|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
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. Common defects occurring during the directional solidification of these single-crystalline blades are long chains of grains that are highly disoriented with respect to the matrix, and are referred to as “freckle chains”. Freckles may cause degradation in the alloy mechanical properties at high-temperatures, and therefore should be avoided. Besides this technological challenge, the persisting effort being made in the technology of jet engines is elevating their working temperature in order to improve their thermodynamic efficiency. The latter implies a demand for improving these alloys high-temperature creep and oxidation resistance. Both problems strongly depend on the alloy composition and morphology in the nanometer length scale. For this purpose, we apply the latest version of the three-dimensional Atom Probe Tomography (APT), namely the Local-Electrode Atom-Probe (LEAP). Our strategy is to produce an array of tips (microposts) from the region of a freckle (several millimeters in size), and to characterized each one of them individually by the LEAP. While a single tip has the lateral dimensions of several micrometers, the typical analysis volume obtained by the LEAP is tens of nm cube (~10M atoms). Additional characterization techniques, such as TEM, are being applied in order to span length scales. In this manner we hope to characterize the morphologies and chemistries of individual defect-rich zones at all pertinent length scales down to the sub-nanometer scale, which may help us understanding the role that each alloying element plays in the nucleation of freckles. This information will be used as input data for ab-initio Molecular Dynamics (AIMD) simulations of the thermodynamics of these alloys, and will enable us to formulate predictive models for the conditions where freckling can be avoided.
Combination of Atom Probe Tomography (APT) and Density Functional Theory (DFT) to investigate atomistic-level phenomena in nickel-based superalloys'
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 30 the ideal materials for turbine blade applications. Ni-based alloys derive their properties from their unique microstructure comprising Ni3Al-g’(L12)- precipitates coherently dispersed in a Ni-based g(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 �/�’ 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 �- and �’-phases determines the lattice parameter mismatch at the coherent �/�’ 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.