Effects of a tungsten addition on the morphological evolution, spatial correlations and temporal evolution of a model Ni–Al–Cr superalloy

Abstract

The effect of adding 2 at.% W to a model Ni-Al-Cr superalloy on the morphological evolution, spatial correlations and temporal evolution of γ'(L1 2 )-precipitates at 1073 K is studied with scanning electron microscopy and atomic force microscopy. Adding W yields a larger microhardness, earlier onset of spheroidal-to-cuboidal precipitate morphological transition, larger volume fraction (from 20 to 30%), reduction in coarsening kinetics by one third and a larger number density (N v ) of smaller mean radii (<R>) precipitates. The kinetics of <R> and interfacial area per unit volume obey t 1/3 and t -1/3 relationships, respectively, which is consistent with coarsening driven by interfacial energy reduction. The N v power law dependencies deviate, however, from model predictions indicating that a stationary-state is not achieved. Quantitative analyses with precipitate size distributions, pair correlation functions, and edge-to-edge interprecipitate distance distributions gives insight into 2D microstructural evolution, including the elastically driven transition from a uniform γ'-distribution to one-dimensional <001>-strings to eventually clustered packs of γ'-precipitates in the less densely packed Ni-Al-Cr alloy.CK Sudbrack, TD Ziebell, RD Noebe, DN Seidman, "Effects of a tungsten addition on the morphological evolution, spatial correlations, and temporal evolution of a model Ni-Al-Cr superalloy" Acta Mater. (2007), in press. I. IntroductionThe widespread use of γ'(L1 2 )-precipitate strengthened Ni-base superalloys as a turbine blade material in high temperature commercial and military jet engines and land-based gas turbines warrants extensive study of the microstructural evolution and γ'-coarsening behavior in model γ(fcc)/γ' alloys, such as Ni-rich Ni-Al, Ni-Al-Cr and Ni-Al-Mo alloys [1][2][3][4][5][6][7][8][9][10][11][12][13]. In these alloys, elastic anisotropy leads to coherent precipitates evolving from spheres-to-cubes to arrays of cubes or to doublets or octets of smaller precipitates [1,8,[14][15][16][17], as well as face-to-face cube alignment along the elastically soft <100>-crystallographic directions [1,2]. In all cases, the morphology is dictated by the minimization of the interfacial free energy and elastic free energy arising from lattice parameter misfit, while the spatial correlations are controlled by elastic and diffusional interactions among neighboring precipitates. To improve its high temperature performance, modern commercial Ni-base superalloys contain as many as 12 or 13 microalloying elements [18,19]. High-melting point refractory element additions, such as Nb, Re, Ru, W, and Mo, are particularly effective in improving creep resistance, alloy strength, and operating temperature [20]. The chemical and microstructural γ/γ' decomposition that controls these properties is strongly dependent on the complex interplay between elastic interactions of misfitting γ'-precipitates and rate-controlling diffusion of these refractory elemental additions with small diffusivities. The research reported he...

On the nanometer scale phase separation of a low-supersaturation Ni–Al–Cr alloy

“…Advanced models of phase separation are needed to account for the interfacial region between phases. The interfacial region, which has a definite width on the order of $2 nm for this alloy (Figure 4) has been measured to be on the order of several nanometers for other model nickel-based superalloys [3,4,14,[73][74][75][76][77]. The diminution of the -matrix supersaturation values approximately follows the t À1/3 prediction of the KV model.…”

Segregation of tungsten at γ′(L12)/γ(fcc) interfaces in a Ni-based superalloy: An atom-probe tomographic and first-principles study

“…As shown in Fig. 23 Our results suggest that W helps to stabilize the ͕100͖ facets of the ␥Ј-precipitates by preferen-tially reducing the interfacial free energy of these facets with respect to facets involving other ͕hkl͖ planes. Conversely, the energies of the ͕110͖and ͕111͖-interfaces increase by 1 and 9 mJ m −2 , respectively, due to the presence of W, thereby predicting that there should not be W segregation at these interfaces but rather a depletion of W. It is noteworthy that the minimum of the ͕100͖ interfacial energy is obtained for W substituting for Ni at the second ͕100͖-plane on the ␥-side ͑in contrast with the symmetric profiles across the other interfaces͒.…”

Precipitation Model Validation in 3rd Generation Aeroturbine Disc Alloys

“…Further comparison of thermodynamic databases is enabled by previous APT and ICP (Inductively Coupled Plasma atomicemission spectroscopy measurement) studies at Northwestern University of isothermal γ' precipitation in simple Ni-Al-Cr-X model alloys [9][10][11][12][13]. The measured time evolution of particle size suggests the precipitation composition trajectory can be modeled as an unstable equilibrium with capillarity.…”