Abstract
The present contribution reviews the recent progress related to the influence of Icosahedral Short-Range Order (ISRO) and icosahedral Quasicrystals (i-QC) formation on the solidification of fcc alloys through minor solute element additions. From intensive crystallographic analysis of multi-twinned regions in as-cast Al-based and Au-based fcc alloys, Kurtuldu et al. have shown recently that a so-called “iQC-mediated” nucleation mechanism occurs when a few hundred ppm of Cr and Ir, respectively, are added to the melt [1] [2]. Similarly, it appears that the growth directions of dendrites in Al-Zn:Cr is also influenced by ISRO in the liquid, thus showing an attachment kinetics effect [3]. In a recent contribution, we have shown that iQC-mediated nucleation also occurs in pink gold alloys with Ir-additions, but two additional phenomena at high solidification speed [4]: (i) a spinodal-type decomposition of the liquid, leading to the formation of twinned Cu precipitates in addition to multi-twinned Au-rich grains; (ii) a change of the microstructure of the Au-rich grains, from 〈100〉 dendrites to 〈111〉 textured cells in the columnar zone.
Highlights
Icosahedral Short Range Order (ISRO) in the liquid phase, conjectured by Frank in 1952 [5], was thought to be responsible for the large undercooling measured in metallic alloys
The corresponding 110 pole figure shows their multiple twin relationships, with the common 110 direction corresponding to the 5-fold symmetry axis of an icosahedron, the common {111} planes being shown with arcs of circle
Addition of small amounts of Ir in pink gold induces the same iQC-mediated nucleation of the fcc phase, to what has been reported in Al-Zn:Cr and in yellow gold with Ir
Summary
Icosahedral Short Range Order (ISRO) in the liquid phase, conjectured by Frank in 1952 [5], was thought to be responsible for the large undercooling measured in metallic alloys. As a matter of fact, Kurtuldu et al discovered this mechanism from the excessive number of twinned grain boundaries in Al-20wt%Zn when 0.1wt% Cr is added They found MT relationships among several nearest-neighbor (nn) fcc grains, respecting the icosahedral symmetry of the iQC template. We have shown that iQC-mediated nucleation occurs in pink gold alloys (Au20.5wt%Cu-4.5wt%Ag) when 0.01wt% Ir atoms are added to the melt [4] While this mechanism could be expected at low solidification speed, since it is used routinely in jewelry for grain refining, two additional phenomena were found at high solidification rate: (i) the formation of fine Cu precipitates, sometimes with twinning relationships, in addition to MT Au-rich grains; and (ii) a change of the internal morphology and texture of columnar grains. These last results are briefly summarized in the present contribution
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