IS CREEP IN DISCONTINUOUS METAL-MATRIX COMPOSITES LATTICE DIFFUSION-CONTROLLED

Creep data for two discontinuous metal matrix composites, namely 3OSiC (p)-Al particulate and 26(Al2O3)(f)Al-5Mg short-fibre composites, were analysed to establish whether their creep is matrix lattice diffusion controlled. It was found that epsilon(m)(1/n)(epsilon(m) is the minim um creep rate) varies with applied stress sigma linearly when the true stress exponent n is set equal to 5. For both composites, the thresho ld stress sigma(TH) was found to decrease rather strongly with increas ing temperature. Good correlation of epsilon(m)/D-L with (sigma-sigma( TH))/G (D-L is the coefficient of lattice diffusion and G is the shear modulus) for both composites strongly suggests matrix lattice diffusi on as the creep-rate-controlling process. However, the origin of thres hold stress has not been identified and thus its temperature dependenc e has not been accounted for. Knowing sigma(TH) as a function of tempe rature and applying a proper constitutive equation of creep, the appar ent activation energy of creep (O) under tilde(c) stress sensitivity p arameter of creep rate m were calculated as functions of applied stres s and temperature. These (O) under tilde(c)(calc) and m(calc) were the n compared with (O) under tilde(c)(exp) and m(exp) following from epsi lon=epsilon(m)(sigma, T) relations as determined experimentally. While for the 3OSiC(p)-Al composite the agreement was found to be good, thi s was not the case for the 26(Al2O3)(f)-Al-5Mg composite.