Interaction of boron with crystal defects in B2-ordered FeAl alloys
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Interaction of boron with crystal defects in B2-ordered FeAl alloys 1
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Anna Fraczkiewicz, Anne-Sophie Gay, Emmanuel Cadel , Didier Blavette ENSMSE, Centre SMS, URA CNRS 1884, 158 Cours Fauriel, F-42023 ST-ETIENNE 1 Université de Rouen, UMR 6634 CNRS, Faculté des Sciences de Rouen, F-76 821 MONT ST-AIGNAN
ABSTRACT Intermetallic alloys are often doped with boron to suppress their intrinsic room-temperature intergranular brittleness. The commonly admitted mechanism of this effect, i.e. an intergranular segregation of boron, seems not to be the only important feature. In this work, boron interactions with numerous kinds of crystal defects (point defects, dislocations, grain boundaries) are studied in B-doped FeAl (B2) alloys containing 40 at. % Al. The intergranular segregation of boron is first characterized. Both an equilibrium and a non-equilibrium (due to a solute atom / thermal vacancy interaction) segregation mechanisms are identified. Strong tendency of boron to segregate to the dislocations lines is shown by direct measurements by atom probe field ion microscopy (AP FIM). This segregation is shown to induce a local depletion in Al in the vicinity of defects. INTRODUCTION Doping with small additions of boron (few hundreds at. ppm) is an efficient way to suppress the intergranular weakness of grain boundaries in many intermetallic alloys, like Ni3Al (L12), Fe3Al (DO3), and FeAl (B2). Thus, in B-doped materials, the room temperature fracture mode becomes mainly transgranular, while a brittle intergranular fracture is commonly observed in B-free alloys. Following observations of B-enriched layer at the fracture surfaces of the former alloys, when analyzed by AES (Auger Electron Spectrometry), it is now currently admitted that this reinforcing effect of boron in intermetallic alloys is due to its intergranular segregation [1]. However, the elementary mechanisms of the action of boron, are not really understood. This question was analyzed a few years ago by George et al. [2] for the case of Ni3Al alloys. Two hypotheses have been proposed, not really contradictory in fact: according to them, the reinforcing effect of boron could consist in (i) an increase of the grain boundary cohesion, or (ii) a modification of conditions of the slip transmission from one grain to another. In this paper, it will be shown that the effect of boron in FeAl (B2) can not be limited to only intergranular phenomena. Experimental evidence of interactions of boron with different crystal defects (point defects, dislocation lines and grain boundaries) will be given. EXPERIMENTAL The B2-ordered FeAl alloys, containing 40 at. % Al (“FeAl” in this text), pure or B-doped (40 - 2000 at ppm), were laboratory prepared by cold-crucible melting. The FeAl alloys are known to retain large concentration of thermal vacancies after quench; therefore, a standard heat treatment was applied to the samples, in accordance to the results of previous work by Rieu and Goux [3]. N3.4.1
The samples were first annealed at 950°C/1 hour, and quenched in air; in this
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