Non-equilibrium solidification, modelling for microstructure engineering of industrial alloys (NEQUISOL)
Résumé
Within NEQUISOL project, crystallisation kinetics and microstructure evolution in undercooled melts of Al-based alloys is investigated. Different techniques are applied for containerless processing of the different alloys. These allow for undercooling a liquid far below its equilibrium melting temperature. An undercooled melt is in a metastable state giving access of different solidification pathways the system can take. Solidification starts with nucleation and is completed by subsequent growth of crystals. The negative temperature gradient in front of the solid-liquid interface and the concentration gradient in alloys destabilize a planar interface leading to dendrite growth. Dendrite growth dynamics and microstructure evolution in undercooled melts is investigated on drops undercooled by Electro-Magnetic Levitation (EML). The speed of the propagating solidification front is monitored by means of a high-speed camera with a maximum frequency of 120 000 pictures per second. Under Earth conditions strong alternating electromagnetic fields are needed to compensate the gravitational force. This, in turn, causes forced convection due to the strong stirring effects. Therefore, equivalent experiments are conducted under microgravity conditions using the TEMPUS facility for electromagnetic levitation in reduced gravity during parabolic flights and during TEXUS sounding rocket missions. Experiments on four selected alloys, Al 40Ni 60, Al 70Ni 30, Al 65Ni 35 and Al 89Cu 11 are in preparation to be performed on board the ISS using the Electro-Magnetic Levitator currently under development by DLR/ESA. In addition atomization facilities are operated that combines containerless processing with large cooling rates and reduced gravity on Earth. Atomization is an industrial processing route to produce metastable materials in large amount. We present a comparison of first experiments conducted in reduced gravity (parabolic flight, TEXUS) and reference experiments on Earth of measurements of the growth velocity as a function of undercooling of the congruently melt-ing Al 50Ni 50 alloy and the Raney type alloy Al 68.5Ni 31.5. The latter one is of special interest for industry because of its extraordinary potency as a catalyst. The experiments clearly demonstrate how important convection is in heat and mass transport processes which control dendrite growth dynamics and, hence, microstructure evolution. A sharp interface theory is presented that takes into account heat and mass transport by forced convection. This mesoscopic model is able to predict the dendrite growth kinetics obtained both on Earth as well as in reduced gravity. In addition, mesoscopic modelling is combined with macroscopic modelling to describe the entire solidification process involving several recalescences and the non-equilibrium solidification of several solid microstructures.