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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). [ABSTRACT FROM AUTHOR]

The inertia friction welding behavior of similar (LSHR to LSHR or Mar-M247 to Mar-M247) and dissimilar (LSHR to Mar-M247) alloy pairs was determined. The relationships between the IFW process parameters and welding response, such as the kinetics and efficiency of the transformation of the mechanical energy of the flywheel into heat of joining samples, weld duration, kinetics of flash formation, and bond quality were identified. Conditions resulting in sound welding were determined. It was found that the mechanistic behavior of each of the alloys during dissimilar welding can be approximated by the behavior of the respective alloy during similar welding. By contrast, the microstructure, microhardness and the thickness of the heat affected zone depended on the type of the alloy pair used during IFW. [ABSTRACT FROM AUTHOR]

Microstructure, tensile properties, and fracture behavior of the inertia friction weld joints of dissimilar superalloys, cast Mar-M247 and wrought LSHR, were studied to assess the weld quality. Tensile tests were conducted at 23 and 704 °C on the samples containing different areas of the weld interface of the same welded material. The stress-strain curves were registered at different axial distances from the weld interface. In all tested samples, plastic deformation was localized on Mar-M247 side, outside the heat-affected zone (HAZ), and the resistance to plastic deformation of Mar-M247 increased with a decrease in the distance from the weld interface inside HAZ. Only elastic deformation occurred on the LSHR side. Fracture occurred on the Mar-M247 side, outside HAZ, or at the weld interface. In the latter case, welding defects in the form of clusters of nanometer-sized oxide and carbide particles were observed at the fracture surfaces. These results revealed that the IFW process is capable of producing the weld joints between Mar-M247 and LSHR with the fracture strength higher than that of Mar-M247. However, optimization of the IFW processing parameters is required to minimize clustering of oxide/carbide particles at the weld interface in this alloy pair. [ABSTRACT FROM AUTHOR]

A methodology for determining the preferred site occupancy of various alloying elements within ordered γ′ precipitates was developed and applied to Rene88 samples. The method utilized atom probe tomography and X-ray diffraction techniques with controlled monochromated synchrotron beams to determine element positions. Samples were solutionized at 1423 K (1150 °C) for 30 minutes and cooled at 24 K/min with subsequent aging at 1033 K (760 °C). The synchrotron X-ray diffraction results indicate that niobium prefers to reside on the aluminum sublattice site of the γ′ phase. Additionally, the results indicate that chromium prefers the nickel sublattice sites, while cobalt is likely to occupy both the aluminum and nickel sublattice sites. The X-ray results on the chromium occupancy disagree with atom probe results from the same alloy that indicate that chromium prefers the aluminum sublattice sites. [ABSTRACT FROM AUTHOR]

This research investigated the strengthening mechanisms associated with the as-welded microstructure developed during inertia friction welding of dissimilar superalloys LHSR and Mar-M247. The weld interface and heat affected regions of the sample were analyzed using hardness indentation techniques and subsequently characterized using SEM, TEM and advanced atom probe tomography. The yield strength of the welded joint was modeled to determine the impact of the gradients in the as-welded microstructure on strengthening mechanisms within the LSHR material. Characterization centered on formation of γ′, γ grain size and chemical segregation within the heat affected regions. Results indicate an increased hardness in the vicinity of the weld interface, resulting from the refined dispersion of γ′ and γ grains. [ABSTRACT FROM AUTHOR]