Papers by Author: Dieter M. Herlach

Abstract: The results on modeling dendritic solidification from undercooled melts processed by the electromagnetic levitation technique are discussed. In order to model the details of formation of dendritic patterns we use a phase-field model of dendritic growth in a pure undercooled system with convection of the liquid phase. The predictions of the phase-field model are discussed referring to our latest high accuracy measurements of dendrite growth velocities in nickel samples. Special emphasis is given to the growth of dendrites at small and moderate undercoolings. At small undercoolings, the theoretical predictions deviate systematically from experimental data for solidification of nickel dendrites. It is shown that small amounts of impurities and forced convective flow can lead to an enhancement of the velocity of dendritic solidification at small undercoolings.

Abstract: Metal Matrix Composites (MMCs) have great technical potential as they combine the ductility of metals with the hardness of ceramics: the reinforced material has improved mechanical properties. MMCs are interesting for the use in automotive or aerospace applications, e.g. in rolling bearings or turbine components. Recent manufacture is done by expensive methods like powder metallurgy to disperse the ceramic particles homogeneously in the metal matrix. In this contribution, we report on an approach to understand the basic mechanisms governing direct casting of MMCs. The basic problem is the particle-solid/liquid-interface interaction during dendritic solidification of metallic melts containing ceramic particles. The experimental idea is to deeply undercool the melt below its melting point. This results in a fast propagating solidification front when the solidification occurs. Due to the rapid solidification, the microstructure is frozen instantaneously and can be investigated post-mortem. It is expected that particles are incorporated in the material in the case of rapid solidification. Experimental techniques are electromagnetic levitation under terrestrial conditions and low gravity conditions during parabolic flight. With the electromagnetic levitation technique, samples were undercooled up to 150 K under its melting points despite the presence of particles. The experiments under low gravity show the importance of the reduction of melt convection on particle distribution within the rapidly solidified sample.

Abstract: Diffraction experiments on electromagnetically levitated Nd-Fe-B alloys during solidification of the undercooled melt have been performed at the European Synchrotron Radiation Facility (ESRF). By using high intensity synchrotron radiation complete diffraction spectra could be detected within a short period of some seconds thus enabling the observation of metastable solidification products that exhibits a limited lifetime. A metastable phase that crystallizes in wide composition range and that initiates the solidification of the stable Nd2Fe14B1-phase (φ-phase) have been observed.