Evolution and microstructural characteristics of deformation bands in fatigued copper single crystals

This work investigated the dislocation arrangements, crystallographic characteristics and fatigue crack initiation of deformation bands (DBs) in a [(5) over bar 12 20] copper single crystal cyclically deformed at high strain amplitude (gamma (pl) = 8x10(-3)). The surface morphology of the fatigued copper crystal was observed to display the following features. (1) There is only one group of fine slip bands (SBs), which seem to carry little plastic strain. (2) Intensive DBs, with a width of 50-60 mum and spacing of 100-110 mum, are homogeneously distributed on the whole surface of the crystal and perpendicular to the SBs. (3) The dislocation patterns within the SBs are often characterized by irregular structures with no persistent feature, indicating that these SBs are not typical persistent slip bands (PSBs). (4) The microstructure of the DBs can be classified into two types. One is the regular, 100% ladder-like PSBs in parallel and can be defined as the developing DB; the one is composed of parallel dislocation walls and is named the well-developed DB. (5) With further cyclic deformation, fatigue cracks always nucleate within the DBs rather than within the SBs or PSBs. Based on the observations above, the crystallographic characteristics and dislocation arrangements of DBs are discussed in combination with the plastic strain distribution and fatigue cracking mechanism within DBs. It is suggested that there is a transformation of deformation mode from slipping on the (111) plane in the developing DBs to shearing on the ((1) over bar 01) plane in the well-developed DBs. Furthermore, the fatigue cracking within the DBs carrying high plastic strain can be attributed to the surface roughness caused by the shearing irreversibility of DBs. (C) 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.