5th International Conference on Advances in Solidification Processes (ICASP-5) 5th International Symposium on Cutting Edge of Computer Simulation of Solidification, Casting and Refining (CSSCR-5)

Frank in 1952 [1] already postulated that Icosahedral Short-Ranger Order (ISRO) in the liquid might explain the large undercoolings measured by Turnbull in fcc of hcp metals. ISRO was later confirmed by several observations such as the formation of quasicrystals or the atomic structure of their approximant intermetallic phases, neutron small angle scattering and atomistic simulations. In contrast to the explanation of Frank, Kurtuldu et al [2,3] have shown recently that ISRO can act as a precursor to what has been called "iQC-mediated nucleation" of fcc alloys. For two alloys based on Al and Au, it has been observed that minute additions of Cr and Ir, respectively, drastically change the final grain size and induce an abnormal fraction of twinned grain boundaries. The occurrence and geometrical configuration of multi-twinned nearest-neighbour grains can only be explained by heteroepitaxy relationships between 5-fold symmetry and fcc phases, where 111 and 112 directions of the fcc phase correspond to 3-fold and 2-fold symmetry axes of the icosahedron or interlocked icosahedron, respectively. The formation of these twins at the onset of solidification can explain the origin of twinned dendrites (or feathery grains), a morphology which has been observed in Al alloys since the mid-fourteens. But ISRO appears to also affect the dendrite growth directions via an attachment kinetics contribution, as first observed by Kurtuldu in Al-20wt%Zn with small additions of Cr [4] and very recently by Zollinger et al in Au-21.5wt%Cu-4.5wt%Ag with Ir traces [5]. This keynote will try to summarize these recent developments and outline future research directions that can confirm the influence of ISRO on nucleation and growth of metallic alloys.