*3.5. Hardness*

Figure 6a shows the Vickers microhardness of as-cast and cooling slope samples, measured after a combination of heat treatment and ECAP processing via route A. It shows that the hardness increased from 61 HV to 125 and 129 Hv, after four passes of ECAPed as-cast and cooling slope samples, respectively. After six passes, the as-cast sample failed to reach six passes, without surface cracks, while the cooling slope sample successfully reached six passes, with a 134 HV microhardness. After heat treatment, the microhardness was enhanced for the as-cast and cooling slope samples by 26% and 36%, respectively.

**Figure 6.** (**a**) Effect of T6 heat treatment and ECAP process on hardness of A356 alloy and (**b**) Mapping of silicon particles distribution in ECAPed samples.

The spheroidization of eutectic Si after T6 heat treatment was found to increase the hardness of samples. Essentially, the spheroidization of Si particles after T6 heat treatment, and the precipitation of magnesium silicide (Mg2Si) particles during the aging process, tend to increase the ultimate tensile strength as well as hardness [47,48].

Since shear force could break down the dendrite arms of the α-Al phase, leading to grain refinement, its microstructure in the rheocasting condition became smaller and denser, where the rheocast sample recorded the highest microhardness, compared to the as-cast sample [12], as well as a change in the morphology of Si particles, from a flake shape in the as-cast to acicular shapes in the cooling slope, which contributed to the microhardness of A356 alloy, as mentioned in Section 3.2.

Four passes of ECAPed-T6 for the as-cast and cooling slope casting samples increased the hardness of both samples due to the fragmentation of globular heat-treated eutectic Si particles, the reduction in grain size, and increase in the density of dislocations. High strain, induced through six passes within the cooling slope sample during the process of ECAP, increases both the dislocation density and grain refinement as well as the fragmentation of eutectic Si particles, which led to a greater improvement in hardness, which is in line with other studies [39,49,50].

The homogenous distribution of fragmented eutectic Si particles and primary α-Al phase of the cooling slope-cast sample plays an integral role in improving the hardness of ECAPed materials. As shown in Figure 4c, the homogeneity of the distribution of the primary α-Al phase and Si particles within the eutectic mixture phase, for the microstructure of ECAPed cooling slope-cast sample, surpassed the homogeneity of a similar distribution of the ECAPed as-cast sample. Figure 6b displays the EDX map of the distribution of Si particles, after the ECAP process for the cooling slope sample. Due to the microstructural evolution, after the processes of semisolid casting, coupled with ECAP, it was expected that the finer formation and homogeneous distribution of α-Al and eutectic mixture phase enhanced the mechanical properties of the material [51].
