Prof Dieter Herlach - DLR, Germany
Dieter Herlach studied physics at the RWTH Aachen and received the doctoral degree as Dr. rer. nat. at the same university. He became private lecturer upon an habilitation at the Ruhr-University Bochum RUB. Presently, he is group leader at the Institute of Materials Physics in Space and Senior Scientist of the German Aerospace Center DLR. At the same time, he is full professor in physics at RUB. He has authored over 300 scientific publications in refereed journals. He is author and editor of six books and co-editor of Advanced Engineering Materials. He educated more than 30 PhD students and acted as supervisor of more than 20 diploma thesis works. Dieter Herlach leaded and leads projects of the German Research Foundation, the German Aerospace Center – Space Management, the European Space Agency and was principal investigator of NASA during three spacelab missions. He initiated and coordinated two priority programs of the German Research Foundation (DFG), and was member of the International Advisory Committee of the Int. Conf. of Rapidly Quenched and Metastable Materials. He is honorary professor of three universities and was granted by the Chinese Friendship Award in Beijing in 2000 and the Lee Hsun Lecture award of the Chinese Academy of Sciences in 2007. He chaired the Division of Metal and Materials Physics of the German Physical Society DPG, and is elected member of the council of DPG. He was elected member of the general review committee of DFG, and deputy chairman of the German Society of Materials Science and Engineering.
Crystal nucleation and dendrite growth in undercooled melts
Dieter M. Herlach
Institut für Materialphysik im Weltraum, Deutsches Zentrum für Luft- und Raumfahrt, Germany
An undercooled melt possesses an enhanced free enthalpy that opens up the possibility to crystallize metastable crystalline solids in competition with their stable counterparts. Crystal nucleation selects the crystallographic phase whereas the growth dynamics controls microstructure evolution. We apply containerless processing techniques such as electromagnetic and electrostatic levitation to containerlesss undercool and solidify metallic melts. Owing to the complete avoidance of heterogeneous nucleation on container-walls a large undercooling range becomes accessible with the extra benefit that the freely suspended drop is direct accessible for in situ observation of crystallization far away from equilibrium. The short-range order in undercooled melts of pure metals and alloys is investigated by combining levitation with elastic scattering of high intensity X-ray synchrotron radiation or neutron radiation. These investigations show the preference of icosahedral short-range order in pure metals and quasicrystal forming alloys but not in Zr-based glass forming alloys. Also the onset of magnetic ordering in deeply undercooled Co80Pd20 alloys is demonstrated that stimulates crystal nucleation. Results of investigations of maximum undercoolability on pure zirconium are presented showing the limit of maximum undercoolability set by the onset of homogeneous nucleation. From these results a lower limit of the interfacial energy is estimated and compared with predictions of simulations and theoretical modelling. Nucleation of metastable solid phases is demonstrated in deeply undercooled melts. Metastable phase diagrams are determined by applying energy dispersive X-ray diffraction on levitation processed samples of alloys with varying concentration. Nucleation is followed by crystal growth. In undercooled melts growth takes place as dendritic growth. Rapid dendrite growth velocity V is measured on levitation-processed samples as a function of undercooling ∆T by using high speed video camera technique. Various non-equilibrium phenomena during rapid dendrite growth are experimentally investigated such as solid trapping of solid solutions and disorder trapping of intermetallic alloys. In addition, the grain refinement process in deeply undercooled melts is studied in detail and is analysed within a model of dendrite fragmentation. Measurements of dendrite growth velocity V in melts of glass-forming Cu-Zr alloy are presented showing a maximum in the V(∆T) relation that is reproduced by dendrite growth theory if a temperature dependent diffusion coefficient is taken into consideration. Anomalous growth behaviour of Al-rich Al-Ni alloys is observed such that the growth velocity decreases with increasing undercooling. The influence of convection on dendrite growth is shown by comparative measurements of dendrite growth velocity as a function of undercooling of Al50Ni50 alloy samples processed both by electromagnetic levitation on Earth and in reduced gravity. The influence of forced convection in electromagnetically processed alloy melts is taken into account by introducing thermal and chemical Peclet numbers for fluid flow. Eventually, faceting of a rough interface by convection is presented by measurements of dendrite growth velocities in undercooled melts of Ni2B alloys, which are processed by different techniques which vary in the level of convection.