Enhancement of Fatigue Damage due to Environmental Variation and its Recovery Treatment

钢的疲劳性能受环境条件的影响很大，而氢在其中起着至关重要的作用。阐明降低易感性的机制和措施一直是许多研究的主题。我们提出了一种新的钢的氢脆模型，认为在塑性变形过程中，氢促进了空位的形成，空位的凝聚导致延性裂纹扩展阻力的降低。基于该模型，本研究的目的是特别研究环境条件变化对氢相关缺陷敏感性的影响。首先，用氢作为探针检测了疲劳过程中产生的缺陷，揭示了空位的形成是恋物癖损伤的影响因素。疲劳损伤和氢之间的相互作用会导致疲劳性能的退化，并增加与氢相关的失效敏感性。在同一个…上更重要的是，在高强度钢的延迟断裂试验中，证实了外加应力和充氢电流密度的循环变化以及它们的协同作用可以提高敏感性。在低至2 0 0℃的温度下对预疲劳样品进行退火热处理可以恢复磁化率的提高。揭示了口腔用形状记忆镍钛合金这一生物医学材料中氢相关失效的演化过程。用聚焦离子束方法从断口正下方制备的样品，用透射电子显微镜直接验证了所提出的模型。与局部非晶化有关的裂纹扩展已被揭示，支持空位密度的增加。氢微印迹技术也被应用于揭示捕获氢的缺陷的局部分布。电子衍射动态观察已成功地应用于揭示纳米尺度样品的表面结构。较少

Fatigue properties of steel are strongly affected by environmental conditions, in which hydrogen plays an essential role. The clarification of the mechanism and measures for reducing the susceptibility have been subjects of many studies. We have proposed a new model for hydrogen embrittlement of steel that claims enhanced formation of vacancies during plastic deformation with the aid of hydrogen and the coagulation of vacancies that leads to the decrease in the ductile crack growth resistance. Based on the model, the aim of the present studies is to investigate particularly die effect of variations of environmental conditions on the susceptibility to hydrogen-related feilure. Firstly, defects created during fatigue have been detected using hydrogen as a probe, revealing the formation of vacancies as a fector of fetigue damage. Interactions between fatigue damage and hydrogen result in degradation of fatigue properties and enhanced susceptibility to hydrogen-related failure. On the same … More basis, in a delayed fracture test of a high strength steel cyclic variations of the applied stress and hydrogen-charging current density and their synergetic effect have been confirmed to enhance the susceptibility. Annealing of pre-fatigued specimens at temperatures as low as 200℃ recovered the enhancement of the susceptibility. Evolution of hydrogen-related failure in a biomedical material, a shape-memory NiTi alloy that is uses in oral cavity, has been revealed. The proposed model has been directly proved with transmission electron microscopy using specimens fabricated by means of a focused ion beam method from just beneath fracture surface. Crack growth associated with local amorphization has been revealed, supporting the increased density of vacancies. Hydrogen microprint technique has been also applied for revealing local distribution of defects that trap hydrogen. Dynamic observation of electron diffraction has been successfully applied for revealing the surface structure of specimens in nanometer-scale. Less