Volume 51, Issue 7

White etching layers have been found at the surface of water submerged cutting tools and have been analyzed by transmission electron microscopy (TEM). For high resolution investigations, samples had been prepared by sandwich and lift-out preparation. The investigations have shown a grain refinement at the surface. The origin martensitic microstructure is transferred into a nano-crystalline microstructure of domain sizes down to 50 nm. The crystallographic structure was identified by electron diffraction as martensite. Moreover, the depth of the grain-refined layer is not as deep as the white etching phenomenon as it is found in conventional optical micrographs. Thus, the white etching layer is not only caused by the grain refinement.

Co-Re-based alloys being developed for high temperature applications and produced by vacuum arc melting have rather small grain size. Large grain sizes are desirable in such alloys for better resistance to high temperature creep. However, no systematic studies on grain coarsening in Co-Re alloys were conducted so far. In this paper grain coarsening at 1000°C and 1200°C were investigated by using three different Co-Re alloys with and without second phase precipitates. Different methods for grain size measurement including EBSD were employed in this study. Co-Re alloys showed inhibition to grain growth, but microstructural features like precipitates or twins are not responsible for this behaviour. Further investigations are needed to find the real cause but slow diffusion of the heavy rhenium atom in Co-matrix may be responsible.

For better observation of the microstructure of cast aluminium alloys under an optical microscope, the contrast obtained for the different micro constituents was enhanced by making use of a copper phthalocyanine based colorant. The proposed method allows distinguishing structural details which would remain hidden or poorly resolved when studying the conventionally etched sample. The method produces high relief for silicon precipitates with the tint further accentuating the outlines, allowing detection of morphology change of the aluminium-silicon eutectic due to chemical modification with strontium. Magnesium micro segregations in the α-aluminium phase, caused by incorrect thermal treatment during ageing, can also be clearly distinguished. The coloured images obtained by optical microscopy are very sharp and provide similar or better information than the corresponding Scanning Electron Microscopy images. The method proved to be suitable for Quality Control in the fabrication of cast aluminium alloy compounds, sourcing down possible errors to different stages of the process.

Cadmium electrodeposited EN24 steel shear bolts were used in space programme. Recently, one of five bolts subjected to wedge test at 18 Nm torque encountered failure. Fracture surface of failed bolt had two distinct regions, when viewed under scanning electron microscope. Outer region consisted of brittle features while inner region had ductile failure. Detailed investigation revealed that the bolt failure was due to hydrogen embrittlement. Electro-deposition of cadmium was main source for hydrogen entrapment, which could not be compelled out by post plating baking treatment.