Anomalous crystal growth in undercooled melts of Al-Ni and Cu-Zr alloy

• Applicant: Professor Dr. Markus Rettenmayr, since 12/2016 (†)
• Subject Area: Thermodynamics and Kinetics as well as Properties of Phases and Microstructure of Materials
• Term: from 2011 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 179096895

Dendrite growth in undercooled melts of Al-Ni and Cu-Zr is anomalous in the sense that the growth velocity decreases with increasing undercooling. Such behaviour has not yet been observed in other metallic systems so far.Within the frame of the proposed project various parameters and processes will be investigated which can clarify such an anomalous growth characteristics. This is forced convection in the undercooled melts which can influence the heat and mass transport at the solid-liquid interface and the atomic impingement kinetics at the solidification front as well. All of these processes are essential for the control of the growth kinetics in undercooled melts. Furthermore, important parameters for modelling of growth kinetics will be determined by independent experiments and MD simulations. These are the temperature dependent diffusion coefficient and specific heat in the regime of the undercooled melt.In particular, the measurements of the specific heat are relevant to study inverse melting as the origin of the anomalous growth behaviour in Al-Ni melts. This process could explain the anomalous dendrite growth. Inverse melting is known for biophysical systems but in case of metallic system it is rather rarely investigated. There are some hints on inverse melting in specific metallic alloys like Cr-Ti.The growth of dendrites takes place both on a mesoscopic and a microscopic scale. Atomic diffusion and atomic attachment kinetics at the solid-liquid interface is controlled by the diffusion coefficient on a microscopic scale. Opposite to atomic diffusion the heat transport takes place on a mesoscopic scale since the thermal diffusivity is by about three orders larger than the atomic diffusion coefficient.Various experimental facilities are available to investigate the anomalous growth kinetics from various sides. Levitation (electromagnetic and electrostatic) allows for large undercoolings due to the complete avoidance of heterogeneous nucleation on container walls. By means of a high speed camera the advancement of the solidification front is measured with high precision. Comparative studies on Earth and in reduced gravity on the International Space Station will lead to an evaluation of the influences of convection on growth kinetics. The parameters needed for modelling of dendrite but also eutectic growth are independently determined in order to narrow the parameter space down. These versatile investigations should lead to an understanding of the anomalous growth kinetics in the alloys under investigations.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Professor Dr. Dieter M. Herlach, until 12/2016 (†)