ÉTATS LIÉS VIRTUELS DANS LES ALLIAGES ORDONNÉS Fe-Co-X (X = Ti, V, Cr, Mn)

Nous avons étudié la résistivité électrique ρ en fonction de l'état d'ordre de l'alliage Fe-Co équiatomique et des alliages ternaires [MATH]-X (X = Ti, V, Cr, Mn). L'ordre de grandeur de la résistivité de ces alliages s'explique dans un schéma d'état lié virtuel dès que l'écart | Z | entre la valence de la matrice et celle de l'impureté est supérieur à 2. L'amplitude et le sens de la variation de ρ en fonction de l'état d'ordre s'expliquent par la déformation de la bande d pour | Z | 2 ; dans ce dernier cas il y a remplissage de l'état lié virtuel lors de la mise en ordre, ce qui entraîne une diminution de la résistivité des alliages [MATH]-Cr et une augmentation de celle des alliages [MATH]-V et [MATH]-Ti.The variations in electrical resistivity ρ as a function of long range order are studied on both binary stoichiometric Fe-Co and ternary [MATH]-X (X = Ti, V, Cr, Mn). The magnitude of the resistivity of such alloys may be interpreted in terms of virtual bound states when the difference | Z | between the matrix valence and the impurity one is larger than 2. The variations in ρ as a function of the ordered state have been connected with the deformation of the d band when | Z | 2 ; in this last case the virtual bound state fills up upon the ordering process, so that the resistivity decreases in [MATH]-Cr and increases in both [MATH]-V and [MATH]-Ti

Mechanical features optimization for α - Cr2 O3 oxide films growing on alloy NiCr30

Mechanical features have been investigated in the case of α - Cr2 O3 thermal oxide films growing on NiCr30 alloy in the temperature range 700-900 °C. From previous experimental results obtained by Raman spectroscopy and giving the growth stress evolution in α - Cr2 O3 films, and based on a modelling of the system, both asymptotic forms and an optimization procedure have been developed which give values for the mechanical characteristic parameters of the system and also its evolution with the oxidation temperature. © 2009 Elsevier B.V. All rights reserved

Graded nitrogen ingress in FCC metallic structures and the related microstructures and high temperature oxidation behaviour

Nitriding by low energy high flux processing has been carried out at about 400°C in fcc metal substrates (pure Ni, Ni-20Cr model alloy and a conventional AISI 304L stainless steel). The gradual ingress of this element into the structures will be shown to depend markedly on the chemical composition of the substrate. The associated expansion of the fcc lattices and surface roughness will be discussed in this work with the support of X-ray diffraction, atomic force, scanning and transmission electron microscopy techniques. In light of the resulting composition, microstructures and thickness of nitrided layers, some preliminary results of their behaviour under isothermal oxidation conditions at high temperatures will be discussed. The high temperatures will provoke decomposition of the expanded austenite into a conventional gamma phase and some chromium nitrides. Trapping of chromium therefore shall explain a reduction of the high temperature oxidation resistance against the untreated substrates. © (2009) Trans Tech Publications

Phosphating of bulk a-iron and its oxidation resistance at 400 °C

Fe bulk specimen have been treated with phosphoric acid in order to obtain a conversion surface layer and the oxidation resistance has been studied by means of in situ thermogravimetric experiments in artificial air (20% O2; 80% N2). For oxidation time of 48 h at 400 °C it shows an important decrease of the oxidation kinetic in comparison with α-Fe. Moreover, the kinetic can be described by two successive parabolic stages. This behaviour is respectively related to an X-ray photoelectron spectroscopy (XPS) analysis of the surfaces and to previous results obtained on the oxidation of α-iron

Determination of the oxidation kinetics of modified a-iron substrate: correlation between TGA and AES

A conversion phosphating treatment has been performed on polycrystalline a-iron in order to modify its thermal oxidation behaviour. This has been studied by means of in situ thermogravimetric experiments and line profile nanoscale AES of the phosphated surface layer, both before and after the oxidation experiments..

Structure and related corrosion behaviour in 1M H2SO4 of b.c.c. 304L films prepared by ion beam sputtering

The corrosion behaviour in a non-deaered 1M H2SO4 solution of 304L stainless steel (SS) in bulk and thin film state was investigated. In the first step, we elaborated based cubic-centred 304L stainless steel thin films onto Si and f.c.c. 304L SS bulk substrates by using a sputter deposition method. Then, the structure and residual macrostress state of the films were studied by X-ray diffraction. It was found that films are in a highly compressive state ( 3 GPa). Finally, corrosion tests revealed an improvement in corrosion resistance of the films with respect to the bulk 304L stainless steel