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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]

The radial temperature gradient developed via direct-resistance heating of round-bar hot-torsion specimens in a Gleeble machine and its effect on the interpretation of plastic-flow behavior were established using a suite of experimental, analytical, and numerical-simulation tools. Observations of the microstructure variation developed within a γ′-strengthened nickel-base superalloy were used to infer the temperature gradient as well as differences between the temperature at the outer diameter and that indicated by thermocouples welded to the surface. At temperatures of the order of 1375 K (1102 °C), the radial variation of temperature was typically ~20 K (~20 °C). Such variations were in agreement with an analytical heat-conduction model based on the balance of input thermal energy and radiation heat loss at the free surface. Using a constitutive model for LSHR, the effect of the radial temperature gradient on plastic flow during hot torsion was assessed via numerical integration of the torque as a function of radial position for such cases as well as that corresponding to a uniformly-heated sample. These calculations revealed that the torque generated in the non-uniform case is almost identical to that developed in a sample uniformly preheated to a temperature corresponding to that experienced at a fractional radial location of 0.8 in the former case. [ABSTRACT FROM AUTHOR]

The effect of microstructure and soft impingement on the kinetics of static dissolution of γ′ precipitates during supersolvus solution treatment of the powder-metallurgy nickel-base superalloy LSHR were determined experimentally and interpreted in the context of a simple diffusion model. The starting material had either a fine duplex structure of γ grains and γ′ precipitates (each of which were ~2 μm in diameter) or a structure comprising ~2- μm-diameter γ′ precipitates lying within large (~15 μm) γ grains. Using a direct-resistance-heated Gleeble machine, the temperature of each sample was first equilibrated at a fixed subsolvus temperature (at which the microstructure was also stabilized) and then quickly raised to and held for a predetermined time in the single-phase γ field. The dissolution of γ′ was found to be approximately five times as fast for the duplex structure in comparison to the kinetics for the intragranular precipitates. The results were interpreted using numerical simulations of dissolution based on the Whelan model modified to account for a distribution of precipitates (rather than a single, isolated spherical particle) and soft impingement of diffusion fields. Using independently-measured values for diffusivity, precipitate composition, and phase equilibria as input, simulation predictions showed good agreement with the observations in terms of the temporal variation of volume fraction and average particle radius. By this means, insight into the retarding influence of soft impingement and the accelerating effect of pipe diffusion (due to stored dislocations) on the rate of dissolution was obtained [ABSTRACT FROM AUTHOR]

It has been widely assumed, but never proven, that the efficiency of the inertia friction welding (IFW) process is independent of process parameters and is relatively high, i.e., ~70 to 95 pct. In the present work, the effect of IFW parameters on process efficiency was established. For this purpose, a series of IFW trials was conducted for the solid-state joining of two dissimilar nickel-base superalloys (LSHR and Mar-M247) using various combinations of initial kinetic energy ( i.e., the total weld energy, E ), initial flywheel angular velocity ( ω ), flywheel moment of inertia ( I), and axial compression force ( P). The kinetics of the conversion of the welding energy to heating of the faying sample surfaces ( i.e., the sample energy) vs parasitic losses to the welding machine itself were determined by measuring the friction torque on the sample surfaces ( M ) and in the machine bearings ( M ). It was found that the rotating parts of the welding machine can consume a significant fraction of the total energy. Specifically, the parasitic losses ranged from 28 to 80 pct of the total weld energy. The losses increased (and the corresponding IFW process efficiency decreased) as P increased (at constant I and E ), I decreased (at constant P and E ), and E (or ω ) increased (at constant P and I). The results of this work thus provide guidelines for selecting process parameters which minimize energy losses and increase process efficiency during IFW. [ABSTRACT FROM AUTHOR]

The flow-stress behavior at hot-working temperatures and strain rates of the powder-metallurgy superalloy LSHR was determined under nominally-isothermal and transient-heating conditions. Two conventional methods, compression of right-circular cylinders and torsion of thin-walled tubes, were used for isothermal tests. A direct-resistance-heating technique utilizing torsion of round-bar specimens in a Gleeble machine was applied for both isothermal and transient-heating conditions. When expressed in terms of effective stress and strain, baseline data determined by the two conventional methods showed good agreement. With the aid of a flow-localization analysis to assess the confounding influence of axial (and radial) temperature gradients on deformation uniformity, the flow stresses determined from nominally-isothermal Gleeble torsion tests were shown to be broadly similar to those from the conventional tests. With regard to transient phenomena, Gleeble tests were also useful in quantifying the effect of rapid heating and short soak time on the observed higher flow stress associated with a metastable microstructure. The present work also introduces two new test techniques using direct-resistance-heated torsion specimens. One involves continuous heating under constant-torque conditions, and the other comprises testing an individual specimen at a series of temperatures and strain rates. Using a single specimen, the former method enabled the determination of the apparent activation energy for plastic flow, which was similar to that determined from the series of isothermal tests; the latter provided a low-cost, high-throughput approach to quantify the flow behavior. [ABSTRACT FROM AUTHOR]

Differences in the elevated temperature mechanical properties of cast Mar-M247 and forged LSHR make it difficult to produce sound joints of these alloys by inertia friction welding (IFW). While extensive plastic upset occurs on the LSHR side, only a small upset is typically developed on the Mar-M247 side. The limited plastic flow of Mar-M247 thus restricts the extent of 'self-cleaning' and mechanical mixing of the mating surfaces, so that defects remain at the bond line after welding. In the present work, the effect of local preheating of Mar-M247 immediately prior to IFW on the welding behavior of Mar-M247/LSHR couples was determined. An increase in the preheat temperature enhanced the plastic flow of Mar-M247 during IFW, which resulted in extensive mechanical mixing with LSHR at the weld interface, the formation of extensive flash on both the Mar-M247 and LSHR sides, and a sound bond. Performed in parallel with the experimental work, finite-element-method (FEM) simulations showed that higher temperatures are achieved within the preheated sample during IFW relative to its non-preheated counterpart, and plastic flow is thus facilitated within it. Microstructure and post-weld mechanical properties of the welded samples were also established. [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 new metallurgical strategy, high-entropy alloying (HEA), was used to explore new composition and phase spaces in the development of new refractory alloys with reduced densities and improved properties. Combining Mo, Ta, and Hf with 'low-density' refractory elements (Nb, V, and Zr) and with Ti and Al produced six new refractory HEAs with densities ranging from 6.9 g/cm to 9.1 g/cm. Three alloys have single-phase disordered body-centered cubic (bcc) crystal structures and three other alloys contain two bcc nanophases with very close lattice parameters. The alloys have high hardness, in the range from H = 4.0 GPa to 5.8 GPa, and compression yield strength, σ = 1280 MPa to 2035 MPa, depending on the composition. Some of these refractory HEAs show considerably improved high temperature strengths relative to advanced Ni-based superalloys. Compressive ductility of all the alloys is limited at room temperature, but it improves significantly at 800°C and 1000°C. [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]

Compression properties of a refractory multi-component alloy, TaNbHfZrTi, were determined in the temperature range of 296-1473 K and strain rate range of 10-10 s. The properties were correlated with the microstructure developed during compression testing. The alloy was produced by vacuum arc melting, and it was hot isostatically pressed (HIPd) and homogenized at 1473 K for 24 h prior to testing. It had a single-phase body-centered cubic structure with the lattice parameter a = 340.4 pm. The grain size was in the range of 100-200 μm. During compression at a strain rate of έ = 10 s, the alloy had the yield strength of 929 MPa at 296 K, 790 MPa at 673 K, 675 MPa at 873 K, 535 MPa at 1073 K, 295 MPa at 1273 K and 92 MPa at 1473 K. Continuous strain hardening and good ductility (ε ≥ 50%) were observed in the temperature range from 296 to 873 K. Deformation at T = 1073 K and έ ≥ 10 s was accompanied by intergranular cracking and cavitation, which was explained by insufficient dislocation and diffusion mobility to accommodate grain boundary sliding activated at this temperature. The intergranular cracking and cavitation disappeared with an increase in the deformation temperature to 1273 and 1473 K or a decrease in the strain rate to ~10 s. At these high temperatures and/or low-strain rates the alloy deformed homogeneously and showed steady-state flow at a nearly constant flow stress. Partial dynamic recrystallization, leading to formation of fine equiaxed grains near grain boundaries, was observed in the specimens deformed at 1073 and 1273 K and completed dynamic recrystallization was observed at 1473 K. [ABSTRACT FROM AUTHOR]

The effect of minor additions of Sc and Zr on tensile properties of two developmental Al-Zn-Mg-Cu alloys was studied in the temperature range -196 °C to 300 °C. Due to the presence of Sc and Zr in a fine dispersoid form, both low-temperature and elevated temperature strengths of these alloys are much higher than those of similar 7000 series alloys that do not contain these elements. After short holding times (up to 10 hours) at 205 °C, the strength of these alloys is higher than those of high-temperature Al alloys 2219-T6 and 2618-T6; however, the latter alloys show better strength after longer holding times. It is suggested that additional alloying of the Sc-containing Al-Zn-Mg-Cu alloys with other dispersoid-forming elements, such as Ni, Fe, Mn, and Si, with a respective decrease in the amounts of Zn and Mg may further improve the elevated temperature strength and decrease the loss of strength with extended elevated temperature exposure. [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]