Hexagonal eutectic solidification patterns operating near a marginal stability point

We study the long-time dynamics of hexagonal directional-solidification patterns in bulk samples of a transparent eutectic alloy using an optical method which permits real-time observation of the growth front. A slow dilatation of the patterns due to a slight curvature of the isotherms drives the system into a permanent regime, close to the threshold for the rod splitting instability. Thus an apparently minor instrumental imperfection suffices to maintain the system near a marginal stability point. This answers the long-standing question of spacing selection in bulk eutectic growth

Experimental study of extruded powder metallurgy γ-TiAl based alloys

International audienceIn the context of research of lighter and/or stronger high temperature materials in order to improve aeroengines, the present work shows an example of a powder metallurgy (PM) manufacturing route for TiAl alloys that could be used for low pressure turbine (LPT) blades processing. In this frame, two different alloys, Ti-47Al-1Fe-1Zr-1Mo-0.2Si (at.%) FZM and Ti-43.5Al-4Nb-1Mo-0.1B (at.%) TNM-B1, were gas-atomized into powders, extruded, heat-treated and mechanically tested.Dans le contexte de la recherche de matériaux haute température plus légers et/ou plus résistants pour l'amélioration des performances des moteurs d'aéronefs, cet article présente un exemple de voie d'élaboration par métallurgie des poudres d'alliages base TiAl. Ces alliages sont potentiellement utilisables pour la fabrication d'aubes de turbine basse pression. Ainsi, l'alliage FZM de composition Ti-47Al-1Fe-1Zr-1Mo-0.2Si (% at.) et l'alliage TNM-B1 de composition Ti-43.5Al-4Nb-1Mo-0.1B (% at.) ont été atomisés par gaz, extrudés, traités thermiquement et testés mécaniquement

Suivi de la cinétique associée à la phase gamma' dans le superalliage N18 en utilisant des mesures de résisitivité électrique in situ

International audienceIn nickel-based superalloys, temperatures related to the formation or the dissolution of the different types of γ' precipitates are important parameters for optimizing the mechanical properties of components but also for developing models which can reproduce the kinetics of their phase transformation. We showed that the electrical resistivity variations during heat treatment of the N18 superalloy was sufficient to monitor the kinetics related to secondary and tertiary γ' precipitates. In particular, the effects of the heating rate and the initial microstructure on the dissolution kinetics of the γ' phase were investigated. Experimental results were also compared to outputs of a precipitation model developed for the N18 alloy showing that in situ electrical resistivity measurements can be used for calibration and validation purposes

Etude expérimentale par observation in-situ de la dynamique non-linéaire des fronts de solidification directionnelle eutectiques fibreux de l'alliage transparent succinonitrile-camphre

Encadrants : Silvère Akamatsu, Sabine Bottin-Rousseau Co-financement par le Centre National de la Recherche Scientifique (CNRS) et le Centre National d'Etudes Spatiales (CNES)We present an experimental investigation of the dynamics of fibrous non-faceted eutectic directional-solidification structures in semi-bulk samples. A transparent succinonitrile-d,camphor alloy is used. The whole structure is followed in real time using a new optical setup, which permits to observe the growth front in oblique view with a micronic resolution. The symmetry of the fibrous structure is basically hexagonal. We show that a residual curvature of the isotherms causes a constant stretching of the structure, and leads to a permanent regime near the upper stability threshold (branching instability). Defects are continually generated by the branching. We also show that the lower stability threshold (fibers elimination) coincides with the Eckhaus instability threshold, as measured in thin-sample experiments. We also studied other instabilities of the fibrous structure (oscillation, ovalization).Nous présentons une étude expérimentale de la dynamique des structures de solidification directionnelle eutectiques fibreuses non-facettées en échantillons semi-massifs. L'alliage transparent utilisé est le succinonitrile-camphre droit. Nous utilisons un dispositif optique original d'observation en vue oblique, qui permet de suivre en temps réel l'ensemble de la structure avec une résolution micronique. La symétrie de base de la structure fibreuse est hexagonale. Nous montrons qu'une courbure résiduelle des isothermes, provoquant un étirement continuel de la structure, conduit à un régime permanent au voisinage du seuil supérieur de stabilité (instabilité de branchement). Les branchements sont alors une source continuelle de défauts. Nous montrons aussi que le seuil inférieur de stabilité (élimination de fibres) coïncide avec le seuil de l'instabilité d'Eckhaus mesuré en échantillons minces. Nous avons étudié d'autres instabilités de la structure fibreuse (oscillation, ovalisation)

γ′ precipitation kinetics in the powder metallurgy superalloy N19 and influence of the precipitation latent heat

The N18 superalloy, powder metallurgy processed, is used for high-pressure turbine disks of the Snecma M88 engine and has been extensively studied. The size distribution of its γ′ precipitates was first modelled in the late 1980's using a binary alloy model [1]. The precipitation model has been recently revised, upgraded in a pseudo-binary alloy model and used in a multi-scale mechanical model for the calculation of the fatigue life in low cycle fatigue conditions [2]. We here present how the new calibration of the pseudo-binary model for the N19 superalloy has been accelerated through the use of the Thermo-Calc® software, its TCNI5 thermodynamic database and its MOBNI2 mobility database. On a more fundamental point of view, the influence of the γ > γ′ latent heat of precipitation on the microstructural parameters of quenched samples of N18 and N19 has been studied using this same model. We conclude on the very importance of taking into account the latent heat in a coupled thermal-microstructural model for correct numerical simulations of the precipitation process at each point of a superalloy sample or part

Small-angle neutron scattering of multiphase secondary hardening steels

International audienceAlloying secondary hardening steels with Ni and Al allows the precipitation of an intermetallic phase B2-NiAl in addition to the classical secondary carbides precipitation, adding up the advantages of both types of precipitation (Erlach et al. Mater Sci Eng A 429:96, 2006; Erlach et al. Int J Microstruct Mater Prop 3:373, 2008). Small-angle neutron scattering experiments were carried out to analyse the nanometric scale precipitation of a martensitic steel containing a double precipitation of carbides and intermetallic phase. The precipitates size, volume fraction and chemical composition for both carbides and intermetallic phases according to the tempering time were estimated and discussed. In addition, experimental cobalt-free grades containing a single precipitation or a double precipitation were manufactured and analysed. Relationship between measured tensile yield strengths and the nanometre-sized particles are suggested, showing that both populations of precipitates have a relevant impact on the mechanical properties

Matériaux haute température pour applications aéronautiques : superalliages base Ni et alliages TiAl

International audienceTwo outstanding aerospace-oriented high-temperature materials, the single-crystal nickel-based superalloys for high-pressure turbine blades and the γ-TiAl-based alloys for low pressure turbine blades, are being presented here. In both cases, the optimisation of their mechanical properties is based on a high knowledge of metallurgy, mixing together different aspects such as processes, alloy design, deformation mechanisms, impact of oxidative environment or interaction between protective layers and protected alloy. Historical evolutions are recalled and put into perspective with more recent research activities.Deux familles de matériaux haute température à finalités aéronautiques, les superalliages monocristallins à base de nickel pour aubes de turbine haute pression et les alliages à base de TiAl pour aubes de turbine basse pression, sont présentées ici. Dans les deux cas, l'optimisation de leurs propriétés mécaniques repose sur une connaissance approfondie de la métallurgie, associant différents aspects tels que les procédés, le développement d'alliages, les mécanismes de déformation, l'impact du milieu oxydant ou l'interaction entre le revêtement protecteur et l'alliage protégé. Les évolutions historiques sont rappelées et mises en perspective avec des activités de recherche plus récentes

Long-time dynamics of the directional solidification of rodlike eutectics

International audienceWe report long-duration real-time observations of the dynamics of hexagonal (rodlike) directional-solidification patterns in bulk samples of a transparent eutectic alloy. A slight forward curvature of the isotherms induces a slow dilatation of the growth pattern at constant solidification rate and triggers the rod-splitting instability. At long times, the rod-splitting frequency exactly balances the dilatation driven by the curved isotherms. The growth pattern is then disordered and nonstationary but has a sharply selected mean spacing. Well-ordered growth patterns can be grown using time-dependent solidification rates

Dynamic instabilities of rod-like eutectic growth patterns: A real-time study

International audienceWe present a real-time experimental study of the rod-like growth patterns formed during directional solidification in a non-faceted transparent eutectic alloy, succinonitrile (d)camphor. Slightly convex isotherms were used to slowly increase the pattern spacing 2 from an appropriate starting value to the threshold spacing for rod elimination or rod-splitting instabilities allowing a quantitative determination of these thresholds as a function of the solidification rate V. We show that the threshold spacing for rod splitting obeys the general lambda similar to V-1/2 scaling law of eutectic growth, whereas the threshold spacing for rod elimination deviates from this law at low V, exhibiting the same overstability effect as previously reported for lamellar eutectic patterns. We demonstrate that topological defects (walls between hexagon domains) play an important role in rod-splitting processes. We also describe a spatially incoherent mode of oscillation that we observed in disordered rod-like patterns. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved