Deformation mechanisms for AC8A/Al2O3(sf) composites over wide ranges of temperature and strain rate

The deformation mechanisms of the AC8A/Al2O3 short-fiber composite and AC8A alloy have been analyzed under the framework of dislocation creep and glid e mechanisms over the wide temperature range 25-450 degrees C and strain ra te range of 10(-5) to 10(3) s(-1). Through careful considerations of variou s characteristic parameters involved in various constitutive equations, suc h as the stress exponent and activation energy, the controlling mechanisms have been assessed and discussed. The deformation of the complex AC8A compo site and alloy seemed to be solely controlled by the dislocation glide + cl imb creep of the Al matrix at 350-450 degrees C and 10(-5) to 10(-3) s(-1), with n similar to 5 and Q(t) similar to 150 kJ mol(-1) governed by lattice diffusion. At lower temperatures of 250-350 degrees C and 10(-5) to 10(-3) s(-1), dislocation glide + climb creep still dominated, with n similar to 5 and Q(t) similar to 83 kJ mol(-1) governed by dislocation core diffusion. At elevated temperatures (250-450 degrees C) and the highest strain rate ( (epsilon) over dot similar to 103 s(-1)), the data could not be explained b y any creep-type process; thermally activated dislocation glide was the gov erning mechanism. As temperature decreased to less than 250 degrees C, ther mally activated dislocation glide was also dominant at all strain rates. In the latter case, the obstacles against traveling dislocations were basical ly immobile dislocations, which can be considered as intermediate-strength, and half-discrete and half-diffuse obstacles. In between these regions, me chanism transition occurred. Even at the highest strain rate of 10(3) s(-1) , the dislocation drag mechanism has not yet occurred for the current mater ials. (C) 1999 Elsevier Science S.A. All rights reserved.