γ′-cutting as rate-controlling recovery process during high-temperature and low-stress creep of superalloy single crystals Original Research Article

In the present study the pairwise cutting of the γ′-phase after high-temperature and low-stress shear creep deformation of superalloy single crystals was investigated using weak-beam and high-resolution transmission electron microscopy. Recently, a cutting process in the single-crystal superalloy CMSX-6 was observed [Acta mater., 45 (1997) 4251] where two γ-channel dislocations with different Burgersʹ vectors (b) jointly shear the γ′-phase in forming a superdislocation with an overall Burgersʹ vector of a[010]. This type of high-temperature and low-stress γ′-phase cutting mechanism was also observed for CMSX-4 in the present work, indicating that this mechanism is relevant for superalloy single crystals in general. Two different configurations have been observed associated with the pure edge a〈010〉 and the 45° a〈001〉 dislocations. The cores of these superdislocations are not compact, but rather are composed of two different a/2〈110〉 dislocations. The distance between the leading and the trailing superpartial dislocation for the pure edge a〈010〉 configuration is of the order of 25 Å. In all cases observed in the present study, the common superpartial is associated with the crystallographic slip system that is directly loaded (Schmid factor 1). The striking feature of the movement of the superdislocations in the γ′-phase is that the two superpartials need to move by a combined process of glide and climb. This requires diffusional exchange of atoms/vacancies between the leading and the trailing superpartial, in which case the process is self-fed and the overall vacancy equilibrium is not disturbed. It is also possible that one dislocation pair produces or absorbs vacancies so that its movement must be coupled to events which maintain overall vacancy equilibrium. Minimum creep rates can be rationalized on the basis of the fluxes associated with the movement of superdislocations in the γ′-phase.