Heterogeneous nucleation and microstructure formation in peritectic alloy systems

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The global goal - set in the initial proposal for the entire funding period - remains the systematic research aiming at the understanding of heterogeneous nucleation and microstructure formation during peritectic solidification of alloys comprising all stages. This work will be systematically continued, while also being inspired by the results of other groups within the SPP. Furthermore, the focus of the project will shift in the next two years, bridging the scale difference between microstructure evolution and nucleation.The specific goals for the second funding period can be summarized as: (i) the generation and utilization of high precision thermodynamic and constitutional data to build refined and consistent thermodynamic descriptions; (ii) further development of the phase-field model and generation of simulated microstructures; (iii) microstructure analysis of directionally solidified samples; (iv) closer coupling of advanced calorimetric and phase-field investigations for a deeper understanding of the role of each of the four stages of peritectic solidification; and (v) clarify the interplay between the four stages on the type of peritectic reaction and the external solidification parameters (temperature gradient G and solidification rate v). Work on the Al-Ni system will be advanced since this is an excellent model system for investigating peritectics of type III and type II. As additional and independent material system Al-Cu has been chosen to investigate type II and type I peritectics. The choice of both alloy systems will, moreover, strengthen the existing network by exchange of important data with colleagues in the SPP.For this work we recognize that the nature of peritectic solidification makes it impossible to directly measure most kinetic properties of the reaction. We believe that by meticulously investigating peritectic solidification down the scale ladder from final microstructure over growth and its three different stages, and by coupling experimentation with simulation, the gap to heterogeneous nucleation can be bridged. By reducing the degrees of freedom for the choice of parameters in the phase-field simulation, the uncertainty range for the parameters of nucleation can be closed. Also, closer coupling of the phase-field model with experimental results makes it possible to jointly analyze a broad range of experimental techniques. Results, which would otherwise not be comparable, can be fed into a unifying phase-field model that is capable of describing all solidification conditions. This is the strength and beauty of this cooperation between our groups in Aachen and Clausthal in this joint project.