The equiaxed solidification of Al-20 wt.% Zn alloys revealed an unexpectedly large number of fine grains which are in a twin, or neartwin, relationship with their nearest neighbors when minute amounts of Cr (1000 ppm) are added to the melt. Several occurrences of neighboring grains sharing a nearly common h1 1 0i direction with a fivefold symmetry multi-twinning relationship have been found. These findings are a very strong indication that the primary face-centered cubic Al phase forms on either icosahedron quasicrystals or nuclei of the parent stable Al 45 Cr 7 phase, which exhibits several fivefold symmetry building blocks in its large monoclinic unit cell. They are further supported by thermodynamic calculations and by grains sometimes exhibiting orientations compatible with the socalled interlocked icosahedron. These results are important, not only because they provide an explanation of the nucleation of twinned dendrites in Al alloys, a topic that has remained unclear over the past 60 years despite several recent investigations, but also because they identify a so far neglected nucleation mechanism in aluminum alloys, which could also apply to other metallic systems.
a b s t r a c tA long lasting controversy on the nature of the reaction between liquid, Al and Al 45 Cr 7 has been ended by planar front solidification experiments of Al-Cr alloy in a Bridgman furnace. Cr depletion in the liquid ahead of the quenched planar interface proves without any ambiguity that the invariant reaction on the Al-rich side of the Al-Cr phase diagram is peritectic. A value of the diffusion coefficient of Cr in liquid Al, D ' ¼ 2:4 Â 10 À9 m 2 =s, was also deduced from the solute composition profile measured in the quenched liquid.
We have recently shown that minute solute element additions to liquid metallic alloys can strongly influence the nucleation of the fcc phase and act as a grain refinement method. Electron back-scattered diffraction observations revealed a concomitant increase in the percentage of nearest neighbor (nn) grains that are in a twin relationship. Furthermore, multiple-twinned (MT) nn grain configurations with a fivefold symmetry around a common h110i direction have been identified, an occurrence that can be explained when the symmetry of the icosahedron is accounted for. It was then conjectured that a new nucleation mechanism occurs in two steps: first, the formation of small icosahedral quasicrystals in the melt, followed by heteroepitaxy of the fcc phase on facets of these quasicrystals. In the present contribution, based on thermodynamics arguments, it is proposed that the first step occurs by spinodal decomposition of the liquid, in a manner similar to Guinier-Preston zones formation in solid state precipitation, while the second step is a transformation of these quasicrystal precursors into MT-fcc nanocrystals once the driving force for this transformation is sufficient to overcome the fcc-liquid interfacial energy and the elastic strains associated with MT-fcc nanoparticles. This explanation sets up guidelines for finding solute elements and composition ranges that favor this grain refinement mechanism.
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