Towards inert cores for investment casting

Improvements in material technology allow gas turbines to be run at ever increasing temperatures and speeds. As a consequence, blades in the high pressure part of the turbine are subject to an extremely high level of oxidative attack. These blades are cast from specialized alloys in ceramic moulds which employ internal ceramic cores. One way in which the blades can be made more resistant to corrosion is by the inclusion of low levels of reactive metal elements such as yttrium and lanthanum in the blade alloy. During casting these reactive elements are easily lost from the alloy by reaction with both the core and shell of the mould. This study tests several candidate core formulations for their ability to retain reactive element levels in the alloy after casting, and also explores the mechanisms of reactive element depletion. Through scanning electron microscope studies, a layer of reaction products from the oxidation of the reactive elements was found at the metal-ceramic interface after a simulated casting procedure. The thickness of this layer was found to correlate with the free energy of formation of the ceramic used, in that using a thermodynamically less stable ceramic led to the formation of a thicker layer and thus greater depletion of reactive elements. Electron dispersive X-ray spectroscopy (EDS) confirmed this result by finding lower levels of reactive elements in metal samples heated with less stable ceramics. The use of yttria (Y\(_2\)O\(_3\)) in the core materials was found to increase the retention of reactive elements in CMSX-4