Papers by Author: Alexis Deschamps

Abstract: The Fe-Si-Ti system is known to show nanoscale precipitation of the Fe₂SiTi Heusler phase with potentially high volume fraction (~4%), very high density and a size ranging from 1 to 20nm after artificial aging. The strong hardening potential of these precipitates make these steels candidates for automotive applications; however no understanding of the precipitation sequence (competition with other phases) nor the precipitation kinetics are available. The present study presents a quantitative study of the precipitation kinetics (size, volume fraction and number density) in a wide temperature range (450-800°C), realised by coupling systematically Small Angle Neutron Scattering (SANS), Transmission Electron microscopy (TEM) and Tomographic Atom Probe (TAP). Tensile tests were also carried out so as to determine the microstructure/properties relationships. Along the complete temperature range, it is shown that a compromise between time for precipitation and small precipitate sizes can be reached around 550°C. At this intermediate temperature, precipitation is shown to occur in two steps, linked with a second nucleation process after nucleation & growth of the first family of Fe₂SiTi has been completed. This second precipitation step results in a temporary decrease in precipitate size and an increase in hardness. The nature of these precipitates is discussed in view of the TEM and TAP observations.

Abstract: Fully austenitic steels of the Fe-Mn-C system can show extensive deformation twinning (TWIP effect). The deformed microstructure of such steels has been analysed using X-ray diffraction at the European Synchrotron Radiation Facility (ESRF). The experimental diffractograms, recorded using a 2D CCD camera, are analysed in terms of Bragg peak profiles (broadening and asymmetry) and position (shift from the reference [undeformed state] position) leading to an estimation of dislocation and stacking faults densities.

Abstract: This paper presents a physically based precipitation model which aims at describing precipitation kinetics when it occurs exclusively on dislocations. We present specific nucleation, growth and coarsening equations, which are integrated in a set of differential equations. This model is successfully applied to the case of precipitation of NbC in a ferritic steel, whose kinetics has been determined by small-angle neutron scattering.

Abstract: In order to improve the understanding of hot tearing during laser welding of aluminium alloys, the rheology of the alloys in the mushy state must be characterized. The present work investigates the mechanical behaviour of the aerospace alloy AA6056 using a specially designed isothermal tensile test in the mushy state. Using a Gleeble thermo-mechanical machine, two different tests have been performed: i) tests during partial remelting and ii) tests after partial solidification at a high cooling rate. These tests have been carried out not only on the 6056 alloy but also on a mix between 6056 and 4047 Al-Si alloy which corresponds to the composition of the nugget of a laser using a filler wire. The increase of the solid fraction results in an increase of the maximum stress and a change on the fracture surface from a smooth dendritic to a more ductile one. Moreover, the alloys exhibit a typical visco plastic behaviour with an increase of the maximal stress with the strain rate. When the test is performed at a particular solid fraction of 0.97, the fracture is more erratic and the ductility is low. The results show the existence of a ductile/brittle/ductile transition with the fraction of solid. The fracture stress is shown to be higher when testing after partial remelting as compared to partial solidification for the same solid fraction. This is due to the difference in microstructure of the mushy zone and more particularly in the connectivity of the solid skeleton. An adapted creep law is used to describe the mechanical behaviour of alloys during the partial remelting test using the fraction of grain boundary wetted by the liquid given by Wray. This law is shown to be irrelevant to the partial solidification tests, as a result of the modified geometry of the liquid phase. From these tests, we have determined a new law relating the solid fraction to the fraction of grain boundaries wetted by the liquid. This law is a useful tool to predict the mechanical behaviour when mechanical loading occurs during solidification.

Abstract: This paper presents a detailed study of the microstructure and mechanical properties of AA7449 alloy during the two step heat treatment leading to the industrial T7651 temper. It is first shown that reproducing the heat treatment without a deformation step as used in the T7651 industrial temper leads to 2-fold decrease of the precipitation kinetics due to the absence of dislocations, while the resulting mechanical properties (if this change in kinetics is accounted for) are very similar. The work hardening rate is shown to strongly evolve during the heat treatment, and this evolution has been correlated to the evolution of microstructure using a Kocks-Mecking-Estrin analysis. Finally, an analysis in terms of activation volume of the strain rate sensitivity allows for the determination of the dislocation / precipitate interaction in the overaged temper.

Abstract: This paper presents two studies illustrating the possibilities of Small-Angle X-ray Scattering for characterising quantitatively the state of precipitation in aluminium alloys. In the first example, maps are presented, of precipitate size and volume fraction in the cross-section of friction stir welds of AA7449 alloy. It is shown that the influence of welding speed on the distribution of mechanical properties can be understood using this microstructural data. In the second example, the precipitation kinetics in an Al-Zr-Sc alloy is evaluated by in-situ small angle X-ray scattering. Evidence is given for the heterogeneous chemical structure of the Al3(Zr,Sc) precipitates, consisting of a Zr-rich shell surrounding a Sc-rich core. It is shown that this particular distribution results in a very good resistance to coarsening of the precipitate microstructure.

Abstract: The microstructural evolution has been investigated in three alloys of the 7000 series possessing increasing zinc contents by combining small-angle X-ray scattering, differential scanning calorimetry and transmission electron microscopy, in order to gain understanding on the evolution of the compromise between yield strength and corrosion resistance. We show that the three materials show qualitatively identical precipitation sequences; however the precipitated volume fraction is shown to increase in parallel to the Zn content. Moreover, the precipitate size evolution is faster in the high Zn alloy. The precipitate composition is inferred to vary in the three materials, and this difference is shown to explain the differences in calorimetric behavior, precipitation kinetics and corrosion resistance.

Abstract: The deformation mechanisms of an Fe-Mn-C TWIP steel have been investigated as a function of deformation and deformation temperature, using synchrotron X-ray diffraction at the European synchrotron radiation facility. Using the Warren theory, it is possible to reach a good qualitative understanding of the deformation mechanisms. We have confirmed that the deformation mechanisms shifted from the formation of martensite at very low temperature, to twinning around room temperature and dislocations at higher temperatures. Although some quantification of the density of crystalline defects can be reached using simple parameters such as peak shift and broadening, the complexity of defects present in this material require the development of more advanced data interpretation models. First results are shown, using shift and broadening of the peak and fit of intensity by a pseudo-voigt function, as well as the study of the asymptotic behavior of the intensity.

Abstract: The solubility limit of copper in iron at temperature lower than 700°C is not precisely known because copper diffusion is too slow to reach an equilibrium with classical experimental techniques involving long range diffusion. However, fine precipitation of copper can lead to an equilibrium in a reasonable ageing time. Hence, coupling ThermoElectric Power and Small Angle X-ray Scattering techniques leads to a precise estimation of this solubility limit in the temperature range 500°C-700°C. Values obtained are confirmed by Tomographic Atom Probe and give results much higher than what is usually extrapolated from high temperature experiments.