Structural Stability of Platinum-Group-Metal-Modified γ plus γ' Ni-Base Alloys

The structural and surface stabilities of two experimental gamma-Ni+gamma'-Ni3Al-base alloys containing Pt or Ir were investigated. These alloys are representative of alloys currently being developed to occupy a unique domain with a good combination of high-temperature strength and resistance to oxidation and hot corrosion. Structural characterization included differential thermal analysis (DTA), transmission synchrotron X-ray analysis, precipitate morphology evolution, phase-partitioning behavior, transmission electron microscopy (TEM), dislocation analysis, and isothermal precipitate-coarsening behavior. Electron microprobe investigations showed that Pt partitions largely to the gamma' phase, while Ir partitions more to the gamma phase. As a consequence, the influence of these two elements on the gamma-gamma' lattice-parameter mismatch was quite different. Specifically, synchrotron X-ray analysis confirmed a positive gamma-gamma' misfit in both the Pt- and Ir-modified alloys in the temperature range 700 A degrees C to 1200 A degrees C; however, the Pt partitioning to the gamma' phase resulted in a much larger misfit. The coarsening kinetics of both alloys followed a cubic time dependence and Pt addition was more effective than Ir in slowing the coarsening rate. Thermodynamic predictions about elemental partitioning and about the solidus, liquidus, and gamma' solvus temperatures were made using the software package PANDAT; the results of these predictions were compared with experimental measurements.