Techniques for Transmission Electron Microscopy of the Microstructure and Dislocation Morphology Associated with Fatigue Crack Nucleation

Abstract

Fatigue crack nucleation studies By transmission electron microscopy have been limited by the lack of a general technique for sample preparation, and while the work described herein was done with 2024-T4 aluminum alloy, the method may be adapted to a host of metals and alloys.A fatigue sample having the geometry shown in Figure 1 was prepared. The shallow dimple shown was machined to a depth of 12 mils after which it was polished with 600 grit powder to remove burrs. The sample was then carefully heat treated to the T4 condition. The sample was painted with microstop to lines A and B and the central area including the dimple was electropolished removing about 2 mils of metal. Then the sample was mounted in a Sontag fatigue machine and cycled, at an appropriate load, until a starting crack was observed by optical microscopy. Due to the sample geometry cracks usually started at the bottom of the dimple. Some samples were stopped before crack nucleation to observe by transmission electron microscopy precrack microstructure and dislocation morphology.