| In recent years,our group has produced a series of Al-based composite materials with biomimetic nanolaminated structure,using a modified powder metallurgy fabrication route.These composites generally have a balanced strength and ductility,showing excellent mechanical property synergy.However,there are two important questions remain to be answered:first,what role does the nano-layered Al2O3 formed in situ on the Al flake surface play in the strengthening and toughening of the Al-based biomimetic nanolaminated composite?Second,in the Al-based composite,the reinforcements?usually nanocarbon such as carbon nanotubes and graphene?are not in direct contact with the Al matrix,then how do the reinforcements exert their strengthening and toughening capability?In this thesis,experiments were carried out to study the effect of in situ formed Al2O3 content and crystal structure on the mechanical properties of Al2O3/Al biomimetic nanolaminated composites.In particualr,we used Al2O3/Al nanolaminated composite prepared in the normal way?without any atmosphere control?as a control group.In one sample set,the composite was made of flaky Al powders oxidized in high temperature to form thicker amorphous Al2O3 layers on the Al surface.In the other sample set,the composite was prepared by our creative glovebox flake powder metallurgy process,which helped reduce the Al2O3 content by controlling the atmosphere during the composite powder processing.In the fourth sample set,the flaky powders were densified and then heated at elevated temperature achieve crystallization of the Al2O3 layers.Then all the samples were subjected to macroscopic tensile tests.Subsequently,microscopic examinations such as SEM,TEM,and TKD were used to characterize the microstructure of the samples,to explain the macroscopic mechanical properties and the strengthening mechanisms of the flaky Al2O3 in the composites.The main findings of this paper are summarized as follows:1.The weight percentages of Al2O3 in the samples were measured to be2.44%?glovebox processing?,3.66%?normal route?and 9.35%?oxidation route?,respectively.Obviously,the glovebox route can considerably reduce the Al2O3 content.2.From the results of tensile test and Vickers hardness test,the samples with lower Al2O3 content?glovebox processing?had higher strength and hardness,while their ductility?uniform and total elongations?showed notable degradation as compared to that of the sample prepared by the normal route.Moreover,surprisingly,the intentionally oxidized samples showed slight decrease in strength and hardness despite their highest Al2O3 content.3.Based on the TKD analysis,we estimated the average Al grain sizes in the four sample sets to be 355±217 nm?glovebox route?,338±194 nm?normal route?,505±200 nm?normal route+Al2O3 crystallization?and595±391 nm?oxidation route?,respectively.According to the Hall-Petch relation,the yield strength increments as a result of the varying grain sizes among different sample sets(??GZ,using the sample set fabricated by the normal route as the benchmark)were calculated to be about-1.7MPa,0 MPa,-12.5 MPa,and-16.9 MPa,respectively.4.Through careful analysis on the TEM images,the average aspect ratio of the Al2O3 layers?length/thickness?in the four sets of samples were estimated to be 138.3,24.5,23.0,and 12.1,respectively.According to the shear-lag model,the corresponding strengthening contributions from Al2O3 load-sharing were then estimated to be 48.7 MPa,18.1 MPa,13.7MPa,and 20.2 MPa,respectively,indicating the load-transfer is the dominant strengthening mechanism in the Al2O3/Al nanolaminated composites.This study provides a novel idea for the design of nanolaminated Al-based composites.Specifically,our results suggest that we can improve the strength of the composites by increasing the continuity?i.e.,the aspect ratio?of the Al2O3 layers by using the glovebox processes.On the other hand,smaller Al2O3 layer aspect ratio can be obtained by deformation processing and may give rise to improved ductility. |
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