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-镍来说，最后一次剪切过程的织构(导致沿最后一次剪切方向的细长晶粒)决定了疲劳行为。ECAP-镍不能通过产生双峰组织转变为抗疲劳组织。PED-镍在双峰变质中具有更好的抗疲劳性能。但对于PED-镍来说，细晶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，本项目第二阶段需要回答以下问题：1.热处理后糖精浓度对组织结构有何影响？尤其是获得双峰组织所必需的异常晶粒长大，与杂质和偏析有很大的关系。2.热处理过程中硫在晶界的偏聚对力学性能有何影响？这可能会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度下也是如此。糖精浓度对双峰NC/UFG镍的疲劳行为有何影响？其中一个重点将是由DFG计划资助的大型科学设备的UDS原子探测器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糖精等杂质的影响。4.如何通过细化和热处理相结合的方法来控制UFG颗粒的比例？现场测量表明，超细晶颗粒的塑性变形降低了双峰NC/UFG显微组织中的裂纹扩展速率。可以认为，纳米晶中大颗粒UFG的含量决定了材料的疲劳行为，并且随着颗粒含量的增加，疲劳抗力也应得到提高。同时，硬度、断裂韧性、屈服强度或断裂应变等力学性能也会发生变化。因此，在不损失基本NC材料优异的其他性能的情况下，确定具有良好抗疲劳性能的理想微观组织是一个目标。简单地说，我们想知道糖精含量和热处理的组合如何产生理想的微观结构，以及哪些物理机制描述了这一点？

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?