OCTAHEDRAL SLIP INSTABILITIES IN GAMMA/GAMMA' SUPERALLOY SINGLE-CRYSTALS CMSX-2 AND AM3

Tensile tests have been carried out on AM3 and CMSX-2 single crystals in the temperature range from 20 to 1050 degrees C, with three orientations of high symmetry of the tensile axis: [001], [011] and [111]. In order to examine the influence of the size and distribution of the gamma' precipitates, three different thermal treatments were used. The present work focuses on the occurrence of unstable plastic flow and on the observation of the resulting strain localizations by scanning electron microscopy and transmission electron microscopy. Plastic instabilities are identified as catastrophic slip on octahedral planes inducing the repeated shearing of the gamma' precipitates by mobile dislocations. The influence of thermal treatments on the occurrence of heterogeneous plastic flow is discussed in terms of a competition between the shearing of the gamma' precipitates and their by-passing by an Orowan process. The plastic instabilities are interpreted as being due to a difficult dislocation generation step in the gamma matrix, followed by an easy propagation step in an octahedral slip plane.