Influence of Ir Additions and Icosahedral Short Range Order (ISRO) on Nucleation and Growth Kinetics in Au-20.5Wt Pct Cu-4.5Wt PctAg Alloy

Abstract

Based on detailed EBSD analyses, Kurtuldu et al [1] have explained the grain refinement of Au-12.5 wt.%Cu-12.5 wt.%Ag (yellow gold) by the addition of minute amounts of Ir in terms of "icosahedral quasicrystal (iQC)-mediated nucleation", i.e., Ir induced the formation of Icosahedral Short Range Order (ISRO) of atoms in the liquid, leading to the formation of iQC on which the fcc-phase forms. In the present contribution, we show that: (i) this mechanism is also responsible of the grain refinement in Au-20.5 wt.%Cu-4.5 wt.%Ag (pink gold) with Ir addition; (ii) ISRO also influences the morphology and growth kinetics of the fcc phase: at solidification rate of a few mm/s, 100 dendrites are replaced by a cellular-type morphology growing along 111 when 100 wt.ppm of Ir is added to the melt; (iii) iQC-mediated nucleation is accompanied by a spinodal decomposition of the liquid, which is revealed at high cooling rate by the formation of Cu-rich particles or dendrites, some of them being also twinned, in parallel to iQC-mediated grain refinement and twin formation.

Influence of Minor Additions on Icosahedral Short-Range Order (ISRO) and Its Consequences on Nucleation and Solidification Growth Kinetics in fcc Alloys

“…This grain refinement is accompanied by a large percentage of twin grain boundaries (7.6%), much larger than the percentage obtained for a population of randomly oriented grains (0.2%). This percentage of grain boundaries is almost independent of the cooling rate, as observed in slowly cooled pink gold specimens with the same amount of iridium [4].…”

“…After cutting and polishing, the grain and microstructure were observed using optical microscopy, SEM and EBSD. Further experimental details are given in [4] and we report below only the results concerning rapidly cooled specimens, with and without Ir addition. Figure 2 summarizes the BSE analysis results of rapidly solidified pink gold droplets without (top) and with (bottom) Ir.…”

“…Recently, we have shown that iQC-mediated nucleation occurs also in pink gold alloys (Au-20.5wt%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 also 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.…”

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

“…This effect was tentatively explained by an "ISRO-induced stacking fault" mechanism, i.e., ISRO motifs attaching to the growing fcc phase with or without annihilation of the induced stacking fault depending on the velocity. In pink gold Au-20.5wt.%Cu-4.5wt.%Ag, a similar effect was observed in the columnar zone of a small specimen which fell onto a copper-chill substrate (free-falling droplet (FFD) technique) [17]. Under such rapid solidification conditions, 100 dendrites are replaced by a 111 cellular-type microstructure growing along the thermal gradient direction.…”

“…In two recent papers [3,17], ISRO has been shown to also influence the growth directions of dendrites during solidification. In Al-20wt.%Zn alloys [3], 100 dendrites are replaced by 110 trunks at intermediate growth rate (<200 µm/s) when 1000 ppm Cr is added.…”