Metastable Solids from Undercooled Melts

Abstract

Undercooled melts of metals and alloys possess an excess free energy. This opens up a variety of solidification pathways from the liquid to the solid with the benefit that a great number of metastable materials all of different physical properties can be directly produced from the undercooled melt. Undercooling is therefore a very efficient experiment parameter for the design of materials of advanced properties. We apply containerless processing by levitation melting of metallic materials to undercool them below their melting temperature. Owing to the complete avoidance of heterogeneous nucleation on container walls deep undercoolings are achieved in the order of 20% of the liquidus temperature of the respective materials investigated. The freely suspended drop gives the extra benefit to combine levitation processing with suitable diagnostic means do directly observe the entire process of non-equilibrium solidification of an undercooled melt starting with the nucleation of different crystallographic phases to rapid crystal growth of metastable microstructures. In the present study, the concept of electromagnetic levitation is introduced to observe the rapid solidification process of an undercooled melt utilizing different diagnostic means. Examples are shown for the formation of metastable phases solidified directly from the undercooled melt. A hardmagnetic intermetallic phase is nucleated in undercooled Nd-Fe-B alloys circumventing the peritectic reaction, which always involves soft magnetic -Fe. Metastable ferritic steels are produced in the regime of the phase diagram of Fe-Ni-Cr in which the austenitic steel is thermodynamically stable. The dendrite growth velocity is measured as a function of undercooling. Such measurements give inside to the conditions of the formation of supersaturated solid solutions and disordered superlattice structures in intermetallics. Undercooling is also a very efficient parameter to produce very grain refined materials as demonstrated by levitation experiments on various metallic alloys. Interestingly, two critical undercoolings are identified at which both grain size and grain morphology changes abruptly like a phase transition.