Undercooling Behavior of Liquid Metals

Abstract

ABSTRACTThe undercooling of liquid metals is observed often, but is restricted by the catalysis of the most potent heterogeneous nucleation site in contact with the liquid. An experimental approach that yields large undercoolings involves the slow cooling from the melt of a dispersion of stabilized fine (5–20μ) liquid droplets. Droplet studies have extended measurements of the undercooling limit for low melting point metals to the range of 0.3 to 0.4 Tm. In droplets, surface coating and size characteristics usually limit the undercooling and control the uniformity of crystallization behavior. A nonuniform undercooling behavior can produce microstructural variations in powder samples with respect to metastable phases and solute segregation, but these variations can be diminished for isenthalpic solidification of hypercooled liquids. In alloys the trend in nucleation temperature can follow a similar pattern as the composition dependence of the liquidus. With highly undercooled droplets and even at modest ΔT (ΔT ≤ 0.lTm) with known heterogeneous sites, structural modifications such as extended solid solutions and metastable intermediate phases can be generated which are similar to those produced by rapid quenching techniques. These features highlight the importance of a high liquid undercooling as well as a high cooling rate in promoting effective rapid solidification processing treatments.