Some electron microscope observations of alumina whisker/nickel composites

Abstract

Alumina whiskers separated from Thermokinetic Fibres sapphire wool mat type 1A have been incorporated into nickel by roll-bonding and by hot-pressing. The roll-bonded composites containing up to 5 vol % whiskers were successfully electrothinned to allow transmission electron microscope examination of alumina whiskers contained in the nickel matrix. In the hot-pressed composites which contained 20 vol % whiskers, examination of the whiskers was only possible after extraction from the composite. The roll-bonding process resulted in whisker break-up but the whisker segments retained their mono-crystalline form. The whiskers in these composites also retained extensive dislocation tangles in the adjacent nickel matrix after annealing at temperatures up to 1400° C. The whiskers extracted from the as-hot-pressed composites had a monocrystalline form and gave no indication of spheroidisation. In hot-pressed samples annealed at 1400° C for up to 100 h the whiskers exhibited evidence of spheroidisation. In some instances the whiskers developed strong crystallographic morphologies with faces parallel to the directions of the approximated close-packed hexagonal lattice ofα-alumina.