INTERNAL-STRESSES DUE TO DISLOCATION WALLS AROUND 2ND PHASE PARTICLES

Nickel base superalloys with large γ′ volume fractions exhibit a high threshold stress during a tensile test, and a large internal stress when tested in high temperature creep. Arrays of regularly spaced edge dislocations which develop during the first stages of creep have been observed. They lie in the γ-γ′ interfaces and form dipolar arrangements on opposite sides of the γ′ cuboids. The various components of the stress tensor are calculated for such a dipolar wall configuration and mapped by drawing equistress lines. The stress field resulting from such dislocation configurations has a very strong component along the tensile axis and opposes the applied stress. The shear component in the glide plane of similar dislocations is also very large between the precipitates and would tend to repel any new dislocation, in the absence of an external stress. The structural instability of these alloys under creep strain is also interpreted by this model. © 1979 American Society for Metals and the Metallurgical Society of AIME.