Fatigue of copper single crystals in vacuum and in air I: Persistent slip bands and dislocation microstructures

Abstract

Copper single crystals oriented for single slip were fatigued (to failure) at room temperature at a constant plastic resolved shear strain amplitude of 2 × 10 −3 in high vacuum (about 10 −5 -10 −4 mbar) and, for comparison, in air. The fatigue life in high vacuum was found to be larger by a factor of 15–30 than that in air. Up to the number of cycles to failure in air (about 10 5) the cyclic deformation behaviours in air and in vacuum were similar in all respects investigated. In particular, no significant differences were observed with regard to the distribution, surface features and dislocation microstructures of the matrix and persistent slip bands (PSBs). At this stage, evidence of secondary slip and dislocation cell formation was observed in most PSBs. After continued fatigue in high vacuum (up to (2–3) × 10 6 cycles), a secondary cyclic hardening stage occurred after about 10 6 cycles, and the surfaces were almost completely covered with PSB traces. The dislocation microstructures consisted almost exclusively of mis-oriented dislocation cells arranged in slabs parallel to the primary glide plane. It was inferred that, during the process of secondary cyclic hardening, old PSBs harden gradually by conversion into dislocation cell structures and are continuously replaced by new PSBs which form out of the matrix until the specimen is filled with (old and new) PSBs. Thus the homogeneous surface coverage with PSBs after extensive fatigue in high vacuum is a consequence of the prolonged fatigue life and not its cause. It is concluded that the major reason for the fatigue life enhancement in vacuum lies not in the impeded development of the crack embryos but in the retardation of the growth of these embryos and in the delayed subsequent crack growth.