Stress-Change Experiments during High-Temperature Creep of Copper, Iron, and Zinc

The steady creep rate, ε s , for polycrystalline copper at 686, zinc at 385, and iron at 923 K varies as the nth power of the applied stress (σ). The value of n was 4.8 for copper and zinc but was 8.3 for iron. Stress-relaxation tests and constant-stress tests in which the stress was progressively reduced gave values of a ‘friction’ stress, σ0, such that for all materials examined ɛ ˙ s ∝ ( − σ 0 ) 4 In all cases, decreasing the stress by a small amount, (∆σ, ≃ 0.1 σ) during steady-state creep resulted in an incubation period of zero creep rate, after which the creep rate increased to a new steady value. When the stress was reduced by > 0.3 σ, negative creep was observed with polycrystalline copper, but negative creep never occurred with single crystals of copper, even on complete unloading. The results are discussed in terms of a recovery model for high-temperature creep involving the generation of pile-ups of dislocations from sources in the three-dimensional dislocation network.