Pinning of dislocations and the origin of the stress anomaly in FeAl alloys

The anomalous stress rise found at intermediate temperatures in FeAl alloys may be caused by the presence of thermal vacancies produced as the temperature rises. Strong support for this hypothesis is provided by the demonstration that the same absolute values of stresses as well as stress increases are found both when testing at high temperatures and when testing at room temperature samples quenched from the same high temperatures. Examination of the superdislocations present after deformation shows strong pinning only at jogs produced by intersection with forest dislocations. Such sessile jogs on screw superdislocations lead to dipole and loop formation as the dislocations continue moving, hence producing the debris observed after deformation. In addition, edge superdislocations show a stepped morphology caused by line instabilities over a certain range of directions. There is no evidence of strong pinning and associated dislocation bowing by vacancy-aggregate type obstacles, and it is, therefore, deduced that the pinning obstacles responsible for the anomalous stress increase are probably relatively weak single vacancies, rather like solute atoms, and not stronger multi-vacancy defect.