| Al-Fe-V-Si heat resista nt aluminium alloys are characterized with their excellent ambienttemperature and elevated temperature strength, good fracture toughness property andcorrosion resista nce. Fine Al12(Fe,V)3Si heat resista nt particulates of high volume fractiondistribute uniformly in Al matrix, the particles are characterized with excellent thermostability,and pla y a dispersion strengthening role. In order to improve the mecha nica l propertiesfurther, SiC particles are added into the alloys to elevate the Young's modulus, abradability,ambient temperature strength and high temperature strength.In this dissertation, development and status quo of rapid ly solid ified heat-resista nt alloy andcomposite, rapid solid ification-powder metallurgica l technology and spray depositionprocesses are commented. Aiming at some problems found in the investigation of SiCP/Al-Fe-V-Si composite, the following researches were done.SiCP/Al-Fe-V-Si composite billets were prepared by multi-la yer spray depositionequipment, and the effects of fluid flow dia meter, atomization gas pressure and jetting heighton microstructure and properties of deposited billets were investigated. Optimized spraydeposition processing parameters were obtained by system research.SiCP/Al-Fe-V-Si composite billets were densified by unia xial hot pressing and hotextrusion. Comparing the performs densified by unia xial hot pressing and hot extrusion, it isfound that, SiC particles la minate vertica l to extrusion direction and aggregate near sheetsuperficia l coat in the extruded sheets . This phenomenon did not appear in the preform as-hotpressed.The processes of rolling after extrusion and after following hot-pressing were investigatedcomparatively, and the different effects of the two processes on microstructure and mecha nica lproperties of the composite sheets were discussed. Compared with microstructure of thesheets as-rolled after extruding, microstructure of the sheets as-rolled after hot-pressing wasmore homogenous. Superficia l aggregation of SiC particles in the extruded sheets wasavoided in the sheets prepared by process of rolling after hot pressing. Strong bonding of SiCparticles and matrix and uniform and fine microstructure contributed to the excellentmecha nica l properties of the sheets as-rolled after hot-pressing. Density and hardness of thecomposite sheets variation versus total reduction during rolling was investigated.Microstructure and mecha nica l properties of rolled SiCP/Al-Fe-V-Si sheets after hotpressingand hot extruding were investigated, and those of Al-8.5Fe-1.3V-1.7Si sheets wereinvestigated comparatively. It is found that the co-depostion of SiC particles increase d the cooling rate, refine d the grain size of the alloy matrix and the dispersion phase ?-Al12(Fe,V)3Si particle and increase its volume fraction. And the stabiliza tion of fine dispersoidparticles during hot working owing to the SiC particles which contributed to the excellentmecha nica l properties. The volume fraction ofα-Al12(Fe,V)3Si dispersoid particles in thematrix went up as Fe content increases, leading to mecha nica l properties improvement as Fecontent increases, but elongation of the composite decreased as Fe content increased.Mechanical properties of SiCP/Al-8.5Fe-1.3V-1.7Si composite sheets as-rolled were moreexcellent when compared with that of Al-8.5Fe-1.3V-1.7Si alloy sheets .SiC-Al interface was investigated. Two ma in kinds of interfaces between SiC particle andthe Al matrix were observed in the composite as-deposited. One was a diffusion bonding witha Si interfacia l film of 3nm in width, Si film improve interfacia l wettability, which resulted ina higher strength bonding at the SiC interface. Some rod-like Al4C3 phases of about 10nm india meter and 40nm in length is distributed in the matrix near the interface. Fine twinnedAl4C3 phases are dispersed in the Al matrix, which strengthen the composite by precipitationstrengthening and improve the mecha nical properties of the composite further. The other wasconstituted by the glassy silica with the thick ness of about 5 nm which could act as aprotective barrier (diffusion barrier)to prevent the formation of massive Al4C3 by reactionbetween Al and SiC. The film improve s wettability, eliminates the contamina nts, and avoid sparticle consumption and increase s the interfacia l strength. Good SiC-Al interfacia l wettabilityassured high bonding strength, contributing to the mecha nica l properties of the SiCP/Al-Fe-VSicomposite.Fracture mecha nism of SiCP/Al-Fe-V-Si composite sheets was investigated. Breakage ofSiC particles and debonding of SiC-Al interfaces were the major fracture mecha nisms, andfracture generation was determined by the strength of SiC-Al interfaces. Strong bondingexited between SiC particles and Al matrix when tensile temperature was low, which resultedin breakage of SiC particles becoming the major fracture mecha nism. Softening phenomenonappeared before the tensile sample broke, which contributed to the good plasticity. Bondingbetween SiC particles and Al matrix became weak when tensile temperature was up to 400℃,and debond of SiC-Al interfaces became the major fracture mecha nism, in which samplefailed before soften ing phenomenon appeared. When SiC-Al interfaces became weaker, SiCparticles broken decreased and those extracted increased, and plasticity became poor as tensiletemperature elevated.Development of dispersions, SiC-Al interface, mecha nical properties and dislocationmultiplication of SiCP/Al-Fe-V-Si composite sheets during heat exposure at high temperaturewas investigated. The SiCP/Al-Fe-V-Si composites prepared by multi-la yer spray deposition technology can provide excellent heat resista nce, dispersoids ma inta ined near sphere shapewith little growth even after 200h at 550oC. The addition of SiC particle prevented the fine?-Al12(Fe,V)3Si particles from coarsening ,decomposition and transforming into coarse phase,subseq uently led to higher microstructure therma l stability. This can be attributed to eleva tionof silicon concentration aroundα-Al12(Fe,V)3Si particulates resulted from partia l dissolutionof SiCP, contriburing to the excellent thermalstability of the composites. Higher Fe/V ratio inthe alloy matrix helped to enhance the hardness and the microstructure stability. Coarse Al4C3phase near the SiC-Al interface precipated during exposure at 640 oC, and disloca tionmultiplication exhibited when SiCP/Al-Fe-V-Si sheets exposed at 550 oC for a long time.Both Al-Fe-V-Si alloy and their composites are characterized with eximious corrosionresista nce, and the addition of SiC can reduce corrosion rate of Al-Fe-V-Si alloys,consequently improved the corrosion resista nce of the alloys. For the SiCP/Al-Fe-V-Si withdifferent Fe content, corrosion rate increased and corrosion resista nce worsened as Fe contentincreased. |
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