Thermodynamic Aspects of Homogeneous Nucleation Enhanced by Icosahedral Short Range Order in Liquid Fcc-Type Alloys

Abstract

We have recently shown that minute solute element additions to liquid metallic alloys can strongly influence the nucleation of the fcc phase and act as a grain refinement method. Electron back-scattered diffraction observations revealed a concomitant increase in the percentage of nearest neighbor (nn) grains that are in a twin relationship. Furthermore, multiple-twinned (MT) nn grain configurations with a fivefold symmetry around a common h110i direction have been identified, an occurrence that can be explained when the symmetry of the icosahedron is accounted for. It was then conjectured that a new nucleation mechanism occurs in two steps: first, the formation of small icosahedral quasicrystals in the melt, followed by heteroepitaxy of the fcc phase on facets of these quasicrystals. In the present contribution, based on thermodynamics arguments, it is proposed that the first step occurs by spinodal decomposition of the liquid, in a manner similar to Guinier-Preston zones formation in solid state precipitation, while the second step is a transformation of these quasicrystal precursors into MT-fcc nanocrystals once the driving force for this transformation is sufficient to overcome the fcc-liquid interfacial energy and the elastic strains associated with MT-fcc nanoparticles. This explanation sets up guidelines for finding solute elements and composition ranges that favor this grain refinement mechanism.

Quasicrystal-Enhanced Nucleation During the Solidification of fcc Metallic Alloys: A Tentative Thermodynamic Approach

“…1). [11] Based on the observations made on the Al-Zn-Cr system and on theoretical considerations, we then proposed the following nucleation mechanism (see Fig. 2): (i) iQC's form in the liquid from ISRO of atoms in the liquid; (ii) They grow, thus depleting the melt in Cr atoms (the system is peritectic [12] ).…”

“…In a recent contribution presented at the occasion of this Hume-Rothery symposium, [11] we have suggested, based on thermodynamic considerations, that the iQC-mediated nucleation of the fcc phase could be viewed in a way similar to precipitation of a stable second phase from a supersaturated solid solution (e.g., a sequence of metastable phases such as h'' fi h' fi h Al 2 Cu in Al-Cu or a¢ rhomb fi a¢ fi Zn in Al-Zn alloys). In such phase transformations, the sequence usually starts from soluterich Guinier-Preston (GP) zones which can be viewed as a spinodal decomposition of the parent phase.…”

“…Once the temperature has decreased and the iQC's have grown, thermodynamic considerations seem to indicate that this is rather a peritectic transformation of the outer layers of the iQC's into a fcc arrangement that promote the transition to fcc grains. [11] Above remarks by: …”

Modeling and characterization of grain structures and defects in solidification

“…Heterogeneous nucleation on foreign particles such as Al on TiB 2 or TiC particles has been shown to be athermal and thus does not follow standard theories, at least at small undercooling [11]. Finally, it has been shown recently that homogeneous nucleation in the liquid might depend on the short range order of atoms in the liquid, itself influenced by minute additions of solute elements [71].…”

Influence of icosahedral short range order on diffusion in liquids: A study on Al-Zn-Cr alloys

“…We have recently shown that icosahedral quasicrystals (iQC's) can also play a major role in the nucleation of the fcc Al phase during solidification of an Al-20wt%Zn alloy [14,15]. Small additions of Cr (1000 ppm) in the liquid metal drastically decrease the size of the fcc grains and induce an abnormally high fraction of twin or near-twin grain boundaries.…”