| Studies indicate that micron particles can improve strength and elastic modulus of aluminum matrix composites(AMCs),but their ductility and toughness will reduce.In contrast,nano-sized particles can enhance the strength of Al matrix composites without sacrificing their ductility and meanwhile remarkably improve elevated temperature mechanical properties.Nano-sized particle-reinforced Al matrix composites have therefore become the preferred choice to develop high performance and lightweight advanced structural materials.Depending on how nano-sized particles are added,the preparation of nano-sized particle-reinforced Al composites material could be divided into ex situ and in situ fabrication methods.Commonly the reinforced particles introduced by ex situ method are susceptible to contamination and tend to agglomerate due to the poor wettability between the matrix and reinforcement,resulting in the degradation of the particle-matrix interface as well as the mechanical properties at both room and elevated temperature.On the contrary,the particles generated by in situ methods are usually fine,relatively uniform dispersed and have cleaner particle-matrix interface.Among many in-situ fabrication methods,combustion synthesis takes the advantages of low energy requirement,simple fabrication process and high purity products,which has attracted many researchers' great attention.However,low density of the prepared particle reinforced Al matrix composites,particles less than 100 nm in size are hard to synthesize,and in situ nano-sized particles are easily aggregated have been the key scientific and technical problems that need to be urgently solved.Meanwhile,the study on the microstructure and mechanical properties of nano-sized particle-reinforced Al matrix composites fabricated by combustion method is less and not deep enough.So it is necessary to further explore and reveal the fabrication process,microstructure,room and elevated temperature mechanical properties of nano-sized particle reinforced Al matrix composites prepared by combustion synthesis.In response to these existing problems,the dense and high performance nano-sized TiC_p/Al-Cu-Mg composites were successfully fabricated by the integrated technology of combustion synthesis and vacuum hot pressing followed by hot extrusion.Carbon nanotubes(CNTs),graphite and C-black were used as the carbon source of combustion synthesis of Al-Ti-C system.The optimal fabrication process and carbon source were revealed.Meanwhile,effects of nano-sized TiC_p volume fraction,dispersion and particle-matrix interface on the microstructure,room and elevated temperature mechanical properties,creep resistance and dry sliding wear resistance were investigated,and the strengthening mechanism of nano-sized TiC_p/Al-Cu-Mg composites with excellent room and elevated temperature mechanical properties,creep resistance and dry sliding wear resistance were revealed.The main results are as follows:1)The dense and high proformance nano-sized TiC_p/Al-Cu-Mg composites were successfully fabricated by the integrated technology of combustion synthesis of Al-Ti-CNTs system and vacuum hot pressing followed by hot extrusion.i)It was revealed that the synthesized TiC_p possessed the smallest size by using CNTs as carbon source,mainly due to the fine size and huge specific surface area of CNTs increased the contact area between CNTs and the reactant,resulting in the fastest dissolution rate.So the carbon atoms were abundant supply during the combustion reaction,and the reacted time was shortest.ii)It was revealed that the mechanism of the promoted nano-sized TiCp dispersion and combustion reaction as well as reduced Al3 Ti phase by using 2 h-milled CNTs as the carbon source could be concluded as: the much finer CNTs enlarged the area of the contact surface between the CNTs and the Al-Ti binary liquid phase,which accelerated the dissolution of CNTs in the Al-Ti-C ternary liquid phase,then promoted the combustion reaction and nano-sized TiC_p dispersion.iii)9 vol.% nano-sized TiCp/Al-Cu-Mg composites prepared by using 2 h-milled CNTs as carbon source possessed the highest yield strength,tensile strength,and fracture strain of 404 MPa,601 MPa,and 8.1%,respectively,which increased by 66 MPa,82 MPa,and 0.4% compared to that of the composites fabricated by using untreated CNTs,and meanwhile the yield strength,tensile strength increase by 135 MPa,160 MPa but the fracture strain decreased compared to that of Al-Cu-Mg alloy.iv)The room temperature strengthening mechanism of nano-sized TiCp/Al-Cu-Mg composites was revealed: the impeded movement of dislocations by relatively uniform distributed nano-sized TiC_p,well particle-matrix interface and the reduced Al3 Ti phase.2)The optimal fabrication processing of nano-sized TiC_p/Al-Cu-Mg composites were achieved: a)the parameters of ball milling CNTs,milling time: 2 h,milling speed: 300 r/min;b)the parameters of ball milling Al,Ti and CNTs powders,milling time: 48 h,milling speed: 50 r/min;c)Al,Ti and CNTs powders were condensed into cylindrical compacts(?45 mm×35 mm)under the pressure of 20-30 MPa;d)the temperature was heated to 900 oC and the reactant was hot pressed after combustion synthesis reaction;e)the parameters of hot extrusion,extrusion temperature: 500 oC,extrusion ratio: 16:1-19:1;f)the parameters of T4 heat treatment,solid solution temperature: 510 oC,solid solution time: 1 h,aging at room temperature for 96 h.3)Effects of in situ nano-sized TiCp on recrystallized microstructure of the composites and the mechanisms of enhanced room temperature strength and plasticity as well as improved creep resistance were revealed.i)Nano-sized TiCp could pin the sub-boundaries,stabilize the substructure and subsequently retard recrystallization as well as the growth of the recrystallized grains,which resulted in a partially recrystallized microstructure containing deformation structure,substructured grains and fine fully recrystallized microstructure.But Al-Cu-Mg alloy showed a fully recrystallized microstructure with equiaxed grain shape(18.8 ?m).Recrystallization fractions of Al-Cu-Mg alloy,5 vol.%,7 vol.% and 9 vol.% nano-sized TiC_p/Al-Cu-Mg composites were approximately determined to 96%,81%,69% and 44%,respectively.ii)It was revealed that the tensile strength of Al-Cu-Mg composites reinforced with 5 vol.%,7 vol.% and 9 vol.% nano-sized TiC_p increased and meanwhile accompanying with a markedly increase in fracture strain,of which the tensile properties at 573 K of 139 MPa,157 MPa,and 17.2% for 9 vol.% nano-sized TiC_p/Al-Cu-Mg composites increased by 29.9%,26.6% and 73.7% compared to that of Al-Cu-Mg alloy.iii)It was found that the creep resistance of 9 vol.% nano-sized TiCp/Al-Cu-Mg composite was almost 4-15 times higher than that of Al-Cu-Mg alloy under applied stresses of 40-100 MPa at temperatures of 493-573 K,of which the creep resistance of the composite was almost 15 times higher than that of Al-Cu-Mg alloy under 40 MPa at 493 K.It was revealed that both the apparent stress exponent and apparent activation energy for the composites display higher values in comparison to the unreinforced matrix alloy,and the creep behavior of both the composite and Al-Cu-Mg alloy was detected by dislocation climb mechanism.iv)The elevated temperature strengthening mechanisms of nano-sized TiC_p/Al-Cu-Mg composites were revealed that:(a)nano-sized TiC_p pinned the dislocation climb motion,(b)nano-sized TiC_p resisted the movement of grain boundaries,including the LAGBs and HAGBs,during elevated temperature deformation,(c)Nano-sized TiC_p could pin the sub-boundaries,stabilize the substructure and subsequently retard recrystallization as well as the growth of the recrystallized grains,resulting to the formation of fine size ?? and S? precipitates in the matrix,which would prevent dislocations from moving.4)It was noted that in situ nano-sized TiC_p/Al-Cu-Mg composites had the better room and high-temperature dry sliding wear resistance than Al-Cu-Mg matrix alloy.i)It was revealed that nano-sized TiC_p/Al-Cu-Mg composites possessed the excellent room temperature wear resistance under the applied load of 20 N-50 N at sliding velocity of 0.63 m/s-1.26 m/s,which increased by 40%-110% compared to that of Al-Cu-Mg alloy.Among them,wear resistance of 15 vol.% nano-sized TiC_p/Al-Cu-Mg composite increased by 110% compared to that of Al-Cu-Mg alloy under the applied load of 40 N at sliding velocity of 0.94 m/s.It was found that the wear resistance of the composites and Al-Cu-Mg alloy decreased with the applied load and sliding velocity increasing,but the decrease rate of the composites was much lower than that of Al-Cu-Mg alloy,which was mainly attributed to the formation of protective mechanically mixed layers enhanced by nano-sized TiC_p and Al2O3 on the worn surface of the developed composites.ii)It was revealed that nano-sized TiCp/Al-Cu-Mg composites possessed the exceptional wear resistance at the temperature of 433 K-493 K under the applied load of 20 N and sliding velocity of 0.94 m/s,which increased by 70%-530% compared to that of Al-Cu-Mg alloy.Among them,wear resistance at 493 K of 20 vol.% nano-sized TiC_p/Al-Cu-Mg composite increased by 530% compared to that of Al-Cu-Mg alloy.The wear resistance at the temperature of 433 K-493 K of 20 vol.% nano-sized TiC_p/Al-Cu-Mg composite was almost unchanged,which indicated that the formation of denser protective mechanically mixed layers enhanced by nano-sized TiC_p and Al2O3 on the worn surface of the developed composites with the temperature increasing.The results above showed that the composites exhibited superb wear resistance at elevated temperature than that at room temperature.iii)The mechanisms of enhanced room and elevated temperature wear resistance for nano-sized TiC_p/Al-Cu-Mg composites were revealed:(a)nano-sized TiC_p pinned the dislocation motion and resisted the plastic deformation of the present composites,(b)the effective transfer load of nano-sized TiC_p prevented the adhesive wear,(c)nano-sized TiC_p resisted the movement of grain boundaries,improved room and elevated temperature strength,hardness,and meanwhile reduced the grain boundary microcrack and delamination,(d)the formation of denser protective mechanically mixed layers enhanced by nano-sized TiC_p and Al2O3 on the worn surface of the developed composites.The present thesis provides experimental data,theoretical and technical references for the development and fabrication of high strength and plasticity,high creep resistance and good dry sliding wear resistance of nano-sized particles reinforced Al composites. |
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