Liquid metal embrittlement studies on model systems wiht respect to the spallation target technology: the importance of nanometre-thick films

Liquid metal embrittlement (LME) is illustrated on the Cu-Bi and Cu-PbBi systems at 300 C using either constant strain-rate tests at 10(-4) s(-1) or constant load tests at 25% of yield stress. Intergranular penetration was studied in the Ni-Bi system at 700 C and was shown to result in the formation of slowly growing micrometre-thick and rapidly growing nanometre-thick films. Both induce very strong intergranular brittleness but only micrometre-thick films are visible by SEM on polished cross-sections. Nanometre-thick films were analysed by Auger electron spectroscopy after "in situ" fractures within the spectrometer; in particular, a series of analyses on an Ni bicrystal proved the constant thickness of this film over several hundreds of microns. The severity of embrittlement due to bismuth penetration was confirmed in the analysis of the Ni-PbBi system. Based on these results, it is underlined that technological systems like T91 steel/Pb or T91/PbBi should work at temperatures below the wetting transition temperature (T-W), i. e. in the temperature range where intergranular films can't form. If it is not the case, nanometre-thick films should be taken into account, both in the evaluation of the kinetics of embrittlement and in the modelling of intergranular penetration and LME

Evidence for a diffusion-based mechanism of liquid metal intergranular penetration : case study of a Ni-Bi model system

A model Ni-Bi system has been used to investigate intergranular penetration (IGP) phenomenon. All experiments have been done on Ni 26° bicrystal at 700°C using bismuth vapour condensation as a source of liquid bismuth. Such a procedure results at room temperature in either partial or total Liquid Metal Induced Embrittlement (LMIE) of a unique grain boundary, depending on the duration of liquid Bi / solid Ni contact at 700°C. Auger Electron Spectrometry (AES) and Rutherford Backscattering Spectrometry (RBS) have been used to measure the Bi concentration profile between the source of liquid bismuth and the penetration front. Two zones have been clearly identified : the first one of almost constant Bi concentration called nanometrethick film which is interpreted in terms of Fowler-Guggenheim multi-layer segregation under local equilibrium conditions and the second one with a progressive decrease of Bi concentration over a distance of the order of 20-200μm. Such a long transition zone, together with parabolic diffusion kinetics indicates diffusion-based mechanism of intergranular penetration as opposed to the direct grain boundary wetting

MODÈLES THÉOTIQUES DE LA STRUCTURE DES JOINTS DE GRAINS.LES MODÈLES DE STRUCTURE DES JOINTS DE GRAINS ET LEUR UTILISATION

Cet article de synthèse examine les fondements, les limitations et les champs d'application des deux grandes catégories de modèles (géométriques et énergétiques). Les deux approches sont en fait complémentaires : les méthodes géométriques peuvent constituer une première étape et sont en tout cas bien adaptées à l'interprétation des micrographies électroniques de joints de grains ; les méthodes énergétiques sont à la fois moins critiquables dans le principe, plus précises et plus puissantes, ouvrant de larges possibilités d'exploitation.In this review paper, the bases, the limitations and the domains of application of the geometric and energetic models are examined. These two approachs are complementary : the geometric methods can be considered as a first approximation and allow in general the interpretation of electron micrographies of grain boundaries ; the energetic methods are on principle less critizisable, more precise and more powerfull, allowing numerous possibilities of utilization

STRUCTURE ET PROPRIETES MECANIQUES DES JOINTS DE GRAINS

On rapporte des expériences faites à l'aide de bicristaux orientés, montrant l'effet de la désorientation relative sur divers comportements mécaniques des joints de grains : transmission des déformations d'un cristal à son voisin, glissement intergranulaire, fissuration. Pour rendre compte de ces propriétés mécaniques des joints de grains, on est amené à prendre en compte aussi bien les divers aspects de la structure des joints (structure idéale et défauts intergranulaires) que le comportement propre de la matrice. Cependant, même dans des cas bien déterminés de bicristaux soumis à une sollicitation simple, la modélisation du comportement mécanique des joints de grains n'est, en général, pas très avancée.We report experiments performed on oriented bicrystals, concerning the effect of the relative misorientation on varied mechanical grain boundary behaviours : transmission of the deformation from a crystal to its neighbour, intergranular gliding, grain boundary cracking. In order to explain these mechanical properties of grain boundaries, it is necessary to take into account the different elements of the grain boundary structure (ideal structure and grain boundary defects) and also the mechanical behaviour of the matrix. However, even in very simple cases (a bicrystal submitted to a simple external stress field), the models are poorly developed in general

OXYGEN SEGREGATION IN 32° [001] MOLYBDENUM TILT GRAIN BOUNDARY II. ENTHALPIES OF SEGREGATION DETERMINATION FROM AES AND GRAIN BOUNDARY GROOVES MEASUREMENTS

On présente des mesures de concentration d'oxygène dans un joint de flexion 32° autour de [001] du molybdène, en fonction de la température de trempe et des mesures de tension intergranulaire en fonction de la concentration intergranulaire, pour le même joint. On propose une description thermodynamique de la ségrégation intergranulaire, prenant en compte les différents sites de ségrégation et on discute la relation entre tension et énergie intergranulaire. L'enthalpie de ségrégation intergranulaire d'oxygène dans les différentes sites du joint étudié a été estimée à -2.4 eV par atome.Measurements of oxygen concentration in a molybdenum 32°[001] tilt grain boundary versus quench temperature and measurements of grain boundary interfacial tension versus grain boundary concentrations are presented. A grain boundary segregation thermodynamic description involving different segregation sites is proposed. The relation between grain boundary tension and energy is discussed. Grain boundary oxygen segregation enthalpies in the different sites are found to be about -2.4 eV per atom

INTERGRANULAR SEGREGATION OF BISMUTH IN COPPER BICRYSTALS

On a analysé, par spectrométrie Auger, des joints fracturés "in situ" dans des bicristaux de cuivre, de flexion autour de , dopés en bismuth. Pour une désorientation donnée, on précise d'abord certains aspects thermodynamiques et cinétiques de la ségrégation intergranulaire du bismuth. Puis, pour des conditions fixées, on a déterminé comment varie cette ségrégation avec l'angle de désorientation.In Cu-Bi tilt bicrystals, "in situ" fractured grain boundaries have been analysed by Auger spectrometry. For a given misorientation, certain thermodynamic and kinetic features of segregation have been evaluated. Then, under a given set of conditions, the influence of misorientation angle on grain boundary segregation of Bi has been determined

High-Temperature Brittleness and Interfacial Segregation in Tin Bronzes

This paper presents preliminary results of Auger electron spectroscopy performed on fractured specimens of copper and tin bronzes at the time of in situ hot tensile tests. They show interfacial segregation of Sn, P and S, according to the materials and the test temperature. In comparing these results and the mechanical behaviour of the materials, we discuss the effect of the interfacial segregations on the drop in ductility at intermediate temperatures and on its restoration by dynamic recrystallisation

FLUAGE INTERGRANULAIRE DE BICRISTAUX D'ALUMINIUM ET DE CUIVRE : ÉTUDE EXPÉRIMENTALE ET INTERPRÉTATION PAR DES DISLOCATIONS INTERGRANULAIRES CALCULÉES PAR ORDINATEUR

Les résultats expérimentaux montrent que le fluage intergranulaire pur, en l'absence de déformations intracristallines, est extrêmement sensible à des facteurs liés à la structure des joints de grains, comme la désorientation et la direction du glissement dans le plan de joint. Ces résultats suggèrent aussi que les phénomènes qui sont à la base du fluage intergranulaire ne sont pas essentiellement lacunaires, mais pourraient être le déplacement de dislocations intergranulaires extrinsèques (ou hors d'équilibre). La structure et certaines propriétés de ces dislocations sont étudiées par une méthode de simulation sur ordinateur, dérivée de celle qui conduit à la structure d'équilibre des joints de grains ; les résultats de ces calculs sont confrontés aux résultats expérimentaux.The present experimental results show that grain boundary sliding, in the absence of intragranular strain, is very sensitive to factors related to the grain boundary structure, such as the misorientation and the sliding direction in the grain boundary plane. These results suggest also that grain boundary sliding is not essentially based upon a movement of vacancies, but could result of the displacement of extrinsic (non-equilibrium) grain boundary dislocations. The structure and certain properties of such dislocations are studied by a method of computer simulation, derived from the method which leads to the equilibrium grain boundaries structure ; the results of the calculations are compared with the experimental results

The influence of the angle between the grain boundary and the precipitate habit plane on the intergranular morphology of thêta' in Al-Cu.

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