On precipitate shearing by superlattice stacking faults in superalloys

Abstract

Superlattice intrinsic stacking faults (SISFs) constitute one of the major features of the deformation microstructure of superalloys. It is pointed out that they may result from the dissociation, in the precipitates, of a single perfect matrix dislocation by a process essentially similar to the formation of SISFs in single-phase Ll₂ ordered alloys. By using transmission electron microscopy under weak-beam conditions, it is demonstrated that SISFs originate mostly at the precipitate corners and that they may then interact attractively with secondary matrix dislocations, in order to form stable junctions. Energy calculations of the growth conditions of a SISF are performed as a function of precipitate size, SISF energy and applied stress, in the case of several alternative crystallographic configurations. It is shown that, although the dislocation dissociation is unfavourable on a per-unit-length basis, the nucleation and growth of SISFs is easy in superalloys since specific geometrical conditions may promote SISF extension, even in the absence of local stress. The conditions of SISF nucleation are briefly discussed.