CREEP-BEHAVIOR OF A 2124 AL-ALLOY REINFORCED BY 20 VOL.PERCENT SILICON-CARBIDE PARTICULATES

The creep in a 2124 Al alloy reinforced by 20 vol.% silicon carbide pa rticulates-2124 Al-20 SiCp particulate composite-is investigated in a temperature interval ranging from 623 to 748 K and at applied shear st resses ranging from 2.5 to 40.0 MPa, depending on the testing temperat ure. The double shear creep test technique is used. it is shown that t he composite exhibits the threshold creep behaviour at least up to a t emperature of 698 K. Up to this temperature, the true activation energ y of creep is very close to the activation enthalpy of matrix lattice diffusion and the value of the true applied stress exponent of minimum creep strain rate is almost exactly equal to five. The threshold stre ss decreases linearly with increasing temperature and, if extrapolated to temperatures above 700 K, it disappears at about 735 K. At 748 K, and even at 723 K, the apparent applied stress exponent of minimum cre ep strain rate increases with applied stress and depends slightly on t emperature. But. most probably, the creep strain rate remains to be ma trix lattice diffusion controlled. It is assumed that the origin of th reshold stress is closely related to its temperature dependence and to its disappearance at a relatively well defined high creep testing tem perature. An attempt is made to account for the origin of threshold st ress, accepting the concept of load transfer to particulates and the l ong range interval back stresses generated in creep in the composite m atrix, which reduce the load transfer to a value approximately indepen dent of applied stress at any given temperature, except at high temper atures where the internal back stress is very close to the applied str ess. Under the latter conditions the threshold stress can be expected to disappear. (C) 1998 Elsevier Science S.A. All rights reserved.