Processing metallic materials far from equilibrium
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Introduction Metals and their alloys are in ubiquitous use, from bicycles to airplane wings to space frames. Generally, we prefer them in their solid state, and in most cases, producing them necessitates cooling down a high-temperature liquid. Indeed, solidification from a liquid is inherently advantageous for the commercial production of bulk materials, due to the greater mass processing rates compared to growth of crystalline layers from a vapor phase (e.g., chemical vapor deposition).1 As noted by Derby,1 this is simply due to the higher density of a condensed-liquid phase, in contrast to the low density of a gaseous phase, especially under near-vacuum conditions employed in molecular beam epitaxial processes. On a practical note, more than 1.8 billion tons of metal was solidified globally in 2019 alone.2 Since many metals are used in their as-cast state (i.e., without further thermal or mechanical processing), it follows that the microscopic structure formed by solidification has a direct consequence on the mechanical properties of the metal product. Castings are produced with dimensions of a few millimeters up to tens of meters, and thus one may infer that the important dimensions to describe solidification cross orders of magnitude. This is true not only in length scale, but also time scale. However, it is the solidification microstructure (on the order of tens to hundreds of micrometers) that ultimately determines the physical properties and performance of metals and alloys.3 Because
solidification is the process by which atoms are transferred from liquid to solid, the distances over which individual atoms diffuse (nanometers) are also important. An accurate description of solidification must therefore reconcile the various processes that occur over several order-of-magnitudes in length scale and also time scale,3 which is an important challenge in its own right. Mastering solidification dynamics is a primary aim in process metallurgy and synthesis science. The end goal is to understand the formation of solid phases and their shapes and patterns that develop from a “disordered” liquid environment (although we note that local ordering in the liquid is an area ripe for further understanding), in addition to growth regime (or morphological) transitions. These microscale patterns span dendrites to cells to labyrinths to spirals.4–9 Questions that arise include: Is each of these diverse patterns the result of unique causes and effects, or are there unifying physical principles that govern their growth and form?10 How do the solidification processing conditions influence phase formation and pattern selection? The past few years have seen the development of such principles, in some cases validating or expanding upon past theoretical treatments. These breakthroughs have been achieved with the aid of in situ and complementary ex situ characterization techniques, new analysis methods, and multiscale modeling approaches. These themes are reflected in the articles in this issue.
Ashwin J. Shahani, Department of Materi
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