A. Galerie

Thermal oxidation of titanium by water vapour

Abstract The thermal oxidation of titanium by water vapour proceeds according to a linear-parabolic rate law resulting from a reaction–diffusion mixed regime. The growth of TiO 2 takes place by rapid diffusion of substitutional hydroxide ions generated at the gas–scale interface. Kinetic calculations based upon these assumptions are in agreement with the experimental observations.

Oxidation of ion-implanted metals☆

Abstract The mechanisms involved in the modification of the thermal oxidation of metals and alloys by ion implantation are reviewed and discussed. Radiation damage is thought to have some influence, but chemical effects always seem to be of greater importance. These can be closely related to the oxidizability of the implanted species. In the very early stages of oxidation, oxidizable elements can act as nucleation promoters and allow the development of more coherent scales. The solubility of the implants in the matrix is an important parameter in the discussion of the blocking of material transport along fast diffusion paths. The presence of particles in the matrix or in the oxide scale is also thought to be of importance either because the particles act as sinks for flowing defects or because they modify the relief of the mechanical stresses resulting from the scale growth.

Revêtements composites TiAl3-TiB2 sur le titane

Resume Nous avons prepare sur le titane un revetement associant le nitrure TiN et laluminiure TiAl 3 . Deux modes operatoires ont ete utilises: aluminisation (par cementation activee) puis nitruration (par lazote) ou nitruration avant aluminisation. Bien quayant des compositions differentes, les revetements obtenus dans les deux cas ont des proprietes tres voisines: microdurete comprise entre 1000 et 1400 HV et tenue dans loxygene presque aussi bonne que celle de TiAl 3 jusqua 850 °C. Pour les temperatures superieures la tenue se degrade tres rapidement soit par destruction de TiAl 3 (formation daluminiures inferieurs de titane), soit par suite de la formation du ternaire de Ti 2 AlN.

The high temperature oxidation of aluminium-implanted iron

Abstract Aluminium was implanted into iron at different doses between 1 × 10 14 and 1.5 × 10 17 atm cm −2 . Oxidation tests were carried out in oxygen during 100–120 h in the temperature range 720–1020 K. Doses of 5 × 10 16 atm cm −2 had to be achieved to observe a notable inhibition of the oxidation process. In these conditions, the formation of two ternary oxides was detected but the spinel phase Fe Al 2 O 4 seemed to be responsible for the observed blocking effect.

Laser surface alloying of Ti-6Al-4V with silicon for improved hardness and high-temperature oxidation resistance

Abstract The surface of Ti-6Al-4V precoated with silicon powders was irradiated by a YAG: Nd 3+ laser to produce a homogeneous and uniform alloy layer. The surface hardness measured was 730 HV 0.1 and the oxidation resistance was greatly improved. This improvement is mostly due to the diminution of the inward diffusion of O 2− and the outward diffusion of Ti 4+ by the presence of silica and a little alumina on the top surface of the oxide scale.

Ion implantation into metals to prevent high temperature oxidation

Abstract The conditions leading to a complete inhibition of the oxidation of implanted metals are discussed using iron and titanium as test metals. This inhibition is thought to be possibly observed with metals oxidizing by outward cation diffusion. In this case, thermodynamic oxidability of the implants is the most important parameter. In the case of metals oxidizing mainly by inward oxygen transport, ion implantation is believed to be unsuited for completely inhibiting the oxidation process.

Elaboration d'Alliages de surface sous irradiation laser

Resume Lincorporation dun ou plusieurs elements etrangers dans un materiau metallique peut etre realisee en irradiant au moyen dun faisceau laser ce materiau prealablement recouvert dune couche de lelement (ou des elements) daddition envisage(s). Le mecanisme conduisant a cet alliage de surface fait intervenir un transport de chaleur et un transport de matiere. Des calculs simples rendant compte de leffet thermique du au faisceau sont possibles, mais nous montrons ici que lon ne peut envisager lobtention de resultats precis quen prenant en compte les proprietes physico-chimiques de toutes les phases presentes dans le systeme. Lhomogeneite de lalliage de surface obtenu peut etre predite a laide dun parametre sans dimensions dont nous avons fixe la valeur minimale. Ce parametre relie les conditions operatoires et les proprietes physicochimiques des elements entrant dans la composition du revetement. Nous proposons egalement une autre approche qui consiste a calculer lacceleration initiale du materiau fondu. Ces approches entrent dans leffort important de comprehension des phenomǹes de mixage sous laser, dont le but est de permettre lelaboration de revetements de qualite susceptibles de resister aux degradations, en particulier a haute temperature.

Oxidation of ion-implanted titanium in the 750–950 °C temperature range

Abstract Ion implantation of various species into titanium is shown to have a strong influence on the oxidation kinetics of this metal in pure oxygen at temperatures between 750 and 950 °C. Phosphorus is particularly active and reduces the parabolic rate constant by a factor of 2 according to the Wagner-Hauffe rules. This element also reduces oxygen dissolution in α-Ti and inhibits stress relaxation in the metal, leading to the formation of highly stratified rutile scales. It is shown to migrate with the metal-oxide interface and therefore its influence is long lasting.

Thermal oxidation resistant surface alloys processed by laser alloying on Z38CDV5 ferritic steel

Abstract New surface alloys were obtained on a low alloyed ferritic steel (Fe-0.38 C-5 Cr-1 Si-0.8 Mo-0.5 V) by laser processing after coating the surface with chromium carbide powder. Three different microstructures were observed depending on irradiation conditions: hypoeutectic, eutectic and hypereutectic, all containing iron solid solutions (austenite and martensite) and iron-chromium carbides (Fe, Cr) 7 C 3 . The oxidation resistance of these alloys was measured in isothermal and cyclic conditions, and was shown to increase with chromium concentration. The hypereutectic alloy, containing 35% Cr, was particularly resistant due to the formation of a chromia scale with excellent behaviour during thermal shocks.

Mechanisms of Corosion and Oxidation of Metals and Alloys

Selected topics in field of the study of the mechanisms of corrosion and of oxidation of metals or alloys are presented. The first part reports a new model for the mechanism of the breakaway oxidation of ferritic stainless steels in water vapour. The second part is devoted to the physico-chemical aspects of oxidation and presents experimental methods useful in the kinetic modelling applied to two alloys, the zircalloy-4 and an AlMg5 % in the liquid state. In the third part the physical and numerical modelling of the stress corrosion cracking behaviour in face-centered cubic (fcc) alloys is detailed, which enables the study of the influence of macroscopic parameters (such as the temperature or hydrogen activity) on the fracture process.