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A study has been made of the relationships between the constitution, microstructure and selected mechanical properties of a series of experimental nickelbase casting alloys designed for high temperature creep,resistance. The investigation started with a study of phase equilibria and selected mechanical properties of a model heat-resistant alloy whose high density precluded its use as turbine blading. Experimental variants were designed by systematically substituting titanium and niobium for tantalum, and molybdenum for tungsten in the parent alloy. From this work two alloys, designated R6 and R8, have shown particular promise with respect to high temperature creep-rupture strength combined with microstructural stability, thermal shock resistance and ability to withstand long-term exposure in high velocity corrosion rig tests. General observations were that the tensile and creep-rupture properties at temperatures up to 104O'C were similar to those of the strongest commercial casting alloys. Unidirectional solidification (UDS) improved the elongation-atfracture values and creep-rupture lifetime of all alloys tested but the level of improvement was dependent on alloy composition and test conditions. The best performance was shown by UDS R8 alloy which at 62 MPa/llOO°C exhibited more than 40°C improvement in 100 hour creep-rupture life over MarM246 alloy. A correlation was obtained between creep-rupture duration and solvus temperature of the gamma-prime precipitate. Regression equations were also developed relating creep-rupture properties to alloy hardness. No correlation was obtained between strength properties and lattice-parameter mismatch between the various matrices and gamma-prime phases. A potentially interesting outcome of the phase equilibria studies has been the testing in bulk form of materials based on alloyed gamma-prime (NisAl). Alloys have been developed that combine good mechanical properties with excellent oxidation resistance and which exhibit good compatibility with aluminidetype coatings.

Experimental Cast Nickel Superalloys and Gamma-Prime Alloys for High Temperature Applications in Gas Turbines