Study on Fatigue Failure of Alloyed Cu Bicrystals by means of in-situ SEM Observation

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It has widely been accepted that a structure of grain boundary (GB) is one of the decisive factors affecting the GB-related phenomena such as the GB sliding, the GB corrosion and the GB stress-corrosion cracking, which are important for engineering purposes. In addition to these phenomena, fatigue behavior of materials is expected to be sensitive to the structure of the GB contained. In order to correlate the fatigue property with the GB structure, both fatigue experiments and simulations of the GB structures were carried out on the copper model bicrystals.1. The results of the fatigue investigation on copper bicrystal having a longitudinal twist boundary are summarized as follows.(1) The incompatibility of plastic strain at the GBs could induce high internal stresses in the vicinity of GBs, and apparently assists the nucleation of an intergranular fatigue crack.(2) The GB affected zone was a strong barrier against the propagation of the intragranular crack that was initiated at a free … More surface.2. In the experiments on the deviation angle dependence of the GB cracking using the copper bicrystals having the orientation relationship close to SIGMA3 (111), the following conclusions were obtained.(1) In the speciments having the deviation angles less than 3ﾟ, no any intergranular crack propagations were observed. On the other hand, the fatigue cracks often propagated along the GBs when the deviation angle increased up to 5ﾟ. Hence, it is can be said that the intergranular fatigue cracking in the copper is sensitive to the GB structure.(2) The slip step dissolution model can be applicable also for the propagation of the intergranular fatigue cracks.(3) Paths of the fatigue cracks were determined not only by the GB structure but also by the slip geometry of the constituent grains.3. The results of the molecular dynamics simulation focused on the GB structure of [110] tilt boundaries can be summarized as follows.(1) Arangements of lattice dislocations within low-angle GBs agreed well with those predicted from the differing at Burgers vector.(2) The estimated GB energy showed strong dependence on the misorientation angle between the grains. The GB energies of the low-angle boundaries were in good agreement with the theoretical values calculated from the Read-Shockley model. Less