Transmission electron microscopy studies of squeeze cast Al-AlN composites

Aluminium-matrix composites containing similar to 45 vol.% AlN particles we re fabricated by melt infiltration of aluminium into an AlN preform under a pressure up to 130 MPa. Three types of aluminium alloy (2024, 6060 and 575 4) were used. The as-prepared composites were studied by light microscopy, scanning and transmission electron microscopies, and energy-dispersive X-ra y spectroscopy, As a result of the melt infiltration process, the composite s are very dense and the microstructure shows a homogeneous distribution of the reinforcement. The interfaces are clean with very little porosity. Com posites with 2024 and 6060 matrices were carefully studied by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) af ter heat treatments, Dislocation density in the matrix of the reinforced ma terial increases due to the difference in thermal expansion coefficients of aluminium alloys and AlN. This can induce an accelerated ageing response o f the coherent and semicoherent precipitations of age-hardened matrices. Th is behaviour has been studied in the 2024 and 6060 composites by using micr ohardness measurements and TEM. Reactions between the AIN reinforcement and aluminium matrices (6060 and 5754) were observed and analysed by TEM. Matr ices containing some of magnesium display a MgAl2O4 spinel formation at the AlN/matrix interface. The spinel formation is probably due to the reaction between magnesium of the matrix and the thin Al2O3 layer on the AIN surfac es. This reaction can affect the mechanical behaviour of the composite infi ltrated with the 5754 matrix. This has been confirmed by overageing some sa mples at high temperatures (300 degreesC and 550 degreesC) for 10 days in o rder to emphasize the interfacial reactions.