**1. Introduction**

Metal-based eutectic composites exhibit periodic microstructures with lamellar, fibrous, degenerate, or other morphologies and are widely studied as model systems for understanding microstructure evolution under solidification conditions [1]. These microstructures typically contain intermetallic or other hard phases, which results in high hardness, suggesting potential for technological applications at ambient and elevated temperatures [2,3]. A limitation for all the as-cast metal-intermetallic eutectic systems has been the lack of plastic deformability [2,3], often even under compression loading. Recent work on a chill cast or laser surface-remelted eutectics has shown that high strength at room temperature can be achieved without loss of plastic deformability in a wide range of refined eutectic microstructures in Al–, Ti–, Ni–, Zn–, and Fe-based alloys [4–16].

In our earlier work, in situ micro-pillar compression testing in a scanning electron microscope (SEM) was used to characterize the stress–strain response of laser surface remelted Al–Al2Cu eutectic alloys [15]. For micro-pillar compression from within a single eutectic colony, the plasticity mechanisms were observed to depend strongly on the orientation of the lamellae with respect to the loading direction. For compression axis 90◦ to the lamellae, the highest yield strength was observed with negligible global plasticity due to localized shear failure, whereas for compression axis close to 0◦ to the lamellae, buckling and kinking occurred. In the case of a loading axis near 45◦ to lamellae, sliding along Al–Al2Cu lamellar interfaces was reported with the lowest yield strength [15]. Polycrystalline eutectics with the inter-lamellar spacing of ~20 nm exhibited a maximum flow strength of approximately 1.6 GPa and plasticity of approximately 17%. Bimodal morphologies in Al–Al2Cu containing degenerate, wavy, morphology dispersed with nanoscale lamellar eutectic resulted in high plasticity that was uniformly distributed throughout the height of the compression

**Citation:** Lei, Q.; Wang, J.; Misra, A. Mechanical Behavior of Al–Al2Cu–Si and Al–Al2Cu Eutectic Alloys. *Crystals* **2021**, *11*, 194. https:// doi.org/10.3390/cryst11020194

Academic Editor: Ronald W. Armstrong Received: 23 January 2021 Accepted: 12 February 2021 Published: 16 February 2021

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sample at a strength level of 1.36 GPa [15]. The role of the mixed and bimodal morphology in promoting high strength and plasticity was not clearly elucidated, particularly for binary vs. ternary systems.

In this work, laser surface remelting was used to refine the eutectic morphology of Al– Al2Cu–Si eutectic and compared with binary Al–Al2Cu, processed and tested with the same approach. A focused ion beam (FIB) was employed to fabricate cylindrical micro-pillars from laser-remelted alloys. Nano-indenter was used to compress the micropillars. The compression behaviors of the micropillars were investigated systematically; the cracking and co-deformation mechanism of the binary and ternary eutectics are discussed below.
