S. Paris

A new experimental setup for the time resolved x-ray diffraction study of self-propagating high-temperature synthesis

A new experimental setup for time resolved x-ray diffraction is described. Designed for the LURE H10 beamline and its 4 (+2) circles goniometer, it allows simultaneous recordings of x-ray patterns with a rate of 30 patterns per second, a maximum 2θ range of 120°, infrared thermography at the same rate, and thermocouples readings at a frequency of up to 3×104 Hz. Preliminary results obtained using this setup are presented, showing how it is possible to analyze a solid–solid or solid-liquid reaction. As an example, an in situ study of phase transformation and temperature evolution during the self-sustaining synthesis of an FeAl intermetallic compound starting from a mechanically activated mixture is investigated. The versatility of the setup was proved and could even be enhanced by the design of new sample holders, thus expanding its area of use at low cost.

Investigation of mechanically activated field-activated pressure-assisted synthesis processing parameters for producing dense nanostructured FeAl

The parameters of the mechanically activated field-activated pressure-assisted synthesis (MAFAPAS) process, which were recently developed and patented for producing dense nanostructured materials, were studied in the case of the B2-FeAl intermetallic. Based on x-ray diffraction (XRD) experiments, residual stresses XRD analysis, relative density measurement, and secondary-electron microscopic observations, the optimal synthesis conditions (time, current intensity, and pressure) were studied. Fe + Al powders were comilled in a specially designed planetary mill to obtain a mixture of reactants at the nanoscale without the formation of any product. The milled mixtures were then subjected to a high density of alternating current (60 Hz ac, total current 1250 or 1500 A), a uniaxial pressure (70 or 106 MPa), and different times (from 2 to 5 min). This work confirms the reproducibility of the MAFAPAS process, showing the essential role of the mechanical activation step to produce a pure nanostructured material. In addition, the composition and the microstructure of MAFAPAS end-products depended on the processing parameters (time, current density, mechanical pressure). In particular, it was observed that the process of simultaneous synthesis and consolidation of the product introduced a high level of residual stresses.

Mechanical Activation as a New Method for SHS

The use of mechanical activation (the elemental powder mixture is milled for a short time at given frequency and impact energy) as a precursor to self-propagating high-temperature synthesis (SHS) results in the formation of nanostructured porous materials. The mechanical activation step was found necessary (i) to modify the thermal parameters of the combustion front (i.e. combustion front velocity, thermal heating rate…) in the cases of Mo-Si, Fe-Al, Ni-Si (ii) to initiate a combustion front in the case of systems having a low exothermicity. Nevertheless, the control of the mechanically activated mixture characteristics and, the understanding of the mechanical activation role on the SHS parameters are essential to produce end-products with expected microstructure.

Effect of Microstructure on the High Temperature Oxidation and Pesting Behaviour of MoSi2

The control of the microstructure (i.e. grain size, defects, texture...) and the surface state of materials is essential for their use under severe conditions, especially during the initial step of high temperature corrosion process where the formation of a protective oxide layer is effective. Very promising new results have been obtained from nano-structured intermetallics such as MoSi2. They are of great interest because they are able to form silica scales under high temperature oxidant atmospheres. The present work shows that the scales formed on the nano-structured materials exhibit very good protective properties compared to the classical micro-structured materials during high temperature oxidation tests. The most original results are obtained at lower temperature (673 K<T<1073 K), for which the classical disastrous “pesting” phenomena (i.e. destruction of the intermetallic compound into powder) never occurs, even after long time oxidation tests (1848 hours = 11 weeks). The high temperature oxidation behavior of the MoSi2 nano-structured materials will be discussed according to careful analyses of the oxide scales grown during exposures under oxidant atmospheres over the temperature range 673-1273K.