Dislocation-depth distribution in high-temperature creep
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I.
INTRODUCTION
I N general, the various theories intended to describe creep phenomena usually assume, without proof, that the dislocation density is uniform throughout the cross section. However, there have been numerous experimental observations that show that during low-temperature creep the dislocation density is not uniform and that a surface layer with a higher dislocation density than the interior exists. In an early experiment, it was noted that the creep rate increased over 100-fold when the surfaces of Au, A1, and Cu were electrochemically polished at room temperature to remove about 15/~m per minute. ~ In these experiments, care was taken to prevent heating effects. Accompanying the increase in the creep rate, there was a large decrease in the apparent activation energy, an increase in the activation volume, and an increase in the dislocation velocity exponent. 2'3 The creep rate can also be decreased markedly by surface alloying treatments. 4'5 By diffusing A1 into the surfaces of specimens of Ti (6 A1-4V) to a depth of 40/xm, the steady state creep rate at a stress of 586 MPa at 563 K was decreased from 1.66 x 10 -7 to 4.3 • 10 9 per minute. Similar effects were observed when Cu was diffused into the surface of aluminum 7075-T6. In this case, the steady state creep rate at 422 K and at a stress of 240 MPa was reduced by a factor of 54. In addition, the transient creep region was decreased and in some cases practically eliminated. When the coating was not diffused into the specimen, there was no change in the creep behavior. From these observations it appeared evident that the creep behavior at elevated and low temperatures was greatly influenced by the reduction in the operation of near surface dislocation sources and/or possibly by the reduction of the egress of dislocation by solid solution hardening of the surface region. The evidence that near surface sources are operative before those in the interior is supported by experimental and theoretical considerations. Using etch pit measurements of lightly deformed Cu, Vellaikal and Washburn 6 clearly demonstrated that plastic deformation occurs in the surface region but not in the interior. Others observed from X-ray line broadening measurements on single crystals of aluminum fatigued at stresses in the Stage I region that the dislocation density increased in the region of the surface and not in the interior. 7 From
I. R. KRAMER, Research Professor, C. R. FENG, Research Associate, and R.J. ARSENAULT, Professor and Director, are all with the Metallurgical Materials Laboratory, University of Maryland, College Park, MD 20742. Manuscript submitted December 5, 1983. METALLURGICAL TRANSACTIONS A
theoretical considerations it was concluded that the stress required to operate dislocation sources near the surface is much lower than the stress required to operate internal sources. 8'9 Alekseev, 8 in a theoretical study, found that the rate of multiplication of dislocations by near surface sources is an order of magnitude greater than the corresponding rate of
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