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The peak of the variation in the critical strain energy release rate G 1Ic with porosity, as previously measured on sintered glass beads, is modelled. The approach is an adaptation of an existing model, based on the description of the actual microstructure as resulting from the densification of an initial stacking of spheres. A previous model made use of a unique reference toughness K Ic0 of a hypothetical bulk material and as such was valid only if the micromechanisms of crack propagation were the same over the whole range of porosity. In the present case, the observed change in fracture micromechanisms with porosity in sintered glass beads is considered. The analysis of the microstructure and crack path morphologies enables specific toughnesses to be attributed to the mechanisms observed at the extreme values of porosity: crack pinning by pores at low porosities and rupture of the sintering necks at high porosities. The transition from one to the other is expressed by an evolution with porosity of the v...

International audience; This study reports on the incorporation of the self-setting polysaccharide derivative hydrogel (silanized-hydroxypropyl methylcellulose, Si-HPMC) into the formulation of calcium phosphate cements (CPCs) to develop a novel injectable material for bone substitution. The effects of Si-HPMC on the handling properties (injectability, cohesion and setting time) and mechanical properties (Young's modulus, fracture toughness, flexural and compressive strength) of CPCs were systematically studied. It was found that Si-HPMC could endow composite CPC pastes with an appealing rheological behavior at the early stage of setting, promoting its application in open bone cavities. Moreover, Si-HPMC gave the composite CPC good injectability and cohesion, and reduced the setting time. Si-HPMC increased the porosity of CPCs after hardening, especially the macroporosity as a result of entrapped air bubbles; however, it improved, rather than compromised, the mechanical properties of composite CPCs, which demonstrates a strong toughening and strengthening effect. In view of the above, the Si-HPMC composite CPC may be particularly promising as bone substitute material for clinic application. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

International audience; Since their initial formulation in the 1980s, calcium phosphate cements (CPCs) have been increasingly used as bone substitutes. This article provides an overview on the chemistry, kinetics of setting and handling properties (setting time, cohesion and injectability) of CPCs for bone substitution, with a focus on their mechanical properties. Many processing parameters, such as particle size, composition of cement reactants and additives, can be adjusted to control the setting process of CPCs, concomitantly influencing their handling and mechanical performance. Moreover, this review shows that, although the mechanical strength of CPCs is generally low, it is not a critical issue for their application for bone repair - an observation not often realized by researchers and clinicians. CPCs with compressive strengths comparable to those of cortical bones can be produced through densification and/or homogenization of the cement matrix. The real limitation for CPCs appears to be their low fracture toughness and poor mechanical reliability (Weibull modulus), which have so far been only rarely studied. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Macroscopic toughness measurements and the Vickers indentation technique have been used to characterise sintered or melt-textured superconducting YBa 2 Cu 3 O 7− x ceramics. A newly developed thermal shock resistance parameter, based on the Vickers indentation technique, has been used. The crack propagation mechanisms observed in sintered ceramics have been analysed with respect to residual stress fields due to the anisotropy in thermal expansion coefficients, or to the difference with that of the Y 2 BaCuO 5 (‘211’) inclusions. The addition of the ‘211’ brittle phase does not increase the toughness and thermal shock resistance of sintered ceramics, but toughens the textured materials. An enhancement of the thermal shock resistance, through an increased R-curve behaviour, has been achieved by introducing a metallic silver ductile phase into the microstructure, both in sintered and textured ceramics. The evaluation of the latter needed the extension of the thermal shock resistance parameter to the case of Palmqvist indentation cracks. Parallelly, a modelling of the thermal shock behaviour of textured ceramics is proposed through the description of the microstructure as a stacking of YBa 2 Cu 3 O 7− x layers cracked along the (001) planes, joined by Y 2 BaCuO 5 particles, corresponding to a low-stiffness material.

A biphasic calcium phosphate (BCP) powder is synthesized, mixed with various amounts of naphthalene particles, pressed and sintered at different temperatures to obtain ceramics with isolated macropores (from 3% to 52% of the specimen volume) and residual microporosity resulting from an incomplete sintering (from 2% to 45% of the ceramic matrix volume). Young’s modulus is measured on a classical three-point bending setup. A good agreement is found between an existing analytical model and the experimental Young’s modulus measurements, on the overall ranges of macro- and microporosity. The Young’s modulus variation as a function of macroporosity is also calculated by a finite element method, by simulating the mechanical response of a periodic tri-dimensional repetition of elemental cubes containing various dispersions of macropores. The contribution of the shape and dispersion (in size and location) of the macropores on the decrease of Young’s modulus as a function of porosity is simulated. Calculated trends are confirmed by experimental results.

YBa 2 Cu 3 O x high- T c ceramic superconductors have been elaborated by the melt-textured-growth method with additions of Ag particles and/or Y 2 BaCuO 5 (211 green phase) in order to study the fracture properties and the thermal shock resistance. The Vickers indentation technique, initially devoted to the sole determination of the toughness has been used in conjunction with a new thermal shock resistance parameter. It makes use of the initial stage of stable propagation of Vickers indentation cracks when submitted to a thermal stress, and results in a generalisation of the fourth parameter of Hasselman. Whereas the toughness increases with increasing 211 content, does the thermal shock resistance decrease. This is explained with respect of the strongly micro-cracked (along the 001 planes) microstructure which is seen as a stacking of dense laminates bound by the 211 particles and subjected to a linear temperature gradient. The higher the number of bounding particles, the stiffer the quenched material, and the higher the induced thermal stress. This micro-mechanical model is implemented into a finite element software. The first results of the calculations confirm the above approach.

Commercial YBa2Cu3O7-x powders have been cold isostatically pressed and pressureless sintered at different temperatures and for various times to obtain porous ceramic samples (5–31 vol.%). The decrease in the mechanical properties with increasing porosity has been studied within the framework of two models, up to now concerned with elastic, that is reversible, behaviours. One model creates porosity from a dense bulk by shrinking randomly its constituting elements and yields a power-law description, whereas the second model describes a densifying microstructure with a given initial stacking of particles and yields a geometrical dependence. Both fit rather well the change in Young's modulus as porosity increases. They were then used to describe the elastic strain energy density of a porous body. After homogenization, a criterion for crack propagation is derived, which describes toughness over the whole domain of porosity. Finally an investigation into the applicability of the indentation technique ...

International audience; A new alloy design procedure is proposed, combining in a single computational tool several modelling and predictive techniques that have already been used and assessed in the field of materials science and alloy design: a genetic algorithm is used to optimize the alloy composition for target properties and performance on the basis of the prediction of mechanical properties (estimated by Gaussian process regression of data on existing alloys) and of microstructural constitution, stability and processability (evaluated by computational themodynamics). These tools are integrated in a unique Matlab programme. An example is given in the case of the design of a new nickel-base superalloy for future power plant applications (such as the ultra-supercritical (USC) coal-fired plant, or the high-temperature gas-cooled nuclear reactor (HTGCR or HTGR), where the selection criteria include cost, oxidation and creep resistance around 750 °C, long-term stability at service temperature, forgeability, weldability, etc.

International audience; The influence of cellulose ether additives (CEAs) on the performance of final calcium phosphate cement (CPC) products is thoroughly investigated. Four CEAs were added into the liquid phase of apatitic CPCs based on the hydrolysis of α-tricalcium phosphate, to investigate the influence of both molecular weight and degree of substitution on the CPCs' properties, including handling (e.g. injectability, cohesion, washout resistance and setting time), microstructure (e.g. porosity and micromorphology) and mechanical properties (e.g. fracture toughness and compressive strength). The results showed that even a small amount of CEAs modified most of these CPCs' features, depending on the structural parameters of the CEAs. The CEAs dramatically improved the injectability, cohesion and washout resistance of the pastes, prolonged the final setting time and increased the porosity of CPCs. Moreover, the CEAs had an evident toughening effect on CPCs, and this effect become more significant with increasing molecular weight and mass fraction of CEAs, inducing a significant tolerance to damage. Overall, the molecular weight of CEAs played a major role compared to their degree of substitution in CPCs' performances.

Calcium phosphate cements have been used in medical and dental applications for many years. However, their low strength and their high brittleness prohibit their use in many stress-bearing locations, which would require an improvement in mechanical properties. The influence of microstructural parameters on the latter has nevertheless barely been investigated in a systematic manner. To this aim, apatite cements have been fabricated through alpha-TCP (alpha-tricalcium phosphate) hydrolysis, and their mechanical properties have been measured (Young's modulus, fracture toughness and compressive strength), as a function of various parameters (particle size, liquid-to-powder ratio, amount and morphology of porosity, including macropores created by mannitol particles used as porogen). Five days following the mixing of phases, identification and microstructural observation indicated the presence of unreacted alpha-TCP particles, exhibiting very weak links with the apatite matrix and often surrounded by microcracks. The latter provoke a decrease in Young's modulus. The coarser the microstructure, the larger the critical flaw size causing fracture. In the case of macroporous materials, the critical flaw size increases with macroporosity. The knowledge gained should allow, in the end, to improve mechanical properties by controlling the microstructure, and to find a better compromise between strength and biological behaviour. (C) 2010 Elsevier B.V. All rights reserved.

Heat resistant steels of the HP-series have widespread uses in the petrochemical industry in pyrolysis and reformer furnaces. The alloys are carbon-rich Fe-Ni-Cr alloys, with additions like Mn, Si, Nb, Ti, W... The typical microstructure of as-cast HP alloys is an austenite matrix with intergranular eutectic-like primary chromium carbides of the M7 C3 type and niobium carbides of the MC type. Upon ageing, phase transformations occur. Intragranular secondary M23 C6 carbides precipitate, which is thought to restrict dislocation motion, and intergranular M7 C3 transforms into M23 C6 . Under certain thermal conditions, a partial transformation of the primary niobium carbides into a nickel-niobium silicide called G phase can occur. These phases may play a critical role during creep, but neither their role on mechanical properties nor the mechanisms of phase transformations are clearly identified. The aim of this study is to understand the role of each phase or phase transformation in the creep resistance of HP alloys. Consequently, a critical review of phase formation and transformations in such alloys is presented using a set of experimental and modelling techniques (electron microscopy, Castaing microprobe, creep tests at high temperature and neural networks modelling of mechanical properties...).Copyright © 2007 by ASME

A clarification is made on the nature of phase transformations dealt with in a recent article [F. Tancret, Computational Materials Science 4 (2007) 13-19]. The two modelled phenomena are in fact of a different nature, i.e. the exclusive melting of γ′ on the one hand, and the eutectic-type constitutional liquation of γ′ on the other hand.

The abilities of artificial neural networks and Gaussian processes to model the yield strength of nickel-base superalloys as a function of composition and temperature have been compared on the basis of simple well-known metallurgical trends (influence of Ti, Al, Co, Mo, W, Ta, of the Ti/Al ratio, γ' volume fraction and the testing temperature). The methodologies are found to give similar results, and are able to predict the behaviour of materials that were not shown to the models during their creation. The Gaussian process modelling method is the simpler method to use, but its computational cost becomes larger than that of neural networks for large data sets.