Abstract
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 <disp-formula> <mml:math> <mml:mrow> <mml:msub> <mml:mrow> <mml:mover> <mml:mi>ɛ</mml:mi> <mml:mi>˙</mml:mi> </mml:mover> </mml:mrow> <mml:mi>s</mml:mi> </mml:msub> <mml:mo>∝</mml:mo> <mml:msup> <mml:mrow> <mml:mo>(</mml:mo> <mml:mo>−</mml:mo> <mml:msub> <mml:mi>σ</mml:mi> <mml:mn>0</mml:mn> </mml:msub> <mml:mo>)</mml:mo> </mml:mrow> <mml:mn>4</mml:mn> </mml:msup> </mml:mrow> </mml:math> </disp-formula> 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.
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