.4ctu rrwler Vol. 45. No. 6. pp. 26Oi- 2620. 1997 mi 1997 Acta Metallure~ca Inc. Published by Elsevier S&e Ltd Printed in Great Britain. All rtphts reserved 1359.6454.‘97 ?I 7.00 + 0.00 Pergamon PII: S1359-6454(96)00367-9 AN INVESTIGATION OF CREEP AND SUBSTRUCTURE FORMATION IN 2 124 Al YONG Lit, STEVEN R. NUTTt and FARGHALLI A. MOHAMED Materials Science and Engineering, Department of Chemical and Biochemical Engineering, Umversity of California, Irvine, CA 92697, U.S.A. (Received IO July 1996) Abstract-The effect of stress on the creep behavior of powder metallurgy (PM) 2124 Al was investigated in the temperature range 618-678 K. In addition, substructure that developed during creep was examined by means of transmission electron microscopy (TEM). The creep data, which extend over seven orders of magnitude of strain rate, show that the apparent stress exponent. n,, for creep is high and variable, and that the apparent activation energy for creep, Q.,, is much higher than that for self-diffusion in aluminum, QD. Analysis of the creep data reveals the existence of a threshold stress, ZO. whose temperature dependence is much stronger than that attributable to the shear modulus. The substructural data inferred from an examination of crept samples suggest that the origin of zo may be related to the interaction between moving 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 equation describing the strong temperature dependence of TO in 2124 Al cannot be accounted for by available threshold stress models, examination of creep data on some dispersion-strengthened (DS) alloys suggests that such an equation may represent general behavior. By considering the effect of n) and its temperature dependence on the creep behavior of 2124 Al, it is shown that the alloy behaves as a class II alloy (metal class). Evidence in support of this finding is provided by two experimental observations: (a) the creep rate in the transient region after a stress increase is faster than the new steady-state creep. and (b) regular arrays of equiaxed subgrains are developed in crept specimens. FJ 1997 Acta Metullqkr Inc. 1. INTRODUCTION Powder metallurgy Al alloys, such as 6061 Al and 2124 Al, have been used as matrices in the development of discontinuous Sic-Al composites. In recent years, the use of discontinuous Sic-Al composites for high-temperature applications has received considerable attention [l-S]. The data obtained in creep studies on these composites have suggested that the high-temperature strength of the matrix may play an important role in determining the strength of the composite [l, 3, 4, 7, 81. Accordingly, systematic investigations on the creep behavior of powder metallurgy Al alloys are essential to the characterization of the creep strength of discontinu- ous SIC-Al composites. In this paper, experimental data obtained in a detailed investigation of the creep behavior of 2124 Al are reported and analysed. 2. EXPERIMENTAL PROCEDURE 2.1. Muterial In the present investigation, 2124 Al manufactured l-Present address: Department of Materials Science and Engineering, University of Southern California, Los Angeles, CA 90089-0241. U.S.A. by powder metallurgy techniques was used. The alloy in the extruded condition was supplied by the Army Materials Technology Laboratory (AMTL). The alloy contained: 4.3 wt% Cu, 1.5 wt% Mg. 0.6 wt% Mn, 0.3 wt% Fe, 0.2 wt% Si, 0.1 wt% Cr, 0.25 wt% Zr, 0.15 wt% Ti, and the balance was Al. The investigation on 2124 Al was motivated by two main considerations. First. the creep characteristics of 2124 Al have not yet been investigated in detail over a wide range of experimental conditions (strain rate and temperature). For example, Nieh et a/. [I] studied the creep behavior of the alloy at a single temperature of 673 K and over a narrow range of strain rates (less than one order of magnitude of strain rate). Second, at the present time there are no data on creep substructure in 2124 Al. Such data are essential to the characterization of the creep behavior of the alloy and identification of rate-controlling deformation processes. 2.2. Mechanical tmting Double-shear specimens of shape and dimensions described elsewhere [9] were used in this investigation. This configuration has three significant advantages: (a) a constant stress condition is maintained under constant load, and a constant strain rate is obtained for a constant rate of cross-head displacement [9], (b) the problem of plastic instability (necking) can be 2607