Authors’ reply

Abstract

a eutectic reaction, and it is indeed observed that the eutectic reaction in hypereutectic Al-Si alloys does not start spontaneously when Al has been nucleated. Instead, Al dendrites develop and grow without establishing cooperation when impinging on primary Si crystals. Evidently, the eutectic reaction has to await the nucleation of a Si crystal with the ability to grow as a plate. For the cubic Si, this can occur by the formation of parallel twin planes, which make possible the classical growth mechanism using re-entrant angles. If this is the growth mechanism of the eutectic Si plates, then the fragments giving rise to new plates must contain the active twin planes. As an alternative, it has been proposed that the appearance of slightly new directions of the Si plates is not caused by fragments breaking off but by low-angle branching during growth of Si through a displacement twinning mechanism. [7] Even in that case it is required that the parallel twin planes be preserved.

Solidification microstructures and solid-state parallels: Recent developments, future directions

“…Examining this issue in hypoeutectic Al-Si alloys, they claim that nucleation of Al 9 Si 2 Fe 2 in the solute boundary layer just ahead of the primary aluminum phase stimulates nucleation of eutectic silicon and serves to initiate the eutectic reaction. They show further that, for Fe levels low enough ($0.0032 wt.%) to preclude Al 9 Si 2 Fe 2 nucleation, a refined Si structure is observed [202], presumably due to the increased undercooling associated with more sluggish nucleation. These assertions have generated some controversy [202][203][204][205] and, while the role of the Al 9 Si 2 Fe 2 phase (compared to the role of other phases, such as AlP) remains unresolved, trace levels of Fe appear to have a distinct effect on the eutectic structure.…”

Localized Recrystallization in Cast Al-Si-Mg Alloy during Solution Heat Treatment: Dilatometric and Calorimetric Studies

“…The procedure for preparing TEM foil using FIB is reported elsewhere. [18] The advantage of using FIB to prepare TEM foil is that large TEM samples can be prepared from an exact region of interest.…”

Reply to discussion on “Nucleation mechanism of eutectic phases in aluminum-silicon hypoeutectic alloys

“…In a discussion, [3] we objected to their proposal that each eutectic Si plate is nucleated by a ␤ particle. They have now replied [4] to our discussion claiming that they did not state or imply that all the eutectic Si plates nucleate on ␤ particles. They now state that the ␤ phase initiates the nucleation event of eutectic Si.…”

“…It should further be emphasized that the title of their article was "Nucleation Mechanism of the Eutectic Phases in Aluminum-Silicon Hypoeutectic Alloys." Even in their Reply [4] to our Discussion [3] they emphasize that their article "addressed the nucleation aspects and not the growth aspects of the eutectic reaction. "…”

“…The importance they attach to their Figure 7 [1] is emphasized by the fact that in their second article, [2] they published it again ( Figure 4) and again in their Reply [4] as Actually, all but one of the figures in the Reply are republications from the first article. Furthermore, if they really believed that only the initial eutectic Si was caused by a ␤ particle, the very extensive experimental study would seem meaningless.…”