MODIFICATION OF THE B2-TYPE MATRIX OF ALUMINIDE DIFFUSION COATINGS ON NICKEL-BASE SUPERALLOYS - BULK ALUMINIDE ANALOGS

Pack aluminide coating is a useful method for conferring oxidation resistance on nickel-base superalloys. Nominally, these coatings have a matrix composed of a Ni-Al based B2-type phase (commonly denoted as beta). However, following high-temperature exposure in oxidative environments, aluminum is depleted from the coating. Aluminum depletion in turn, leads to destabilization of the beta phase, resulting in the formation of a characteristic lathlike beta-derivative microstructure. This article presents a transmission electron microscopy study of the formation of the lathlike beta-derivative microstructure using bulk nickel aluminides as model alloys. In the bulk nickel aluminides, the lathlike microstructure has been found to correspond to two distinct components: L1(0)-type martensite and a new beta derivative. The new beta derivative is characterized and the conditions associated with the presence of this feature are identified and compared with those leading to the formation of the L1(0) martensitic phase.