Microscopic densification mechanisms of metallic systems by spark plasma sintering

This study reports on the microscopic mechanisms accounting for the fast densification kinetics of metallic materials processed by spark plasma sintering (SPS). Metallic systems have been chosen in reason of their elevated electric conductivity. Hence, an important fraction of the electric current of the SPS passes through the powder, and electrically induced new mechanisms, or acceleration of conventional ones (e.g. plasticity, diffusion), can be expected. TiAl powder and Ag-Zn diffusion couple have thus been chosen to study the influence of the electric current on plasticity and on diffusion, respectively. An originality of this work was to observe by transmission electron microscopy (TEM) the phenomena occurring at the necks between TiAl powder particles during densification. For this purpose, a method of extraction of TEM thin foils at the necks by focused ion beam (FIB) has been developed (Figure 1). High dislocation densities and recrystallization mechanisms have been observed at these locations. The influence of the electric current on the densification kinetics and on the superplastic behavior of the TiAl powder has been evaluated by comparisons between the SPS and hot pressing techniques. The influence of the current on diffusion has been studied with Ag-Zn diffusion couples by carrying out experiments with and without the SPS current, using in the last case special setups to electrically insulate the couples. Though diffusion in this system has been previously shown to be sensitive to current densities above 200 A/cm2 in dedicated diffusion experiments, by SPS, no influence of the current was observed. This was also the case if the SPS current density was artificially increased, using special devices, to ≈1000 A/cm2, that is, five to twenty times the values obtained in typical SPS conditions (50-200 A/cm2)

Inverse radiative design in human thermal environment

International audienceThis work investigates inverse boundary design for radiative heat transfer applied to human thermal environment. The problem consists in finding the temperatures of certain surfaces in a complex configuration around the human being that satisfy both the temperature and the heat flux prescribed on his body. Such a mathematical procedure is called inverse modeling which is described by an ill-conditioned system of linear equations based on the absorption factors method. The solution is obtained by regularizing the system of equations by the Tikhonov method. As a result we obtain optimized conditions for a complex human thermal system

In situ high temperature TEM tensile testing of pseudo single crystalline Si for PhotoVoltaic applications

Single crystalline Silicon has the highest efficiency to convert sunlight into electricity. Its production is however costly. On the other hand, cheap polycrystalline Si cells can be produced, with a 10% lower PV conversion efficiency. A promising technique, dubbed mono-like Si consists in growing pseudo-single crystalline Si ingots from a tile of single crystalline seeds aligned at the bottom of the crucible. At the present time, this technique is confronted to the high density of defects that multiply during solidification, fueled by the thermal gradients generated in the furnace. Some of these defects have small impact on the electrical properties but others are heavy recombination sites and should be avoided. Here, we focus on dislocations, micro twins and grain boundaries. Their mutual interaction may act as stress concentrators or sinks. We are working at various scales (Fig. 1a-b) to understand these interactions that occur both at long ranges and atomic-scale processes. To gain insight about the later, we have started to tensile strain dedicated micro-samples in situ in the TEM at temperatures between 900 and 1000°C (the melting temperature of Si is 1414°C). We have shown that multiplication processes are initiated at existing GBs and that mobile dislocations are poorly affected by Peierls stresses at this temperature. Interactions of dislocations with twins (Fig 1c) are currently investigated, along with twin terminations and initiation sites

Nanoindentation cartography in Al/Al-Cu-Fe composites: Correlation between chemical heterogeneities and mechanical properties

During the last two decades, nanoindentation testing has become a commonly used technique for measuring surface mechanical properties such as hardness or elastic modulus. With devices equipped with a motorized X-Y table, it is now possible to perform large regular nanoindentation arrays in order to make an accurate statistics of the mechanical properties. This method is particularly interesting to study heterogeneous materials. The statistical analysis, associated to mathematical deconvolution methods allows identifying the properties of each individual phase. Furthermore, hardness or elastic modulus maps can be then established and compared to other local properties such as microstructure, crystallographic orientation or chemical composition. The nanoindentation cartography method has been used to study the mechanical properties of a metal matrix composite (Aluminum matrix with ω-Al-Cu-Fe reinforcement particles, synthesized by sparking plasma sintering) (cf. figure 1). Emphasize has been placed on the Aluminum matrix properties, where the detailed analysis of the individual nanoindentation curves shows serrated behavior characteristic of Portevin-Le Chatelier effect associated to dislocation pinning by solute atoms. The comparison between chemical (SEM – EDXS analysis) and hardness maps as well as the quantitative analysis of the deformation curves gives evidence of a strong correlation between the chemical heterogeneities and mechanical properties of the Aluminum matrix

Microstructural Characterization of Graphite Spheroids in Ductile Iron

The present work brings new insights by transmission electron microscopy allowing disregarding or supporting some of the models proposed for spheroidal growth of graphite in cast irons. Nodules consist of sectors made of graphite plates elongated along a hai direction and stack on each other with their c axis aligned with the radial direction. These plates are the elementary units for spheroidal growth and a calculation supports the idea that new units continuously nucleate at the ledge between sectors

Theoretical study of oxygen insertion and diffusivity in the g-TiAl L10 system

This work is a first-principles study of the insertion and diffusivity of oxygen in the -TiAl L10 system. Five interstitial positions were identified as stable. One, however, the 2h site a pyramid composed of a Ti square topped by an Al atom, was found more stable than the others. The oxygen interactions with the TiAl system were thus studied and analyzed in detail using vibrational, elastic and electronic properties. The results show that the O atom prefers to be surrounded by Ti atoms and tries to minimize the number of bonds with aluminum. The diffusion mechanism is subsequently studied at the atomic scale, by analyzing displacements between stable interstitial sites. The oxygen diffusivity is found to be anisotropic and the components in the x and z direction, Dx and Dz, are then calculated and compared with those of O diffusion into other Ti–Al alloys. The analysis of results shows two effects. First, the stability of sites is related to the number of O–Al bonds, the fewer there are, the more stable the site is, and second, the diffusion is faster when the content of interstitial sites composed of many Ti atoms is lo

An electron microscopy study of graphite growth in nodular cast irons

Growth of graphite during solidification and high-temperature solid-state transformation has been investigated in samples cut out from a thin-wall casting which solidified partly in the stable (iron–graphite) and partly in the metastable (iron–cementite) systems. Transmission electron microscopy has been used to characterize graphite nodules in as-cast state and in samples having been fully graphitized at various temperatures in the austenite field. Nodules in the as-cast material show a twofold structure characterized by an inner zone where graphite is disoriented and an outer zone where it is well crystallized. In heat-treated samples, graphite nodules consist of well-crystallized sectors radiating from the nucleus. These observations suggest that the disoriented zone appears because of mechanical deformation when the liquid contracts during its solidification in the metastable system. During heat-treatment, the graphite in this zone recrystallizes. In turn, it can be concluded that nodular graphite growth mechanism is the same during solidification and solid-state transformatio

Editorial

Monchoux André. Editorial. In: Littératures 9,1961. p. 4