A Study on Mechanisms of Environmental Brittle Behavior by Single-and Bicrystal Experiment.

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The environmental effect on fatigue crack propagation in copper and copper-aluminium compact tensio (CT) single and bicrystals was examined. Single and bicrystals with either SIGMA41 (338), SIGMA9 (221) and random [110] tilt boundaries were grown by Bridgman technique and used to the fatigue test. The role of the initial grain boundary (GB) structure is anlyzed for the bicrystals. Fatigue tests were done at 296 (]SY.+-。[)2K and humidity of (50 (]SY+-[)5)% in air or in vacuum of 1.3*10<@D1-3@>D1 Pa in a chamber of a scanning electron microscope (SEM). It is shown that the GB is not necessary a preferential site for crack initiation and propagation in vacuum. A crack introduced integranulary in air may change its mode of growth to transgranular in vacuum. The relative importance of GB cracking compared with persistent slip bands (PSB) cracking in vacuum is examined. The dominant mechanism of fracture is determined by two factors : The GB structure and the geometry of slip with respect to the GB orientation. The model of PSB-GB crack initiation proposed and modified for cyclic deformation explains satisfacctorily most of the experimental results of fatigue crack propagation.