Icosahedral quasicrystal-enhanced nucleation of the fcc phase in liquid gold alloys

Although the grain refinement of yellow gold alloys with Ir has been used in industry and known for nearly half a century, the basic mechanism is still unknown. The present contribution shows that the mechanism is the same as that evidenced recently in Al-Zn alloys, when small amounts of Cr (1000 ppm) are added to the melt (Kurtuldu et al., 2013). The reduced face-centered cubic (fcc) crystal size, the abnormal fraction of twin, or near-twin, grain boundaries and the 5-fold symmetry crystallographic orientation of multiple nearest-neighbor grains reproduce the symmetry of icosahedral quasicrystals (iQCs) with the following heteroepitaxial relationships: {1 1 1}fcc/〈110〉fcc⊥ 3-fold/2-fold symmetry axes of iQCs. While iQCs and the approximant stable Al45Cr7 phase, which contains several 5-fold symmetry building blocks in its unit cell, are known to exist in Al-Cr alloys, no such phases have been reported for yellow gold +Ir. Nevertheless, when minute amounts of Ir (≲200 ppm) are added to the gold alloy melt, it is shown that the grain refinement from 248 to 30 μm is accompanied by a spectacular increase in the fraction of twinned grain boundaries, i.e. from less than 1% without Ir to 11% with 200 ppm Ir. Furthermore, up to 9 grains have been shown to reproduce the six 5-fold symmetry axes of the icosahedron, while many other grain configurations exhibit this heteroepitaxial relationship with the icosahedron or interlocked icosahedron. This confirms that fcc crystals can form in a supercooled liquid by heteroepitaxial growth from an iQC template.