Dislocation deformation mechanisms during fatigue of the nickel-based superalloy CMSX-4.

This paper presents an overview of deformation mechanisms during the fatigue in the Nickel-based Superalloy CMSX-4. Details of creep processes in Nickel based superalloys have been studied in great detail but the dislocation configurations developed during fatigue and of these alloys have received comparatively little attention. Low cycle fatigue tests have been performed at 750 degrees C at stresses above and below the nominal yield stress and the microstructures examined at rupture and following early interruption of the test. Below the yield stress dislocation activity is largely restricted to the gamma phase, whilst at higher stress the dislocations break through into the gamma prime as single dislocation loops. These can penetrate several precipitates, and when sectioned perpendicular to the tensile axis, show up as dislocation dipoles. In addition to these defects, there is also a range of defects formed by partial dislocations trailing stacking faults similar to those observed during creep, (Superlattice Stacking Fault shear or SSF). At higher temperatures deformation is entirely by perfect dislocations in the gamma and also in the gamma prime above the yield stress.