- Transmission Electron Microscpopy
- Mechanical characterization
- High Entropy Alloys (HEA)
- Compositionnally Complex Alloys (CCA)
The research themes that I am currently conducting in the research group “Alloy Design and Microstructures” (CAM), in the department “Metallurgy and Inorganic Materials” (M2I) are mainly in line with the ICMPE priority axis “Materials for Structures and Energy (MSE)”.
Since 2013, a pioneering activity in France on high entropy configuration alloys has been developed in the CAM group and led to the creation of the GDR HEA in January 2019. High Entropy Alloys (HEA), also called multi-elemental alloys or complex composition alloys, have been receiving increasing attention since their conceptualization in 2004. Previously, the fear of forming complex mixtures with numerous intermetallic compounds had hindered the search for new alloys of (quasi-)equiatomic composition. A new class of refractory alloys with high mixture entropy based on the elements Ti, Zr, Nb, Hf and Ta has been explored. The mechanical behaviour of the equimolar alloy was characterised at room temperature by complex compressive and tensile tests (load/relaxation/discharge and multi-relaxation). The results and observations in TEM, show that the modes of deformation are identical to those of conventional cc alloys at low temperature and that the high yield strength is to be related to the obstacles at short distance therefore to the existence of the disordered solid solution. In order to increase the strain-hardening rate without loss of ductility, a chemical design strategy, developed for titanium alloys, was implemented and led to the formulation of an AHE TRIP, Ti35Zr27.5Hf27.5Nb5Ta5, which exhibits phase transformation hardening (Thesis L. Lilensten 2013-2016).
The thesis of G. Bracq (2015-2018, Labex MMCD funding) allowed to identify the limits of the CrMnFeCoNi solid solution domain using intensive Calphad calculations and to study the evolution of mechanical properties in this large solid solution domain through nano-indentation tests. In addition, work carried out within the framework of the ANR (TURBO-AHEAD) focused on the development of new AHE alloys for low-pressure aircraft engine turbines (SAFRAN collaboration). Two types of strategies have been implemented: firstly, a cfc solid solution rich in nickel and hardened by an L12 phase, and secondly, a cc solid solution hardened by an orthorhombic phase of the Ti2AlNb type (ONERA collaboration). For cfc alloys, massive thermodynamic calculations have shown the continuity of the existence of the L12 phases of type Ni3Al to Co3Ti and of the two-phase fcc plus L12 domain in the NiCoFeCrTiAl stranded system, the thermodynamic databases being reliable for this system. This modeling allowed to select some compositions for which the L12 phase is stable above 900°C (Thesis T. Rieger, 2017-2020). As the databases are incomplete for refractory alloys, another strategy, based on the study of diffusion couples, has been implemented to determine the stability of the orthorhombic phase as a function of certain substitutional elements (Mo, Ta, V, Cr). This original approach led to the formulation of one or two compositions with improved temperature stability (Thesis A. Lacour-Gogny-Goubert, 2017-2020).
On the other hand, an alloy design strategy, integrating thermodynamic calculations, diffusion couples and composition gradients obtained by laser projection, has been set up in the framework of a thesis with the CEA in order to develop an alloy or alloys with good tribological properties likely to replace cobalt-based alloys in nuclear applications (Thesis G. Huser, 2017-2020). This aspect will to be developed in the European project M-ERA.NET (cladHEA+, Thesis Clément Vary, 2020-2023).
A second research theme associated with the thematic axis “Materials for Structures and Energy” is centered around the role of hydrogen and oxygen on the mechanical behavior of titanium and zirconium alloys, behavior that depends on the solubility limit of hydrogen in these alloys. For low contents of interstitial elements, H and O, a multi-scale study, both experimental and numerical, of the influence of hydrogen and oxygen contents in solid solution on the viscoplastic behaviour of alpha titanium has been undertaken within the framework of the ANR “FLUTI” (Thesis B. Barkia 2011-2014, then Post-doctorate 2015). The deformation mechanisms were observed in real time during tensile tests under optical microscope or SEM preceded by EBSD measurements and accompanied by measurements of the deformation fields by image correlation. The analysis is also based on TEM observations of dislocation arrangements and twins. Oxygen in solution increases sensitivity to velocity, making the deformation more homogeneous. It causes dynamic strain aging at low strain rates, which slows creep at low stress, resulting in an incubation phenomenon before creep. Hydrogen in solution, much more mobile at room temperature, “screens” the interactions of dislocations with oxygen, lowering the creep threshold stress and accelerating creep. Finally, in situ tensile tests under TEM allowed to determine the sliding planes as well as their deviation as a function of the oxygen content of the titanium. It is widely believed that small additions of oxygen induce a strong hardening effect leading to a significant increase in strength coupled with a drastic decrease in ductility. Additions of zirconium, leading to a series of Ti-4.5Zr-O type alloys with an oxygen content ranging from 0.15 to 0.80 wt%, cause a classical hardening effect but without a dramatic decrease in ductility. The strength/ductility compromise of Ti-Zr-O alloys is therefore exceptional compared to that of other Ti alloys and opens up promising prospects in various applications such as the biomedical, aerospace and nuclear fields. It constitutes the focus of the TiTol project selected by the ANR in 2019 and which started in January 2020 (Thesis Fabienne Amann, 2020-2023).
Finally, a collaboration with the CEA began in 2018. The vessels of pressurized water reactors (PWR) currently in operation in France are made of 16MND5 type steels containing many alloying elements (Mn, Ni, Cu, Cr, Mo, Si…), as well as carbon, limited to 0.15 wt%, due to the dependence of the toughness of the steel with the carbon concentration. However, recent measurements carried out in samples from the lid and tank of the Flamanville 3 EPR have revealed higher contents, up to 0.29 wt%. In addition, some steam generator (SG) bottoms in the EPR park show even higher concentrations. The objective is to better understand the links between the microstructure, particularly the evolution kinetics of carbides under tempering conditions, and the toughness properties of these high-carbon steels. In order to study the structure and composition of the carbides, as well as their stability as a function of heat treatments (a few hours at 640°C and 300°C over long periods), model grades of the FeXC ternary system, with X= Mo, Mn, Cr have been developed and are characterized at very fine scales. In particular, in-situ temperature ageing in transmission electron microscopy is planned (Thesis A. Benarosch, 2018-2021).
Teaching and tutoring
Professor at the Faculty of Science and Technology (FST) of UPEC :
- First semester:
- Chemical equilibria in solution (S3 – L2, FI), in charge of this course, 13h30 of lectures
- Degradation and protection of metals in their environment, (S3 – M2, FI): UE of the master specialty “Heritage Materials in the Environment” (MAPE) of the SGE master, 2h of lectures
- Second semester:
- Origin and structure of materials (S2 – L1, FI), in charge of this course, 9h00 of lecture, 10h30 of tutorials
- Physics and chemistry of materials (S6 – L3, FI), in charge of this course, 13h30 of lecture, 13h30 of tutorials
- Throughout the year:
- Metallurgy, diffusion, mechanics and material characterization, oxidation, (L3 – FA), Professional degree in Chemistry and Physics of Materials, option Metal and Alloy Treatments, 19h of courses and 21h of tutorials
- Deputy Director of ICMPE (UMR 7182),
- Director of the research federation “Fédération pour l’Enseignement et la Recherche sur la Métallurgie en Ile de France” (FERMI, FR 3701),
Deputy Director of GDR N°2048: “High Entropy or Complex Composition Alloy Metallurgy (HEA)”.