Formation of locked-lamellar grains in a slightly hypoeutectic Al-Al₂Cu alloy during thin-sample directional solidification

Abstract

We studied the formation and growth of locked-lamellar microstructures in a thin sample of a slightly hypoeutectic Al-Al2Cu alloy. The coupled-growth dynamics, including early stages and steady-state regimes, was observed optically in real time during directional solidification. The orientation of the α (Al) and θ (Al2Cu) crystals was measured ex situ in a series of eutectic grains by X-ray Laue microdiffraction. A nucleation event of a θ crystal on a pre-existing α crystal, and the subsequent growth of a eutectic grain with a type-C orientation relationship, that is, with a coincidence of {123}-α and {100}-θ planes, were observed in situ. In type-C eutectic grains, lamellar locking occurred parallel to the low-energy coincidence plane. A regular (floating) coupled-growth dynamics was observed in misoriented eutectic grains.

On the Effect of Interphase Boundary Energy Anisotropy on Morphologies: A New Type of Eutectic Grain Observed in a Three-Phase Eutectic System

“…Remarkably, since the c(Sn) phase covers the sample wall, In 2 Bi and b(In) phases lose their contact with one side of the sample wall, which in turn results in the In 2 Bi/c(Sn) boundary being parallel to the X-axis and the sample wall. This is in strong contrast with the eutectic microstructures observed in thin sample directional solidification experiments reported until now [3][4][5]9,11,14,[16][17][18][19][20] wherein the interphase boundaries are always in contact with both sample walls and mostly parallel to the Y direction. In both of these LMR grains, the b(In)/c(Sn) boundary does not exist as it was already reported for the other grain types observed in this system.…”

“…A new eutectic grain type is discovered and characterized in this system during DS experiments in thin samples. In this eutectic grain, in strong contrast to all the 2D lamellar eutectic microstructures reported in the literature, [3][4][5]7,9,11,14,[16][17][18][19][20] at a given location, while one side of the sample is covered by large c(Sn) phase, the alteration of the In 2 Bi and b(In) phases exists on the other side, as shown in Figure 4. Thanks to the double-sided microscope, which is a unique equipment used to visualize the solidification front in real time from both sides of the sample, we could identify this difference indicating that the microstructure is completely 3D, despite the highly confined sample geometry.…”

“…In all these aforementioned grain types reported experimentally and numerically in two-phase CBr 4 -C 2 Cl 6 , [2][3][4]16] In-In 2 Bi, [3,7,16] Al-Al 2 Cu [11,17] eutectic systems, three-phase b(In)-In 2 Bi-c(Sn) [5,9,[18][19][20] eutectic system, and simulations, [15,21,22] irrespective of the anisotropy type and strength, the growth morphology of all the phases was always lamellar in thin samples. This means that all the interphase boundaries are nearly normal to the sample walls (glass plates), which results in forming essentially two-dimensional (2D) lamellar microstructure.…”