Francis Delannay

The wetting of solids by molten metals and its relation to the preparation of metal matrix composites

This review aims at making a bridge between the fundamentals of the wetting of solids by liquid metals and the practice of the preparation of metal-matrix composites. One recalls first the significance of concepts such as surface tension, work of adhesion, adsorption and the relation between these concepts, the phenomenon of wetting and the process of liquid metal infiltration. Thereafter, the wetting of various types of solids is considered: metals, oxides, carbon and carbides. !n the !light of this body of science, one proposes finally a critical evaluation of the literature concerning the preparation of metal-matrix composites by liquid metal infiltration techniques. Particular emphasis is devoted to reinforcements made of graphite, alumina or silicon carbide multifilament fibres; the use of coatings and the addition of alloying elements to the metal are successively discussed.

Metallographic methods for revealing the multiphase microstructure of TRIP-assisted steels

Classical etching techniques used for the investigation of steel microstructures allow the simultaneous observation of only a restricted number of phases. So far, this limitation has not been too detrimental, because most low-carbon steel grades possess a quite simple microstructure. The recent interest in the so-called TRIP-assisted multiphase steels characterized by complex microstructures requires new developments in metallographic methods. This paper proposes an extension of already known techniques to allow the study of four kinds of TRIP-aided steels. The actual restrictions justifying the development of an improved method are emphasized. In spite of its simplicity, the procedure has the advantage of allowing the simultaneous observation of the four phases that generally compose the microstructure of TRIP-assisted steels; that is, ferrite, bainite, austenite, and martensite. Light and electron microscopy as well as diffraction techniques are used to demonstrate the interest of the method.

Experimental and numerical comparison of void growth models and void coalescence criteria for the prediction of ductile fracture in copper bars

Ductile fracture of round copper bars is investigated within the scope of the local approach methodology Two damage models are analysed: the Rice and Tracey model and the Gurson-Leblond-Perrin model. A comparative study of four coalescence criteria is conducted: a critical value of the damage parameter, the Brown and Embury criterion, the Thomason criterion and a criterion based on the reaching of the maximum von Mises equivalent stress in a Gurson type simulation. Ellipsoidal void growth and void interaction are accounted for. As far as possible, all the parameters of the models are identified from experiments and physical observations. The effect of stress triaxiality is studied using specimens presenting a wide range of notch radii. The effect of strain-hardening is analysed by comparing the behaviour of the material in the cold drawn state and in the annealed state

The influence of high sintering temperatures on the mechanical properties of hydroxylapatite

The kinetics of the thermal decomposition of stoichiometric hydroxylapatite (HA) has been studied up to 1500°C for the purpose of determining the maximum admissible combinations of temperature and time for sintering HA. The influence of the sintering temperature on shrinkage, density and grain growth is then investigated in the temperature range from 1000 to 1450°C. Nearly theoretical density was achieved above 1300°C. A maximum fracture toughness is obtained for the samples sintered at 1300°C whereas hardness increases up to a sintering temperature of 1400°C. These results are discussed in terms of the roles of porosity and grain size.

On the sources of work hardening in multiphase steels assisted by transformation-induced plasticity

The mechanisms effectively responsible for the enhancement of the work-hardening capabilities of multiphase steels assisted by transformation-induced plasticity are highlighted. Different microstructures, some containing a proportion of retained austenite with various mechanical stabilities, are studied. The dislocation density generated within ferrite by the mechanically induced martensitic transformation of retained austenite is shown to scale with the incremental work-hardening exponent. The acoustic emission generated during tensile straining was also measured. The acoustic emission was revealed to result mainly from dislocation motion, especially from the motion of the additional dislocation density generated in intercritical ferrite by the strain-induced martensitic transformation.

On the measurement of the nanohardness of the constitutive phases of TRIP-assisted multiphase steels

The nanohardness of the phases present in the microstructure of two TRIP (for TRansformation Induced Plasticity)-assisted multiphase steels differing by their silicon content was measured by nanoindentation in an atomic force microscope. It is observed that the softest phase in both steels is the ferritic matrix, followed by bainite, austenite and martensite. It is also shown that the silicon content of the steel grades is responsible for an increase of the hardness of the ferritic matrix due to solid solution strengthening. Finally, the influence of the preparation mode of the surface prior to the nanoindentation measurements has been investigated. An electropolishing stage after mechanical polishing is acceptable to allow valuable nanohardness measurements

On the role of martensitic transformation on damage and cracking resistance in trip-assisted multiphase steels

The damage resistance, fracture toughness and austenite transformation rate in transformation-induced plasticity (TRIP)-assisted multiphase steel sheets were comparatively characterised on two steel grades differing by the volume fractions of the phases (i.e. ferrite, bainite, retained austenite) and by the mechanical stability of retained austenite. The influence of stress triaxiality on austenite transformation kinetics and the coupling between martensitic transformation and damage were investigated using double edge notched (or cracked) plate specimens tested in tension. The map of the distribution of transformation rates measured locally around the notch (or the crack) was compared with the map of the effective plastic strains and stress triaxialities computed by finite element simulations of the tests. The mechanically-activated martensitic transformation was found to progress continuously with plastic straining and to be strongly influenced by stress triaxiality. Fracture resistance was characterised by means of J(R) curves and crack tip opening displacement (CTOD) measurements using DENT specimens. The fracture toughness at cracking initiation was found to be lower for the steel with higher tensile strength and ductility. The contrasted influence of the TRIP effect, which improves formability by delaying plastic localisation but reduces fracture toughness at cracking initiation, is shown to result from parameters such as the volume fraction of non-intercritical ferrite phases or the mechanical properties of martensite

High-resolution Electron-microscopy of Hydrodesulfurization Catalysts - a Review

Abstract A review is made of the state of the art of HREM of unsupported and alumina supported HDS catalysts. Unsupported catalysts usually have an amorphous structure made by stacking of disordered MoS 2 or WS 2 slabs. In supported catalysts, the optimum activity is obtained when Mo or W is completely sulfided so as to form finely dispersed MOS 2 or WS 2 -like crystallites. These crystallites lie flat on the support. High resolution imaging has failed to resolve, up to now, the nature of group VIII metal in the catalyst. Electron-optical limitations are discussed.

Bainite transformation of low carbon Mn–Si TRIP-assisted multiphase steels: influence of silicon content on cementite precipitation and austenite retention

Studies dealing with TRIP-assisted multiphase steels have emphasized the crucial role of the bainite transformation of silicon-rich intercritical austenite in the achievement of a good combination of strength and ductility. The present work deals with the bainite transformation in two steels differing in their silicon content. It is shown that both carbon enrichment of residual austenite and cementite precipitation influences the kinetics of the bainite transformation. A minimum silicon content is found to be necessary in order to prevent cementite precipitation from austenite during the formation of bainitic ferrite in such a way as to allow stabilisation of austenite by carbon enrichment

Nature and Properties of a Potassium-promoted Nimo/al2o3 Water Gas Shift Catalyst

Abstract A comparative study has been performed of a conventional NiMo Al 2 O 3 catalyst and of this same material after impregnation with K2CO3. The catalysts were characterized by conventional and microprobe Raman spectroscopy, diffuse reflectance and X-ray photoelectron spectroscopy, analytical electron microscopy and temperature-programmed decomposition. The chemisorption of carbon monoxide was studied by transmission infrared spectroscopy. The results indicate a very strong interaction of K+ ions with the catalyst surface. The original octahedral coordination of Mo6+ in the NiMo Al 2 O 3 catalyst is transformed into a tetrahedral coordination and the reducibility of Mo6+ is strongly decreased in the presence of K+ ions. Also the degree of sulfidation seems to be reduced in KNiMo Al 2 O 3 , this phenomenon being accompanied by a stabilization of the Mo5+ oxidation state. CO chemisorption leads to the formation of carbonates and formates on both catalysts in their oxidized and sulfided states. On reduced KNiMo Al 2 O 3 CO polymer anions also seem to be formed. The effect of K+ ions on the catalytic performance of the NiMo Al 2 O 3 catalysts was tested for thiophene hydrodesulfurization (HDS), ethylene hydrogenation, CO methanation, and water gas shift (WGS). A significant reduction in activity was observed for HDS in the presence of K+ ions, while the activity for WGS was simultaneously enhanced. An attempt is made to correlate the changes of the catalytic performance with the changes which are induced by K2CO3 impregnation on structure, symmetry, and valence state of the molybdenum species.