EFFECT OF THERMOMECHANICAL PROCESSING ON THE MICROSTRUCTURE AND MECHANICAL-PROPERTIES OF AL-MG (5083) SICP AND AL-MG (5083)/AL(2)O(3)P COMPOSITES .3. FRACTURE MECHANISMS OF THE COMPOSITES/

In this paper, the last of three articles; the fracture mechanisms of as-cast composites, extruded composites and extruded monolithic 5083 a luminum alloy deformed in uniaxial tension are discussed. Dimple fract ure is observed in both the monolithic 5083 aluminum alloy and the ext ruded composites, whereas the fracture of the as-cast composites is ma inly caused by decohesion at matrix/particle interfaces and crack prop agation along dendrite boundaries. For extruded composites the fractur e is mainly nucleated by particle cracking and final failure occurs by shear coalescence of the small void sheets of matrix between the larg e reinforcing particles (or between clusters of particles). The large voids caused by fracture/decohesion of reinforcing particles induce lo calization of deformation in the matrix which accelerates the nucleati on, growth and coalescence of the small voids in the matrix. Localizat ion of deformation is one of the important factors which decrease the ductility of the composites. Three types of composites reinforced by S iC particles, artificially oxidized SiC particles prior to their incor poration and Al2O3 particles are examined. The effects of the interfac ial reactions on the fracture of the composites are also presented.