THERMOMECHANICAL PROPERTIES OF NI-AL2O3 METAL-MATRIX COMPOSITES PRODUCED BY ELECTROFORMING

Ni-Al2O3 metal matrix composites have been prepared by an electroformi ng process. Dispersed oxide particles greatly increase the strength of these materials over those of pure electroformed nickel. We have foun d that the strengthening mechanisms depend on the annealing treatment applied. Without treatment, the composite matrix exhibits compressive internal stresses which are related linearly to the yield strength. Du ring the electrocrystallization process, the particles probably genera te these internal stresses, which increase the dislocation density fol lowing in a way similar to classical cold rolling. When heat treatment s up to 1000 degrees C are applied, the strengthening phenomenon is du e to a particle-induced inhibition of matrix grain growth. This grain growth inhibition causes a strengthening according to the Hall and Fet ch law. This effect is all the more effective as the particle size is smaller. Heat treatment at very high temperature(1200 degrees C) in so me cases results in a solid state reaction between the particles and t he metal matrix, which allows a partial load transfer as commonly occu rs in composites.