Persistent Slipbands in Fatigued Face-Centered and Body-Centered Cubic Metals

The cyclic stress-strain response, the development of persistent slipbands (PSB's), and the related dislocation microstructures have been investigated on single crystals of face-centered-cubic (fee) copper, nickel, and silver and of body-centered-cubic (bee) α-iron and niobium at room temperature. The formation of PSB's as a bulk phenomenon appears to be a general feature in fee metal single crystals and can be described remarkably well in a simple model based largely on Winter's work. The present work clarifies and emphasizes the special roles of cross slip and of long-range internal stresses in the formation of the matrix structure, in the nucleation of PSB's, and in the steady-state deformation occurring in the PSB's. In pure bee metals, on the other hand, PSB's are not found to be a feature of comparable generality. Under typical testing conditions, cyclic deformation of bee metals occurs by the low-temperature deformation mode, characterized by the low mobility of the screw dislocations and impeded dislocation multiplication. However, a deformation mode similar to that in fee metals, accompanied in some cases by the formation of PSB's as a bulk feature, is found under special conditions: low strain rate, elevated temperature (below the regime of self-diffusion), and the presence of interstitial impurity atoms. In conclusion, some general requirements that appear essential to the development of PSB's in fatigued metals are formulated.