Strain softening, grain boundary migration and dynamic recrystallization of Ni during high temperature low cycle fatigue

Abstract

The flow stress behavior and microstructural development of nickel polycrystals during high temperature low cycle fatigue were studied. All specimens exhibited a flow stress decrease at large cycle numbers, even at total strain amplitudes as low as 0.4% but distinctly before crack initiation. The microstructure revealed a tendency toward grain coarsening due to grain boundary migration, but locally grain refinement owing to dynamic recrystallization is the major softening mechanism at large cumulative strains and the cause of the observed flow stress maximum. Observations on Al, which does not recrystallize dynamically support this conclusion. After large cycle numbers the dislocations were arranged in a well recovered cell structure, and the average cell size scaled with the reverse flow stress. The results substantiate that dynamic recrystallization can be set off at drastically lower stress in cyclic deformation than in monotonie deformation. This supports the stipulation that occurrence of dynamic recrystallization is nucleation controlled.