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

Based on the detail reviews of the interfacial research progress of discontinuously reinforced aluminium-based metal matrix composites, this thesis investigated both the microstructure of short mullite fibers and the interfacial microstructure and microchemistry of Mullite/Al-Cu, Mullite/Al-Cu-Mg as well as Mullite/Al-Cu-Si composites by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM, including high resolution TEM and analytical TEM, both of them have energy disperse spectrum (EDS) functions). X-ray diffraction (XRD), respectively. The composites were produced by squeeze casting. The main results are showed as follows:1. The mullite fibers impeded in the Mullite/Al composites are composed of uneven scale crystal particles with different orientation arrangement. It is found that the grain microstructure of mullite fiber is compact but its distribution is not so homogenous. It is also found that partial mullite monocrystalline particles with spheric or dumbbell shape are much larger than surroundings. The chemical composition of mullite fiber is 3Al2O3 .2SiO2 and its crystal structure is recognized as orthorhombic (ao = 0.7490nm, bo -0.9269nm, co = 0.5814nm) which is further verified by HRTEM. There exists a thin discontinues SiO2 film on the surface of the fibers in the composites.2. The solidification of Mullite/Al-Cu composite is non-equilibrium with large amount of (Al2Cu) phase being deposited either at the surface of mullite fibers or at the boundary of -Al. There exists much denser dislocation in the near vicinity of the interface (aluminium side) in as quenched Mullite/Al-Cu composite and a precipitate-free-zone (PFZ) at the interface of aged Mullite/Al-Cu composite. In the Mullite/Al-Cu composite,interfacial reactions have taken place, resulting in the formation of C11Al2O4 and/or CUAIO2 which, along the surface of mullite fiber, grow up toward the matrix. It is found that the magnitude of CUAI2O4 spinels are about 500nm and they are liable of being cracked.3. Non-equilibrium phases of SCACuMg) and (CUAI2) easily deposited at the surface of fibers and/or -Al in as cast Mullite/Al-Cu-Mg composite. There also exists much denser dislocation in the near vicinity of interface (aluminium side) in as quenched Mullite/Al-Cu-Mg composite. MgAC spinels which are the products of interfacial reactions were observed at the fiber/matrix interface of the testing composite.4. Si crystals and (CuAl) phases easily segregated at the surface of fibers and/or a-Al grains in as cast Mullite/Al-Cu-Si composite. It is also found that much denser dislocation exists in the near vicinity of both fiber/matrix interface and Si/matrix interface (aluminium side) in as cast Mullite/Al-Cu-Si composite. Free crystal Si is an in-situ precipitated phase. The bonding of Si to matrix is much firmer in point of crystallization so the interface is clear. Inside crystal Si phase deformed twins appeared after the composite being quenched. Because of relatively higher Si content, the fiber/matrix interfacial reaction in Mullite/Al-Cu-Si composite was restrained, resulting that non-equilibrium eutectic (CUAI2) other than CUAI2O4 spinels were the dominating phases which can be easily observed at the composite interface.