Anomalous Internal Friction Associated with the Precipitation of Copper in Cold-Worked Al—Cu Alloys

Anomalous internal friction at very small stress levels has been observed in cold-worked aluminum containing one-half percent of copper. This internal friction, which was not observed in high purity aluminum after similar treatment, can be eliminated by annealing at high temperatures so that the specimen completely recrystallizes. This anomalous behavior is shown to be associated with an early stage "precipitation" of copper from the solid solution with aluminum. The temperature range within which such anomalous internal friction was observed is -5^∘{}C to 125^∘{}C when the cold-worked specimen was annealed at 300^∘{}C for one hour. At each temperature within this range the anomalous behavior first increases and then decreases with the aging time at that temperature.The internal friction associated with this behavior is anomalous in the following aspects: (1) At a given temperature the internal friction first increases and then decreases with an increase of stress amplitude even when the stress amplitude is extremely small. There is an optimum stress amplitude at which the internal friction is a maximum. (2) At each stress amplitude there is an optimum temperature of measurement at which this anomalous internal friction is a maximum.These observed phenomena are consistent with the concept that atmospheres of foreign atoms are formed around the dislocations created in the specimen by cold-working. Such an "atmosphere" concept was originally suggested by Cottrell for the explanation of the yield points in iron containing carbon or nitrogen.