NANOSTRUCTURAL TEMPORAL EVOLUTION AND SOLUTE PARTITIONING IN MODEL NI-BASED SUPERALLOYS CONTAINING RUTHENIUM, RHENIUM, AND TUNGSTEN

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1 Title of Publication Edited by TMS (The Minerals, Metals & Materials Society), Year NANOSTRUCTURAL TEMPORAL EVOLUTION AND SOLUTE PARTITIONING IN MODEL NI-BASED SUPERALLOYS CONTAINING RUTHENIUM, RHENIUM, AND TUNGSTEN Dieter Isheim 1, Gillian Hsieh 1, Ronald D. Noebe 2, and David N. Seidman 1 1 Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, IL , USA 2 NASA Glenn Research Center; Brookpark Rd., Cleveland, OH 44135, USA Keywords: nickel-based superalloy, ruthenium, rhenium, tungsten, coarsening, transmission electron microscopy, atom-probe tomography Abstract Transmission electron microscopy (TEM) and atom-probe tomography (APT) are employed to study the growth and coarsening kinetics of γ -precipitates and the solute partitioning behavior in a series of model superalloys based on a Ni-8.5Cr-10Al at.% composition with Ru, Ru-Re, or Ru-Re-W additions, during isothermal aging at 800 o C up to 256 h. Substituting Ru for Ni is found to retard the growth and coarsening kinetics of the γ -precipitates; Re and W additions decrease the coarsening rate constant further by an order of magnitude. The exponent of the temporal power law for the precipitate radius is n = (3.3 ± 0.1) and the time exponent for the number density is m = (-0.82 ± 0.06) for all three alloys, independent of coherency strain energy differences between the three alloys. The aluminum concentration in the γ -precipitates is found to decrease continuously with aging time, which is consistent with a nucleation and growth precipitation process. Introduction Refractory metal additions are known to strongly affect the properties of Ni-based superalloys and are employed in commercial alloys to increase solid-solution strengthening, retard precipitate coarsening kinetics, and improve high-temperature creep-resistance [ 1 ]. After the early seminal work by Lifshitz and Slyozov [ 2 ] and Wagner [ 3 ], modeling of coarsening processes has been extended to multicomponent systems [ 4 ] and finite volume fractions [ 5-6 ]. One important result is that the exponent n of the power law of the evolution of the average precipitate radius, <R> n <R 0 > n = kt, (1) is n = 3, independent of the number of components and the volume fraction [4,]. In this article, we study the growth and coarsening kinetics of γ -precipitates and partitioning behavior, in a series of model superalloys containing Ru, Ru plus Re, or Ru plus Re plus W. Ruthenium has been used in Ni-base superalloys as a newer alloying addition with the benefits of increased solid-solution strengthening of the γ-matrix and concomitant reduction in the tendency to form detrimental topologically close-packed (TCP) phases induced by Re and W additions [ 7 ]. This study is part of a systematic experimental program to determine the effects of Ru, Re, W, Ta, and Nb, on the decomposition kinetics of a ternary base alloy, Ni-8.5Cr-10Al at.% [ 8, 9 ].

2 Experimental Methods Three alloys with nominal compositions Ni-8.5Cr-10Al-2Ru at.%, Ni-8.5Cr-10Al-1Ru-1Re at.%, and Ni-8.5Cr-10Al-0.5Ru-0.5Re-1W at.% were prepared by vacuum induction melting and chill cast in 19 mm diameter copper molds. The alloys are referred to in this article by their respective refractory alloying contents as the Ru, the Ru-Re, and the Ru-Re-W alloy. Alloy compositions were verified by ICP atomic emission analyses and found to be close to the nominal compositions. After a homogenization anneal at 1300 o C for 20 hrs, the ingots underwent a solution anneal at a temperature ca o C above the respective solvus temperatures and water quenched. The purpose of the solution anneal was to reduce the concentration of quenched-in vacancies as much as possible. The samples were then aged at 800 o C for times ranging up to 256 h, followed by a water quench. Tips for APT were prepared by electropolishing. APT data evaluation was performed using ADAM, a software package developed at Northwestern University [ 10 ]. TEM was performed employing a Hitachi H8100 analytical TEM at 200 kv. Foils for TEM were electropolished by standard methods [9,10]. The γ -precipitates were imaged by centered dark-field microscopy using a superlattice reflection of the ordered L1 2 structure. TEM micrographs were processed with commercial software, Adobe Photoshop 6.0. Precipitate sizes were determined digitally from the processed negatives using the NIH Image. To obtain true three-dimensional precipitate size distributions (PSDs) and average precipitate radii, a stereological correction procedure outlined by [ 11 ] was applied. The foil thickness, required for performing the stereological correction, was measured at several locations of each foil employing the Kossel-Möllenstedt fringes in convergent-beam electron diffraction patterns [ 12 ]. Results and Discussion Figure 1 displays dark-field TEM micrographs of γ -precipitates in the Ru containing alloy after aging at 800 o C for 4, 16, 64 or 256 h. The average precipitate radius, <R>, increases significantly with aging time, and after 256 h <R> = (82.6 ± 2) nm. The precipitate number density, N V, decreases with aging time, indicating that nucleation is not occuring and that the decomposition is in a growth and coarsening regime. The precipitates in the Ru-Re and Ru-Re-W alloys are significantly smaller than the ones shown in Fig. 1, for all aging times, indicating that Re and W additions retard both growth and coarsening of the γ -precipitates. Figure 2 compares the precipitate morphologies of all three alloys after aging for 256 h where <R> = (36.5 ± 0.9) nm for the Ru-Re alloy and <R> = (32.2 ± 0.8) nm for Ru-Re-W. Since the Ru alloy contains the largest precipitates, aging this alloy for 256 h represents the most advanced decomposition stage investigated. Figure 1. Centered-dark field TEM micrographs of γ -precipitates in Ni-8.5Cr-10Al-2Ru at.% after aging at 800 o C for: (a) 4 h; (b) 16 h; (c) 64 h; and (d) 256 h.

3 Figure 2. Centered-dark field TEM micrographs of γ -precipitates after aging at 800 o C for 256 h for: (a) Ni-8.5Cr-10Al-2Ru at.%; (b) Ni-8.5Cr-10Al-1Ru-1Re at.%; and (c) Ni-8.5Cr-10Al- 0.5Ru-0.5Re-1W at.%. By comparing the precipitate morphologies for all three alloys after 256 h aging, Fig. 2, it is evident that the addition of Re and in particular W leads to the formation of more cuboidal precipitates, Figs. 2 (b) and (c). A cuboidal precipitate morphology indicates the influence of coherency strain energy resulting from a lattice parameter misfit between precipitate and matrix. This effect becomes more pronounced at larger precipitate sizes [ 13 ]. The precipitates in the Ru alloy, in contrast, remain spheroidal, even at larger precipitate sizes, and their morphology therefore is dominated by interfacial energy, indicating a much smaller lattice parameter misfit of the γ -precipitates in the Ru alloy than in the Ru-Re and Ru-Re-W alloys. Strain energy also causes precipitate alignment into chain-like aggregates in the Ru-Re and Ru-Re-W alloys, along the elastically soft <100> directions, Fig. 2 (b) and (c). The Ru alloy, however, does not exhibit precipitate alignment, Fig. 2 (a). The Ru-Re alloy, Fig. 2 (b), represents an intermediate stage with only a slight tendency toward a cuboidal morphology and precipitate alignment. Finally, based on the morphological evidence, W has the largest influence on the lattice parameter mismatch between the precipitates and the matrix. The histogram displayed in Fig. 3 is the stereologically corrected, scaled, and normalized precipitate size distribution (PSD) of the Ru alloy after 256 h aging, with the dotted curve being the PSD expected from the numerical modeling of a system with a 20 % volume fraction, following Akaiwa and Voorhees [ 14 ]. The PSD of this alloy, in an advanced stage of decomposition, exhibits good agreement with the PSD predicted by [14]. Figure 3. Measured particle size distribution of γ -precipitates in Ni-8.5Cr-10Al-2Ru at.% after aging at 800 o C for 256 h (solid line) and the predicted PSD [14] (dotted line) for a volume fraction of 20%. The experimental PSD is based on measurements of 343 precipitates.

4 The poorest agreement between the model and experimental PSDs is found for the Ru-Re-W alloy at the shortest aging time investigated, 4 h. This indicates that the slowest evolving alloy (Ru-Re-W) has, for the shorter aging times, not yet reached a stationary-state coarsening regime and is still in a growth and coarsening phase. The Ru-Re and the Ru alloys exhibit better agreement with the predicted PSD throughout the aging period investigated and thus are closer to a quasi-stationary state coarsening. In general, the experimentally observed PSDs tend to be more symmetrical than the model PSD. A similar behavior has been observed for the ternary Ni-8.5 Cr-10 Al at.% alloy [8] and implies that a true stationary-state coarsening regime is not yet achieved. Even though stationary-state coarsening is only approached asymptotically, we discuss in the following the measured decomposition parameters in the framework of the coarsening models, which assume stationary-state coarsening. Figures 4 (a) and (b) are plots of the cubed average precipitate radius, <R> 3, versus time. A linear regression procedure permits the determination of the coarsening rate constant, k, in Eq (1). For comparison, the data obtained from the ternary Ni-8.5Cr-10Al at.% alloy [8] are plotted in Fig. 4 as well. Substituting 2 at.% Ru for Ni reduces k from (2.9 ± 0.6) x m 3 s -1 [8] by a factor of five to k = (6.0 ± 0.9) x m 3 s -1, Fig. 4(a). Replacing half of this Ru content by Re further reduces k by an order of magnitude to k = (5.1 ± 0.4) x m 3 s -1. Since the <R> 3 values for the Ru-Re and Ru-Re-W alloys are barely visible on the scale of Fig. 4(a), the ordinate in Fig. 4(b) is magnified by a factor of 50. Additionally, substituting W at the expense of Ru and Re decreases k to a value of k = (3.5 ± 0.3) x m 3 s -1. To verify the validity of the temporal power law for the radius evolution, the time exponent is determined employing a linear regression analysis of a double-logarithmic plot of <R> versus aging time. The time exponent is identically n = (3.3 ± 0.1) for the three alloys investigated. This value is close to the ideal time exponent n = 3 for stationary-state coarsening. Since the γ -precipitates in the alloys containing Ru, Re or W span a range of coherency strain energies, it is interesting to note that they all have the same time exponent for the radius evolution, at least for the aging time range investigated. This implies that the time law for radius evolution in these alloys is not strongly dependent on strain energy. In fact, this is frequently the case for Ni-base alloys with γ -precipitates, but not necessarily valid in general [6]. Figure 4. Temporal evolution of the cube of the average precipitate radius, <R> 3, versus aging time at 800 o C for times up to 256 h for Ni-8.5 Cr-10 Al at.% [8] (squares); Ni-8.5 Cr-10 Al-2 Ru at.% (triangles); Ni-8.5 Cr-10 Al-1 Ru-1 Re at.% (circles); and Ni-8.5 Cr-10 Al-0.5 Ru-0.5 Re-1 W at.% (diamonds). The slope of the linear fit for each alloy is the coarsening rate constant. Plot (b) is an enlarged version of (a), magnified by a factor of 50 in <R> 3. The precipitate number density, N V, is predicted to follow a temporal power law with an exponent of m = 1 for stationary-state coarsening [2-6]. Values of m for the three alloys are

5 determined from the slopes of a linear regression in a double-logarithmic plot of N V versus aging time. The time exponents for the Ru, Ru-Re, and Ru-Re-W alloys are identical with m = ( 0.82 ± 0.06). This indicates that the variation in strain energy among the three alloys does not influence strongly the time exponent for the evolution of N V, similar to the temporal evolution of <R>. The measured value is, however, significantly smaller than the value of m = 1, predicted by coarsening models. The evolution of the precipitate and matrix chemistry with aging time was followed by APT. Figure 5 displays the time dependence of the aluminum concentration in the precipitates. Within experimental error, the γ -precipitates of all three alloys show a decrease in Al concentration between the as-quenched state (0 h aging time) and 256 h. The effect is most pronounced for the Ru alloy, where the as-quenched state contains only very small precipitates. The alloys containing Re and W, due to their larger equilibrium volume fraction and driving force for precipitation, have attained in the as-quenched specimens a more advanced decomposition stage with larger precipitates than the Ru alloy. Thus, the excess Al found in the precipitates of these alloys at shorter aging times is not as pronounced. Overall, since the precipitate Al concentration decreases continuously during the course of the decomposition, it is concluded that a nucleation and growth mode is operative, and not a spinodal or spinodal ordering process. Figure 5. Temporal evolution of the Al concentration in the cores of the γ -precipitates, measured by APT, versus aging time at 800 o C for times up to 256 h, for Ni-8.5 Cr-10 Al-2 Ru at.% (circles); Ni-8.5 Cr-10 Al-1 Ru-1 Re at.% (triangles); and Ni-8.5 Cr-10 Al-0.5 Ru-0.5 Re-1 W at.% (squares). Summary The evolution of γ -precipitates in three model nickel-based superalloys, Ni-8.5 Cr-10 Al-2 Ru at.%, Ni-8.5 Cr-10 Al-1 Ru-1 Re at.%, and Ni-8.5 Cr-10 Al-0.5 Ru-0.5 Re-1 W at.%, at 800 o C, has been followed for aging times up to 256 h by TEM and APT. TEM reveals that the precipitates evolve the fastest in the Ru containing alloy, whereas Re and W additions retard the growth and coarsening kinetics by decreasing the rate constant by more than an order of magnitude. Cuboidal precipitate morphology and precipitate alignment along the <100>-type directions indicate the influence of coherency strain energies on precipitate morphology for the Re and W containing alloys. But the time exponents found for precipitate radius evolution, n = (3.3 ± 0.1), is identical for the three alloys and thus not influenced strongly by the elastic strain energy. This time exponent is close to the value of n = 3 predicted by coarsening models. The time exponent for the number density is m = ( 0.82 ± 0.06) for the three alloys and hence

6 unaffected by the variation in strain energy among them. This value is significantly smaller than m = 1 predicted by stationary-state coarsening models. The aluminum concentration in the precipitates is found to decrease continuously with aging time, which is consistent with a nucleation and growth precipitation process, but not with a spinodal process. Acknowledgements Research supported by the National Science Foundation, Division of Materials Research, Grant No. DMR G.H. is an NSF Research Experience for Undergraduate student.

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