Yaron Amouyal

Research:	Freckle formation in Ni-based superalloys
Education:	Ph.D., Technion - Israel Institute of Technology M.Sc., Technion, Israel
Publications:	Publications by Amouyal in our database
Contact
Dr. Yaron Amouyal Materials Science and Engineering 2220 North Campus Drive Evanston, IL 60208 Phone: 847.467.5698 Email: y-amouyal@northwestern.edu Fax: 847.467.2269

Combination of Atom Probe Tomography (APT) and Density Functional Theory (DFT) to investigate atomistic-level phenomena in nickel-based superalloys

Upon graduation from the Technion – Israel Institute of Technology (Ph.D.), I joined Prof. David N. Seidman group in August 2007 as a post-doctoral fellow. My current research activity is investigating the formation of defects during the solidification of Ni-based superalloys applied for turbine blades in aeronautical jet engines.

One of the most efficient energy conversion devices is the turbine engine, which is utilized for either aeronautical jet engines or natural-gas fired land-based electrical power generators; a single unit can produce up to 500 Mega-Watt. Owing to their excellent high-temperature strength as well as creep and oxidation resistance, nickel-based superalloys are the ideal materials for turbine blade applications. Ni-based alloys derive their properties from their unique microstructure comprising Ni3Al-gamma’(L12)- precipitates coherently dispersed in a Ni-based gamma(f.c.c.)- matrix. The continuing efforts made to increase the thermodynamic efficiency of the turbines, i.e., to obtain a high ratio of energy yield to fuel consumption,require high working temperatures (>1200 ºC). Elevating the service temperature of a turbine engine implies improving the high-temperature properties of these superalloys. In this context,there are several phenomena that are critical for high-temperature performance that are studied by us:

1) Segregation of refractory elements at the gamma/gamma' interfaces correlate with the interfacial free energy, thus related to the precipitates temporal evolution at high temperatures, and affecting the alloys’ mechanical properties.

2) Partitioning behavior of refractory elements to the gamma- and gamma’-phases determines the lattice parameter misfit at the coherent gamma/gamma' interface, thus affecting the alloys’ mechanical properties at high temperatures.

3) A major factor limiting the operating temperature is the formation of chains of misoriented grains, called “freckles”, on the surface of the single-crystal turbine blades during their solidification. Freckles introduce internal interfaces and serve as nucleation sites for micro-cracks as well as short-circuit diffusion paths, thereby reducing creep resistance. Their formation is associated with the solid/liquid partitioning of elements during solidifications, which affects the liquid local density. Eliminating the formation of freckles can be achieved by completely characterizing the alloy's crystallography, morphology, and composition at the micrometer to nanometer length scales.

We apply the latest version of the laser-pulsed three-dimensional Atom Probe Tomography (APT), namely the Local-Electrode Atom-Probe (LEAP) in combination with first-principles calculations based on the density functional theory (DFT).

1) Interfacial segregation of tungsten at the gamma/gamma' interfaces in a Ni-based superalloy

We studied a multi-component alloy called ME-15 having the composition Ni-15.1 Al-7.73 Cr-7.31 Co-1.97 Ta-0.9 Mo-0.75 W-0.46 Re-0.67 C-0.05 Hf (at.%). Transmission electron microscopy (TEM) observations reveal that all of the detected flat gamma/gamma' interfaces possess the same {100} crystallographic orientation. Taking the advantages of both high mass-resolution and detectability for low concentration elements (<500 at. ppm) along with high spatial resolution (<0.5 nm),LEAP analysis enables us to distinguish between different geometries of interfaces and to detect interfacial excess values as small as 1 at/nm^2. LEAP results reveal two classes of gamma/gamma' interfaces: flat, {100}-type and curved, non-{100} interfaces. It is found that the {100}-type interfaces exhibit 1.2 ± 0.2 at/nm^2 Gibbsian interfacial excess of tungsten, corresponding to a 5 mJ/m^2 decrease in their interfacial free energy. In a special case, a double gamma/gamma' interface embracing a gamma-phase layer as thin as 4 nm with two interfacial excess peaks was detected, demonstrating the high spatial resolution that can be obtained using LEAP. Additionally, DFT calculations for a model Ni-Al-W alloy with a {100} gamma/gamma' interface yield a concomitant decrease in the interfacial energy, when a W atom is placed as close as 1 to 3 atomic planes from the interface. Conversely, no measurable segregation of W is detected in LEAP analyses of the curved, non-{100} gamma/gamma' interfaces. Similar calculations for the {110} and {111} gamma/gamma' interfaces predict an increase of 1 and 9 mJ/m^2 in their energies, respectively. This demonstrates how interfacial segregation of W can play a significant role in the microstructural evolution of the gamma’-precipitates. Also, this study indicates how DFT calculations can supplement experimental observations made by LEAP, helping us interpreting them.

Further information:

Y. Amouyal, Z. Mao, D.N. Seidman: "Segregation of tungsten at gamma'(L12)/gamma(f.c.c.)interfaces in a Ni-based superalloy: An atom-probe tomographic and first-principles study". Appl. Phys. Lett. 93 (20), 201905 (2008).

2) Phase partitioning of tungsten and hafnium in Ni-based superalloys [[1]]

We investigate the partitioning behavior of tungsten in several model and multicomponent (>10 elements) Ni-based alloys using APT and first-principles calculations. The alloys investigated are generally comprised of highly-curved gamma/gamma’ interfaces, with complex topologies, separating large gamma’-precipitates and tiny regions of the gamma-matrix, as narrow as 4 nm. Moreover, the elements that we are interested in quantifying have typically low concentrations reaching 0.05 at.%. These conditions require both high spatial resolution (<0.5 nm) in parallel with high mass-resolution and detectability (<500 at. ppm), which are obtained utilizing APT. Furthermore, the analysis of the 3D results obtained from APT enables us studying topologically complex interfaces, projecting the elemental concentrations across these interfaces onto 2D plots, utilizing proximity histograms (“proxigrams”). Proxigrams acquired from 3D-APT observations of the ternary Ni-14 Al-3 W (at.%) alloy indicate that W partitions preferentially to the gamma’-phase. Its partitioning behavior is, however, reversed in favor of the gamma-phase in the ME-9 alloy having the composition Ni-14.6 Al-8.18 Cr- 7.74 Co-1.95 Ta-0.95 Mo-2.31 W-1.47 Re-0.63 C-0.05 Hf (at.%) as well as in many multi-component Ni-based alloys. Having a strong preference for the gamma’-phase as demonstrated by APT, tantalum is assumed to significantly affect the partitioning behavior of W. To supplement our experimental APT results, we utilized DFT-based first-principles calculations as an integral part of our study. We calculated the substitutional formation energies of W and Ta in a model simulation cell, predicting that Ta has a larger driving force for partitioning to the gamma’-phase than does W, suggesting that Ta atoms displace W atoms from their Al-sublattice sites of the gamma’- precipitates into the gamma-matrix in multi-component alloys, in agreement with our 3D-APT results.

Additionally, we focused on understanding the electronic nature of interaction between these two solute atoms and their neighboring atoms, and conclude that hybridization between the p-orbitals of these solute atoms and the d-orbitals of their neighboring Ni atoms in the gamma’- phase is the governing mechanism in their site-substitution. We reached this conclusion based on DFT calculations of the electron localization functions (ELF) and the electronic DOS of the pure and W- and Ta-microalloyed lattices.

Further information:

Y. Amouyal, Z. Mao, C. Booth-Morrison, and D. N. Seidman: "On the interplay between tungsten and tantalum atoms in Ni-based superalloys: An atom-probe tomographic and first-principles study". Appl. Phys. Lett. 94 (4), 041917 (2009)

Y. Amouyal, Z. Mao, and D. N. Seidman: "Effects of tantalum on the partitioning of tungsten between the gamma- and gamma’- phases in nickel-based superalloys: Linking experimental and computational approaches". Acta Mater. 58 (2010) 5898

In a related project, we studied the partitioning behavior of the minority element hafnium (0.05 at.%) to the gamma- and gamma’- phases, and how it is affected by the presence of majority elements. APT results indicate strong partitioning of Hf atoms to the gamma- phase. DFT calculations of the substitutional formation energy of Hf for a model gamma(Ni) / gamma’(Ni3Al) system indicate Hf partitioning to the gamma-phase, which seems to contradict our APT observations. We perform further DFT calculations for the Hf-Cr binding energy by calculating the total energies of Hf- and Cr- doped cells, and subtracting these energies from the total energy of a cell containing a Hf-Cr dimer; these calculations were performed for different nearest-neighbor (NN) positions. The calculated Hf-Cr binding energies suggest, indeed, that Cr atoms, which partition to the gamma- phase, have strong attractive binding interaction with Hf atoms in the gamma-phase, whilst in the gamma’- phase these two atoms repel each other. The result predicted by these DFT calculations is a reversal of the Hf partitioning in favor of the gamma-phase due to alloying with Cr.

Further information:

Y. Amouyal, Z. Mao, and D. N. Seidman: "Phase partitioning and site-preference of hafnium in the gamma'(L12)/gamma(fcc) system in Ni-based superalloys: An atom-probe tomographic and first-principles study". Appl. Phys. Lett. 95 (16), 161909 (2009).

3) Employing APT for studying freckle formation in Ni-based superalloys

We investigate the multi-component alloy ME-15, which was directionally-solidified. The microstructure of the directionally-solidified alloys comprises a 3-level hierarchy. First, the misoriented freckles have a length scale of 0.1-1 mm. Second, both the single-crystalline matrix (SC) and freckles consist of dendritic cores and interdendritic regions with a mean periodicity of ca. 50 μm. Third, these four regions of interest (ROIs) are sub-divided into gamma-matrix/gamma’-precipitates with an average diameter 0.1-1 μm. To study the formation of freckles in this alloy, the composition of these four ROIs should be completely analyzed. To this end, we proposed and applied a novel methodology enabling us to get advantage of the two analytical methods x-ray energy dispersive spectroscopy (EDS) and APT. EDS is capable of analyzing relatively large areas of 10-500 μm, but lacks the adequate detectability for low-concentration (<0.2 at. %) elements. On the other hand, APT has a superior detectability, but is limited to relatively small analysis volumes up to 1000×200×200 nm3. First, we performed compositional analysis of these four ROIs using EDS in the SEM. Only elements having concentration above a certain limit, typically, 0.5 at. %, could be quantified. Second, we employed the lift-out technique using a dual-beam focused ion beam (FIB) to selectively access each of the above ROIs, and prepared specimens for APT analysis. Subsequently, a complete composition analysis of all elements in the gamma- and gamma’- phases at the four ROIs is performed using APT. This work has been sent for publication in Acta Mater.