Solidification of Metallic Alloys: Does the Structure of the Liquid Matter?
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Solidification of Metallic Alloys: Does the Structure of the Liquid Matter? M. RAPPAZ, PH. JARRY, G. KURTULDU, and J. ZOLLINGER In 1952, Frank (Proc R Soc Lond Ser-Math Phys Sci 215:43–46, 1952) already postulated that Icosahedral Short Range Order (ISRO) of atoms in the liquid could possibly explain the large nucleation undercoolings measured in metallic alloys by Turnbull and Fisher (J Chem Phys 17:71–73, 1949). About thirty years later, this conjecture was proven to be key for the understanding of Quasicrystals (QC) formation (Shechtman et al. in Phys Rev Lett 53:20, 1951–3, 1984). More recently, it has been found that a few tens to thousand ppm of solute elements in Al-base and Au-base alloys can influence the nucleation and growth of the primary fcc phase via mechanisms involving ISRO and QC formation. ISRO has also been found to limit the mobility, and thus diffusion, of atoms in the liquid. This can lead to out-of-equilibrium conditions, e.g., the formation of metastable phases or supersaturated solid solution, at reduced velocity compared to alloys where ISRO is not predominantly present. Finally, there are several experimental evidences that ISRO is also responsible for twinned dendrites formation in Al alloys. The present contribution summarizes these recent findings and points out the implications that these might have in the field of solidification and additive manufacturing. https://doi.org/10.1007/s11661-020-05770-9 The Minerals, Metals & Materials Society and ASM International 2020
I.
INTRODUCTION
SOLIDIFICATION of metallic alloys is usually assumed to occur from a fully disordered liquid. The solid–liquid interface at the atomic scale is considered to be diffuse over a thickness, ds‘, on the order of nanometer, with many sites where atoms in the liquid can attach to the solid.[1,2] Provided that the velocity v* of the solid-liquid interface is one or two orders of magnitude smaller than D‘/ds‘, where D‘ is the solute diffusion coefficient in the liquid, the solid–liquid interface is close to equilibrium and solidification microstructures are governed by diffusion of solute elements in the liquid and capillarity effects.
M. RAPPAZ is with the Ecole Polytechnique Fe´de´rale de Lausanne, Institute of Materials, Station 12, 1015 Lausanne, Switzerland. Contact e-mail: michel.rappaz@epfl.ch. Ph. JARRY is with Constellium C-TEC, Centr’alp, 725 rue Aristide Berge`s, CS10027, Voreppe, 38341, France. G. KURTULDU is with the Laboratory of Metal Physics and Technology, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland. J. ZOLLINGER is with the Department of Metallurgy & Materials Science and Engineering, Universite` de Lorraine, Institut Jean Lamour, Alle`e Andre´ Guinier ARTEM Campus, 54000, Nancy, France. Manuscript submitted October 24, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Dendrites of the primary phase grow along minima of what is called the solid–liquid interface stiffness, an entity which is deduced from the orientation-dependent solid–liquid interfacial energy, cs‘(n), where n is the un
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