Dislocation Structure in Fatigue

Examples of dislocation structures after fatigue deformation appear in a series of microstructures (Figs. 7.55-7.59) obtained by TEM. Subramanian [27] claims that these dislocation substructures are similar to that of unidirectionaUy-stressed MgO. The micrographs presented below are of single-crystal magnesia which underwent a large number of cycles (in the millions) of low strain amplitude. The maximum strain was about 0.1 % per cycle. The characteristics of the dislocation structure in MgO, having a rock-salt structure, is as follows  

It was further claimed, based on Figs. 7.59 and 7.60, that the dislocation substructure in fatigued single-crystal MgO specimens is independent of the number of cycles of repeated stress. Thus, in Fig. 7.59, the number of cycles is 1 x 10 and, in Fig. 7.60, the number of cycles is 4 x 10 , but the dislocation substructures are similar in both these micrographs. The TEM data further indicate that the dislocation density in specimens after fatigue should be less than in unidirec-tionally-strained specimens and, thus, the dislocation density is independent of the cycles after a few initial testing cycles. 

Since MgO has a structure similar to that of NaCl crystal, its slip systems are also of the type 110 (110). There are six such planes in a cubic structure. In a cubic structure, four of the six 110 slip-plane projections onto the (001) slip planes leave traces along the (100) direction and lie at 45° to the surface these are known as 45° slip planes . Two have traces along the (110) direction, lie at 90° to the surface and are referred to as 90° slip planes . The shortest Burgers vector for a perfect dislocation in the NaCl structure is a/2 (110). This is the operative slip direction. There is only one (110) slip direction in each 110 slip plane. Therefore, there can be only one kind of mobile dislocation in each slip plane. The secondary slip systems are of the type 001 (110). Since the primary and secondary slip planes have a common Burgers vector, this is the cross-slip plane. No two primary slip planes have the same Burgers vector. 

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