**3. Experimental Results**

Figure 1 shows the cross-section view of the laser-remelted molten and local regions in Al–Al2Cu and Al–Al2Cu–Si eutectics. As shown in Figure 1a, the as-cast Al–Al2Cu eutectic structures can be divided into lamellae and degenerates, resulting from different heat transfer characteristics during solidification [9]. However, only lamellae Al–Al2Cu eutectics can be observed in the laser-remelted sample (Figure 1b). In the backscattered electron (BSE) images (Figure 1a,b), the phases in the bright and dark contrasts are Al2Cu and Al, respectively. The inter-lamellar spacing (λ) was statistically analyzed and was observed to decrease from 1400 ± 200 nm (as-cast) to 39 ± 4 nm (laser-treated region). The refined Al–Al2Cu eutectics should be ascribed to the high heating and cooling rate of the laser surface processing technique [7,8,13]. In the Al–Al2Cu–Si eutectics, Al (dark), Al2Cu (bright), and Si (gray) were detected in the as-cast Al–Al2Cu–Si samples (Figure 1c), while there were bimodal structures with dendrite structure of Al (dark) + Al2Cu (bright) and lamellae structure (Al–Al2Cu–Si) in the laser-remelted Al2Cu–Si eutectics (Figure 1d).

Four Al–Al2Cu–Si micropillars cut from the laser-treated (LT) region and as-cast base are named laser-remelted and as-cast lamellae, respectively. Three Al–Al2Cu micropillars cut from the laser-treated (LT) region and as-cast base are labelled as-cast lamellae, as-cast degenerate, and laser-remelted lamellae. The applied compression force (F) and the immediate height (hI) of the pillar were recorded. An average diameter (d) of the top, middle, and bottom of each pillar was employed to calculate the stress and the strain. The engineering stress (σE) was computed by dividing the applied force (F) by the average cross-section area (π × d × d/4), σ<sup>E</sup> = F/(π × d × d/4). The engineering strain (εE) was obtained by dividing the height

variation of the pillar by the original height of the pillar (h0), ε<sup>E</sup> = (hI–h0)/h0. The engineering stress (σE) versus engineering strain (εE) curves of the Al–Al2Cu–Si and Al–Al2Cu eutectic micro-pillars are depicted in Figure 2, in which the as-cast Al–Al2Cu + Si eutectics exhibit ultimate engineering stress of 480 MPa and a total strain to fracture of 4.8%, while the lasertreated Al–Al2Cu + Si eutectics with bimodal structures exhibit both the highest ultimate engineering stress of 1586 MPa and the highest fracture strain of 28.5%. The as-cast Al– Al2Cu lamellae and degenerate eutectics exhibit ultimate engineering stress of 742 MPa and 1180 MPa and total strains to fracture of 3.3% and 5.6% respectively. The laser-treated Al– Al2Cu lamellae exhibit ultimate engineering stress of 2075 MPa and a total strain to fracture of 26.5%, as summarized in Table 1. This suggests that laser surface processing can effectively improve both the strength and plastic deformability of Al–Al2Cu–Si and Al–Al2Cu eutectics. The high strength of laser-treated Al–Al2Cu lamellae eutectics should be ascribed to the nanoscale lamellar spacing. Decreasing the inter-lamellar spacing results in more Al–Al2Cu phase boundaries (PB) being introduced, facilitates the dislocation-PB interactions, and affords more room for dislocation storage, which sustains more pronounced strain hardening in the Al–Al2Cu eutectics [16,19].

**Figure 1.** Backscattered electron (BSE) images of the microstructure of as-cast Al–Al2Cu and Al– Al2Cu–Si eutectics; (**a**) as-cast Al–Al2Cu; (**b**) laser-remelted Al–Al2Cu; (**c**) as-cast Al–Al2Cu–Si; (**d**) laser-remelted Al–Al2Cu–Si.

**Figure 2.** Compressive engineering stress—engineering strain curves of micropillars with different microstructures: (**a**) Al–Al2Cu–Si eutectic; (**b**) Al–Al2Cu eutectic. The strain to fracture and the maximum compress flow stress before fracture are presented.


**Table 1.** Microstructure and compressive properties of the studied eutectic alloys.

Figure 3 shows the macro- and micro-structures of the Al–Al2Cu–Si pillars before and after compression tests. Microstructures of the as-cast Al–Al2Cu–Si eutectic micropillars before and after the compression test are shown in Figure 3a1,a2. Microstructures of the laser-treated Al–Al2Cu–Si eutectic micropillars before and after the compression test are shown in Figure 3b,c. Micro-cracks were detected in the compressed as-cast Al–Al2Cu–Si eutectic micropillars (Figure 3a2–a4). The detected micro-cracks on the Si and Al2Cu phases indicated that the stress concentration occurred on the boundaries between Si and the other as-cast eutectics, while there were no micro-cracks detected in the laser-treated Al–Al2Cu+Si eutectic micropillars (Figure 3b2–c4). The laser-treated Al–Al2Cu–Si eutectics presented good deformation capability.

**Figure 3.** Marco- and micro-structures of the Al–Al2Cu–Si pillars before and after compression test; (**a1**–**c1**) the as-cast, laser treated, refined laser remelted pillars before compression; (**a2**–**c2**) the pillars after compression; (**a3**–**c3**) the cross-section of the pillars after compression; (**a4**–**c4**) the high magnification images of the images from a3–c3. All the images were captured in the SEM with a tilt angle of 52◦.

Figure 4 shows the macro- and micro-structures of the Al–Al2Cu pillars before and after compression tests. The phases in the bright and dark contrasts are Al2Cu and Al in the BSE images (Figure 4) and TEM image (Figure 4c3). Before compressive tests, the size

and shape of these micro-pillars are nearly identical. Both lamellae and degenerate as-cast eutectics show the distinct interface of α–Al and θ–Al2Cu phases (Figure 4a1,b1); however, the interface cannot be distinguished clearly for the laser-treated lamellae eutectics because of their nanoscale inter-lamellar spacing (Figure 4c1). The microstructures of the as-cast Al–Al2Cu lamellar eutectics micropillars before and after the compression tests are shown in Figure 4a1,a2, and Figure 4a3 is the section view of the compressed pillar. Micro-cracks were detected on the Al2Cu lamellae inside the compressed micropillar (Figure 4a2,a3), and two voids were formed inside the Al2Cu (Figure 4a3). The microstructures of the lasertreated Al–Al2Cu+Si degenerate eutectics micropillars before and after the compression tests are shown in Figure 4b1,b2. Micro-cracks were also detected on the Al2Cu lamellae. The microstructures of the laser-treated Al–Al2Cu+Si eutectics micropillars before and after the compression test are shown in Figure 4c1,c2. There were no micro-cracks detected in the laser-treated Al–Al2Cu eutectics micropillars (Figure 4c2). Further TEM characterization in Figure 4c3 revealed that cracking in θ–Al2Cu phase was suppressed in the laser-treated nanoscale Al–Al2Cu eutectics; α–Al and θ–Al2Cu phases were observed to co-deform together with some distinct dislocation transmission across Al–Al2Cu interfaces, as pointed out by red arrows. After compressive tests, apparent cracks could be observed in θ–Al2Cu layers for as-cast lamellae eutectics with 3.22% reduction and as-cast degenerate eutectics with 4.69% reduction. A similar phenomenon was observed in our previous works, in which after indention tests, Al–Al2Cu eutectics with microscale inter-lamellar spacing exhibited cracking in the θ–Al2Cu phase, owing to their brittle nature at room temperature [2,3]. The angles of the crack propagation direction were detected to be 48.5◦ and 47.7◦ tilted along the loading direction for as-cast lamellae and as-cast degenerated, respectively (Figure 4a2,b2). However, no crack was detected in the laser-treated lamellae eutectics with a 10.36% reduction. The microstructure of the compressed micropillars indicated that the laser-treated Al–Al2Cu eutectics present much higher deformation capability than the as-cast Al–Al2Cu eutectics. The high strength of laser-treated lamellae should be ascribed to the nanoscale lamellar spacing. Decreasing the inter-lamellar spacing results in more Al– Al2Cu phase boundaries (PB) being introduced, facilitates the dislocation-PB interactions, and affords more room for dislocation storage, which sustains more pronounced strain hardening in the Al–Al2Cu eutectics [19].

**Figure 4.** *Cont*.

**Figure 4.** Macro- and micro-structures of micropillars before and after compressive tests from the Al-Al2Cu eutectics with different structures of (**a**) as-cast lamellae, (**b**) as-cast degenerate, and (**c**) laser-remelted lamellae; (**a1**–**c1**) the micropillars before compression; (**a2**–**c2**) the micropillars after compression; (**a3**,**b3**) the cross-section views; (**c3**) the TEM image of the cross-section view. All images except c3 were captured in the SEM with a tilt angle of 52◦.
