Papers by Author: Véronique Massardier-Jourdan

Abstract: C-Mn steel welds of the secondary circuit of pressurised water reactors might be subjected to static strain ageing (SSA) due to the segregation of free interstitial atoms, especially nitrogen, on dislocations at temperatures of 250°C-300°C in operating conditions. Thus, several parameters such as the initial free nitrogen content and the amount of prestrain are likely to affect the sensitivity of the material to SSA, both influences being investigated in this paper. To this end, a broad experimental campaign based on internal friction (IF) measurements was conducted to follow the evolutions of the solute content. Preliminary studies emphasized the presence of nitrogen solutes in the as-welded material, whose content tends to increase from 250°C to 300°C due to the dissolution of ε carbonitrides. Regarding the parametric study of SSA, it was shown that the amount of segregated nitrogen first increases with the amount of prestrain or the initial concentration of nitrogen solutes before saturating from a critical threshold. Its value depends on the other parameter, leading to either no effect, a continuous increase or an increase before saturation of segregation. Therefore, a cross influence of both parameters was highlighted.

Abstract: Spark plasma sintering has been used for decades in order to consolidate a wide variety of materials and permitting to obtain fully dense specimens. This technique has been mainly applied to ceramics. This paper concentrates on an unusual use of spark plasma sintering system: obtaining innovative materials especially architectured ones. Different applications are presented. Firstly, the SPS technique has been used to elaborate nanometers grain size materials or containing nanoscale microstructure. This is possible since the sintering temperature and the holding time are far lower in the SPS compared to other techniques. Then SPS has been used to realize diffusion bonding. In that case again, bonding can be realized at low temperature and for short time. It permits for example to realize bonding between two copper layers which is of a great importance for microelectronic applications. It is worth noting that this bonding can have the same mechanical strength as pure copper even for diffusion time of a few minutes. Secondly, bonding has been also carried out between a metallic layer and a ceramic one. This could lead to design of new layered materials combining interesting properties in terms of mechanical strength but also in terms of electrical resistance. The SPS machine has also been used to obtain porous materials (cobalt alloys or copper) with an adapted microstructure (porosity, tortuosity,). These structures could open new perspectives for biomedical or for microelectronic applications. All these examples lead to a better understanding of the physical processes which happen during spark plasma sintering.

Abstract: Ultra rapid annealing cycles were conducted on two low carbon Al-killed steel sheets differing mainly by their coiling temperatures (600°C or 700°C). For the lowest coiling temperature, the mean grain size of the steel was found to gradually decrease with an increase of the annealing temperature from 700°C to 920°C. A more complex grain size evolution was detected in the case of the steel coiled at high temperature. This led us to the conclusion that the size and the distribution of the iron carbides present before annealing, which is mainly governed by the coiling temperature, plays a very important role on the mechanisms involved in the grain refinement of extra-mild steels during ultra-rapid annealing cycles.

Abstract: In this study, Spark Plasma Sintering has been used to sinter pure iron with an initial crystallite size around 100 nm. The process parameters for sintering pure iron have been optimized in order to obtain fully dense materials and avoid excessive grain growth. Archimede's method has been used to calculate the relative density of the sintered samples. It appears that almost fully dense materials can be obtained (95%). X Ray diffraction applied to the sintered samples shows the presence of iron and of the wustite oxide FeO (around 6% wt) formed during the sintering process. Peak enlargement measurements show that the grain size after sintering is around 200nm. This is confirmed by TEM observations showing a dual distribution of grain size. Finally, mechanical characterization has been carried out. The sintered compact exhibits a very high hardness of about 400 Hv. Compression test reveals a very high maximal stress of about 1.2 GPa and that the ductility in compression is non negligible. Using the Hall and Petch law, the calculated grain size should be around 450 nm which is in accordance with direct observations.

Abstract: Advanced High Strength Steels (AHSS) are key materials in the conception of car body structures, permitting to reduce their weight while increasing their behavior in crash conditions. Nevertheless, the weldability of AHSS presents some particular aspects, in that complex failure types involving partial or full interfacial failure can be encountered more often than with conventional mild steels during destructive testing, despite high spot weld strength levels. This paper aims at characterizing the behavior of different AHSS spot welds under two quasi-static loading conditions, tensile shear and cross tension, often used in the automotive industry for the determination of their weldability. Interrupted cross tension and tensile shear tests were performed and spot welds failure was investigated with optical micrographs, SEM fractography and 3D-tomography in order to follow the three-dimensional crack paths due to the complex loading modes. A limited number of failure zones and damage mechanisms could be distinguished for all steel grades investigated. Moreover, numerical simulation of the tests was used to better understand the stress state in the weld and the influence of geometrical features such as weld size on the occurrence of the different failure types.

Abstract: The thermoelectric power (or TEP) technique was used to determine the segregation kinetics of the interstitial atoms (C or N) to the dislocations in various extra-mild steels submitted to a heavy deformation by cold-rolling when substitutional atoms (Mn or Cr) are simultaneously in solid solution. It was shown that the substitutional atoms (Mn or Cr) have almost no influence on the segregation kinetics of carbon and on the activation energy associated with the segregation of this element. In contrast, these elements tend to delay the segregation kinetics of nitrogen to the dislocations all the more so as their content in solution is high. In the mean time, the activation energy associated with the segregation of nitrogen is increased.

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.

Abstract: In Fe-C-Mn steels, the carbon atoms in solution can be either completely free in the iron matrix or in interaction with manganese atoms. In this context, a methodology based on the combined use of thermoelectric power and internal friction measurements was developed in order to evaluate quantitatively these two populations of carbon atoms. This methodology was used to determine the binding energy of the C-Mn dipoles and to follow the precipitation kinetics of the two populations of carbon atoms and/or their segregation kinetics to the dislocations during an isothermal treatment. Lastly, the influence of each population of carbon atoms on the strain aging of extra-mild steels was discussed.