Françoise Nardou

Time-resolved X-ray diffraction study of SHS-produced NiAl and NiAl-ZrO2 composites

NiAl and NiAl-ZrO2 intermetallic composites were prepared from pressed elemental powders by SHS in a dynamic vacuum. The effect of diluent properties (such as conductivity and grain size) on the parameters of the SHS reaction has been investigated by time-resolved X-ray diffraction (TRXRD). By decreasing the reactivity of the green mixture, these nonreactive additives not only slow down the velocity of combustion front propagation but can also give rise to local melting of the product. Synchrotron radiation was also used to elucidate the reaction mechanism. It has been found that the combustion synthesis of NiAl is triggered by aluminum melting and proceeds via the formation of two transient intermetallic phases, NiAl3 and Ni2Al3. In the final product, only the NiAl phase has been detected.

Suivi cinétique de la relaxation des contraintes de croissance au cours de l'oxydation d'un alliage FeCrAl☆

The oxidation behaviour of FeCrAl alloy in O2 was studied in the temperature range 800–1200°C for two types of sample: platelets and wires. In both cases the oxidation rate is controlled by a diffusion process. However, the induced growth stresses and their relaxation are very sensitive to the geometry of the oxidized sample. They have been evaluated as a function of time and temperature by following the resulting differences observed in the rate constants.

Equipment for studying hot corrosion processes in dynamic gas mixtures containing water vapour

Abstract A Setaram unit including a symmetric oven and a MTB 10 8 microbalance was chosen as a base equipment in order to reduce the impulse variations frequently occurring in dynamic regimes. The thermobalance and its surroundings were thoroughly thermostated owing to water vapour. To pull up and let down the sample quickly without damage for the torsion strip, a telehandling airtight apparatus with visual control has been worked out, allowing to make optional the connection between the suspension chain and the microbalance beam. The equipment here described has been conceived for hot corrosion kinetic studies (round 1000°C) concerning low weight samples (10–100mg) in such gaseous flows as X/H2O containing air, oxygen and hydrogen in the presence of water vapour (1–75 %).

In situ monitoring of the kinetics of solid-gas SHS reactions: Application to the Zr-O2 system

The combustion of a powder mixture of zirconium and zirconia in a pure oxygen atmosphere was chosen to present a new approach of the solid-gas-type SHS process. Indeed, this study is the first attempt to monitor the reaction rate in situ by pressure measurements of the reactive gas Two kinetic modes have thus been defined: (1) a surface reaction which corresponds to the combustion wave propagation and (2) a parabolic consumption of oxygen which occurs during the after-burn reaction. Some specific phenomena such as the trigger mechanism of the self-sustaining combustion have also been highlighted by this method. The latter, easy to implement, should provide a better insight into reaction mechanisms of any solid-gas reaction.

Relaxation of the Mechanical Stresses Developed Through Oxide Scales During Oxidation of Metals

The oxidation behaviour of a FeCrAl alloy has been studied in various atmospheres containing water vapour (O2/H2O mixtures) in the temperature range 800–1200°C. The growth stresses developed through the oxide scale and their relaxation have been followed by comparing kinetics obtained from planar and cylindrical samples.