Crystal nucleation and dendrite growth of metastable phases in undercooled melts

Kurzfassung

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 of 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 feely suspended drop is direct accessible for in situ observation of crystallization far away from equilibrium. Results of comparative investigations of maximum undercoolability on pure zirconium are presented showing the limit of maximum undercoolibiliy set by the onset of homogeneous nucleation. Nucleation of metastable solid phases is demonstrated, if the undercooling is exceeding a critical value. Rapid dendrite growth is measured as a function of undercooling by a high speed camera and analysed within extended theories of non-equilibrium solidification. In such both supersaturated solid solutions and disordered superlattice structure of intermetallics are formed at high growth velocities. A sharp interface theory of dendrite growth is presented capable to describe the non-equilibrium solidification phenomena during rapid crystallization of deeply undercooled melts. Eventually, anomalous growth behaviour of Al-rich Al-Ni alloys are presented, which maybe caused by gravity related phenomena. This is demonstrated by comparative experiments performed in the reduced gravity environment during parabolic flight missions of ESA/DLR and the sounding rocket mission TEXUS launched at Esrange in Sweden.