Construction of Dislocation Structure Diagrams for Life Assessment of Materials

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Strain-controlled cyclic deformation tests for pure Cu, Cu alloys, A1 and A1 alloys (mostly single crystals) were conducted. Although the crystal structure of these metals and alloys are all fcc, the fatigue behavior was quite different. For example, pure Cu showed cyclic hardening to saturation, whereas pure Al showed hardening followed by distinct softening. Observation of dislocation microstructure has revealed that the PSB ladder structure develops in Cu and it is absent in Al. Through extensive investigation, it has been found that the differences in both the fatigue behavior and microstructure can be explained by the relative feasibility of cross slip that is determined by the magnitude of stacking fault energy. Furthermore, from experiments using a Cu-Fe alloy with dispersed small Fe precipitate particles in Cu, it has been found that the Fe particles are effective in preventing the development of fatigue dislocation structure. With this knowledge, dislocation structure diagrams that indicate fatigue dislocation structure as a function of size and distribution of second-phase particles were successfully constructed.Dislocation structures developed during fatigue tests are found to be much more stable (both thermally and mechanically) compared with those developed after cold rolling. Owing to the to-and-fro motion during cyclic deformation, dislocations arrange themselves into an energetically stable configuration resulting in the formation of characteristic fatigue microstructures that eventually lead to fatigue failure.In conclusion, we could obtain various new results and findings, most of which are very valuable for life assessment of structural materials.