EFFECT OF HEAT-TREATMENTS ON PHASE CHEMISTRY OF THE NICKEL-BASE SUPERALLOY SRR-99

The average phase composition of the single-crystal nickel-base alloy SRR 99 after solution treatment, single and double stage annealing, and overaging has been determined by energy dispersive X-ray spectrometry (EDX) (HPGe detector) in a transmission electron microscope (TEM). Calculations of hypothetical nominal compositions for the alloy, using measured compositions of gamma' phase and matrix as a function of the volume fraction of gamma' phase, allow determination of the volume fraction of gamma' phase at the minimum error between nominal composition and hypothetical nominal composition. After single-stage annealing, the gamma' concentrations of W, Al, and Co are higher, and those of Cr, Ta, Ti, and Ni are lower near the phase boundary compared to the average concentration of a precipitate. These concentration gradients and the integral concentrations of element in the gamma' phase are interpreted by the temperature-dependent solubility of elements in the gamma' phase.