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

• Applicant: Privatdozent Dr.-Ing. Michael Marx
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: from 2009 to 2016
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 160994633

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?

• DFG Programme: Research Grants