Microstructure and mechanical properties of aluminium based nanocomposites strengthened with alumina and silicon carbide
Al-Al₂O₃ and Al-4wt%Cu-SiC metal matrix nanocomposites were studied because these materials have a potential for offering good ductility, high strength, and high electrical and/or thermal conductivity, which make them ideal for engineering applications such as aerospace and automobile components. In order to achieve these goals the reinforcement phase needs to be in a particulate form, and the size of the particles needs to be small. Samples of aluminium based nanocomposites were produced with different volume fractions, ranging from 2.5-10 vol.% of alumina (Al₂O₃) and silicon carbide (SiC) nanoparticles. High energy mechanical milling (HEMM) with various milling times ranging from 6-12 hours was used to produce these samples. Optical microscopy, XRD, SEM, TEM and microindentation were used to characterize the milled powder and bulk samples. Bulk solid Al-(2.5-10) vol. % Al₂O₃ and Al-4wt%Cu-(2.5-10) vol. %SiC nanocomposites samples were produced using different powder consolidation techniques such as powder compact forging and powder compact extrusion. The microstructure of the composite powder/balls/granules produced was studied in details to understand the morphology, macrostructure and microstructural evolution during the HEMM and with changing volume percent of the reinforcements in the matrix. The nano SiC and Al₂O₃ were imbedded into the aluminium matrix due to the high forces and strains affecting particle surfaces during milling and the very small size of the reinforcement relative to the size of the Al particles. The average microhardness was increased with increasing volume fraction of reinforcement within the matrix. HEMM was used to fabricate Ultra-Fine Grained (UFG) and nanostructured Al- (2.5-10) vol. %Al₂O₃ composites with a dispersion of nano alumina within the matrix and Al-4wt%Cu- (2.5-10) vol%SiC with two different sizes of SiC in the micro and nano ranges. A UFG structure in the Al and Al-(2.5-10)vol.% Al₂O₃ nanocomposites can be synthesized by a combination of high energy mechanical milling and severe plastic deformation used to consolidate the powder compacts into nearly fully dense forged discs and extruded bars. No significant microscopic yielding was found in the Al-2.5 and 10 vol. %Al₂O₃ composites produced by powder compact forging. However, Al-5vol. % Al₂O₃ showed plastic yielding of 8%, and the best fracture strength of 343 MPa. No significant microscopic yielding was noticed for the Al- 10 vol. %Al₂O₃ composite produced by powder compact extrusion. Al-2.5vol. % Al₂O₃ showed plastic yielding of ~1% with the highest tensile strength of 364 MPa while Al-5vol. % Al₂O₃ showed plastic yielding of 8% with a yield strength of 318 MPa. The average microhardness of the extruded bars for Al-4wt%Cu-(2.5-10)vol.% SiC increased from 104 HV to 205 HV with increasing the volume fraction of SiC nanoparticles from 2.5 to 10%. The ultimate tensile strength increased from 168 MPa to 400 MPa with increasing volume fraction of SiC nanoparticles from 2.5 to 5% while the ductility dropped from 6.8% to 1.2 %. The fracture strength of the Al-4wt%Cu-micro-SiC was increased from 225 MPa for Al-4wt%Cu-2.5vol. %SiC to 412 MPa for Al-4wt%Cu-10vol. % SiC. The Al-4wt%Cu-2.5vol. %SiC forged disc did not show any macroscopic plastic yielding, while the Al-4wt%Cu-(7.5 and 10)vol. %SiC forged disk showed macroscopic plastic yielding with a small plastic strain to fracture (~1%).
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