Solidification of Metallic Alloys: Does the Structure of the Liquid Matter?

Abstract

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.

Multi-scale microstuctural investigation of a new Al-Mn-Ni-Cu-Zr aluminium alloy processed by laser powder bed fusion

“…Finally, we would like to comment on the peculiar morphology of the dendrites in the CZ as well as on the presence of some twinned dendrites (see example in Figure 4c-d). This unusual palm-tree like morphology of dendrites in FCC-metals has already been reported in some cases [35], [36], [48]. The recent viewpoint paper published by Rappaz et al [36] suggests that our observations are consistent with possible Icosahedral Short Range Order (ISRO) of the atoms in the liquid.…”

“…This unusual palm-tree like morphology of dendrites in FCC-metals has already been reported in some cases [35], [36], [48]. The recent viewpoint paper published by Rappaz et al [36] suggests that our observations are consistent with possible Icosahedral Short Range Order (ISRO) of the atoms in the liquid. Experimental evidence has been provided in favor of this hypothesis that icosahedral quasicrystal (iQC-) mediated nucleation could trigger twinned dendrites growth in some Al-alloys [48], [49].…”

Influence of Minor Cr Additions on Crystal Growth in Rapidly Solidified Al-20Zn Alloys

“…It means that, rather than having the fcc phase nucleating from iQC or ISRO motifs, here we have rather an fcc phase growing on which ISRO motifs attach, then leading to the formation of a "new" fcc grain with a twinning relationship with the parent grain. As explained in a forthcoming paper [18], the formation of a twin during growth induced by ISRO motifs in the liquid will be called "ISRO-induced stacking fault", and will be further discussed in Section 4.…”

Microstructure and Indentation Properties of Single-Roll and Twin-Roll Casting of a Quasicrystal-Forming Al-Mn-Cu-Be Alloy

“…The addition of Mn and, especially, Be enhance icosahedral short-range order (ISRO) in the aluminium melt [9], which seems to play an essential role in other aluminium alloys, too [34,35]. At higher cooling rates, the primary IQC phase can form [11].…”