Yaron Amouyal

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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


Dr. Yaron Amouyal
Materials Science and Engineering
2220 North Campus Drive
Evanston, IL 60208
Phone: 847.467.5698
Fax: 847.467.2269

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

I graduated from the Technion – Israel Institute of Technology (Ph.D.) and joined Prof. David 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 hightemperature 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 were 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 mismatch 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 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.%). TEM observations revealthat 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/nm2. 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 �- phase layer as thin as 4 nm with two interfacial excess peaks was detected, demonstrating the high spatial resolution that can be obtained by the 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 3D APT 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. This study indicates how DFT calculations can supplement experimental observations made by LEAP, helping us interpreting them. For further information see:

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).