Papers by Author: Muriel Véron

Abstract: EBSD is a well known technique that allows orientation and phase mapping using an SEM. Although the technique is very powerful, has serious limitations related with a) special resolution limited to 50 nm (SEM-FEG) and b) specimen preparation issues as is not possible to obtain EBSD signal from rough surfaces or strained materials , nanoparticles etc.. To address those difficulties , a novel technique has been developed recently (EBSD-TEM like) allowing automatic orientation and phase mapping using template matching analysis of acquired diffraction patterns in TEM. Electron beam is scanned through the sample area of interest ; the acquired electron diffraction patterns from several sample locations are compared via cross-correlation matching techniques with pre-calculted simulated templates to reveal local crystal orientation and phases. The dedicated device (ASTAR) allows orientation and phase identification of crystallographic orientation in a region of interest up to 10µm2, with a step size ranging from 1nm to 20nm depending on the transmission electron microscope setting (FEG or LaB6).

Abstract: Duplex stainless steels (DSS) are alloys made of ferrite and austenite, with a proportion of each phase around 50%. Their main advantage in comparison with other austenitic and ferritic stainless steels is the attractive combination of high strength and corrosion resistance together with good formability and weldability. Unfortunately, DSS often present a poor hot workability. This phenomenon can stem from different factors associated to the balance of the phases, the nature of the interface, the distribution, size and shape of the second phase, and possibly also from difference in rheology between ferrite and austenite. In order to determine the specific influence of phase morphology on the hot-workability of DSS, two austenite morphologies (E: Equiaxed and W: Widmanstätten) with very similar phase ratio have been generated using appropriate heat treatments. It was checked that the latter treatments generate stable microstructures so that subsequent hot mechanical tests are performed on the microstructures of interest. One microstructure consists of a ferritic matrix with austenitic equiaxed islands while the other microstructure is composed of a ferritic matrix with Widmanstätten austenite. The latter morphology corresponds to the morphology observed in as-cast slabs.