College of Engineering
Materials Engineering Department
BS in Materials Engineering
PCC Structurals, an industry leader in superalloy investment castings, has observed inconsistencies in the stress rupture performance of polycrystalline nickel-based superalloy PWA 1455. PCC has changed their casting parameters to reduce the thermal gradient during cooling but have been unable to correlate these changes with an increase in stress rupture elongation. Metallographic examination of past samples indicated microstructures composed of non- equiaxed dendritic grains with mean diameter of .021 inches along the test axis. A similar study on polycrystalline superalloys has indicated that excessive superheat temperatures above the liquidus can result in large grains identical to those observed, limiting the stress rupture ductility. Additionally, the study connected low superheats above the liquidus to an equiaxed, small grain structure. Conclusions from metallographic examination and literature review point to the necessity of lowering the pour temperature and increasing the mold temperature during casting to lessen the average grain size, promoting better stress rupture ductility. The new processing parameters produced an average grain diameter of .014 inches and resulted in sufficient stress rupture elongations, with all samples passing the minimum required by specification. Specimens with the lowest superheat temperature showed a mixed grain structure with equiaxed grains on the boundaries of the test bar and columnar grains on the interior. The observed grain morphology signals the need for additional experiments to produce a fully equiaxed structure and reach peak stress rupture performance.