation of impurities (P or S) in engineering alloys, the present approach provides a practical tool, as it can be coupled to other finite element simulations (heat transfer and/or mechanics). Thus, it becomes possible to predict the risk of synergetic segregation and thermomechanical damage during service or processing (forging, welding,...).

It is here demonstrated that combinations of various modeling techniques can be used to predict the service behaviour of heat resistant alloys, and sometimes to design “made-to-measure” alloys for specific high temperature applications. A first example is given, where an affordable creep-resistant nickel-base superalloy for operation around 750°C has been designed without any experiment. Based on the analysis of huge databases on existing alloys, Gaussian processes were used to predict its thermomechanical properties. Thermo-Calc allowed to design its fabrication process and to assess its weldability and its thermodynamical stability at service temperature. The alloy has then been tested and the validity of the modeling approach verified a posteriori, in particular a creep rupture life of 100 000 h at 750°C under 100 MPa. A second example is given, in the case of high-carbon Fe-Ni-Cr based alloys for reformer tube applications (HP steels). Their mechanical properties are predicted through the analysis of existing data with artificial neural networks. Parallelly, their thermodynamical stability in operating conditions is assessed using Thermo-Calc in combination with Dictra, to simulate the precipitation of carbides in the austenite matrix during service. It is therefore tried to understand microstructural evolution in service, damage mechanisms, and durability.

One of the most promising technologies for future applications of solide oxide fuel cells (SOFC)is the so-called “anode-supported” configuration: a dense yttria-stabilised zirconia (YSZ) ceramic electrolyte is deposited as a thin film over a porous Ni / YSZ cermet substrate anode, followed by a porous ceramic cathode such as La1-xSrxMnO3 (LSM). In this paper, a short review is made of the current technologies available to achieve this particular architecture, and to optimise the service behaviour. Then, alternative materials and fabrication technologies, as well as their possible impact on performance, are proposed and investigated.

Macroporous biphasic calcium phosphate bioceramics, for use as bone substitutes, have been fabricated by cold isostatically pressing and conventional sintering, using naphtalen or saccharose particles to produce macropores. The resulting ceramics, composite materials made of hydroxyapatite and b-tricalcium phosphate containing ~ 45% macropores and ~ 25% micropores, have been submitted to compression and three-point bending tests, toughness tests by single-edge-notched-bending, and spherical indentation tests. A new model is established to describe mechanical properties as a function of the amount and morphology of porosity, and propositions are made to optimise the fabrication procedure. Finally, those highly porous ceramics, although very brittle, exhibit a damage-tolerant contact behaviour, due to the compaction of the porous body under the indenter.

Heat-resistant steels of HP series (Fe-25Cr-35Ni) are used as reformer tubes in petrochemical industries. Their composition includes Nb and Ti as strong carbide formers. In the ascast condition, alloys exhibit an austenite matrix with intergranular MC, M23C6 and/or M7C3 eutectic carbides. During exposure at high temperature, phase transformations occur: chromium carbides of M7C3 type transform into the more stable M23C6 type, intragranular M23C6 carbides precipitate, and a silicide, the G-phase (Ni16Nb6Si7), forms due to the instability of MC carbides (NbC). Thermodynamic simulation is of great help for understanding precipitate formation and transformations. Thermo-Calc and Dictra are used to simulate the precipitation of carbides in the austenite matrix during service. However, from an experimental point of view, M23C6 and M7C3 are not easy to distinguish in bulk alloys. Indeed, backscattered scanning electron microscopy does not bring any contrast between the two phases, and energy dispersive spectroscopy (EDS) analysis does not lead to carbon content and consequently to the distinction between M23C6 and M7C3. With transmission electron microscopy (TEM), sample preparation is difficult and the observed area is extremely small. In the present work, HP alloys are investigated by electron backscatter diffraction (EBSD) coupled to EDS. Carbides are identified on the basis of crystal structure, in the bulk, within their microstructural context, and the experimental procedure is both simpler and cheaper than TEM. Precipitates (M23C6, M7C3) could be identified by orientation mapping and single spot analysis.

Biphasic calcium phosphate (BCP) bioceramics, for use as resorbable bone substitutes, containing both isolated macropores and interconnected micropores, have been fabricated by sintering, using naphtalen particles as a porogen to produce macropores. The resulting ceramics contain ~ 45% macropores and various amounts of microporosity. Mechanical properties (compression and bending strength, toughness and hardness) have been measured and modeled by combining two approaches, at two different scales: the one describes the mechanical properties of a partly sintered stacking of grains, supposed to account for the interconnected microporosity, the other one holds in the case of closed and isolated macropores within a continuous matrix. The material is then represented as a quasi-continuous matrix containing macropores, the matrix being itself microporous. The model also considers that fracture always initiates on a macropore, which allows to set a correspondence between fracture toughness and fracture stress equations. The mechanical tests performed on the sintered ceramics tend to validate the modeling approach.

Macroporous biphasic calcium phosphate bioceramics, for use as bone substitutes, have been fabricated by cold isostatic pressing and conventional sintering, using naphtalen particles as a porogen to produce macropores. The resulting ceramics, composite materials made of hydroxyapatite and β-tricalcium phosphate (TCP) containing ∼45% macropores and with various microporosities, have been submitted to compression and three-point bending tests, toughness tests by single-edge-notched-bending (SENB), and spherical indentation tests. By combining two approaches at two different scales, one for closed porosity and one for open porosity, a model is established to describe mechanical properties as a function of the amount and morphology of porosity. The model assumes a quasi-continuous matrix containing macropores, the matrix being itself microporous, and considers that fracture always initiates on a macropore. The preliminary mechanical tests performed on the sintered ceramics tend to validate the modelling approach.

The reaction mechanism describing the formation and surface decomposition of Ni 3 P has been investigated. Ni 3 P was synthesized as a thick film on a Ni foil by a simple solid-state reaction. X-ray diffraction confirms that the layer produced, on the surface of the Ni foil, is pure Ni 3 P. The solid-state reaction did not occur at 400 °C or lower temperatures. At 600 °C or higher temperatures the solid-state reaction occurred by a nucleation mechanism of synthesis and later sintering of Ni 3 P particles at the surface of the Ni foil. The surface of the Ni 3 P–Ni foil showed to be stable in air up to 300 °C. At higher temperatures, Ni 3 P decomposed trough a complex mechanism, including the loss of phosphorus, the formation of the phases Ni 12 P 5 and Ni 2 P, and finally, oxidation of the nickel phosphides. Samples heated at 800 °C for 4 h produced a NiO layer on the surface of the Ni 3 P–Ni foil.

Copper phosphide (Cu 3 P) was produced as thick films over copper foils. The synthesis was performed by solid-state reaction at low temperature (400 °C). Similar attempts were carried out for other transition metals of the first series without success. Scanning electron microscopy (SEM) revealed the formation mechanism of the Cu 3 P thick films. First, phosphorus diffuses into the copper foil followed by the subsequent formation of the binary compound. During this process, the Cu 3 P particles seem to dig the copper foil, producing holes, where the Cu 3 P crystallites nucleate and growth. Then, the thick films are formed by the conjugation of several agglomerates and their morphology is not homogeneous. Oxidation of Cu 3 P occurs to a small extend on the top surface of the films. The electrochemical behaviour of the thick film was compared with a standard Cu 3 P composite electrode, in which the active material is mixed with carbon and a binder. Although the two different electrodes presented some differences in their electrochemical behaviour, both electrodes showed promising qualities to be used as anode materials in lithium ion batteries or hybrid devices.

Cu3P is studied as a potential material to be used as anode in a Li-ion battery. Depending on the synthetic route, solvothermal, ball-milling (with or without annealing), spray method or ceramic, used for its preparation, Cu3P shows various particle sizes and crystallinities. The electrochemical reactivity towards lithium of these various Cu3P powders is discussed through galvanostatic and potentiodynamic measurements, electron microscopy techniques, and X-ray diffraction on powder. Electrochemical performances, especially initial capacity and capacity retention, are shown to strongly correlate to the powder morphologies: small particle size favors high capacity values and the operation scan rate affects the capacity depending on the degree of crystallinity of the powder. On other hand, the battery capacity retention is better with microsized powders.

Air stable copper phosphide was synthesized as a thick film over a copper foil by a very simple solid-state reaction at low temperature. X-ray diffraction confirms that the layer is pure Cu3P. Scanning electron microscopy reveals a porous microstructure consisting of agglomerated particles with 10 μm of diameter. Electrochemical reaction of Cu3P with lithium leads to an amorphisation of the thick film during the first discharge, and Cu3P does not seem to recover upon subsequent charge. Structural and microstructural analyses coupled with electrochemical tests emphasize a complex behavior of the copper phosphide material. An initial capacity of 415 mAh/g has been measured with a stable reversible capacity close to 200 mAh/g on subsequent cycles without the help of binder and/or conductive additives. Although the gravimetric capacity values obtained with Cu3P are slightly below the graphite capacity, the volumetric capacity of Cu3P (1473 Ah/L) is 80% higher than those of graphite (800 Ah/L). Keywords: Air stable phosphides, Copper phosphide, Lithium ion batteries, Solid-state reaction

Models have been developed and used as tools to design a new 'made to measure' nickel base superalloy for power plant applications. In Part 1 Gaussian processes were used to model the mechanical pr...

A semi-industrial scale rolled bar of the creep resistant Ni base superalloy designed in Parts 1 and 2 has been fabricated. The influence of heat treatment on microstructure and phase formation has been investigated using both optical and electron microscopy, and X-ray diffraction. Mechanical properties at ambient and high temperature have been measured by hardness testing, compression testing, and tensile creep testing. These experimental results are in good agreement with the predictions of Parts 1 and 2 concerning phase formation, yield stress and creep rupture stress. The target of a lifetime of 100 000 h at 750° C under 100 MPa seems attainable. The design procedure is therefore mainly validated, and results in a promising new alloy for power plant applications.

Models have been developed and used as tools to design a new 'made to measure' nickel base superalloy for power plant applications. In Part 1, Gaussian processes are used to model the tensile and creep rupture properties of superalloys as a function of their composition and processing parameters, making use of large databases on existing alloys. The models are able to estimate the actual influence of alloying elements on the mechanical properties over a wide range of temperature and stress. They have been used, in conjunction with general metallurgical concepts and industrial requirements, as a basis for the design of a new Ni-Cr-W-Al-Ti-Fe-Si-C-B superalloy with desirable properties. It is estimated that the proposed forgeable and weldable alloy should have a creep rupture life at 750° C of 100 000 h under a stress of 100 MPa, with a huge reduction in price compared to existing commercial alloys with similar properties. In a following paper, Part 2, an attempt is made to design against the format...

Classical descriptions of the Vickers indentation technique, used for toughness measurements in dense ceramics, model the indent as an expanding sphere which induces the extension of median radial cracks. They result in the use of a proportionality factor hr relating the toughness of the material to the indentation load and to the radial crack length. Both analysis and experiment yield a value of hr depending on hardness and Young's modulus for dense ceramics. However, experimental data from Vickers indentation toughness measurements on porous YBa 2 Cu 3 O 7 mx ceramics have shown that this proportionality factor diminishes as porosity increases. This is seen as a loss of efficiency of the indent and is interpreted as the sharing of its volume with densification of the porous material. The ratio of a virtual effective indent volume to the actual volume is used to define an indentation efficiency factor, which is then measured as a function of porosity in ceramics. For cross-checking and experimental verif...