Microscale heat and mass transfer and non-equilibrium phase change in rapid solidification

Many advanced materials processes such as pulsed-laser surface treatment and thermal spray deposition involve extremely fast heat and mass transfer at very small length scales. Under such extreme conditions, the accompanying phase change processes involve complex non-equilibrium phenomena. As a result, the non-equilibrium kinetics of crystalline growth becomes as important as heat and mass transfer in the determination of the materials microstructure and properties. Several rapid solidification processes that involve non-equilibrium melting and solidification under the conditions of microscale heat and mass transfer are presented. Attention is focused on the processes that can achieve either large melt undercooling prior to solidification as in the case of electromagentic levitation process and thermal spray, or a fast rate of heat transfer and thus high-speed phase change as in pulsed laser surface melting. In both cases, melting and/or solidification are characterized by non-equilibrium behavior at the fast moving solid/liquid interface where microscale and non-Fourier diffusion of heat and mass may take place. Various theoretical treatments and models for microscale heat and mass transfer processes are discussed. Recent progress in rapid solidification modeling that takes into account both the microscale heat and mass transfer and non-equilibrium kinetics is summarized, and some typical results are presented. (C) 2000 Elsevier Science S.A. All rights reserved.