Transmission electron microscopy studies of squeeze cast Al–AlN composites

Abstract

SummaryAluminium-matrix composites containing ,45 vol.% AlN particles were 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 5754) were used. The as-prepared composites were studied by light microscopy, scanning and transmission electron microscopies, and energy-dispersive X-ray spectroscopy. As a result of the melt infiltration process, the composites are very dense and the microstructure shows a homogeneous distribution of the reinforcement. The interfaces are clean with very little porosity. Composites with 2024 and 6060 matrices were carefully studied by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM) after heat treatments. Dislocation density in the matrix of the reinforced material increases due to the difference in thermal expansion coefficients of aluminium alloys and AlN. This can induce an accelerated ageing response of the coherent and semicoherent precipitations of age-hardened matrices. This behaviour has been studied in the 2024 and 6060 composites by using microhardness measurements and TEM. Reactions between the AlN reinforcement and aluminium matrices (6060 and 5754) were observed and analysed by TEM. Matrices containing some of magnesium display a MgAl 2 O 4 spinel formation at the AlN/matrix interface. The spinel formation is probably due to the reaction between magnesium of the matrix and the thin Al 2 O 3 layer on the AlN surfaces. This reaction can affect the mechanical behaviour of the composite infiltrated with the 5754 matrix. This has been confirmed by overageing some samples at high temperatures (300 8C and 550 8C) for 10 days in order to emphasize the interfacial reactions.

TEM Investigation of Metal/Ceramic Interfaces in AA7475/AlN or Al₂O₃ Nano-Composites

“…As an example, reaction between aluminum metal matrix and SiC ceramic particles may cause formation of a brittle and unstable Al 4 C 3 particles deteriorating composite final properties [4]. In case of 2XXX or 7XXX aluminum alloys reinforced with AlN or Al 2 O 3 the possible presence of brittle oxides containing magnesium at the interface also needs further investigation [5].…”