Unloading yield effects in aluminum alloys
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I.
INTRODUCTION
AN unloading yield effect has been observed previously 1-5 for both fcc and hcp metals. The effect, which is illustrated in Figure 1, can be produced by simply unloading and reloading a sample in the course of plastic deformation. Most previous studies of this effect have been conducted using high purity metal single crystals, e.g., Ni, 1 Cu, 2-s A1,1 Zn,: and Mg.Z Haasen and Kelly 1 interpreted their experimental results in terms of the formation of Lomer-Cottrell sessile dislocations during unloading. Birnbaum, 2 on the other hand, proposed a forest type dislocation interaction which was assumed to be associated with dislocation back stresses. Brydges3 suggested that the effect could be attributed to a nonuniform flow stress distribution in plastically deformed single crystals, and some support for this was provided by Fourie. 4'5 The unloading yield effect was first observed in an aluminum alloy by Greetham and Honeycombe6 in their study of single crystals of A1-4.5 pct Cu. They argued that the effect may be due to precipitation during deformation. Bolling7 also observed a similar phenomenon in alpha brass, but no particular mechanism was advanced to explain the effect. In the present paper we show that the unloading yield effect can also be observed in heat treated polycrystalline aluminum alloys. The plan of the paper is as follows. First, some of the basic characteristics of the effect are described and demonstrated for three alloys. Then a qualitative model is presented to account for these experimental observations. Finally, additional evidence which supports the model is presented and discussed. The terminology that has been used previously to describe the unloading yield effect has not been standardized. The effect has been called a yield phenomenon,1 a yield point effect, ~ a yield drop, 8'9 an unloading yield point, 4 and an unloading yield point effect, z'3 Because the stress does not necessarily fall during reloading, we prefer not to use the term "yield drop". Also, since the effect may occur at any point on the stress-strain curve, we prefer to avoid the use of the term "yield point". Instead we use the term "unloading yield effect" to describe the phenomenon.
T.G. NIEH is Research Scientist with Lockheed Missiles and Space Company, 0/9310, B/204, 3251 Hanover Street, Palo Alto, CA 94304. W. D. NIX is Professor, Department of Materials Science and Engineering, Stanford University, Stanford, CA 94304. Manuscript submitted January 24, 1985.
METALLURGICAL TRANSACTIONS A
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/! I I eu S T R A I N - - ~ Fig. 1 - - T h e unloading yield effect. A sample is unloaded at the point ~,,~r, and subsequently reloaded. On reloading the flow stress rises above the monotonic stress-strain curve, showing the unloading yield effect,
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II.
EXPERIMENTAL
The materials under study are commercially available 2024, 7075, and 6061 wrought aluminum alloys obtained from Aluminum Company of America. Materials were obtained in sheet form with a thickness of 1.6 mm. Tensile samples, with a gage
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