Plastic anomaly of the B2 ordered Fe-40Al alloy

The plastic behaviour of Fe-40Al (B2-ordered) alloys was studied. This material has the particularly of exhibiting a yield stress that increases in a given temperature range ('yield stress anomaly'). This anomaly is usually associated with a zero strain rate sensitivity in the very same temperature range. These two peculiarities can be explained as a whole by a thermally activated exhaustion of the mobile dislocations. In this work, the macroscopic mechanical behaviour of the FeAl alloys was first characterised. Then, slip geometry was studied in a large temperature range using oriented single crystals. Finally, the elementary deformation processes were studied through an analysis of dislocation structures in deformed materials using TEM post-mortem and in-situ techniques. Our results show that the yield stress anomaly stems from superdislocations exhaustion through the formation of antiphase boundary tubes, due to vacancy absorption. The number of antiphase boundary tubes produced during dislocation motion depends on vacancy concentration, that in turn increases with temperature. This is believed to be the main reason for thermally activated exhaustion. In addition, the anomaly peak and the related stress fall-off at higher temperatures can be ascribed to superdislocation decomposition, which provides the material with ordinary dislocations that cannot generate antiphase boundary tubes. Superdislocations exhaustion seems to be the catalyzing factor for decomposition. We have proposed a basis for an exhaustion/multiplication model. Considering a classical Frank-Read type multiplication mechanism, we express the yield stress as a function of temperature and we show that this results in an anomaly. Moreover, supposing that boron modifies vacancy migration energy, we suggest that the influence of boron on the stress anomaly stems from an increase of vacancy capture radius by mobile superdislocations. (authors)