Enthalpies of formation of L12 intermetallics derived from heats of reordering

A new method is proposed for estimating the enthalpies of formation of L12 (fcc-ordered) intermetallics from the heat release measured during ordering of their disordered polymorphs. The method is applied to Cu3Au, Ni3Al, and Ni3Si. The resulting estimates of enthalpies of formation are close to values obtained by high temperature dissolution calorimetry. They also appear to be more precise than estimates based on Miedema's correlations provided that care is taken to account properly for the magnetic and lattice stability contributions to the formation enthalpies in the ordered and disordered states

Debye temperature of disordered bcc-Fe-Cr alloys

Debye temperature, TD, of Fe100-xCrx disordered alloys with 0<x<99.9 was determined from the temperature dependence of the centre shift of 57Fe Mossbauer spectra recorded in the temperature range of 80-300K. Its compositional dependence shows an interesting non-monotonous behaviour. For 0<x<~45 as well as for ~75<x<~95 the Debye temperature is enhanced relative to its value of a metallic iron, and at x=~3 there is a local maximum having a relative height of ~12% compared to a pure iron. For ~45~95 the Debye temperature is smaller than the one for the metallic iron, with a local minimum at x=~55 at which the relative decrease of TD amounts to ~12%. The first maximum coincides quite well with that found for the spin-waves stiffness coefficient, D0, while the pretty steep decrease observed for x>~95 which is indicative of a decoupling of the probe Fe atoms from the underlying chromium matrix is likely related to the spin-density waves which constitute the magnetic structure of chromium in that interval of composition. The harmonic force constant calculated from the Debye temperature of the least Fe-concentrated alloy (x>99.9) amounts to only 23% of the one characteristic of a pure chromium.Comment: 15 pages, 7 figures, 26 reference

Evolution originale de la radiolyse de l'eau porale dans un matériau cimentaire comportant du sulfure : couplage expériences/simulations

International audienc

Phase transition of pure zirconia under irradiation: A textbook example

One of the most important goals in ceramic and materials science is to be able to design materials with specific properties. Irradiation seems to be a powerful tool for the design of advanced ceramics because of its ability to modify over different scales the microstructure of solids. Nowadays, it is clearly proved that irradiation induces order-disorder phase transitions in metallic alloys and in some ceramics. In this paper, we show that a displacive phase transition can also be induced by irradiation. Based on many experimental facts, a microscopic model is proposed to explain the displacive phase transition observed in this material after irradiation. Defects, produced in the oxygen sublattice, induce important strain fields on a nanometric scale. This strain field can be handled as a secondary order parameter within the Landau theory approach, leading to a decrease of the phase transition temperature and thus quenching the high temperature tetragonal phase. (c) 2006 Elsevier B.V. All rights reserved

Nano structuration of zirconia under irradiation: A way to enhance the mechanical stability of zirconia layer

Zirconia is one of the most important ceramic materials because of the large range of industrial applications (catalysis, coatings, spacecraft shielding, paint additives, oxygen sensors, fuel cells, nuclear fuel matrix, alternative high permitivity material to replace silicon oxide as gate dielectric in MOS devices...). It is now well established that a monoclinic to tetragonal phase transition occurs in this material. This transition can for instance be trigged by the grain size. The mechanism of this phase transition is now clearly understood. Zirconia can be considered a textbook example for describing these effects. In this paper, we will discuss the mechanism of the tetragonal to monoclinic martensitic phase transition induced by irradiation within the Landau theory framework, pointing out the peculiar effects related to nanostructuration of this material by irradiation. Such a work is an illustrative example of structural modification of solids induced by irradiation

Synthesis of alumina-metal nanocomposites by mechanical alloying

The synthesis of nanometer-sized [MATH]-Al2O3-metal composites can be performed by room temperature ball-milling of mixtures of metal-oxides and aluminium as shown by Matteazzi and Le Caër. The average crystallite size of the alumina-metal composite so obtained is in general about 10nm. Such composites may also be prepared by direct grinding of a mixture of [MATH]-Al2O3 and of a metal or an alloy. The present work is devoted to the study of the reaction mechanisms by X-Ray diffraction and 57Fe Mössbauer spectroscopy for the [MATH]-Al2O3-Cr, [MATH]-Al2O3-Fe, [MATH]-Al2O3-(Fe-Cr) and [MATH]-Al2O3-Ti systems. Mössbauer spectroscopy shows that non-completely reduced Fe, partly in the form of Fe2+, still exists at the end of the synthesis and belongs to a ternary Al-Fe-O oxide in [MATH]-Al2O3-Fe composites although it cannot be indexed by X-Ray diffraction. The Fe2+ component is mainly associated with iron environments in or similar to those of hercynite. Species like Fe3+ in Al2O3 or in grain boundaries are observed, Fe4+ and Feo cannot be excluded. Differences are obtained by reactive milling and by direct milling the reaction products. Reactive milling of Al-TiO2 (anatase) yields [MATH]-Al2O3-Ti nanocomposites. The transient formation of a high-pressure orthorhombic form of TiO2 of the [MATH]-PbO2 type is observed