Design of the next generation of solid solution strengthened ductile cast iron

• Applicants: Professor Dr.-Ing. Christoph Broeckmann; Professor Dr.-Ing. Andreas Bührig-Polaczek
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 459577017

Silicon solid solution strengthened ductile cast irons (SS-FDI) provide outstanding mechanical properties by an improved combination of strength and ductility compared to conventional ductile cast irons. However, it is known that an excessively high Si content can also have a detrimental effect on the mechanical properties and, in particular, the notched bar impact strength of SS-FDI. This is attributed to the restriction of dislocation movement due to the formation of B2 and DO3 superstructures. Therefore, the damage mechanisms of SS-FDI were investigated in the previous project using a coupled experimental and simulative approach. The knowledge gained will be used to develop new SS-FDI grades. The embrittlement caused by superstructure formation at elevated Si contents is to be avoided by targeted adjustment of the Si segregations. It is assumed that the current upper limit for the Si content in SS-FDI (4.3 wt.%) can be increased by adding suitable alloying elements and adapting the casting process, so that embrittlement can be reduced and the strength can be increased. Taking into account the first project phase, the proposed project aims at developing the next generation of SS-FDI alloys that overcome the current compromise between strength and ductility and compensate for the drastic drop in toughness due to the increased Si content. The aim is to develop a new SS-FDI grade for the first time in which the solid solution strengthening effect is exploited to the greatest possible extent by increasing the Si content, without impairing the mechanical properties by forming a superstructure. This is also achieved by adding further alloying elements and adjusting the cooling conditions. In order to achieve this goal in the most effective way, several new innovative methods are being developed that will also support future research in the field of GJS. The work follows a coupled approach of numerical calculation and experimental investigation: the presence of superstructures is to be predicted qualitatively by a thermodynamically coupled microsegregation modeling for SS-FDI. TEM investigations of the formation of superstructures are correlated with nanoindentation mappings. The results are combined with residual stress simulations to exclude process influences. The correlation between superstructure formation and micromechanical properties determined in this way is then used to optimize cooling conditions. The simulated microsegregation profiles are also compared with real segregation profiles in order to develop a simulation method for predicting the microstructures. Finally, the microstructures and the mechanical properties of an optimized SS-FDI grade are characterized under static and cyclic loading.

• DFG Programme: Research Grants