Cross-slip and fatigue in metals at low temperatures

Abstract

The mutual annihilation of jogged screw dislocations approaching one another on non-parallel slip planes is shown to give rise to a type of recovery which accounts satisfactorily for the pronounced decline in the rate of work-hardening observed in f.c.c. metal crystals above a certain shear stress, frequently denoted by τ_III(T). The relation τ_III = (τ_III)o[1 - (mkT/Q)], deduced from the model, where (τ_III)a = τ_III(0), m =30 and Q the energy of vacancy formation, agrees well with the experimental data for aluminium, copper, gold and nickel up to about 0·15 T _m, where T _m (°k) is the temperature of melting. It is suggested that the heat evolved during such dislocation annihilations facilitates the clustering of slip into bands observed at stresses above r _III by catalyzing further destructive interactions, and that condensation of vacancies formed in the process of cross-slip provides the crack nuclei in slip clusters from which fatigue fractures spread. The temperature dependence of the fatigue limit appears to be the same at that of τ_III.