In-Situ Measurement of Deformation Induced Formation of Martensite in Austempered Ductile Iron (ADI)

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Austempered Ductile Iron (ADI) is a cast iron material which is heat-treated in a multistage process. Thus a microstructure called ausferrite forms, consisting of metastable retained austenite, ferrite sheaves and spheroidal graphite. This microstructure is the reason for the excellent mechanical properties of ADI compared to the initial state. From TRIP-steels, which are used in sheet metal forming processes, it is known that metastable retained austenite can transform to martensite under external loads.Based on the results of the DFG research project the phase transformation kinetics of selected ADI-materials during the heat treatment will be investigated in-situ using neutron diffraction. The phase compositions and the carbon contents of the retained austenite will be determined together with the microstructures and mechanical material properties depending on the respective heat treatment parameters.The formation of martensite in heat-treated specimens will be measured in-situ using neutron diffraction during tension and compression tests. The experimental data will be used for testing existing mathematical models for calculation of stress and strain-induced formation of martensite in TRIP-steels. These models will be validated and extended so that they can be used for ADI.