Marius Graf

• This work is concerned with two often separated disciplines. First, experimental studies in which the effect of cooling rate on martensite transformation and the resulting microstructure in a low-alloy steel is investigated. From this, a possible transformation mechanism is derived. Second, the development of a simulation model which describes the martensitic morphology and its evolution. In this context, a phase field model is presented introducing order parameters to simulate the material state, namely austenite and martensite. The evolution of the order parameters is assumed to follow the time-dependent Ginzburg-Landau equation. A major extension to previous models is the consideration of twelve crystallographic martensite variants corresponding to the Nishiyama-Wassermann orientation relationship. To describe the ordered displacement of atoms during transformation and to account for the martensitic substructure, the well-known phenomenological theory of martensite crystallography is employed. The presented experiments as well as thermodynamic calculations are used as a basis in the identification of model parameters. With the presented model, basic features of the martensitic transformation can be reproduced. These include the martensite start temperature and the hierarchical microstructure consisting of blocks and packets. The sizes of the blocks are in good agreement with the real sizes of the experimental database.