Character of dislocations at the γ/γ′ interfaces and internal stresses in nickel-base superalloys

It is experimentally observed in Ni-based superalloys that the dislocation substructure after deformation at elevated temperatures is different on horizontal and vertical gamma/gamma' interfaces. Edge dislocation networks are predominantly observed at horizontal interfaces while screw dislocation segments are seen mostly on the vertical interfaces. A crystallographic model of the critical shear stress for dipole expansion for screw and 60 degrees dislocation segments in the gamma/gamma' interfaces is used to explain the observations in high gamma' volume fraction (similar to 70%) alloys. However, the same model is found to be inadequate in explaining the substructure observations in those alloys with lower gamma' volume fraction (similar to 40%) which have wider matrix (gamma) channels (approximate to 180 nm). It is shown by superimposition of the dislocation's stress field, the internal coherency stresses and the externally applied stress that between 60 degrees and screw dislocation segments the former increase the internal stresses in vertical interfaces more than the latter. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.