Study On The Interfacial Microstructure And Ageing Characteristics Of Short Mullite Fiber Reinforced Aluminium-based Composites

Based on the detail reviews of both abroad and at home literatures about interfacial reaction and/or interfacial microstructure as well as ageing characteristics of discontinuously reinforced aluminium-based metal matrix composites, this dissertation systematically and deeply investigated the interfacial microstructure and precipitation behaviors of short mullite fiber reinforced heat treatable aluminium alloy (Al-Cu, Al-Cu-Si, Al-Cu-Mg and Al-Mg-Si) composites by means of hardness measurement (HB), differential scanning caloremitry (DSC) and transmission electron microscope (TEM) respectively. The composites and their monolithic base alloys were produced by squeeze casting simultaneously. The main innovate results are presented as follows.The microstructure of fiber, interface and precipitate of Mullite/Al composites was extensively studied by TEM (including high resolution TEM and analytical TEM). It is found and also verified by HRTEM that mullite fibers in the composite are composed of un-even scale crystal particles with different orientation arrangement. The chemical composition of mullite fiber is 3Al2O3·2SiO2, in which grain structure is sound enough and the crystal structure is recognized as orthorhombic. There exists a thin discontinues SiO2 film on the surface of the fiber and much denser dislocation in the near vicinity of the interface (aluminium side) in as quenched Mullite/Al composites. The interfacial reaction took place in the fabrication and/or heat treatment processes are different depending on the aluminium alloys used. CuAl2O4 and/or β-CuAlO2 particles were observed at Mullite/Al-Cucomposite interface but MgAl2O4 spinels were observed at the interfaces of Mullite/Al-Cu-Mg composite and/or Mullite/Al-Mg-Si composite. There were not any interfacial reaction products at fiber/Al interface in Mullite /Al-Cu-Si composite, however, only a few θ (CuAl2) phases were observed that were deposited at the interface during non-equilibrium solidification of the composites.It is shown from deep and wide ageing experiments that the aged hardness of Mullite/Al composites is always higher than that of their monolithic base alloys, which indicates that short mullite fibers are able to reinforce (harden) heat treatable aluminium alloys. The ageing progress underwent the same procedure in both of the composites and the base alloys, that is, hardness increasing, reaching peak and slowing down. The higher the ageing temperatures, the shorter the time needed to reach peak ageing and the lower the peak hardness of both composites and base alloys. There were not any changes in the general ageing procedure of heat treatable aluminium alloys when embedded with short mullite fibers.There were also not any changes in the nature of ageing precipitates of aluminium alloy comparing the composite with its monolithic base alloy. The orientation of θ' , S' or β ' precipitates along with the mother a -Al phases is always the same wherever in the alloys or in the reinforced composites:It is found, however, that the GP zone formation is suppressed and delayed remarkably while the nucleation and growth of θ " and θ ' precipitates are accelerated when Al-Cu alloy is embedded with short mullite fibers. But there is little fiber influence on the formation of θ phases in Mullite/Al-Cu composites. It is also found that if copper content is about 4.0 wt% and magnesium content is less than 1.85 wt% , the GPB zone formation is then suppressed and delayed considerably while the nucleation and growth of S' precipitates are accelerated but there is little fiber influence on the formation of S phase in Mullite/Al-Cu-Mg composites. Much attention should be paidwhen concluding the results of ageing characteristics of Mullite/Al-Mg-Si composites, where it is found that there is neither any remarkable influence on the formation of Mg, Si clusters nor efficient effect on the nucleation and growth of GP zones. Short mullite fibers can only do a moderate acceleration to the formation of β' precipitates when...