The Effect of Cooling Rate on High-Temperature Precipitation in a Powder-Metallurgy, Gamma/Gamma-Prime Nickel-Base Superalloy

The effect of cooling rate in the range of 10 K/s to 250 K/s (10 °C/s to 250 °C/s) on the precipitation of secondary gamma prime following supersolvus solution treatment of the powder-metallurgy superalloy LSHR was determined via a suite of critical experiments, analytical and FEM analysis of local temperature transients in test samples, and fast-acting numerical simulations based on classical (homogeneous) nucleation and growth. Using high-resolution scanning-electron microscopy, average 2D precipitate diameters were found to range from approximately 10 to 100 nm in various regions of small cubes that had been water quenched, oil quenched, or air cooled. After applying a stereological correction to estimate the equivalent 3D diameters, the precipitate sizes were plotted as a function of cooling rate deduced from analytical/numerical heat-transfer simulations that had been validated using selected thermocouple measurements. This plot revealed an approximately linear dependence of size on the inverse square root of the cooling rate within the temperature range for which nucleation was initiated and essentially completed. However, the present size-dependence on cooling rate was approximately 60 pct higher than that based on an extrapolation of the trend deduced from previous measurements for slower cooling rates and precipitation simulations over the entire cooling-rate range. Several sources of this difference, including the effect of small local plastic straining on nucleation, were hypothesized. The effect of stored work on precipitation was also underscored in a comparison of laboratory observations and the size of precipitates developed near the joint in inertia-friction-welded LSHR samples whose cooling rate after local supersolvus exposure had been of the order of 150 K/s (150 °C/s).