Role of Nickel in Al-10 Percent Si Composites Containing Nickel-Coated Sapphire Whiskers

The role of nickel in regard to whisker-matrix bonding in a composite of nickel-coated sapphire whiskers inserted into a matrix of aluminum-10 percent silicon alloy by means of liquid phase hot-pressing was investigated. The study was carried out with the aid of optical and electron microscopy, electron probe microanalysis, and microhardness measurements. Results show that most of the nickel is distributed within the matrix alloy. Some of the nickel apparently interacts with the matrix and forms NiAl₃. The presence of NiAl₃ in this form increases the average hardness of the composite but apparently does not contribute significantly to strengthening of the alloy. Occasionally, clusters or clumps of nickel-rich material which also contains aluminum are found at or very near whisker-matrix interfaces. It is concluded that if any bonding of the nickel to the sapphire occurred, it was in these regions. Finally, a heat treatment to improve nickel to sapphire bonding and hence bonding of the entire composite is suggested.