## 3 Results and Discussion  
### Microstructural Analysis-Hardness  
Figure 1 presents a panoramic view of the HEA particulates after the milling process, through SEM examination. It also presents the mean actual composition of the powder particles in at.% after energy dispersive X-ray spectroscopy (EDS) analysis at various particle areas. It can be observed that most of the particles show an angular morphology. The first estimation of their size shows that it lays within the range of 1 \(\upmu\)m to 30 \(\upmu\)m.  
Figure 2 presents the X-ray diffractograms for the different materials produced in the present effort. It can be seen that in all cases, two different phases can be distinguished- an FCC phase of the higher intensities that dominates, corresponding to Al, and a BCC phase of significantly lower peak intensities that corresponds to the HEA-reinforcing particles. These significantly lower peak intensities of the reinforcing phase particles are expected since their overall content is relatively small (up to 5 vol.%). MoTaNbVW produced in the form of bulk material consists of two BCC solid solutions- one rich in MoTaW and one rich in NbV. This is the result of a low-extend segregation of these two phases during the solidification process. It is well understood that the segregation of phases is inevitable, and it is acceptable as long as these phases are solid solutions rather than brittle intermetallic phases. It should be mentioned that MoTaNbVW was one of the pioneering refractory HEAs that has been developed and it meets the prerequisites of simple solid solutions formation. A thorough evaluation of the microstructure of MoTaNbVW can be found in previous efforts [50; 51]. Figure 1: Microstructure and composition (average of six different measurements) of MoTaNbVW powder under SEM (Secondary electron mode). (**a**,**b**) represent two different areas where point analysis has been performed in 6 points in total. The particle distribution within the Al matrix for all the different composite systems is shown in Figure 3. As a first observation, it can be noticed that the actual particle content is in agreement with the nominal compositions: the higher the nominal composition, the more increased the actual reinforcing particle content. This observation refers to all the different particle sizes--as the particle content increases, the number of both the bulk and the sub-micron particles increases. As far as the particle distribution is concerned, it can be observed from Figure 3 that the distribution appears to be almost homogeneous for all the different particle sizes. Such a homogeneous distribution is expected since powder metallurgy processing route, due to the primary powder mixing process, ensures this type of microstructural homogeneity at the final sintered products. Another important observation that rises from Figure 3 is related to the quality of the actual sintering process. It can be noticed that in all cases there is no significant evidence of pores or other defects (flaws, micro-cracks, micro-voids) which implies that the sintering process was fully developed and concluded, leading to fully dense and rigid composite systems. Figure 3: Microstructure of the produced composites (Back scattered electron mode): (**a**) Al-1 vol% HEA, (**b**) Al-3 vol% HEA, and (**c**) Al-5 vol% HEA. Figure 2: X-ray diffractograms of Al-HEA composites. Figure 4 illustrates an EDS mapping on a selected region around a bulk reinforcing particle in order to reveal the elemental distribution. It can be observed that the reinforcing particle consists of the elements related to the refractory HEA system adopted in the present effort. Figure 4 also reveals useful information on the nature and integrity of the matrix-reinforcement interfacial area. Indeed, no signs of pores or interfacial detachment can be observed. Additionally, there is no significant evidence- at least at the observed magnification of severe reactivity between the Al matrix and the HEA system elements at the interfacial areas.