A debris mechanism of cyclic strain hardening for F.C.C. metals

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Abstract
Prismatic dislocation loops (dislocation debris), formed by fatigue or cyclic straining, have been observed in a number of metals. To account for these observations a mechanism of cyclic strain hardening is proposed which depends on the ease of loop formation and the behaviour of the loops after they are formed. In the initial rapid hardening stage, the rate of cyclic strain hardening is determined by the rate of formation of debris obstacles. Since the debris is thought to be formed by some variant of the double cross slip mecachism, changes of variable which lead to easier cross-slip result in higher hardening rates (because the rate of debris obstacle formation is increased). As the crystal fills up with debris, the motion of screw dislocations diminishes and the enforced strain is accommodated by the motion of the prismatic loops already present in the crystal. The basic motion of the prismatic dislocation loops is a flip-flop motion from one stable equilibrium position to another which is reversible and results in a nearly zero work hardening rate. This mechanism provides an explanation for the saturation stage during which the stress no longer increases with continued cycling. A small temperature dependence of the stress associated with saturation may be attributed to prismatic dislocation loop-point defect interactions. The debris model has been found to be consistent with the results of experiments on the effect of temperature on the cyclic strain hardening behaviour of copper single crystals.

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