AN INVESTIGATION OF CREEP AND SUBSTRUCTURE FORMATION IN 2124-AL

The effect of stress on the creep behavior of powder metallurgy (PM) 2 124 Al was investigated in the temperature range 618-678 K. In additio n, substructure that developed during creep was examined by means of t ransmission electron microscopy (TEM). The creep data, which extend ov er seven orders of magnitude of strain rate, show that the apparent st ress exponent, n(a), for creep is high and variable, and that the appa rent activation energy for creep, Q(a), is much higher than that for s elf-diffusion in aluminum, Q(D). Analysis of the creep data reveals th e existence of a threshold stress, tau(0), whose temperature dependenc e is much stronger than that attributable to the shear modulus. The su bstructural data inferred from an examination of crept samples suggest that the origin of tau(0) may be related to the interaction between m oving dislocations and dispersion particles; such dispersion particles most likely represent oxide particles that are introduced in 2124 Al as a result of processing the alloy by powder metallurgy. While the eq uation describing the strong temperature dependence of tau(0) in 2124 Al cannot be accounted for by available threshold stress models, exami nation of creep data on some dispersion-strengthened (DS) alloys sugge sts that such an equation may represent general behavior. By consideri ng the effect of tau(0) and its temperature dependence on the creep be havior of 2124 Al, it is shown that the alloy behaves as a class II al loy (metal class). Evidence in support of this finding is provided by two experimental observations: (a) the creep rate in the transient reg ion after a stress increase is faster than the new steady-state creep, and (b) regular arrays of equiaxed subgrains are developed in crept s pecimens. (C) 1997 Acta Metallurgica Inc.