7. Consolidation of Micro- and Nano-Sized Al Powder

In this final set of experiments, Al NPs were employed in the as-received and ball-milled condition to produce in situ reinforced MMnCs. The NPs possess higher surface than the micro-sized counterpart. This means that they may lead to the production of nanocomposites reinforced with a much higher content of in situ reinforcement and, as a limiting and desired condition, highly reinforced composites could be produced even without relying on ex situ addition of oxide NPs. For comparison, Al micro-sized powder was consolidated in the as-received condition and after ball milling as well. Furthermore, a mix of the two above-mentioned powders was also employed to complete the frame of experimental conditions. A ball-to-powder weight ratio r = 10:1 was adopted for grinding the metal powder for 16 h using 1.5 % of stearic acid as PCA. Powder consolidation was performed by BP-ECAP. It was expected that the higher content of non-metallic compound made the consolidation of powder rather difficult. It was also known that in SPD processes, more ductile and bigger particles ease the consolidation process [3] since the driving force is the severe plastic deformation of metal powder particles. On the contrary, nano-sized particles cannot accommodate high shear strains and are inclined to slip on each other instead of being deformed. Since the back-pressure (BP) revealed to be able to more efficiently consolidate powders by ECAP, it was applied for producing the nanocomposite billets. After preliminary attempts at different temperatures, 600 °C was selected as a suitable temperature for producing the following full dense bulk samples: (1) As-received Al micro-powders consolidated by BP-ECAP, (2) As-received Al nano-powders consolidated by BP-ECAP, (3) Ball-milled Al micro-powders consolidated by BP-ECAP, (4) Ball-milled Al nano-powders consolidated by BP-ECAP, (5) Ball-milled Al micro-(50 wt%) and nano-powders (50 wt%) consolidated by BP-ECAP.