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

This work investigated the dislocation arrangements, crystallographic chara cteristics 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 c opper 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 100-11 0 mum, are homogeneously distributed on the whole surface of the crystal an d perpendicular to the SBs. (3) The dislocation patterns within the SBs are often characterized by irregular structures with no persistent feature, in dicating that these SBs are not typical persistent slip bands (PSBs). (4) T he microstructure of the DBs can be classified into two types. One is the r egular, 100% ladder-like PSBs in parallel and can be defined as the develop ing DB; the one is composed of parallel dislocation walls and is named the well-developed DB. (5) With further cyclic deformation, fatigue cracks alwa ys nucleate within the DBs rather than within the SBs or PSBs. Based on the observations above, the crystallographic characteristics and dislocation a rrangements of DBs are discussed in combination with the plastic strain dis tribution and fatigue cracking mechanism within DBs. It is suggested that t here is a transformation of deformation mode from slipping on the (111) pla ne 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 carryi ng high plastic strain can be attributed to the surface roughness caused by the shearing irreversibility of DBs. (C) 2001 Acta Materialia Inc. Publish ed by Elsevier Science Ltd. All rights reserved.