Phase Transformations and Microstructures

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Team leaders : Frédéric Danoix (CR) & Williams Lefebvre (PR)

Team members : F. Danoix (CR), R. Danoix(MCF), A. Guillet (MCF), W. Lefebvre (PR), B. Lefez (MCF), C. Pareige (PR), X. Sauvage (DR), A. Barbier (MCF), F. Petit (MCF), S. Moldovan (IR)

The study of phase transformations in metallic alloys and the resulting microstructures is a historical activity of the materials physics group. At first, it was mainly focused on the study of precipitation and segregation, in particular with the tomographic atom probe. Taking advantage of the development of the instrumental park (in particular electron microscopy, SEM-FIB-EBSD and TEM), it has since strongly developed, to treat both fundamental and applied problems in collaboration with many industrial partners. The efforts are now mainly focused on the understanding of solute/crystalline defect interactions, process/structure and structure/properties links, design of new alloys and aging.

1. Phase transformations in steels: 
   
   The objective is to better understand the basic mechanisms involved in very fine scale phase transformations in metallic alloys, in order to apply them to industrial grades. In steels, the main phenomena studied concern on the one hand the redistribution of carbon between the different phases, such as during the quenchingand partitioning treatment or within the martensite (spinodal decomposition and precipitation of transition carbides), and on the other hand the interfacial phenomena, whether it be thermodynamic conditions at the grain boundaries (ferrite-austenite transformation interfaces or equilibrium and non-equilibrium segregation phenomena) in collaboration with the thematic modelling team. Another important theme is the study of phase transformations within the low temperature miscibility gap of Fe-Cr alloys and stainless steels, in order to better understand the embrittlement phenomena of this family of alloys. This theme is at the interface between our thematic team and the "nuclear materials" team for which the problem of the embrittlement of austeno-ferritic and ferritic-martensitic steels is strongly linked to the precipitation of Cr. Our activities around nitriding (steels and nickel bases), and more particularly on the nanostructure developed in sub-surface, will also be continued. On the "ageing" theme, efforts will continue on the refractory steels developed in the framework of the joint laboratory with Manoir Industries. In particular, models will be established and validated to predict microstructural changes in service over very long periods (several years) and the corresponding creep behavior.

    

2. Metal Additive Manufacturing:
   
   Metal additive manufacturing opens today vast perspectives in the field of designing a microstructural state in response to a set of targeted properties. This process, emerging industrially, is at the confluence of different themes: rapid solidification, powder metallurgy, aging, mechanical behavior. Focusing on the link between the elaboration process and the genesis-aging couple of microstructures, our studies concern for example aluminium alloys specially developed for metal additive manufacturing with a particular interest on the formation of non-equilibrium microstructures (supersaturated solid solutions, amorphous phases) generated by an extremely fast cooling (103-106 K/s). Depending on the fabrication parameters, the microstructural gradients (e.g. work hardening gradients, precipitation gradients) can be studied at all scales with the GPM instrumental park. The knowledge thus developed can be used to design materials with gradient properties.

    

3. Materials for high temperature applications:

   The activities on materials for "high temperature" applications are mainly those carried out by the joint laboratory IPERS. This was founded between the GPM and the Manoir Pîtres company (a subsidiary of the Manoir Industries group), the world's third largest producer of refractory steel tubes by centrifugal casting for applications in the petrochemical industry with severe environmental constraints (oxidation, carburization, nitriding and coking) at temperatures up to 1100°C.

   
   Three lines of work are developed to address the major scientific issues of the in-service behavior of these refractory steels: creep resistance, environmental aging, monitoring of materials in service. In all cases, the approach involves a detailed understanding of the aging phenomena, supported in particular by microstructural studies down to the atomic scale and supplemented by modeling. On the basis of this knowledge, it is then a question of proposing, testing and then developing (beyond the laboratory scale) innovative solutions for high added value steels.