Phase Selection in Undercooled Liquids of Pulsed-Laser Melted Alloys
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PHASE SELECTION IN UNDERCOOLED LIQUIDS OF PULSED-LASER MELTED ALLOYS D. M. FOLLSTAEDT,* P. S. PEERCY* AND J. H. PEREPEZKO** *Sandia National Laboratories, Division 1112, Albuquerque, NM 87185-5800 **University of Wisconsin, Department of Metallurgical and Mineral Engineering, 1509 University Ave., Madison, WI 53706 ABSTRACT The use of thermodynamic analysis of phase relationships to identify metastable conditions for generating undercooled liquids by pulsed laser from melting, and to interpret the resulting microstructures is illustrated Phases with simple, disordered structures are studies of Mn, FeV and TiAl. found to nucleate and grow within tens of nanoseconds in liquids undercooled An extended transformation depth is found in which the stable by 50-100 K. high-temperature a phase replaces the metastable a phase in FeV due to the The formation of the heat release from the rapidly forming a phase. disordered bcc structure and of a metallic glass in the melt of the ordered compound -- TiAl indicate that regrowth of this phase is sufficiently slow to generate undercoolings of - 1000 K. INTRODUCTION Many alloy systems can exist in more than one solid phase at a given composition. As a result, melts can solidify into different phases under The fundamental principles and mechanisms governing different conditions. phase selection during solidification of liquid metals are currently being examined in detail because of increasing interest in rapid quenching of Rapid solidification processing molten alloys to produce new materials. (RSP) is being used to produce alloys with refined microstructures as well as a variety of metastable phases, including extended solid solutions of stable phases, metastable crystalline structures, and noncrystalline structures such Basic understanding of the as quasicrystals and amorphous phases [1]. formation rates of potential solid phases is needed to exploit RSP more fully in the production of alloys with specified microstructures and properties. The rate at which a solid phase forms is determined by two kinetic The processes: the nucleation rate and the subsequent growth rate. microstructure of a quenched alloy is thus determined by competition between the combination of these two rates for different possible solid phases. Thermodynamics can be used to assess the driving force for nucleation and The free energies of the liquid and solid phases, growth of these phases. including metastable structures and metastable extensions of equilibrium phases, can be properly discussed in terms of reversible equilibria between them [2]; however, the extent to which the equilibria are a-hieved during solidification depends upon the quench rate, which can be > 106 K/s, relative to rates of kinetic processes such as diffusion in the alloy. aspect of the nucleation and growth from the liquid is A central "undercooling": the difference between the liquid temperature (T) and the For small values, the undercooling melting point of the solid phase (Tm). AT - T - T is proportional to the free energy differenc
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