High resolution in situ SEM observations of early stages of fatigue crack growth in IN 9052 aluminium alloy

Abstract

A study of the early stages of fatigue crack growth in vacuum from a notch in an ultrafine grain size aluminium alloy (IN 9052) has been carried out. Specimens were tested under cyclic loading at afrequency of 0.2 Hz in the vacuum chamber of a field emission gun scanning electron microscope (SEM) which provides a high spatial resolution of 25 nm. The notch, ∼20 μmlong by 1 μm wide and 2 μm deep, proved to be effective in initiating fatigue cracks in this alloy. The notch was formed by melting the material and ejecting it as vapour. At the bottom of the notch, defects resulting from the rapid solidification of the molten material were found. The combination of these defects and the geometrical stress concentration resulting from the presence of the notch ensure crack formation in association with the notch. Direct in situ SEM observations suggest that the transition from initial mixed mode (stage I) crack growth to mode I (stage II) crack growth in vacuum is discontinuous and intermittent. In addition, the transition can occur when the crack is very small (≤ 500 nm). The observations also suggest that interactions with an aggressive environment are not essential for stage I fatigue crack growth in this material. It was found that fatigue crack rewelding, blunting, and propagation are associated with the fatigue crack growth process. A sequential development of the crack tip during fatigue crack growth in vacuumfor the ultrafine grain size aluminium alloy (IN 9052) is suggested. The implications of these observations for the underlying mechanisms of fatigue crack growth in vacuum for such alloys are discussed.