Microstructural Changes Produced by Fatigue in High-Purity Copper Severely Deformed by Cryogenic Rolling

High-purity Cu (99.995 pct pure) rods were cryogenically rolled through a 90 pct reduction in area and given a very brief low-temperature anneal. The resultant material was highly textured, had very low internal strains, and had a broad distribution of grain sizes with an average grain size of 150 to 300 nm. Samples were tested in stress-control tension-tension fatigue. Fatigue life was strongly improved over that of coarse-grain (CG) copper. The cycling caused significant softening and grain growth, especially at the lower stress amplitudes. Plastic strain was small but creep was evident. Clusters of long parallel protrusions that were quite uniformly spaced about 100 nm apart formed on the sample surfaces during fatiguing. Among the various microstructures seen in the fatigued specimens, transmission electron microscopy (TEM) pictures taken just inside the surface showed parallel dislocation lines many micrometers in length spaced about 100 nm from one another. It is possible that these dislocations are involved in the formation of the protrusions.