Development of Platinum-Group-Metal Superalloys for High-Temperature Use
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Development of
Platinum-GroupMetal Superalloys for HighTemperature Use
L.A. Cornish, B. Fischer, and R.Völkl Abstract Superalloys based on platinum-group metals are being developed for hightemperature applications. These alloys have two-phase structures comprising either ordered precipitates in a matrix analogous to the nickel-based superalloys or a fine dispersion of oxide particles in a matrix analogous to oxide-dispersion-strengthened nickel-based alloys. Currently, alloys based on iridium, rhodium, and platinum have been obtained. This article reviews the rationale of developments and the progress made in this area. Oxidation and compression tests as well as characterization with scanning electron microscopy and transmission electron microscopy were undertaken. These tests showed encouraging results, and further work is being done on new alloying additions and tensile testing. Keywords: intermetallic alloys, jet engines, platinum, polycrystals, structure, superalloys, ultrahigh-temperature materials.
Introduction Nickel-based superalloys (NBSAs) have been successfully used in turbine components since the late 1940s. Approximately 70% of the weight in a modern jet turbine comprises NBSAs, and their success is due to their high yield stresses and excellent resistance to environmental attack at high temperatures.1 The high yield stresses are possible because the microstructure is composed of fine, coherent, ordered precipitates in a compatible matrix. The precipitates are coherent with the matrix since they have a very similar structure, so that the misfit between the lattice parameters of the matrix and precipitate phases is very low. The precipitates are ordered (specific atom types, e.g., Ni or Al, prefer specific sites), whereas the matrix is random or disordered (the atoms have no site preference). In NBSAs, the precipitates have the L12 structure: they are ordered fcc and are thus very closely related to the fcc matrix phases.
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Alloy deformation is slowed because dislocations must move through many precipitates to pass through the component as stress is applied. Two matrix dislocations with the same Burgers vector, 12101, have to associate in order to form a 101 superdislocation of the L12 ordered precipitate. Inside the precipitates, the superdislocations dissociate again into 12101 superpartials with an antiphase boundary between them. This takes energy, so the dislocations are slowed even further; thus, the alloys are further strengthened. The coherent nature of the precipitates means that the particles are very stable, even at high temperatures, because there is very low surface energy per unit area of precipitate/matrix interface. The surface energy is always low, even for the high surface areas of fine precipitates, and there is very little driving force for particle coarsening. This is very advanta-
geous because the precipitates remain fine and the high strengths of the alloys are retained. If the precipitates did coarsen, the dislocations would move through the material more easily
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