Construction of Dislocation Structure Diagrams for Life Assessment of Materials

对纯铜、铜合金、铝和铝合金(主要是单晶)进行了应变控制循环变形试验。虽然这些金属和合金的晶体结构都是面心立方的，但疲劳行为却有很大的不同。例如，纯铜表现为循环硬化到饱和，而纯铝表现为先硬化后明显软化。位错显微组织观察表明，在铜中有PSB梯形结构，而在Al中没有。通过广泛的研究发现，疲劳行为和微观组织的差异可以用层错能大小决定的交叉滑移的相对可行性来解释。此外，用铜中弥散分布的细小铁析出物颗粒进行实验，发现铁颗粒对防止疲劳位错结构的发展是有效的。利用这些知识，成功地建立了位错结构图，该图显示了疲劳位错结构与第二相粒子尺寸和分布的函数关系。与冷轧后形成的位错结构相比，在疲劳试验中形成的位错组织在热和力学上都要稳定得多。由于循环变形过程中位错的往复运动，位错形成能量稳定的构型，形成具有特征的疲劳组织，最终导致疲劳失效。综上所述，我们可以得到各种新的结果和发现，其中大多数对结构材料的寿命评估非常有价值。

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.