Thermally Stable Nanometals by Predictive Atomic Scale 

Interfacial Energy Reduction

by Dr. Kris Darling, Dr. Laszlo Kecskes, Professor Zi-Kui Liu,

Professor Elizabeth Dickey, and Dr. Jing Li

ARL-TR-5490 March 2011

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Army Research Laboratory

Aberdeen Proving Ground, MD 21005-5069

ARL-TR-5490 March 2011

Thermally Stable Nanometals by Predictive Atomic Scale 

Interfacial Energy Reduction

Dr. Kris Darling and Dr. Laszlo Kecskes

Weapons and Materials Research Directorate, ARL

Professor Zi-Kui Liu, Professor Elizabeth Dickey, and Dr. Jing Li

Dept. of Materials Science & Engineering

The Pennsylvania State University, University Park, PA 16802

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Thermally Stable Nanometals by Predictive Atomic Scale Interfacial Energy 

Reduction

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Dr. Kris Darling and Dr. Laszlo Kecskes, Professor Zi-Kui Liu, 

Professor Elizabeth Dickey, and Dr. Jing Li

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14. ABSTRACT

Nanocrystalline iron (Fe) alloys doped with 1 at.% zirconium (Zr), tantalum (Ta), and nickel (Ni), produced through high 

energy mechanical alloying, were investigated for microstructural stability. The results were consistent with theoretical 

predictions, indicating a decrease in stability with solute type in the following order: Zr, Ta, and Ni. Preliminary transmission 

electron microscopy (TEM) investigations of the most stable system, Fe-Zr, revealed the presence of 2.5 at.% Zr within the 

grain boundaries of the sample, confirming a thermodynamic tendency for stability. More specifically, Zr segregation was 

found to be inhomogenously distributed between grain boundaries of nanostructured regions. Such a distribution accounts for 

and provides a potential mechanism for the appearance of abnormal grain growth at elevated temperatures. Additional 

compositional profiling uncovered a new FCC phase, within the abnormally grown grains. This phase is coherent with the 

BCC Fe substrate matrix and has a lattice parameter of 8.5 Å.

15. SUBJECT TERMS

Solute Segregation, Nanocrystalline, Thermodynamic Stability, Abnormal Grain Growth 

16. SECURITY CLASSIFICATION OF:

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Kris Darling

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iii

Contents

List of Figures iv

List of Tables v

1. Objective 1

2. Approach 1

2.1 Production and Initial Characterization of the Nanocrystalline Alloys ..........................2

2.2 Advanced Micro Structural Analysis..............................................................................2

2.3 Atomistic Modeling.........................................................................................................3

2.4 Milestones .......................................................................................................................3

3. Results 3

4. Conclusions 16

5. References 17

6. Transitions 18

List of Symbols, Abbreviations, and Acronyms 19

Distribution List 20
iv

List of Figures

Figure 1. Plot of Fe interfacial energy vs. interfacial solute concentration for various solutes.....4

Figure 2. Grain size vs. annealing temperature for various Fe alloys.............................................5

Figure 3. Optical micrographs for pure Fe after annealing 1 h at the given temperatures. 

Percent area abnormally grown is indicated, respectively.........................................................6

Figure 4. Focused ion beam contrast images for pure Fe after annealing at 616 °C. .....................6

Figure 5. Optical micrographs for Fe 1 at.% Ni after annealing 1 h at the given temperatures. 

Percent area abnormally grown is indicated, respectively.........................................................7

Figure 6. Focused ion beam contrast images for Fe 1 at.% Ni after annealing at 700 °C. .............7

Figure 7. Optical micrographs for Fe 1 at.% Ta and Fe 1 at.% Zr after annealing 1 h at the 

given temperatures. Percent area abnormally grown is indicated, respectively. ......................8

Figure 8. Focused ion beam contrast images for Fe 1 at.% Ta and Fe 1 at.% Zr after 

annealing 1 h at the given temperatures....................................................................................8

Figure 9. Plot showing the percent area remaining nanocrystalline for the various alloys vs. 

annealing temperature................................................................................................................9

Figure 10. Bright and dark field TEM images of Fe 1 at.% Zr annealed at 900 °C. Images 

show a single micron-sized grain and a region containing nanocrystalline grains.................10

Figure 11. High resolution TEM image of Fe 1 at.% Zr annealed at 900 °C giving regions 

where EDS was performed to quantify Zr solute content in the boundary region. ................11

Figure 12. Corresponding solute profile for figure 11 (Series 1). ................................................11

Figure 13. High resolution TEM image of Fe 1 at.% Zr annealed at 900 °C giving regions 

where EDS was performed to quantify Zr solute content in the boundary regions between 

nanograins................................................................................................................................12

Figure 14. Corresponding solute profile for figure 13 (GB2).......................................................12

Figure 15. Bright and dark field TEM images of Fe 1 at.% Zr annealed at 700 °C. Images 

show elongated grain with a single micron-sized grain and a region containing 

nanocrystalline grains with an average aspect ratio of 10:1. ...................................................13

Figure 16. High resolution TEM image of Fe 1 at.% Zr annealed at 700 °C giving regions 

where EDS was performed to quantify Zr solute content in the boundary region. ................13

Figure 17. High resolution TEM images of Fe 1 at.% Zr annealed at 900 °C giving figure 17: 

Precipitates contained within abnormally grown grain. (a) Precipitates showing average 

size of 5 nm, (b) a larger precipitate showing coherency, and (c) a Fourier filtered high 

resolution TEM image using the second phase pattern in (b), showing a superlattice of 

FCC <110> (a0 = 8.5 Å), which is overlapped on the -Fe <001> lattice. .............................14

Figure 18. High resolution TEM images of Fe 1 at.% Zr annealed at 900 °C giving 

precipitates contained within abnormally grown grain. EELS spectra (a) and (b) for 

nanocrystals given in the high resolution TEM image above..................................................15
v

List of Tables

Table 1. Thermodynamic parameters for select solutes in Fe. .......................................................4
vi

INTENTIONALLY LEFT BLANK.
1

1. Objective

Our objective is to understand the electronic contribution to solute/solvent systems to enable a 

more robust methodology to predict future alloy systems, which cannot be achieved with existing 

strategies. Particularly, we intend to use state-of-the-art atomic-level characterization techniques 

to identify and determine the electronic contributions responsible for reducing and minimizing 

the interfacial energy of high-temperature nanocrystalline alloys relevant to U.S. Army 

applications. It is the predictive power of this new model, made possible by knowledge of the 

electronic structure, which, in turn, will allow for a non-empirical approach to alloy 

development.

Initially, the transmission electron microscopy (TEM) and analysis aspects of the effort were to 

be contracted to Oak Ridge National Laboratory. The initial kick off meeting at Oak Ridge took 

place and contract processing was initiated. Samples were sent such that work could begin once 

the contract was in place. After a few months of inconsistent communications and the failure to 

complete the contract initiation forms, a new source was identified. As such, funds were 

diverted to Prof. Zi-Kui Liu at Pennsylvania State University, and the contract process was 

restarted. Additionally, because the samples from Oak Ridge could not be recovered, new 

samples had to be made. Funds were released to Pennsylvania State University on August 27,

2010. The objectives have remained the same and experiments are currently underway.

2. Approach

Interfacial energy reduction has been suggested to be a superior method to prevent grain growth 

in nanocrystalline materials, as compared to kinetic pinning of grain boundaries by secondary 

phases. Since grain boundary mobility follows an Arrhenius behavior, all kinetic pinning 

processes are temperature dependent and temperature limited. Recently, it was reported by Krill 

et al. (1) that the addition of zirconium (Zr) to palladium (Pd) led to an alloy that retained its 

nanocrystalline microstructure (<100 nm) up to ~1500 °C, or 95% of its melting temperature. A 

large interfacial energy reduction of approximately 70% was cited as the reason for the thermal 

stability. In particular, the release in elastic enthalpy upon Zr solute segregation to the interfacial 

boundaries was identified as the actual mechanism responsible for the large energy reduction. 

Additional experimental studies on interfacial energy reduction, via grain boundary segregation 

of solutes, have been reported, e.g., niobium (Nb)-copper (Cu), ruthenium aluminum (RuAl)-iron 

(Fe), yttrium (Y)-Fe, titanium oxide (TiO2)-calcium (Ca), etc. (2–8). However, these experiments 

have little impact on the interests of the U.S. Army.
2

Instead, as a first principles demonstration of the solute prediction method, we proposed to 

develop stable nanocrystalline alloys by the additions of select solutes to Fe, which would

exhibit extraordinary thermal stability. Elemental additions to the selected base metals would

result in engineering alloys that will exhibit a large interfacial energy reduction as the sole means 

of thermal stability (excluding kinetic effects). For comparison, further elements were to be 

added to the base metal(s) to demonstrate that little or no interfacial energy reduction would 

result if certain selection rules are ignored. Specifically, other parameters such as the electro 

negativity difference, free surface energies difference, solubility limits, heats of mixing (+/-), and 

atomic size difference are not taken into consideration. However, in this study, they will be 

varied. More importantly, the electronic contributions of the solute at the interface, as it pertains 

to interfacial energy reduction, have largely been ignored. To our knowledge, there are no 

ongoing experimental efforts that have investigated the effect of electronic contributions to 

interfacial energy reduction. This is despite the large influence of these contributions on grain 

boundary attributes. In part, these effects have been ignored because the characterization of 

electronic structure, due to solutes in grain boundaries at the nanoscale, is an extreme challenge 

in itself. Specifically, the prediction of the solute atom in any base metal system entails an 

intricate knowledge of the mechanisms contributing to the total free energy change of the 

system, including characterization of the bulk/surface impurity sites, analysis of possible 

relaxation states around the impurity atom, core level binding energy shifts, and partitioned 

electron density and orbital hybridization. 

2.1 Production and Initial Characterization of the Nanocrystalline Alloys

High energy mechanical milling was used to alloy and reduce the microstructure to the nanoscale 

(<50 nm) regime. Isothermal annealing studies were performed, along with x-ray line 

broadening measurements and TEM in order to estimate the thermal stability. 

2.2 Advanced Micro Structural Analysis

The grain boundary and interfacial structure was analyzed using electron energy loss 

spectroscopy (EELS). More specifically, energy loss near edge structure (ELNES) analysis, in 

combination with Z-contrast imaging, is used as the main characterization method in addition to 

energy dispersive spectroscopy (EDS). Such analysis is an effective way to separate differences 

in electronic structures at the interface and how it relates to the overall interfacial energy 

reduction of the specific alloy system. Additionally, such techniques can give information on

how much solute resides in the grain boundaries; how the solute is distributed 

(homogeneously/in homogeneously) among the grain boundaries; and if Zr solute is distributed 

uniformly, whether the interfacial energy reduction is homogenous.
3

2.3 Atomistic Modeling

In upcoming work, atomistic models are to be used in combination with density functional 

theory (DFT) to yield the energy and atomic configurations of the grain boundaries, their 

associated electron distribution (charge density) site, and momentum resolved density of states

and ELNES structures. Modeling and calculations of ELNES structures will be performed for all 

experimental cases, using supporting EELS data and will be compared to the electronic 

structures of the grain boundaries, with or without the presence of solute atoms.

2.4 Milestones

The following are the milestones for this effort:

• First Quarter: Fabricate solute-stabilized metal specimens (completed).

• Second Quarter: Conduct atomic characterization to extract electronic contributions to 

interfacial energy reduction; interim report (ongoing).

• Third Quarter: Build a DFT simulation using experimentally derived electronic

contributions (to be performed).

• Fourth Quarter: Match the model to experimental results and use the model to predict 

new system; final report (to be completed).

3. Results

The current model, equation 1, used to estimate the effect of various solutes on decreasing grain 

boundary energy for a given system, is given below. The model takes into consideration the 

elastic enthalpy change, ∆Eel, and the change in cohesive energy for both the lattice and grain 

boundary area: 

*

2 * 1

17

6 3 2

s s

A A Fe mix

o A el

x H

x E    



                   

. (1)

γ is the effective grain boundary energy after solute segregation; γ0 is the initial grain boundary 

energy;

*

A

x

is the interfacial solute concentration;



is the molar interfacial area and is equal to 

vB

2/3 NAVG, where vB is the atomic volume and NAVG is the Avogadro number; and ∆Hmix is the 

equimolar (xA = xB) enthalpy of mixing in the liquid state. Based on equation 1, solutes were 

selected (table 1). Using these values (7) in equation 1 provides the graph shown in figure 1. 
4

Table 1. Thermodynamic parameters for select solutes in Fe.

Figure 1. Plot of Fe interfacial energy vs. interfacial solute concentration for 

various solutes.

Figure 1 depicts the effectiveness of the given solutes for decreasing the grain boundary energy 

in nanocrystalline Fe and suggests the addition of Ni to Fe does not stabilize the microstructure. 

However, tantalum (Ta), Zr, and Y appear to have a large effect on reducing the interfacial

energy, and hence, stabilizing the microstructure. Y is excluded experimentally, as grain 

boundary embrittlement would be a concern; this is based on Hondros’s correlation (8). 

Mechanical alloying was used to create solid solutions of select alloys, those being Fe 1 at.% 

nickel (Ni), Fe 1 at.% Ta, and Fe 1 at.% Zr. The idea being that upon annealing, solute would 

either be incorporated into the grain boundaries as they expanded or be interfacially segregated

there. Figure 2 gives the x-ray estimated grain size of the given alloys versus annealing 

temperature. Fe with 1 at.% Ni follows the same grain growth trend as pure Fe, whereas
5

Fe 1 at.% Ta and Fe 1 at.% Zr show significant reduction in grain growth. These experimental 

results are in good agreement with the theoretical predictions that were given graphically in 

figure 1.

Figure 2. Grain size vs. annealing temperature for various Fe alloys.

To investigate the stability of the alloys further, additional investigations were conducted. 

Optical images of polished and etched cross sections of the samples revealed a bimodal grain 

size distribution in the form of dark gray regions and white regions. Micrometer-scale grains

could be distinguished in the white region, whereas no grain structure could be resolved in the 

darker regions (these regions are marked by white and black arrows, respectfully). The latter 

were assumed to be submicrometer and/or nanoscale regions, where the high density of etched 

grain boundaries produced darker contrasts. This was also confirmed in focused ion beam 

contrast images. 

Figure 3 depicts the microstructural evolution for pure Fe at increasing temperatures. The 

annealing temperature and the percentage of the microstructure, which underwent abnormal 

growth, are indicated for each micrograph. Figure 4 gives a focused ion beam contrast image for 

nanocrystalline pure Fe annealed at 616 °C. Here, it can be seen that between the large 

abnormally grown grains are regions of coarse nanocrystalline grains (approximately 175 nm in 

diameter). Figures 5 and 6 are the same type of images; however, this time, data for the Fe 1

at.% Ni samples are shown. Again, figures 7 and 8 show the microstructure evolution for 

Fe 1 at.% Ta and Fe 1 at.% Zr at various temperatures and respective focused ion beam contrast 

images. What can be taken from images 3–8 can be summarized as follows. Changing the 

solute type in the following order, Fe with Ni, Ta, and Zr, increases the temperature at which 

abnormal grain growth occurs. Furthermore, the temperature and area fraction of existing
6

nanocrystalline grains increase with solute type in the same order (figure 9). Additionally, there 

exists a distinct difference in the stability observed between Fe with Ni and Fe with Ta or Zr,

with Zr solute additions giving the highest stability. 

Figure 3. Optical micrographs for pure Fe after annealing 1 h at the given temperatures. 

Percent area abnormally grown is indicated, respectively.

Figure 4. Focused ion beam contrast images for pure Fe after annealing at 616 °C.
7

Figure 5. Optical micrographs for Fe 1 at.% Ni after annealing 1 h at the given temperatures. Percent 

area abnormally grown is indicated, respectively.

Figure 6. Focused ion beam contrast images for Fe 1 at.% Ni after annealing at 700 °C.
8

Figure 7. Optical micrographs for Fe 1 at.% Ta and Fe 1 at.% Zr after annealing 

1 h at the given temperatures. Percent area abnormally grown is 

indicated, respectively.

Figure 8. Focused ion beam contrast images for Fe 1 at.% Ta and Fe 1 at.% Zr after annealing 

1 h at the given temperatures.
9

Figure 9. Plot showing the percent area remaining nanocrystalline for the various alloys vs.

annealing temperature.

It can then be stated with some confidence that figure 1 predicts which solute would more 

effectively reduce the effective grain boundary energy in a dilute Fe alloy. Additionally, the 

relative depth of the free energy minima in figure 1 corresponds to the relative stability of the 

nanocrystalline alloys. 

Due to the setbacks, the more advanced microscopy and modeling has been delayed but is 

currently under way. However, the initial results collected thus far are exciting. The remaining 

results briefly describe the preliminary aspects of this ongoing work.

Figures 10–12 summarizes current TEM findings on the Fe 1 at.% Zr system, annealed at 

900 °C. Figure 10 shows low and high magnification bright field and dark field TEM images 

taken from a region that contained both large micrometer- and nanocrystalline-sized grains. The 

upper left-hand corner of each image shows the boundary between a large micrometer grain and 

a region containing the nanocrystalline grains. Figure 11 is a high resolution TEM image 

showing the location of EDS scans taken to quantify the interfacial solute concentration. 

Figure 12 is the solute profile for the given scanned region labeled series 1. Note that the 

boundary region contains a high level of Zr solute. Figures 13 and 14 give the location and 

solute profiles within a region of the nanocrystalline area. Also note that region GB2 shows an 

enrichment of Zr between the nanograins, while GB1 does not. This may indicate that solute 

distributions are inhomogeneous in the nanocrystalline region and could be a potential 

explanation for figures 15 and 16, which show the existence of regions within the microstructure 

which consist of high elongated grains. Additionally, inhomogeneous solute distributions within 
10

the grain boundaries could be the potential source for abnormal grain growth. Results of 

ongoing compositional profiling will be used to understand how and why these phenomena are 

observed.

Figure 10. Bright and dark field TEM images of Fe 1 at.% Zr annealed at 900 °C. 

Images show a single micron-sized grain and a region containing 

nanocrystalline grains.
11

Figure 11. High resolution TEM image of Fe 1 at.% Zr annealed at 900 °C giving 

regions where EDS was performed to quantify Zr solute content in the 

boundary region.

Figure 12. Corresponding solute profile for figure 11 (Series 1).
12

Figure 13. High resolution TEM image of Fe 1 at.% Zr annealed at 900 °C 

giving regions where EDS was performed to quantify Zr solute 

content in the boundary regions between nanograins.

Figure 14. Corresponding solute profile for figure 13 (GB2).
13

Figure 15. Bright and dark field TEM images of Fe 1 at.% Zr 

annealed at 700 °C. Images show elongated grain with a 

single micron-sized grain and a region containing 

nanocrystalline grains with an average aspect ratio of 10:1.

Figure 16. High resolution TEM image of Fe 1 at.% Zr annealed at 

700 °C giving regions where EDS was performed to 

quantify Zr solute content in the boundary region.
14

Additional compositional profiling has uncovered a new FCC phase, which is coherent with the 

BCC Fe substrate matrix and has a lattice parameter of 8.5 Å (figure 17). The new phase has 

been observed to have two particle size distributions: particles with an average diameter of 5 nm 

and larger particles with a diameter greater than 20 nm. Even more intriguing is the observation 

that the particle with an average diameter of 5 nm appears to be pure Fe or with very low Zr 

content, while the larger particles are rich in Zr content (figure 18). A possible explanation is 

that this new FCC phase is actually the Fe2Zr intermetallic, where the strain caused by 

maintaining coherency with the BCC matrix is expanding the lattice parameter from 7.1 to 8.5 Å. 

Additionally, it can be assumed that Fe may be occupying the sublattice sites. As the Zr 

concentration builds up in the lattice due to the expulsion of Zr solute from grain boundaries 

during grain growth, Zr can then replace the Fe to form the stoichiometric structure. 

Understanding how and why this phase forms may allow methods for kinetically hindering 

precipitation, thereby extending the temperature of stability in these systems.

Figure 17. High resolution TEM images of Fe 1 at.% Zr annealed at 900 °C giving figure 17: Precipitates contained 

within abnormally grown grain. (a) Precipitates showing average size of 5 nm, (b) a larger precipitate 

showing coherency, and (c) a Fourier filtered high resolution TEM image using the second phase pattern 

in (b), showing a superlattice of FCC <110> (a0 = 8.5 Å), which is overlapped on the -Fe <001> lattice.
15

Figure 18. High resolution TEM images of Fe 1 at.% Zr annealed at 900 °C giving precipitates contained 

within abnormally grown grain. EELS spectra (a) and (b) for nanocrystals given in the high 

resolution TEM image above.

A B

A B
16

4. Conclusions

The thermal stability of nanocrystalline Fe alloys is determined by the amount of heat release 

associated with grain boundary segregating solutes and was correlated with experimental 

observations of grain growth. As suggested by the model for segregation used in this work, the 

addition of Ni to Fe did very little to stabilize against grain growth, while Zr had a more 

significant effect than Ta. In all samples, abnormal grain growth was observed to occur. The 

amount of abnormal grain growth increased with temperature and decreased with solute type in 

the following order: Ni, Ta, and Zr. The experimental observation of the increase in onset 

temperature for abnormal grain growth in these alloys coincided with the predicted stability. 

New microstructural analysis made possible by advanced microscopy techniques including high 

resolution imaging, EELS, and EDS analysis has lead to some interesting and important findings. 

Thus far, Zr solute profiles have been observed, which may yield an understanding of how grain 

morphology and abnormal grain growth occurs in these materials. Additionally, a new FCC 

phase has been identified and may hold the key on kinetically inhibiting secondary phase 

formation and extending thermal stability in these systems. Furthermore, current studies 

conducted thus far have only scratched the surface. This is such a new field and there is much to 

learn. It is fully expected that several ground-breaking papers will result from this work.
17

5. References

1. Krill, C. E.; Ehrhardt, H.; Birringer, R. Z. Metallkd. 2005, 96, 1134. 

2. Abe, Y. R.; Holzer, J. C.; Johnson, W. L. Mater. Res. Soc. Symp. Proc. 1992, 238, 721. 

3. Abe, Y. R.; Johnson, W. L. Mater. Sci. Forum 1992, 88–90, 513. 

4. Liu, K. W.; Muchlich, F. Acta Mater. 2001, 49, 395. 

5. Weissmuller, J.; Krauss, W.; Haubold, T.; Birringer, R.; Gleiter, H. Nanostructured Mater.

1992, 1, 439. 

6. Terwilliger, C. D.; Chiang, Y. M. Acta Mater. 1995, 43, 319. 

7. de Boer, F. R.; Boom, R.; Mattens, W.C.M.; Miedema, A. R.; Niessen, A. K. Cohesion in 

Metals: Transition Metal Alloys. Amsterdam: North-Holland, 1988.

8. Hondros, E. D.; Seah, M. P. in: R. W. Cahn, P. Haasen (Eds.), Physical Metallurgy, 3rd 

edition, Elsevier Science Pub. BV, 1983, 856–931.
18

6. Transitions

6.1 Journal Publications

• Stabilized Nanocrystalline Iron-based Alloys: Guiding Efforts in Alloy Selection; 

submitted to Materials Science and Engineering A.

6.2 Presentations

• A Regular Solution Model for Estimating Solute Segregation and the Stability of 

Nanocrystalline Fe Alloys, presented at TMS 2010, Seattle, WA, February 2010.

• Grain Size Stabilization in Nanocrystalline Fe Alloys, presented at TMS 2010, Seattle, 

WA, February 2010.
19

List of Symbols, Abbreviations, and Acronyms

Ca calcium

Cu copper

DFT density functional theory

EDS energy dispersive spectroscopy

EELS electron energy loss spectroscopy

ELNES energy loss near edge structure

Fe iron

Nb niobium

Ni nickel

Pd palladium

RuAl rubidium aluminum

Ta tantalum

TEM transmission electron microscopy

TiO2 titanium oxide 

Y yttrium

Zr zirconium 
20

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