With the aim of understanding the complex dynamics of crystal–melt interfaces and associated structural evolution, investigators in the field of Solidification Science are presented with the substantial problem of discerning the various relaxation processes that occur on multiple length and time scales, elucidating the fundamental physical mechanisms that permit them, quantifying the relevant energetic and kinetic contributors, and, ultimately, developing various analytical and numerical models for prediction and control of these phenomena. In recent years, the field of Solidification Science, a mature research area with its roots firmly established in physical metallurgy, has experienced a renaissance of sorts, marked by a great influx of researchers from physics, chemistry and mathematics. Whilst there has always been a clear connection between Solidification Science and the pure sciences, recent developments in theoretical approaches, experimental techniques and computational capabilities have contributed towards a renewed and widespread interest in the fundamentals of solidification phenomena, and to the emergence of new and promising avenues of investigation. Most notably, the continued development of phase-field methods and the emergence of powerful atomistic modelling strategies have given rise to an increased level of understanding and a new set of questions to be addressed regarding the stability and dynamics of crystal–melt interfaces. As new and more fundamental questions are posed, it becomes increasingly clear that continued advancement in this field will require a higher level of interaction between researchers conducting theoretical and experimental investigations, and that the barriers presented by such distinctions must be overcome.
In the spirit of this multidisciplinary multi-approach outlook, a symposium, titled Frontiers in Solidification Science, was held on February 14 and 15, 2005, in San Francisco, California. The motivation for this symposium was to bring to the forefront those questions which are critical to the advancement of the field and to discuss the benefits and limitations of various new experimental and theoretical approaches which are driving current discovery in Solidification Science in the areas of morphological stability and selection, nucleation phenomena, chemical partitioning and segregation, microstructural evolution, multiple length-scale phenomena, intrinsic properties of crystal–melt interfaces, impurity effects at crystal–melt interfaces, growth mechanisms, and interface kinetics. Included in this special issue of Philosophical Magazine are selected papers from that symposium.
Over the past several years, atomistic simulation has emerged as an important tool for examining the behaviour of crystal–melt interfaces and, motivated largely by the need for accurate physical quantities in larger-scale modelling schemes, various atomistic techniques have been developed for the investigation and theoretical prediction of interfacial free energy and mobility, and their dependence on crystallographic orientation. In the first paper of this issue, Hoyt, Asta and Sun present a molecular dynamics investigation of crystal–melt interfaces for two bcc metals and consider the anisotropy of interfacial free energy and interfacial mobility by examining several low-index orientations.
The next two papers in this issue focus on nucleation phenomena and are motivated by the fundamentals of grain refining and glass formation, respectively, two areas which are currently receiving considerable attention on both scientific and commercial fronts. In the first of these papers, Greer and Quested examine heterogeneous nucleation and assert that overcoming the free growth barrier is the critical step for grain initiation. Thus, for a realistic innoculant size distribution, they show that nucleation is athermal, with the number of active nucleants depending only on the undercooling. In the subsequent article, Perepezko and Hildal examine the utility of wedge-shaped castings in the quantitative experimental investigation of solidification microstructure. Specifically, they consider the applicability of an instrumented wedge-casting technique to the problem of characterizing the solidification behaviour of glass forming alloys.
In the next four articles, fundamental studies of eutectic and dendritic solidification are reported. Indeed, understanding the stability and dynamical evolution of these prevalent growth modes is a longstanding challenge within the field of Solidification Science. In this issue, Akamatsu et al. report on an experimental investigation of the lamellar eutectic growth mode in a transparent metallic analogue, where they identify the limits of stability for the parallel regular mode with respect to several types of instabilities. Next, Liu and co-workers use specimen size as a means for systematically controlling thermal convection during directional solidification of Al–Cu alloys. They measure dendrite tip radii in the limit of complete diffusion control and show that selection is well characterized by the microscopic solvability theory. They continue to propose a conceptual model for spatiotemporal evolution under conditions where convection is significant. In the next article, Athreya et al. use a phase-field modelling approach to examine the influence of geometric constraint on dendritic growth. Indeed, they find that the selection of dendrite tip-operating state is affected when the system size approaches a critical dimension, which may be the thermal diffusion length for equiaxed dendrites growing in an undercooled melt or the primary dendrite spacing for the directional dendritic solidification of an alloy melt. Gránásy et al. also use phase-field modelling to examine both nucleation and spherulite formation. The nucleation studies match parameters to interfacial properties, including widths and free energies, determined from simulation, and show that the phase-field results may then be directly compared to atomistic simulations of nucleation rates. To model polycrystalline spherulite formation, they include preferential orientational mismatch energies, modelling low-energy grain boundaries that may occur. Including only a few model parameters allows the authors to demonstrate various spherulite morphologies similar to those seen in experiment.
The final set of papers includes two articles on mushy zone phenomena. In the first paper, Vernède and Rappaz develop a model to describe the conditions deep within the mushy zone where the last liquid to freeze, present as a continuous thin intergranular film, gradually disappears during grain coalescence. In the final paper of this issue, Matson and Hyers present a compelling analysis of a two-stage solidification process, where the initial growth of a metastable phase is superceded by the growth of the stable phase. Their analysis, which is supported by experimental results for the Fe–Ni–Cr system, shows that the remelting of the metastable phase is an important mechanism for the absorption of heat, increasing the growth rate of the stable phase.
The symposium, Frontiers in Solidification Science, and the selected papers included in this special issue are dedicated to emerging developments in the field of Solidification Science and the associated experimental, analytical and computational techniques used for their investigation. With 21 invited presentations, six contributed talks, 22 poster presentations and a great deal of stimulating discussion, the symposium was widely viewed as a valuable one and, due to its great success, will be continued on a biennial basis as a multidisciplinary forum for new developments in Solidification Science.
As organizers of the symposium and guest editors for this special edition, we would like to thank all those who participated, with a special thanks to M. Asta, M. I. Baskes, M. E. Glicksman, J. J. Hoyt, A. Karma, S. Liu, D. M. Matson, J. H. Perepezko, M. Rappaz and P. Voorhees for serving as symposium session chairs. We would also like to thank the Minerals Metals and Materials Society for their gracious sponsorship of the Frontiers in Solidification Science symposium. Finally, we would like to acknowledge the sponsorship from the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, US Department of Energy under contracts W7405-Eng-82 with Iowa State University and DE-AC05-00OR-22725 with UT–Batelle.