## 4 Discussion  
The above findings are indicative of the important effect of the cooling rate from the homogenization temperature, which was accomplished in this project for simulating the hot working temperature of the extrusion process, on the mechanical properties of extruded profiles. Water quenching led to profiles which had superior properties regarding strength. The main reason for this behavior is that by very rapid cooling a high percentage of the alloying elements were retained in a solid solution (Si and Mg in this case) and the formation of the maximum possible Mg\({}_{2}\)Si precipitates quantity was allowed during aging. The maximum grain boundary precipitate particles length was 150 nm, but the lengths of the majority were finer than 30 nm. On the AC sample, the precipitation behavior was characterized by the presence of up to microscopic-size \(\beta\) (Mg\({}_{2}\)Si) particles with an elongated morphology located on the grain boundaries. These did not contribute to strength, as they had non-coherent phases. In addition, they were preferable sites for stress concentration limiting the overall ductility. Therefore, even after three hours of aging, the AC sample exhibited low strength levels and unacceptable ductility in tensile testing. As a result, the sample is expected to exhibit unacceptable crashworthiness and formability performance in general. One of the main targets of this project was to observe the precipitation behavior in different boundaries. In this frame, the SC sample was examined. The precipitates (mainly \(\beta^{\prime}\)) were nucleated in the areas which were considered as energetically favored (grain boundaries). The electrolytic polishing process enabled the observation of the precipitates on the various grain boundaries types (HAGBs, LAGB, and SGBs), and they could be easily measured at magnifications higher than 25 k. The ones that were located on triple grain boundaries were generally quite coarser than those found on the other grain boundary interfaces exhibiting a higher length and thickness. In most cases, the precipitates on the triple boundaries extended on all double boundaries, rendering them a star-like morphology. The texture of the profiles was formed at the end of the extrusion process. The SC sample was selected for viewing the texture in the subsurface zone and the mid-thickness region. The texture differed significantly between these two areas. It was observed that the mid-thickness region was strongly textured with a (001) single fiber texture while the subsurface zone exhibited a more random texture. However, the biggest difference between these two areas lied in the grain boundaries characteristics. Even though the sample was recrystallized, in the mid-thickness region, SGBs were observed as a result of the extrusion process and the respective flow of the material through the die. These boundaries were not removed after solution and aging heat treatments. Extrusion conditions and more specifically a high amount of shear were, on the other hand, adequate for the elimination of SGBs and the limitation of LAGBs in subsurface zones. In terms of crack initiation, texture analysis revealed with high accuracy where this would be observed. The largest cracks after three-point bending testing were formed in the transition zone between the more random subsurface and the mid-thickness region. At this area, the grains which were more favorably oriented towards rotation during bending created a minute discontinuity which evolved to a crack during the stress increase. The Taylor factor maps clearly indicated that in the examined samples such interfaces and a lot of boundaries were present. The surface roughening of the bent SC sample was linearly connected with slip lines development and grain boundaries with favorable orientation for cracking. The amounts of precipitation were different on HAGBs, LAGBs, and SGBs. Coarser Mg\({}_{2}\)Si precipitates in terms of length and thickness were located on HAGBs.