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INTRODUCTION

THE purpose of this research is twofold. One objective is to characterize the dislocation substructure of copper deformed with the five-power-law regime. The second and primary objective is an investigation of the existence of internal stresses in creep-deformed Cu and Al. A. Creep of Copper Cu was investigated in the past, and it appears that this material obeys a classic five-power-law behavior. Figure 1 reflects much of the early creep work on Cu.[1–8] Several conclusions were apparent from the early work. Usually, the activation energy for metals and alloys over the five-powerlaw regime corresponds to lattice self-diffusion, and a decrease in the activation energy for creep is observed during power-law breakdown (PLB)[9] at roughly 0.5 to 0.6 Tm . It has often been suggested that, within PLB, the decrease in Qc corresponds to a switch to dislocation pipe diffusion with an activation energy of Qp . The rate-controlling mechanism for creep is still dislocation climb. In the case of Cu, the same M.E. KASSNER, Northwest Aluminum Professor, Department of Mechanical Engineering, Oregon State University, Corvallis, OR 97331, is Adjunct Professor, Mechanical and Aerospace Engineering Department, University of California at San Diego, La Jolla, CA 92093-0411. M.T. PE´REZ-PRADO, Researcher, CENIM, CSIC, Madrid, Spain, is Research Associate, Mechanical and Aerospace Engineering Department, University of California at San Diego. M. LONG, formerly Research Assistant, Department of Mechanical Engineering, Oregon State University, is Engineer at Hewlett Packard, Corvallis, OR 97331, and K.S. VECCHIO, Professor, is with the Mechanical and Aerospace Engineering Department, University of California at San Diego. This article is based on a presentation made in the workshop entitled “Mechanisms of Elevated Temperature Plasticity and Fracture,” which was held June 27–29, 2001, in San Diego, CA, concurrent with the 2001 Joint Applied Mechanics and Materials Summer Conference. The workshop was sponsored by Basic Energy Sciences of the United States Department of Energy. METALLURGICAL AND MATERIALS TRANSACTIONS A

arguments by Barrett et al.[2,3] have been applied, except that the transition to climb control by vacancy diffusion via dislocation pipes occurs at relatively high temperatures well within five-power-law behavior, at 0.7 Tm . More recent work by Raj and Langdon[6] suggests that there is a more gradual and less dramatic decrease in Qc that is approximately equal to Qsd over the entire five-power-law regime. Curiously, in Figure 1, both the data above and below 0.7 Tm are reasonably described by five-power-law behavior using a single activation energy, Qsd . The PLB in Figure 1 occurs at about that combination of temperature and strain-rate expected, according to Sherby and Burke.[38] It is recognized that creep experiments using copper are difficult for at least two reasons. One is that discontinuous dynamic recrystallization (DRX) easily occurs at higher temperatures within five-power-law creep.[2,5,10] Thus, re