Influence of Cr on the nucleation of primary Al and formation of twinned dendrites in Al–Zn–Cr alloys: Can icosahedral solid clusters play a role?

Abstract

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.

Influence of icosahedral short range order on diffusion in liquids: A study on Al-Zn-Cr alloys

“…Another intermetallic phase, Zn 13 (Cr x Al 1Àx ) 27 (x ¼ 0.34e0.37), has recently been shown to exhibit icosahedral building blocks in its rhombohedral system [51]. These crystallographic observations strongly support the interpretation of grain refinement induced by Cr during equiaxed solidification of Al-Zn-Cr alloys in terms of quasicrystalmediated nucleation [14]. The present measurements of the influence of Cr on the diffusion coefficient of Zn in Al-Zn-Cr alloys is another experimental fact that supports the formation of ISRO in this liquid alloy.…”

“…The quasicrystal-enhanced nucleation mechanism discovered recently in Al-Zn-Cr alloys [14] assumes that Cr promotes the formation of ISRO in the liquid. If such interpretation is correct, Cr should also influence diffusion in the liquid as a result of shortrange covalent-type interactions.…”

“…11 of Ref. [14]. These icosahedral building blocks are linked together by sharing a vertex, an edge or a face, or by interlocking.…”

“…We have recently shown that icosahedral quasicrystals (iQC's) can also play a major role in the nucleation of the fcc Al phase during solidification of an Al-20wt%Zn alloy [14,15]. Small additions of Cr (1000 ppm) in the liquid metal drastically decrease the size of the fcc grains and induce an abnormally high fraction of twin or near-twin grain boundaries.…”

“…

a b s t r a c tIt has been shown recently that minute additions of Cr in Al-Zn alloys refine the solidified grain structure and induce multiply-twinned grains which can only be explained if the icosahedral symmetry is considered (Kurtuldu et al, 2013). It has been suggested that Cr addition to Al-Zn alloys promotes the formation of icosahedral short-range order (ISRO) in the liquid, leading to the formation of icosahedral quasicrystals (iQC) acting then as a template for the nucleation of the fcc Al-phase.

…”