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Microstructure, Thermal Stability and Consolidation of Nanostructured and Ultrafine Structured Cu based Metal Matrix Composite and Alloy Powders Produced by High Energy Mechanical Milling

Abstract
Ultrafine grained and/or nanostructured Cu and Cu-(2.5-10)vol.%Al2O3 composite balls/granules/powder particles were produced using two high energy mechanical milling (HEMM) routes respectively. The microstructural evolution of the as-milled Cu and Cu-Al2O3 composite balls/granules/powder particles produced using Route 1 (12hours) and Route 2 (Route 1 + 12 hours milling under another condition) of milling was studied using scanning electron microscopy (SEM), transmission electron microcopy (TEM), scanning transmission electron microscopy (STEM) and energy dispersive X-ray (EDX) mapping. The study confirmed that HEMM can be effectively used to disperse (2.5-10)vol.%Al2O3 nanoparticles into a ultrafine grained or nanocrystalline Cu matrix after both routes of milling. The as-milled Cu and Cu-Al2O3 composite balls/granules/powder particles were heat treated at 150, 300, 400 and 500 C for 1 hour, respectively, to determine the thermal stability of the microstructure and corresponding microhardness change as a function of annealing temperature. It was found that for Cu and Cu-2.5vol.%Al2O3 composites after heat treatment at 150 C, the Cu grain sizes decreased due to recrystallisation, and increasing the annealing temperature to 300 C causes slight coarsening of the Cu grains. Further increasing the annealing temperature to 500 C caused significant coarsening of the Cu grains and the Al2O3 nanoparticles. With increasing the volume fraction of Al2O3 nanoparticles, (i) the thermal stability of the Cu-Al2O3 composite increases, (ii) the microstructure of the Cu matrix became finer, and (iii) the coarsening of Cu grains in the composite powder particles after annealing at 500 C become less severe. The average microhardness of the Cu-Al2O3 composites decreased after annealing at 150 C due to decrease of dislocation density, then remained almost unchanged with increasing the annealing temperatures to 300 C and 400 C. Further increasing the annealing temperature to 500 C caused significant decrease in average microhardness due to reduction in dislocation density and grain coarsening, suggesting that Cu-Al2O3 composites are thermally stable at temperatures up to 400 C. Pure copper powder and Cu-Al2O3 composite powders produced using Route 2 were compacted by hot pressing at 350 C followed powder compact forging. Increasing the volume fraction of Al2O3, the average microhardness increased for the forged Cu-Al2O3 composites. A decrease in tensile fracture strength was examined for the Cu-Al2O3 composites with the increase of the volume fraction of Al2O3 as 2.5% to 10%. Nanostructured Cu-(1-4)at.%Pb alloy powder particles were produced using Route 1 of high energy mechanical milling. The microstructural evolution and thermal stability of microstructure of powder particles as a function of annealing temperature were examined. It was found that heat treatment at 150 C caused slight coarsening of the Cu grains, and increasing the annealing temperatures to 300 and 500 C caused significant coarsening of the Cu grains. The average microhardness of the Cu-Pb alloy powder particles decreased after annealing at 150 C due to decrease of dislocation density, and then remained almost unchanged with increasing the annealing temperature to 300 C. Further increasing the annealing temperatures to 400 C and 500 C caused significant decrease in average microhardness due to reduction in dislocation density and grain coarsening, suggesting that Cu-Pb alloy powders are thermally stable at temperatures up to 300 C.
Type
Thesis
Type of thesis
Series
Citation
Mukhtar, A. (2010). Microstructure, Thermal Stability and Consolidation of Nanostructured and Ultrafine Structured Cu based Metal Matrix Composite and Alloy Powders Produced by High Energy Mechanical Milling (Thesis, Doctor of Philosophy (PhD)). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/4393
Date
2010
Publisher
The University of Waikato
Supervisors
Rights
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