Vacancy winds in cyclic deformation

Abstract

From these calculations it is shown that if a deformation induced vacancy flux should occur as hypothesized, it will result in a Zn concentration differrence between the center of a dislocation cell and the dislocation cell walls of at least 10 −3 atom fraction, or possibly more in cylically deformed CuZn alloy. The zinc concentration should decrease near the vacancy sinks and increase near the sources. Hence, if such concentration gradients can be shown to develop during steady state fatigue, they will provide evidence as to the dominant direction of vacancy flow during fatigue. Such measurements will also provide insight into the magnitude of the vacancy generation rate and should lend support to one of the theories mentioned in the introduction. Such measurements require concentration determinations at the sensitivity of 10 −4 over a spatial dimension approximately 1/10 of the cell size, or approximately 0.2 μm. A step counting energy dispersive x-ray technique such as available on modern electron microprobes with measurement of Cu and Zn peaks or the use of STEM with energy dispersive x-ray capability should provide the answers. We are in the process of developing collaborative efforts to carry out these experiments. These calculations also estimate the vacancy flux at the cell wall due to the creation of 1 ppm vacancies per second [7] during cyclic deformation as sufficient to generate one unit climb of every dislocation in the cell wall during 35 cycles under typical low cycle fatigue deformation conditions. The rapid dislocation rearrangement and recovery in the cell walls produced by this flux of vacancies lends additional credibility to the cell shuttling model.