Abstract
Nanocrystalline (Average grain size ∼200nm) bulk AA2124 alloy was produced through high energy ball milling of microcrystalline powder followed by spark plasma sintering (SPS) at 480°C with a holding time of 10min. The effect of initial particle and grain size on the microstructural evolution as well as on the relative density and mechanical properties of the specimens consolidated through SPS and hot pressing (HP) at the same temperature for 60min was investigated for ball milled nano-powders (NP), as well as as-received micro-powders (MP). Results showed that the NP specimens consolidated with SPS had the highest microhardness values compared to the other specimens despite not achieving full densification. On the other hand, a general increase in density, hardness, and compressive strength was observed for all SPS consolidates compared to HP. The presence of aluminum oxide and its influence on the consolidation process as well as the resulting mechanical properties of the bulk specimens is also discussed.
Highlights
Over the last few decades, there has been an ever increasing interest in nanocrystalline (NC) materials due to their extremely high strength and hardness [1,2], high diffusion rates [3], high fatigue resistance [4], corrosion resistance [3] and improved tribological properties [5] compared to conventional microstructured materials
Microhardness and compressive tests were carried out on the consolidated near full density specimens of both unmilled and milled powders and the results showed significant increase in both hardness and compressive strengths for the milled nanocrystalline powders as a result of the very fine grain size
Field Emission Scanning Electron Microscope (FESEM) imaging of the powders before and after milling revealed that the high energy of milling imposed on the powder during the 36 h milling time led to the refinement of the micron-powders
Summary
Over the last few decades, there has been an ever increasing interest in nanocrystalline (NC) materials due to their extremely high strength and hardness [1,2], high diffusion rates [3], high fatigue resistance [4], corrosion resistance [3] and improved tribological properties [5] compared to conventional microstructured materials. SPS has evident superiority over other conventional sintering techniques due to the higher heating rates, lower sintering temperatures and much shorter holding times required for full/ near full density consolidation, this in turn leads to enhanced materials characteristics especially in the sintering of NC materials as little time is allowed for grain growth, which is a major concern when it comes to synthesis of NC bulk products. Results showed that the NC powder (that have previously gone through high energy ball milling) was strain hardened and was only consolidated at higher temperatures (480 °C and prolonged holding times between 60 and 90 min), while specimens processed at lower sintering temperatures lacked integrity. The discrepancy in phase formation and its subsequent effect on the mechanical properties and the microstructure for both set of powders under each processing condition is discussed
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