Understanding, Developing and Exploiting Cobalt Superalloys for Discs

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In 2005, the discovery of a new intermetallic was announced in Science, Co3(Al,W), together with evidence Co-Co3(Al,W) alloys might have a higher temperature capability than in the Ni-base superalloys used in jet engines. It was claimed that this would allow a new generation of gas turbine alloys to be developed beyond Ni-base superalloys. However, these alloys and those developed since have densities that are too high and oxidation resistance that is too poor for engineering application. Over the past two years, as part of the Rolls Royce - EPSRC Strategic Partnership, two PhD students at Imperial College have demonstrated that a lower density, oxidation resistant Co-base superalloy can be developed, which still has a superior temperature capability.The aim of the current work is to develop the underpinning science underlying the effect of alloying on the creep behaviour of these alloys, which is principally a function of the way in which deformation-related defects called dislocations move through the structure and in particular the way they interact with the intermetallic precipitate. Our recent work has demonstrated that this interface is not sharp and instead has a width that varies with alloying from 0.4 to over 2nm, a factor of over 5X. It is expected that this will increase the rate at which these intermetallic precipitates grow in size - from 20nm to over 200nm - in service, which in turn is a major driver for strength and creep resistance.