Volume 53, Issue 4

The use of focused ion beams, whilst permitting the targetted preparation of thin specimens for Transmission Electron Microscopy, also results in modification of the material to be investigated as a result of energy being transferred into the material. This undesirable effect is normally limited to the surface of the material, which is particularly unfavourably orientated towards the impinging ion beam. If the crystal structure and composition of areas close to the surface of such specimens need to be characterised, protective layers may be used. However, those layers, depending on the applied deposition technique, may interact with the sample surface as well thus affecting the results of the analysis. In the work presented here, the possible interactions which might occur between the various protective coatings of ion-beam deposited Platinum, electron beam deposited Platinum, Silicon Oxide or adhesively bonded Gold foil and the subsequent FIB-preparation of the oxide layers on Ni-Ti alloys are investigated, with respect to and how these might affect the TEM-images obtained of areas close to the surface of such specimens. It is shown that the use of adhesively bonded Gold foil as a protective coating, in particular, permits comprehensive characterisation of the surface, including the use of high-resolution TEM, to be carried out, up to the surface of the Oxide layer itself.

This work presents a comparison of selected nickel alloys of the same condition which were treated by means of specifically chosen etching techniques. Microstructures on microscope images of wrought Alloy 617, a casting variant of Alloy 625, a polycrystalline casting alloy IN-738 LC, as well as of a monocrystalline superalloy CM 247 LC SX, respectively, are juxtaposed and evaluated. This approach allows for a comprehensive optical microscopy characterization of the characteristic microstructural features.

Ion beam polishing and ion beam etching have proven their worth in examinations for which mechanical and wet chemical methods can not be applied. The analysis of voids in steel microstructures requires a sample preparation which provides for a surface ablation without mechanical impact on the sample surface. Otherwise, smearing effects prevent the revealing of voids. These voids arise during cold forming. They provide information on the ductile damage which is characterized by the formation, growth, and the merging of cavities in the microstructure. Taking the example of deep drawing steels, modified preparation methods for the imaging of microvoids and nanovoids by means of electron microscopy are presented. The specific resulting surface topography, which is influenced by the ion beam's angle of incidence, the accelerating voltage, and the processing time, is examined in a parameter study. It will be analyzed to what extent the relief structures created by ion beam processing are helpful for or interfere with the examinations. In addition to that, their suitability for the interpretation of microstructural processes in the context of the occurrence and evolution of ductile damages is discussed.

There was a fire incident at a combined cycle engine after almost 100 000 operating hours. The engine was operating on fuel oil at the time of failure. The fire was located at one burner only. An extensive visual inspection revealed that the fire was apparently caused by a leakage due to cracking in that burner's fuel oil premix (FOPM) manifold. The subject component failed due to high cycle fatigue (HCF) cracking. The failure mechanism is purely mechanical. No corrosion was involved, nor any form of stress corrosion cracking (SCC). No evidence of any material or weld imperfections was found. The cyclic loading that caused the cracking was partly reversed bending.