Microstructure based crack initiation and propagation mechanisms in bimodal UFG and nanoscaled materials

在项目的第一阶段，最重要的结果是：对于ecap -镍，最后剪切过程的织构（导致沿最后剪切方向的拉长晶粒）决定了疲劳行为。ecap -镍不能通过产生双峰组织而转变为抗疲劳组织。双峰改性的ped -镍具有较好的抗疲劳性能。然而，对于PED-Nickel，晶粒细糖精对晶粒生长和疲劳行为的影响尚不清楚。结合最新的文献，在这个项目的第二阶段需要回答以下问题：1。热处理后糖精浓度对微观结构有何影响？特别是获得双峰组织所必需的异常晶粒长大，在很大程度上取决于杂质和偏析。2. 热处理过程中晶界处硫的偏析如何改变合金的力学性能？这可能导致完全不同的性能，如硬度，断裂韧性，屈服强度或断裂应变，即使在相同的标称晶粒尺寸。糖精浓度如何影响双峰NC / UFG镍的疲劳行为？其中一个重点将是由DFG计划资助的大型科学设备UdS的原子探针对硫分布的超高分辨率测量。我们自己制作样品，因此我们有独特的可能性来详细研究像糖精这样的杂质的影响。4. 如何通过晶粒细化和热处理相结合来控制UFG晶粒的比例？通过现场测试表明，UFG晶粒的塑性变形降低了双模态NC / UFG显微组织的裂纹扩展速率。预计大晶粒在NC基体中的比例决定了其疲劳行为，随着大晶粒比例的增加，其抗疲劳性能也会提高。同时，硬度、断裂韧性、屈服强度或断裂应变等力学性能也发生了变化。因此，确定具有良好的抗疲劳性能而又不丧失基本数控材料优异的其他性能的理想微观组织是一个目标。简而言之，我们想知道糖精含量和热处理的结合是如何产生理想的微观结构的，哪些物理机制可以描述这一点？

In the first period of the project the most important results are:For ECAP-Nickel the texture of the last shearing process (resulting in elongated grains along this last shearing direction) determines the fatigue behavior. ECAP-Nickel cannot be turned into a fatigue resistant microstructure by producing a bimodal microstructure.PED-Nickel is more fatigue resistant in the bimodal modification.However, for PED-Nickel the influence of the grain finer saccharine on grain growth and fatigue behavior is unknown.In combination with the newest literature there are following questions which should be answered during the second period of this project:1. How is the microstructure influenced by the concentration of saccharin after the heat treatment? Especially the abnormal grain growth, which is necessary to get a bimodal microstructure, depends very strongly from impurities and segregation. 2. How does the segregation of sulfur at the grain boundaries during the heat treatment change the mechanical properties? This could result in completely different properties like hardness, fracture toughness, yield strength or strain at fracture even at the same nominal grain size.3. How does the saccharin concentration influence the fatigue behavior of bimodal NC / UFG Nickel? One focus will be ultra-high resolution measurements of the sulfur distribution by the atom probe at UdS financed by the DFG program for large scientific equipment. We produce the specimens ourselves therefore we have the unique possibility to investigate the influence of impurities like saccharine in detail. 4. How can the fraction of UFG grains be controlled by a combination of grain finer and heat treatment? By in-situ measurements it was shown, that the crack propagation rate in bimodale NC / UFG microstructures is reduced by plastic deformation of the UFG grains. It is expected that the fraction of large UFG grains in the NC matrix determines the fatigue behavior and with increasing fraction of UFG grains the fatigue resistance should also improve. Simultaneously the mechanical properties like hardness, fracture toughness, yield strength or strain at fracture are changed. Therefore it is one aim to determine the ideal microstructure with a good fatigue resistance without a loss of the excellent other properties of the basic NC material.Shortly summarized, we want to know how a combination of saccharine content and heat treatment results in an ideal microstructure and which physical mechanisms do describe this?