Jeudi 28 Janvier 2010, à 14h00
One of the main characteristics of B2-ordered FeAl alloys is its "yield strength anomaly (YSA)", a complex phenomenon resulting in an increase of the alloy’s yield stress (YS) in a domain of temperatures (400-700°C) where its decrease could rather be expected. The YS origin is not fully understood. It seems to be related to the unique ability of FeAl to form (and to retain) high concentrations of thermal vacancies, but is associated with complex dislocations processes.
In this work, new configurations, named "fork-shaped" are observed in single-crystal foils of a B2-FeAl (40 at. %Al, 3.8 at. % Ni, 400 ppm B) alloy, strained at 600°C (i.e. near the YSA peak) in an in-situ dedicated transmission electron microscope (TEM). In situ straining experiments were carried out using a dedicated JEOL 3010 TEM (LTPCM/SIMAP, Grenoble), operated at 300 kV. Mechanical straining of thin foils (estimated strain rate about 10-4 s-1) was performed after a slow heating of the sample to 600°C. Dynamic images were recorded using a digital video camera.
The analysis of the “fork-shaped” dislocation configurations located within the in-situ deformed foils is performed by coupling TEM techniques (in-situ, post-mortem) to dislocation contrast simulation, both with crystallographic considerations coming from the single crystal geometry.
Thus, from the combination of TEM techniques and dislocation contrast simulation, the following conclusions are proposed :
"Fork shaped" dislocations configurations, observed during in situ straining TEM experiments at 600°C in an FeAl alloy containing boron and nickel result from a reaction of two different a<111> superdislocations gliding on crossing 112 planes. This reaction leads to a junction, formed by a pair of a<001> dislocations.
The geometry of observed junctions is different from those previously observed in post mortem studies of similar materials. The different chemical composition of the studied alloy may be responsible for that feature. In particular, boron additions seem to promote the 112 slip in B2-ordered FeAl.
Even if all individual dislocations involved are glissile, the complex geometry of the a<001> junction gives it a sessile character. The locking role of this type of junction will be discussed.