Inhomogeneous positron trapping in fatigued aluminum single crystals

Abstract

Surface fatigue phenomena have been studied in 99.9999% aluminum single-crystal cantilever beams subjected to reverse bending, by using positron annihilation and X-ray diffraction techniques. The coincidence of about the same penetration depth of the surface fatigue damage and of the positrons resulted in a very high sensitivity of the positron technique which, combined with X-ray diffraction, yielded information after single fatigue cycles for a fatigue life of about 107 cycles. A linear positron trapping model based on an inhomogeneous distribution of two different types of traps has been developed by using and extending the principles of the simple linear trapping model for one type of traps. The lifetimes and the relative integrated intensities at annihilation of positrons in pairs of aluminum single-crystal specimens were measured as functions of the number of fatigue cycles during the fatigue hardening stages I and II, the two crystals being subjected to the same strain amplitude of 0.17%. Correlation between the positron results and the corresponding spots of von Laue back-reflections was obtained. In early stages of fatigue, two characteristic position lifetimes, the first being (215±15) ps and the second varying during fatigue from 230 ps to (260±15) ps, have been found and have been interpreted to originate from trapping of positrons at dislocation jog elements and voids/vacancy dipoles, respectively.