Correlation of stacking fault energy and mechanically induced martensite formation and adjustment of both to enable the targeted use of transformation induced plasticity (TRIP-effect) in ausferritic ductile cast iron

• Applicant: Professorin Dr.-Ing. Babette Tonn
• Subject Area: Mechanical Properties of Metallic Materials and their Microstructural Origins ; Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Primary Shaping and Reshaping Technology, Additive Manufacturing
• Term: since 2025
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 557288079

The aim of the research project is to investigate mechanically induced martensite formation and the stacking fault energy (SFE) of austenite in ausferritic ductile iron (ADI). The results will allow the controlled activation of strain induced martensite formation in ADI. On this basis, hardening, energy absorption and plasticity resulting from transformation induced plasticity (TRIP effect) can be harnessed in ADI. By varying the chemical composition and heat treatment in a statistical design of experiments, austenite of different stability is produced and its deformation behaviour is investigated in tensile tests in the temperature range of -196°C to 200°C. The SFE and austenite stability influence thermally and mechanically activated martensite formation in a temperature dependent manner. Stress-strain and strain-hardening curves show the temperature and stress dependency of martensite formation. XRD is used to measure the transformed austenite fraction and the SFE. EBSD studies of martensite crystals in mechanically stressed areas of the specimen show the crystallographic transformation mechanism. In steels that take advantage of the TRIP effect (e.g. FeMnSiAl), the SFE has a crucial influence on mechanical behaviour and martensite formation. The influence of compositional and manufacturing parameters on the SFE, onset and extent of mechanically induced martensite formation are unknown in ADI. By investigating these relationships, it is possible for the first time to tailor the production of ADI to enable the TRIP effect to be utilised. The conditions, extent, effects, and microstructural basis of mechanically induced martensite formation in ADI can be described by combining mechanical and crystallographic investigations. The TRIP effect enables ADI to make better use of parts cross-sections along with increased energy absorption during deformation, both enhancing its potential for lightweight design. This can potentially open up new areas of application for ADI.

• DFG Programme: Research Grants