Showing papers by "Mines ParisTech published in 1974"

Sintering behaviour of ultrafine NbC and TaC powders

Abstract: Compositional and microstructural changes upon firing ultrafine (300 to 400 A) stoichiometric NbC and TaC powder lots have been studied up to a temperature of 1600° C. Substantial amounts of oxygen impurities, mostly oxide particles or layers are eliminated by reductions with hydrogen, free carbon or the carbides themselves. TGA showed these reactions to take place at 700 to 1400° C with maxima around 1000 to 1100° C. Low temperature sintering is inhibited by this impurity and its removal is thus essential. Other impurities (Ni, Cr, Fe) were also found in the starting powders in total concentration 0.5 to 1%. They give rise to a liquid phase located at grain edges at temperatures as low as 1100° C which then controls microstructure development. It dissolves to some extent in the carbide matrix at high temperature, and has a tendency to rise to the free surface of the samples. Compositional and structural heterogeneities are thus produced between bulk and surface at high temperatures. Owing to these impurity effects, it was not possible to clearly evaluate the influence of powder granulometry.

Temper embrittlement and intergranular segregation of antimony: A quantitative analysis performed with the backscattering of energetic ions

Abstract: The intergranular segregation of antimony associated with temper embrittlement in a low carbon manganese steel was quantitatively studied through the backscattering of MeV12C12+ and1214N12+ ions. The principles of the technique and its application to interface segregation problems are briefly explained and its main advantages discussed. The influence of various heat treatments was investigated and shown to strongly influence the segregation taking place in the α field. Segregation could not be detected in the γ field. The kinetics of the phenomenon in the critical range (400° to 600°C) is described. The role of the micro-structure was studied and it is shown that segregation does not occur only at the previous austenitic grain boundaries but at all the disordered high angle boundaries of the structure. The grain boundary Sb content after a reversion and a resegregation treatment was also studied. The results are interpreted in terms of a reversible type of segregation taking place entirely in the α phase.

Reductibilite des systemes hereditaires

Abstract: Resume On considere, dans R n , des systemes evolutifs hereditaires. On donne d'abord une condition necessaire et suffisante pour qu'ils soient reductibles a des systemes dynamiques (c'est a dire regis par des equations differentielles ordinaires), ce qui permet egalement de montrer qu'il existe des systemes hereditaires irreductibles. On montre ensuite que tous peuvent cependant etre ramenes a des systemes regis par des equations non hereditaires, a condition d'introduire un champ interne regi par des equations aux derivees partielles. Deux exemples, l'un tire de la rheologie, l'autre de l'electromagnetisme, illustrent ce dernier point.

Capillary effects in the growth of thin Pb-Sn eutectic specimens

Abstract: The growth structure of thin Pb-Sn eutectic specimens has been investigated. The major features observed were the orientation of the lamellar structure by the external surfaces, the stability of lamellae at very low growth rates, the occurrence of perfect lamellar regions within eutectic grains and of a new type of disturbance in the structure due to holes or inclusions. The predictions based on the influence of capillary effects during growth were qualitatively in good agreement with the experimental evidence.

Résistivité électrique d'un niobium de haute pureté de 20 K à la température de fusion. II. Analyse des résultats

Abstract: Resume Les auteurs ayant presente dans une publication precedente 1 leurs mesures de resistivite du niobium de 20 K a la temperature de fusion, se proposent d'analyser les resultats obtenus. Il est d'abord montre que le niobium verifie bien, aux temperatures elevees, la theorie de Mott; en particulier, la courbure observee sur la courbe de resistivite traduit la variation de la densite d'etats au niveau de Fermi lorsque la temperature s'eleve, mettant ainsi en evidence la preponderance des transitions s-d. D'autre part, de 20 a 300 K, un ajustement de la courbe experimentale avec une loi de Bloch-Gruneisen ou de Wilson est impossible et plusieurs arguments conduisent a penser que la determination, meme approchee, d'une temperature de Debye est sans signification physique. Il est en effet impossible de negliger les processus U.

B-A1 Composite Failure Under Combined Torsion and Tension Loading:

Abstract: An essentially experimental investigation was made regarding the effect of combined torsion and tension loading on the fracture characteristics of an aluminium matrix, unidirectional boron fiber reinforced composite. Three types of loading path were considered. The experiments were performed on notched specimens thereby allowing the fracture behaviour to be tentatively interpreted in terms of linear elastic fracture mechanics.The combined mode fracture loads did not appear to obey a constant energy of fracture criterion. This conclusion is supported by a substantial difference in fracture surface morphology for pure tension and pure torsion loading. However, the lack of agreement between theory and experiment may be due to data reduction errors caused by the tendency for stable crack growth and plastic shear between the fiber and matrix to increase as the ratio of torsion to tension loading is increased. The fracture loads were found also to be loading path dependant with radial loading being the most severe.

Relations entre magmas volcaniques et zones séismiques dans les arcs Honshu et Hokkaido

Abstract: The relationship between chemical composition of volcanic rocks and vertical depth from the volcano to the inclined seismic zone has been investigated by some authors only recently.