On the influence of stress state, stress level and temperature on γ-channel widening in the single crystal superalloy CMSX-4

The present investigation describes the kinetics of gamma-channel widening in the single-crystal superalloy CNISX-4 at temperatures above 1000degreesC and at stress levels between 50 and 100MPa. A miniature tensile creep specimen (uniaxial loading in <001>-direction) and a double shear creep specimen (biaxial loading of the macroscopic crystallographic shear system {011} <011>) were used for interrupted creep testing. The volume fractions of the gamma- and gamma'-phases and the kinetics of gamma-channel widening were investigated using high resolution scanning electron microscopy in combination with quantitative metallography. It was found that rafting always occurred perpendicularly to the direction of the maximum principal stress (i.e. perpendicular to the tensile direction in the tensile specimens and in 45degrees to the shear stress for the shear tests). Over the whole stress range, there was no significant difference between gamma-channel widening in <001>-tension and {110} <011>-shear creep deformation, when the experiments were performed at the same maximum principal stress. gamma-Channel widening is an important process during creep of gamma/gamma'-microstructures because dislocations propagate in the thin gamma-channels. Our present study shows how temperature, stress level and stress state affect this microstructural coarsening process. Superimposed stresses favour the gamma-phase, and therefore channel widths increase faster with increasing superimposed stresses. (C) 2004 Elsevier B.V. All rights reserved.