| The creep properties and microstructural stability of single crystal nickel-base superalloys are of primary importance in the development of new alloys for turbine blade and vane applications. Ruthenium additions to nickel-base superalloys may provide improved high temperature performance relative to current commercial superalloys. As these alloys are required to operate for longer times at higher temperatures, a better understanding of creep and the sources of creep resistance in the high temperature, low stress creep regime will be essential. The objective of this study has been to examine the influence of ruthenium on the high temperature behavior of superalloy single crystals, while gaining an improved understanding of creep mechanisms at high temperatures of single crystal superalloys. This study is unique compared to other recent studies on Ru-containing alloys with respect to the broad matrix of alloys with varying chemical compositions that have been investigated. One result of the compositional variations and Ru additions in the experimental alloys was the considerable difference in gamma' precipitate morphologies, which ranged in shape from spherical to intermediately-shaped to cuboidal. Furthermore, the experimental Ru-containing superalloys had misfits which ranged from positive to near zero to negative. Large variations in the Re partitioning ratios are believed to explain the lattice misfits and resulting precipitate shapes, strongly suggesting that they are influenced by Ru and Cr additions through changes in the gamma-gamma' phase equilibrium. Not surprisingly, large variations in creep behavior between the experimental alloys are observed during creep testing at 290 MPa and 950°C. Investigations of the equilibrium and crept interfacial dislocation networks reveal an excess of dislocations at the gamma-gamma' interface beyond the amount that is necessary to relieve the lattice misfit stresses in the matrix channels. The excess interfacial dislocations and the high temperature lattice misfit on the high temperature creep resistance suggest the strength of the gamma' precipitates is of primary importance in the high temperature creep resistance. |
