gamma '-cutting as rate-controlling recovery process during high-temperature and low-stress creep of superalloy single crystals

In the present study the pairwise cutting of the gamma'-phase after high-te mperature and low-stress shear creep deformation of superalloy single cryst als was investigated using weak-beam and high-resolution transmission elect ron microscopy. Recently, a cutting process in the single-crystal superallo y CMSX-6 was observed [Acta mater., 45 (1997) 4251] where two gamma -channe l dislocations with different Burgers' vectors (b) jointly shear the gamma' -phase in forming a superdislocation with an overall Burgers' vector of a[0 10]. This type of high-temperature and low-stress gamma'-phase cutting mech anism was also observed for CMSX-4 in the present work, indicating that thi s mechanism is relevant for superalloy single crystals in general. Two diff erent configurations have been observed associated with the pure edge a[010 ] and the 45 degrees a[001] dislocations. The cores of these superdislocati ons are not compact, but rather are composed of two different a/2(110) disl ocations. The distance between the leading and the trailing superpartial di slocation for the pure edge a(010) configuration is of the order of 25 Angs trom. In all cases observed in the present study, the common superpartial i s associated with the crystallographic slip system that is directly loaded (Schmid factor 1). The striking feature of the movement of the superdisloca tions in the gamma'-phase is that the two superpartials need to move by a c ombined process of glide and climb. This requires diffusional exchange of a toms/vacancies between the leading and the trailing superpartial, in which case the process is serf-fed and the overall vacancy equilibrium is not dis turbed. It is also possible that one dislocation pair produces or absorbs v acancies so that its movement must be coupled to events which maintain over all vacancy equilibrium. Minimum creep rates can be rationalized on the bas is of the fluxes associated with the movement of superdislocations in the g amma'-phase. (C) 2000 Acta Metallurgica lac. Published by Elsevier Science Ltd. All rights reserved.