Microstructural studies on the initiation and growth of small fatigue cracks at 298, 77 and 4.2 K in polycrystalline copper

The temperature dependence of the initiation and growth of small fatigue cracks has been studied by microscopic observations in polycrystalline copper during the early stages of fatigue. The primary procedure was to examine the same site of the specimen by SEM and TEM using surface replicas taken periodically during the fatigue tests. A periodical array of pit-shaped fatigue cracks as small as 0.1 ,um in depth forms along slip bands at 298, 77 and 4.2 K, even in an inert environment. A dislocation gliding mechanism of dislocation dipoles in the slip bands has been suggested as a possible process for the formation of these cracks. It was found that the configuration of small fatigue cracks is related to the periodically spaced dislocation regions in fatigued Cu. The growth rate of small fatigue cracks during the early stages of fatigue was measured and found to be approximately 0.1 nm/cycle at 298 and 77 K. A quantitative analysis of the net effects of temperature and environment on the fatigue crack initiation was performed.