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.