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

International audienceBased 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