CREEP OF METALS CONTAINING HIGH-VOLUME FRACTIONS OF UNSHEARABLE DISPERSOIDS .1. MODELING THE EFFECT OF DISLOCATION PILE-UPS UPON THE DETACHMENT THRESHOLD STRESS

The high creep resistance of dispersion-strengthened metals is the res ult of a threshold stress, which is determined in existing models by c onsidering the interaction of a single dislocation with dispersoids. T his paper presents a new model which takes into account the effect of dislocation pile-ups upon the detachment threshold stress of dispersio n-strengthened metals. First, it is shown that dislocation pile-ups ar e expected to form at dispersoids when the volume fraction and/or size of the dispersoids is large. Then, the equilibrium dislocation positi ons within the pile-ups are calculated and the resulting shear stress exerted upon the detaching dislocations pinned at the dispersoids is d etermined. Finally, this pile-up stress is added to the athermal detac hment threshold stress determined with existing models to find a total threshold stress. Calculations for aluminum containing 25 vol.% alumi na dispersoids show that the magnitude of the pile-up stress is compar able to the athermal threshold stress, and thus contributes significan tly to the total threshold stress. The model also predicts a creep act ivation energy much higher than that of the unreinforced metal as a re sult of the temperature dependence of the number of dislocations in th e pile-ups. (C) 1997 Acta Metallurgica Inc.