Phase equilibria in the AlCr system

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?

“…All these findings are fairly strong evidences that a fivefold symmetry template similar to that of iQC promotes the formation of the fcc phase, even though no icosahedral or Al 45 Cr 7 phase was observed in the present study. This novel "nucleation" mechanism (but is it nucleation?…”

“…or should we rather talk about heteroepitaxial growth on a template?) is further supported by several considerations: (i) icosahedral short range order in undercooled metallic liquids is proven; (ii) icosahedral solid clusters in Al-ZnCr melts tend to form due to the presence of stable phases such as Al 45 Cr 7 and Zn 13 (Cr x Al 1-x ) 27 (x = 0.34-0.37) in this system, which have icosahedral building blocks in their unit cells; (iii) there are many examples in the literature for the growth of multiple twins of intermetallics from an icosahedral phase and the same can well be possible for the multiple twin formation of stable a-Al crystals; (iv) the conditions under which iQCs form are quite similar to those favoring the appearance of twinned dendrites in industrial Al alloys (high cooling rate, presence of Fe impurities which are inherent to aluminum and can favor QC formation).…”

“…As can be seen in Fig. 11a, the composition of our alloy (red dot) falls within the liquid-Al 45 Cr 7 3 two-phase region at this temperature, before the fcc phase forms. This intermetallic phase has a monoclinic structure, with a very large unit cell containing 14 Cr and 90 Al atoms [43,44] (Fig.…”

“…They can in fact contribute significantly to their formation by acting as nucleation sites with some mutual crystallographic relationships [38]. In rapidly solidified and subsequently annealed Al-Cr alloys, Zhang et al [39] reported that the [1 0 1] direction of the monoclinic Al 45 Cr 7 phase is parallel to the fivefold axis of the icosahedral phase and found multiple twinning of Al 45 Cr 7 around this axis. In as-quenched Al alloys, iQCs are frequently surrounded by the a-Al matrix.…”

“…Al and Cr have the appropriate atomic radii to be arranged in the icosahedral configuration and besides Al 45 Cr 7 , other Al-rich intermetallic compounds formed by peritectic reactions, Al 11 Cr 2 and Al 4 Cr, have also icosahedral atomic arrangements in their unit cell [45]. These complex intermetallics formed under equilibrium conditions can coexist together with iQCs in rapidly solidified Al-Cr alloys when there is insufficient time for long range ordering [21,39,46].…”