Showing papers by "Mines ParisTech published in 1973"

Morphology of y’ and y” precipitates and thermal stability of inconel 718 type alloys

Abstract: The precipitation of the γ’ (Ll2) and γ" (DO22) phases has been studied in four alloys Fe-Ni-Cr-Ti-Al-Nb containing a higher Ti + Al/Nb ratio than that of the INCONEL 718 alloy. For these alloys, the precipitation microstructure varies rapidly with aging temperature and composition. Bct γ"particles have always been found to precipitate on γ’ phase. Moreover, by aging three alloys above a critical temperature, a “compact ntorphology” has been observed: cube-shaped γ’ particles coated on their six faces with a shell of γ" precipitate. This microstructure has proved to be very stable on prolonged aging. A thermal stability better than that encountered in nominal INCONEL 718 alloy can thus be achieved. The influence of composition and aging temperature on the conditions that bring about this “compact morphology” has been investigated. A minimal Ti + Al/Nb ratio between 0.9 and 1 has been determined, allowing the “compact morphology” to be obtained.

Systèmes de lévy des processus de markov

Abstract: L'existence des syst6mes de L6vy est intimement li6e ~ la repr6sentation des fonctionnelles additives purement discontinues h sauts totalement inaccessibles. Nous nous proprosons dans cet article de donner, dans un cadre par ailleurs plus g6n6ral, une nouvelle d6monstration tr+s simplifi~e de l'existence du syst+me de L6vy et du th6or~me de repr6sentation des fonctionnelles additives. Passons rapidement en revue les diff6rentes m6thodes employ6es pour arriver aux syst6mes de L6vy: a) Motoo et Watanabe [11, 14] 6tudient les premiers ces deux probl6mes pour les processus de Hunt v6rifiant l'hypoth+se (L): les deux questions sont trait6es presque parall+lement h l'aide des int6grales stochastiques, qui donnent en outre d'autres r~sultats (voir aussi Meyer [7,1 et Benveniste [1]). b) Weil [15,1 d6montre ensuite que l'existence des syst+mes de L6vy entraine une propri6t~ de Markov pour le pass6 strict, par rapport aux temps totalement inaccessibles.

The effect of loading mode on hydrogen embrittlement

Abstract: Hydrogen embrittlement is shown to occur very easily in notched-round bars under opening modeI (tension) but not under antiplane shear modeIII (torsion). The stress tensor invariants under modeI,II, andIII loadings and how these affect interstitial diffusion are discussed. It is suggested that long range diffusion of hydrogen down orthogonal trajectories to the vicinity of the crack tip, which can occur under modeI but not modeIII, is a key part of any hydrogen embrittlement mechanism. This premise was evaluated with AISI 4340 steel heat treated to ultrahigh strength levels. It was found that an initial modeI stress intensity level of 17,000 psi-in.1/2 produced failure in several minutes. ModeIII stress intensity levels three times this produced no crack initiation in 300 min. Further analysis of the time-dependent hydrogen concentrating effect utilized a stress wave emission technique. This produced plausible critical hydrogen concentrations even though the present elastic analysis is a first order approximation of the stress field.

Flow stress and microstructure in superplastic 60/40 brass

Abstract: Stress-strain curves have been obtained for a superplastically deformed industrial 60/40 brass tested in tension under constant true strain-rate conditions at 600‡C. This alloy produced by extrusion shows a fibrous structure with elongated grains. It is shown that the flow stress is influenced by two microstructural factors, i.e. the change of phase shape which becomes approximately equiaxed and the variation of the phase size during superplastic deformation. These factors are of different relative importance according to the strain-rate.

Etude en microscopie électronique du contraste des dislocations décorées dans des structures cubiques

Abstract: Une etude experimentale en microscopie electronique des dislocations decorees dans deux structures cubique centree et cubique a faces centrees a mis en evidence, comme principales caracteristiques de la decoration, un elargissement de l'image et l'apparition de deux ou plusieurs franges noires. Pour expliquer ces phenomenes, nous avons utilise un modele dans lequel le precipite est represente par un cylindre superpose a la dislocation et introduisant dans la matrice un champ de deplacement supplementaire. Les profils d'intensite et les images simulees calcules sur ordinateur a partir de ce modele sont en bon accord qualitatif avec les observations experimentales, notamment en ce qui concerne l'elargissement de l'image et l'apparition des franges. An experimental investigation of decorated dislocations in body and face centered cubic structures, using transmission electron microscope, has shown the presence of two or more dark fringes and an increased image width. The experimental observations are explained on the basis of a model postulating a cylindrical precipitate surrounding the dislocation and the accompanying additional strain field in the matrix. Computed intensity profiles and simulated images are in good agreement with the experimental observations of image broadening and fringe number.

Failure mechanisms of aluminide and silicide coatings on a niobium alloy subjected to thermal cycling in air

Abstract: Failure during thermal cycling in air of silicide and aluminide coatings on a niobium-base alloy has been investigated and failure mechanisms proposed. Most failures occurred at or very near geometrical transitions between the curved and flat sections of the specimen. In the neighborhood of these transitions, cracks form in the coating at oblique angles to the specimen surface due to non-uniform deformation induced by the thermal stress. These oblique cracks allow easy detachment of coating segments and exposure of the substrate to the oxidizing environment. Cracks in aluminide coatings were found to multiply much more rapidly during cycling than in suicide coatings. This leads to the development of numerous oblique cracks and consequently the protective life of aluminide coatings is well below that of silicide coatings.

Étude du système ternaire Niobium-Oxygène-Soufre et estimation des propriétés thermodynamiques des sulfures de niobium

Abstract: Resume Le systeme ternaire Nb-O-S a ete etudie dans la region delimitee par NbO, Nb 1− x S , NbS 2 et Nb 2 O 5 . Seules les phases connues dans les deux systemes binaires Nb-S et Nb-O ont ete observees. Dans sa configuration la plus stable la phase Nb 1− x S est en equilibre avec NbO ou NbO 2 suivant sa composition. La transition NbO 2 -Nb 2 O 5 a lieu pour la limite riche en soufre de 2s Nb 1+ x S 2 . Les phases 3s Nb 1+ x S 2 , 3s NbS 2 et Nb 2 O 5 sont en equilibre vers 750°C pour p S 2 = 1 atm. A partir de ces resultats et de ceux de Larson et Elliott, l'enthalpie libre de formation de Nb 1,12 S 2 a ete calculee et celle des autres sulfures estimee a 1000°C.

Study on the Size and Location of Pores During Grain Growth in Hydrogen Sintered Al2O3

Abstract: Possible mechanisms limiting the rate of grain growth in porous compacts are briefly recalled. Experimental results obtained with an automatic “Texture Analyzer” on pore size distributions in hydrogen-fired, undoped Al2O3 are presented. Effects of the position of an inclusion relative to the grain boundary network are discussed with special reference to energetic considerations and identification of the elementary processes that are necessary for the average grain diameter to increase.

Évolution des indications de l'exsanguino-transfusion chez le nouveau-né

Abstract: Resume Les indications de l'exsanguino-transfusion du nouveau-ne ont evolue ces dernieres annees. Il apparait aux auteurs a l'analyse de 1 200 cas personnels (1969 a 1972) que la principale indication est desormais la prematurite et l'insuffisance hepatique. Cependant, un certain nombre d'interventions est encore justifie par l'allo-immunisation fœto-maternelle anti-Rhesus et par le conflit immunologique mere-enfant dans le systeme ABO. Les interventions sont renouvelees (dans la regle 2 fois) dans un quart des cas. Une analyse des parametres biologique et clinique (poids de l'enfant — taux initial de bilirubine — titre de l'anticorps anti D — positivite plus ou moins forte du test de C.M.R., etc...) permet dans une certaine mesure de prevoir si, chez un enfant, une ou plusieurs exsanguino-transfusion est necessaire.