THE FORMATION OF EITHER TONGUE-LIKE OR RIBBON-LIKE EXTRUSIONS IN FATIGUED COPPER POLYCRYSTALS

The surface morphology of polycrystalline copper fatigued at temperatures of 107, 298 and 533 K was investigated by scanning electron microscopy (SEM). Both ribbon- and tongue-like extrusions were observed except after fatigue at 533 K, where ribbon-like, complex and needle-like surface structures were apparent. The dependence of the formation of either ribbon- or tongue-like extrusions on the crystallographic orientation of the grains in the direction of the stress axis was investigated by selected area electron channeling patterns in the SEM. The results suggest strongly that the morphology of extrusions formed at emerging PSBs is controlled by the type and the Schmid factors of the activated slip systems. A model suggested by Polak et al. [Mater. Sci. Engng 74, 85 (1985)] to explain the formation of the tongue-like extrusions was checked for 20 different grains and found not to be valid in general.