Microstructure and High Temperature Creep of Platinum Group Metal Modified Nickel Base Superalloys.

Abstract

Increasing efficiency in aeropropulsion and energy generation systems drives the development of higher temperature structural materials with good mechanical properties and environmental resistance. Platinum group metal (PGM) additions are known to be beneficial for the oxidation and corrosion resistance of superalloys and Ni rich intermetallics. However, there is limited understanding of how they influence the microstructure and high temperature mechanical properties of γ-γ´ superalloys. The objective of this research is to examine experimental Ni-base compositions that systematically alter PGM and non-PGM elements. The PGMs used for this study are Pt, Ir, and Ru. The non-PGM elements are ones commonly used in conventional Ni-base compositions, such as Re, W, and Ta. Partitioning studies indicates that platinum preferentially partitions to the γ´ phase, resulting in alloys that exhibit positive misfit behavior – quite uncommon for Ni-base superalloys. Iridium influences the partitioning behavior of PGM and non-PGM elements, which could be useful in manipulating alloying chemistry. The unique partitioning behavior of the PGMs results in a wide range of lattice misfit and, therefore, γ´ morphology. Coarsening studies demonstrate that Pt slows γ´ coarsening and helps maintain unusually high volume fractions at temperatures up to 1200 °C. Such high volume fractions at these elevated temperatures could lead to impressive creep properties. High temperature creep tests at 1000 °C demonstrate variations in creep strength between PGM alloys. Alloys with precipitate boundary strengthening elements boron, carbon, and zirconium, can increase the creep resistance by at least a factor of three. For alloys that directionally coarsen, the formation of dense, interfacial dislocation networks increases creep strength. Dislocation density measurements within positive misfit alloys indicate, in some cases, that a majority of the dislocations are deposited within the precipitate phase rather than the matrix phase – a very unusual feature for high temperature deformation. The calculation of a creep stress exponent establishes that creep is carried out in the power-law regime. A climb velocity is calculated that can be applied not only to the PGM alloys in this investigation, but other Ni-base systems as well.