Secondary cyclic hardening in fatigued copper monocrystals and polycrystals

Abstract

Copper single crystals oriented for single slip and copper polycrystals were deformed cyclically at constant plastic strain amplitudes (in the range about 10 −3–10 −2) up to large cumulative plastic strains. It was noted for monocrystals and polycrystals that, during prolonged cycling in the so-called cyclic saturation regime, slow but significant microstructural changes continued to occur in the bulk, giving rise to “secondary” cyclic hardening. The observed microstructural changes result from a slow but persistent enhancement of secondary (multiple) slip which leads ultimately to the formation of misoriented dislocation cells. In single crystals the enhancement of secondary slip begins in the persistent slip bands (PSBs) at the PSB-matrix interfaces, causing a gradual transformation of the PSB-wall structure into misoriented cells. In parallel to the microstructural changes, the hysteresis loops become more pointed and, in some cases, a renewed increase in the peak stress occurs. The described secondary cyclic hardening was found to be a general bulk phenomenon whose occurrence can be described by an empirical law. It occurs the earlier the larger the plastic strain amplitude is. In polycrystals, secondary cyclic hardening is observable at lower plastic strain amplitudes and at an earlier stage than in single crystals oriented for single slip.