Difference between revisions of "Yaron Amouyal"

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1) Interfacial segregation of tungsten at the gamma/gamma' interfaces in a Ni-based superalloy
+
'''[[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
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
 
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
 
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
Line 42: Line 41:
  
 
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).
 
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]]'''
 +
We investigated 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 �/�’ interfaces, with complex
 +
topologies, separating large �’-precipitates and small regions of the �-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
 +
32
 +
these interfaces onto 2D plots, utilizing proximity histograms or “proxigrams” for short.
 +
Proxigrams acquired from 3D APT observations of the ternary Ni-14 Al-3 W (at.%) alloy
 +
indicate that W partitions preferentially to the �’-phase. Its partitioning behavior is, however,
 +
reversed in favor of the �-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 multicomponent
 +
Ni-based alloys, see Fig. 1 in Ref. 73 Having a strong preference for the �’-phase
 +
as demonstrated by APT, tantalum is assumed to significantly affect the partitioning behavior
 +
of W, see Fig. 2 in Ref. 73 To supplement our experimental APT results, we utilized firstprinciples
 +
calculations as an integral part of our study. We calculated the substitutional
 +
formation energies of W and Ta in a model simulation cell depicted in Fig. 3 of Ref. 74,
 +
predicting that Ta has a larger driving force for partitioning to the �’-phase than does W, see
 +
Fig. 4 of Ref. 74 , suggesting that Ta displaces W from the Al-sublattice sites of the �’-
 +
precipitates into the �-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 �’-
 +
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, see Figs. 10 and 11 of Ref. 74 This topic was
 +
presented in several scientific conferences, Part C, and was also published in Applied Physics
 +
Letters, 73 and more extensively in Acta Materialia. 74
 +
In a related project, we studied the partitioning behavior of the minority element hafnium
 +
(0.05 at.%) to the �- and �’- phases, and how it is affected by the presence of majority
 +
elements. APT results indicate strong partitioning of Hf atoms to the �- phase, see Fig. 1 of
 +
Ref. 60 and also Fig. 6 in this proposal. DFT calculations of the substitutional formation
 +
energy of Hf for a model �(Ni) / �’(Ni3Al) system indicating Hf partitioning to the �-phase,
 +
see Fig. 2 of Ref. 60, 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 Crdoped
 +
cells, and subtracting these energies from the total energy of a cell containing a Hf-Cr
 +
33
 +
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 �-
 +
phase, have strong attractive binding interaction with Hf atoms in the �-phase, whilst in the �’-
 +
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 �-phase due to alloying with Cr. This topic was
 +
published in Applied Physics Letters, 60 and, in an extended form, was accepted for
 +
publication in Acta Materialia. 62

Revision as of 11:01, 30 January 2011

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:
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 superalloyWe 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).

2) Phase partitioning of tungsten and hafnium in Ni-based superalloys We investigated 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 �/�’ interfaces, with complex topologies, separating large �’-precipitates and small regions of the �-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 32 these interfaces onto 2D plots, utilizing proximity histograms or “proxigrams” for short. Proxigrams acquired from 3D APT observations of the ternary Ni-14 Al-3 W (at.%) alloy indicate that W partitions preferentially to the �’-phase. Its partitioning behavior is, however, reversed in favor of the �-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 multicomponent Ni-based alloys, see Fig. 1 in Ref. 73 Having a strong preference for the �’-phase as demonstrated by APT, tantalum is assumed to significantly affect the partitioning behavior of W, see Fig. 2 in Ref. 73 To supplement our experimental APT results, we utilized firstprinciples calculations as an integral part of our study. We calculated the substitutional formation energies of W and Ta in a model simulation cell depicted in Fig. 3 of Ref. 74, predicting that Ta has a larger driving force for partitioning to the �’-phase than does W, see Fig. 4 of Ref. 74 , suggesting that Ta displaces W from the Al-sublattice sites of the �’- precipitates into the �-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 �’- 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, see Figs. 10 and 11 of Ref. 74 This topic was presented in several scientific conferences, Part C, and was also published in Applied Physics Letters, 73 and more extensively in Acta Materialia. 74 In a related project, we studied the partitioning behavior of the minority element hafnium (0.05 at.%) to the �- and �’- phases, and how it is affected by the presence of majority elements. APT results indicate strong partitioning of Hf atoms to the �- phase, see Fig. 1 of Ref. 60 and also Fig. 6 in this proposal. DFT calculations of the substitutional formation energy of Hf for a model �(Ni) / �’(Ni3Al) system indicating Hf partitioning to the �-phase, see Fig. 2 of Ref. 60, 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 Crdoped cells, and subtracting these energies from the total energy of a cell containing a Hf-Cr 33 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 �- phase, have strong attractive binding interaction with Hf atoms in the �-phase, whilst in the �’- 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 �-phase due to alloying with Cr. This topic was published in Applied Physics Letters, 60 and, in an extended form, was accepted for publication in Acta Materialia. 62