Thermal Stability of Milled Nanocrystalline Tungsten 

Powders

by Brady Butler, Eric Klier, Matt Kelly, and Micah Gallagher

ARL-TR-5541 May 2011

Approved for public release; distribution unlimited. 
NOTICES

Disclaimers

The findings in this report are not to be construed as an official Department of the Army position 

unless so designated by other authorized documents.

Citation of manufacturer’s or trade names does not constitute an official endorsement or 

approval of the use thereof.

Destroy this report when it is no longer needed. Do not return it to the originator.
Army Research Laboratory

Aberdeen Proving Ground, MD 21005

ARL-TR-5541 May 2011

Thermal Stability of Milled Nanocrystalline Tungsten 

Powders

Brady Butler, Eric Klier, Matt Kelly, and Micah Gallagher

Weapons and Materials Research Directorate, ARL

Approved for public release; distribution unlimited. 
ii

REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the 

data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the 

burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302. 

Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently 

valid OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

May 2011

2. REPORT TYPE

Interim

3. DATES COVERED (From - To)

1 January 2009 to 31 December 2010

4. TITLE AND SUBTITLE

Thermal Stability of Milled Nanocrystalline Tungsten Powders

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Brady Butler, Eric Klier, Matt Kelly, and Micah Gallagher

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

U.S. Army Research Laboratory

ATTN: RDRL-WMM-F

Aberdeen Proving Ground, MD 21005

8. PERFORMING ORGANIZATION

 REPORT NUMBER

ARL-TR-5541

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR'S ACRONYM(S)

11. SPONSOR/MONITOR'S REPORT

 NUMBER(S)

12. DISTRIBUTION/AVAILABILITY STATEMENT

Approved for public release; distribution unlimited.

13. SUPPLEMENTARY NOTES

14. ABSTRACT

The development of nanocrystalline and ultrafine-grained bulk materials through powder metallurgy processes has long been 

constrained by the thermal stability of nanostructures. Many difficulties arise due to the fact that grain growth and 

densification occur within the same temperature regime. Additionally, the mechanisms supporting densification require some 

degree of grain coarsening in order to achieve full density. This study outlines the development of a thermally stable 

nanocrystalline tungsten powder and discusses the influence of grain size stability on the overall densification rate during 

pressureless sintering in hydrogen atmosphere. Furthermore, the activation energy of sintering is calculated to determine 

whether the mechanisms of grain growth and densification can be decoupled through the addition of thermal grain stabilizers.

15. SUBJECT TERMS

Tungsten, nano, thermal stability

16. SECURITY CLASSIFICATION OF:

17. LIMITATION

 OF 

 ABSTRACT

UU

18. NUMBER

 OF 

 PAGES

24

19a. NAME OF RESPONSIBLE PERSON

Brady Butler

a. REPORT

Unclassified

b. ABSTRACT

Unclassified

c. THIS PAGE

Unclassified

19b. TELEPHONE NUMBER (Include area code)

(410) 306-0835

Standard Form 298 (Rev. 8/98)

Prescribed by ANSI Std. Z39.18
iii

Contents

List of Figures iv

List of Tables iv

Acknowledgments v

1. Introduction 1

2. Experimental Procedure 2

2.1 Master Sintering Curve Calculations ..............................................................................2

2.2 Determining the Apparent Activation Energy for Densification ....................................4

3. Results and Discussion 5

4. Summary and Conclusions 11

5. References 12

List of Symbols, Abbreviations, and Acronyms 14

Distribution List 15
iv

List of Figures

Figure 1. Arrhenius plot for the determination of the apparent activation energy of 

densification...............................................................................................................................5

Figure 2. Grain size measurements acquired by x-ray diffraction for each of the thermally 

stabilized nano-tungsten powders.............................................................................................6

Figure 3. Electron micrographs, grain sizes and densities of three tungsten samples 

processed under different milling conditions.............................................................................7

Figure 4. Densification behavior of milled nanostructured tungsten powders. ..............................7

Figure 5. Apparent activation energy for densification of milled tungsten powders exhibiting 

different degrees of microstructural stability. Note that the differences in initial density 

are a result of different green packing densities resulting from dissimilar milling 

conditions...................................................................................................................................8

Figure 6. Plot of the measured apparent activation energy as a function of grain pinner 

concentration and grain size......................................................................................................9

Figure 7. Master sintering curves for tungsten powders under different milling conditions. 

Each master sintering curve represents the dilatometry results from three different heating 

rates (3, 10, and 25 °C/min.)....................................................................................................10

List of Tables

Table 1. Measured activation energies for self diffusion based on three mass-transport 

mechanisms in pure tungsten (20, 28). ......................................................................................9
v

Acknowledgments

This research is supported by mission funding from the Depleted Uranium Replacement Program 

and is the result of collaborative efforts from a number of team members. Special thanks are

given to Bradley Klotz for excellent work on the microstructural characterization of these 

materials. 
vi

INTENTIONALLY LEFT BLANK.
1

1. Introduction

This report presents factors identified that influence the thermal stability and sinterability of 

nanocrystalline tungsten. In addition to conventional methods of determining the stability of 

ultrafine and nanocrystalline grain structures, these tungsten materials were analyzed using a 

novel in-situ dilatometry technique, which identifies the thermal stability and densification 

behavior concurrently. This was accomplished by identifying the activation energy for 

densification and developing master sintering curves.

Nanocrystalline materials have long held the promise of demonstrating improved performance 

over their microcrystalline counterparts. While recent years have seen much advancement in 

producing bulk nanocrystalline materials through various novel processing techniques (1–3), the 

commercial application of these materials requires a significant reduction in processing costs to 

be viable in all but a few niche markets. In particular, bottom-up powder processing and 

consolidation techniques have shown promise for developing a cost-effective method for 

creating bulk nanocrystalline materials. While significant research has been performed in 

consolidating nanocrystalline powders (4–6), there remains some difficulty in achieving full 

density while retaining the nanostructure of the milled powders. Unstable nanostructures have 

significantly higher atomic mobility than their coarse-grained counterparts. This increased 

mobility tends to reduce the temperature required to sinter to a desired density; however, this 

mobility also causes rapid grain growth to occur during the densification.

Nanocrystalline materials have a very large volume fraction of grain boundary area, making

these materials inherently unstable as a result of incomplete bonding at the interface between 

grains. Within the past decade, there has been significant research to improve the thermal 

stability of nanostructures by limiting the propensity of grain growth through kinetic inhibitors

(7, 8) or thermodynamic stabilizers (9–11). The most commonly used method for improving 

stability is a kinetic method termed Zener pinning, where the mobility of the grain boundaries is

impeded by the addition of small insoluble particles that sit preferentially at the grain boundary 

interface and must be dragged along the path of the growing grain. While the methods and 

mechanisms for increasing thermal stability have been described in some detail (12), proper 

analysis of this stability requires a thorough inspection of the rate dependence for grain growth. 

This is often achieved through Kissinger analysis of calorimetry curves (13) or by analysis of the 

grain size at different stages in the sintering process using x-ray diffraction (14, 15) or electron 

microscopy (16) of specimens annealed to different temperatures. These techniques tend to be 

very time consuming and most researchers are content to describe the thermal stability of a 

material by referring to the grain size after sintering (17–19).
2

While thermal stability is critical for retaining the nanostructure of the as-prepared powders, the 

densification behavior is equally important to the final properties of a sintered material. In many 

cases, increasing the thermal stability of a material can drastically reduce the ability to achieve 

full density by inhibiting the diffusion required for densification. Although densification factors 

are very important for identifying sintering guidelines and achieving a quality final product, it is 

often presented independently from the thermal stability of grains.

2. Experimental Procedure

Three tungsten powders labeled High Stability (HS), Moderate Stability (MS), and Low Stability 

(LS) were prepared by milling tungsten powder in a high energy mill. Milling parameters were 

selected so that essentially pure tungsten powders were prepared with various levels of grain 

pinning impurities leading to varying degrees of thermal stability in each sample. Powder

samples were analyzed in a Panalytical X’Pert PRO Materials Research Diffractometer (MRD)

to determine particle size and lattice strain. Direct current plasma emission spectroscopy was

used to identify the concentration of trace grain pinning elements in each sample. 

Milled tungsten samples were compacted using a cold isostatic press and sintered in a 99.999% 

hydrogen atmosphere using a Thermal Technologies furnace or a Netzsch 402E Dilatometer. 

Each sample was reduced for 3 h at 700 °C in order to remove surface oxidation that can inhibit 

densification while increasing the propensity for grain growth (20). Dilatometry data were

collected at three constant heating rates: 3, 10, and 25 °C/min. The data from these 

measurements were used to determine the apparent activation energy for sintering and develop 

master sintering curves. The final density of the sintered samples was tested using the 

Archimedes method. Grain size was analyzed using x-ray diffraction (XRD), scanning electron 

microscopy (SEM), and transmission electron microscopy (TEM). 

2.1 Master Sintering Curve Calculations

Classical sintering theory distinguishes three different stages in the sintering process based upon 

the dominant sintering mechanism and diffusion processes that are occurring in the sample (21). 

Stage one begins at low temperatures where surface diffusion connects particles and creates 

regions of necking with minimal increases in density. Stage two shows the greatest increase in 

density and has grain boundary diffusion and possibly volume diffusion as the predominant mass 

transport mechanisms. The removal of small pores and coarsening of grains are characteristic of 

the third stage of sintering where volume diffusion tends to dominate. 

Many sintering theories have been developed to describe the densification behavior of a material 

based on the diffusion mechanisms that lead to densification in the sample. A combined stage 

sintering model developed by Su and Johnson (22) shows the linear shrinkage rate as a function 

of the microstructural and diffusion properties of a material:
3











 



 

 

3 4 G

D

G

D

Ldt kT

dL V V b b

. (1)

This equation relates the normalized instantaneous shrinkage rate of the specimen 

dL

Ldt

     

to the 

surface tension (γ), grain size (G), and the coefficients for volume and grain boundary diffusion 

(Dv and Db, respectively) at each of the three stages of the sintering process, where Ω is the 

atomic volume; k is Boltzmann’s constant; T is the absolute temperature; Гv and Гb are constants 

for scaling the microstructure of volume and grain boundary diffusion, respectively; and δ is the

grain boundary thickness. The surface diffusion term is generally neglected because it does not 

generally contribute to the densification process, though it can be responsible for significant 

coarsening in the specimen. Equation 1 can be further modified to represent the instantaneous 

density by substituting the following relation, which is valid for isotropic shrinkage during

densification:

dt

d

Ldt

dL





3

  , (2)

where ρ is the instantaneous density of the specimen. If there is only one dominant diffusion 

mechanism contributing to the densification process, the diffusion terms can be simplified and 

expanded as shown in equation 3:

















 









 





RT

Q

G

D

G

D

G

D

n

V V b b

exp

[ ( )]

[ ( )] 0

3 4



 

, (3)

where R is the universal gas constant, Q is the apparent activation energy, and n is 3 or 4 

depending on whether the process is volume or grain boundary diffusion controlled. Combining 

equations 1 and 2 and substituting the result from equation 3 leads to















 



RT

Q

kT G

D

dt

d

n

exp

[ ( )]

[ ( )]

3

0



 





. (4)

In equation 4, the terms that describe microstructural evolution of the material as a function of 

density can be separated from the diffusion terms, which are dependent on the time and 

temperature profile of the processing as shown by equation 5:

 





















t

n

dt

RT

Q

kT

D

d

G

0

0

exp

3 ( )

[ ( )]

0





 

 



. (5)

The integration of equation 5 requires the microstructural characteristics of the specimen be

independent of the time-temperature profile of the system and solely a function of the total 

density of the specimen. While this assumption is not adequate for complex sintering processes 

(23), it is justifiable for certain constant heating rate experiments and has been proven accurate 
4

for both metal (24, 25) and ceramic (22, 26, 27) systems in general. Equation 5 can be 

rearranged to separate the functions for density and time/temperature dependence; the results are 

shown as equations 6 and 7, respectively.

  

 







 







0 3 ( )

[ ( )] ( )

0

d

G

D

k

n

(6)













   

t

dt

RT

Q

T

t T

0

exp

1

( , )

(7)

In equation 7, Θ(t, T) can be described as the total amount of thermal work that is put into the 

system, while Ф(ρ) describes the resulting density, grain size, and microstructural characteristics

of the system. The foundation of the master sintering curve is shown in the relationship between 

these parameters as described by equation 8:

()  (t,T). (8)

For any given energy input described by Θ(t, T), there will be a resulting density, grain size, and 

microstructural parameter as described by Ф(ρ).

2.2 Determining the Apparent Activation Energy for Densification

The activation energy of the system can be estimated from constant heating rate experiments if Г 

and G are independent of the heating rate. Furthermore, if there is only one dominant diffusion 

mechanism active during the measured portion of the sintering cycle, the activation energy will 

be close to the tabulated value for the activation energy of diffusion. Conversely, the changes in

the grain size or microstructural properties that result from different heating rates will become 

apparent in the activation energy term. Г and G are never entirely independent of thermal 

processing conditions and the activation energy measured in these experiments is referred to as 

the “apparent activation energy for densification” to mark this distinction. In certain systems, 

specifically unstable nanocrystalline powders, the grain size and microstructural terms clearly 

change when sintering; however, thermally stable nanostructures are inherently less dependent 

on temperature and these terms can be considered to remain constant.

The apparent activation energy for densification can be determined from dilatometry curves by 

using an Arrhenius plot for constant heating rate of 













T T

dt

d

ln * *





VS 

T

1

, where 

T



is the 

heating rate, T is the absolute temperature, and 

dt

d

is the instantaneous densification rate. The 

slope of the fitted lines at each density is equal to the activation energy divided by the negative 

of the universal gas constant as shown in figure 1.
5

Arrhenius Plot for Activation Energy 

1

1.5

2

2.5

3

3.5

4

4.5

5

0.0007 0.00072 0.00074 0.00076 0.00078 0.0008 0.00082 0.00084

1/T

ln(dP/dT*T*T)

Lines of Equal 

Density

Figure 1. Arrhenius plot for determining the apparent activation energy of densification. 

The activation energy was measured using this method so that the variation in activation energy

could be measured as a function of density in order to identify the mechanisms operating at 

different stages of the sintering process. Other methods commonly use a least squares fit of the 

master sintering curve to describe the average activation energy. While the activation energy 

does vary as a function of density, the parameter remains fairly consistent during the 

intermediate sintering stage, where the densification rate is greatest. 

3. Results and Discussion

Upon sintering, the grain size increases significantly to achieve a lower energy microstructure. 

The grain size versus temperature plot shown in figure 2 reveals a gradual increase in grain size 

for the low stability powder throughout the temperature range from 800 to 1400 °C. The 

moderate and low stability tungsten show reduced grain growth between 1000 and 1200 °C. 

This is likely a result of Zener pinning from the dispersoid phase, which causes a reduction in 

grain boundary mobility. The high stability tungsten actually shows a decrease in grain size,

which could result from coagulation of the dispersoid phase within the pure tungsten grains. 
6

Anneal Temperature VS Grain Size (Scherrer)

0

2 0

4 0

6 0

8 0

100

120

140

160

0 200 400 600 800 1000 1200 1400 1600

Annealing Tempearture (°C)

Grain Size (nm) - Scherrer

High Stability

Moderate

Stability

Low Stability

Figure 2. Grain size measurements acquired by x-ray diffraction for each of the thermally stabilized 

nano-tungsten powders.

Figure 3 shows the microstructures of the three different tungsten powders. While each sample 

shows a similar density, the grain size of each material indicates that the thermal stability 

changes significantly based on milling conditions.
7

Figure 3. Electron micrographs, grain sizes, and densities of three tungsten samples processed under different 

milling conditions.

Figure 4 shows the density and densification rate of tungsten as a function of temperature for the 

HS tungsten sample. These in-situ density measurements were acquired by measuring the linear 

displacement as a function of temperature at three different heating rates. Similar curves were 

acquired for the MS and LS samples, though the plots are not shown in this report. Density was 

calculated assuming isotropic shrinkage, a fair assumption for isostatically pressed compacts.

Dilatometry Measurements

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

700 800 900 1000 1100 1200 1300 1400 1500 Temperature (C) dP/dt

3 C/min

10 C/min

25 C/min

Dilatometry Measurements

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

0.04

700 800 900 1000 1100 1200 1300 1400 1500 Temperature (C) dP/dt

3 C/min

10 C/min

25 C/min

Figure 4. Densification behavior of milled nanostructured tungsten powders.

Material Properties Material Properties Material Properties

Mean Grain Size (nm) 135 Mean Grain Size (nm) 240 Mean Grain Size (nm) 640

Standard Deviation (nm) 38 Standard Deviation (nm) 113 Standard Deviation (nm) 276

Density (%) 94.6 Density (%) 95.9 Density (%) 94.6

SEM – Secondary Electron

HAADF 

STEM

SEM – Secondary Electron

HAADF

STEM

SEM

Backscatter 

High Stability Moderate Stability Low Stability

SEM – Secondary Electron

SEM – Secondary Electron
8

The density of the tungsten is shown to be a function of both the temperature and the heating 

rate. The densification rate plot indicates that this is a thermally activated process where the 

peak shifts with increasing ramp rate. Furthermore, the densification plot shows local maxima at 

875–900 °C, indicating a shift in densification mechanisms based upon heating to high 

temperatures. 

The activation energy plotted as a function of density for the HS, MS, and LS samples shows a 

significant difference in the recorded activation energy corresponding to the degree of thermal 

stability. The result is shown in figure 5.

Activation Energy 

0

200000

400000

600000

800000

1000000

1200000

1400000

1600000

40% 45% 50% 55% 60% 65% 70% 75% 80%

Percent of Theoretical Density

Activation Energy (J/mol)

High Stability

Moderate Stability

Low Stability

Figure 5. Apparent activation energy for densification of milled tungsten powders exhibiting different 

degrees of microstructural stability. Note that the differences in initial density are a result of 

different green packing densities resulting from dissimilar milling conditions.

The activation energy for the highly stable tungsten powder does not fluctuate significantly from 

the measured activation energy for grain boundary diffusion. The apparent activation energy for 

densification increases drastically for samples exhibiting moderate to low stability. This increase 

cannot be described by a different mass transport mechanism operating on the sample as the 

activation energies are much higher than the standard values listed in table 1. While the 

implications of the activation energy thermal stability relationship are not immediately apparent 

from standard chemical kinetics theory, it is important to realize that this number represents the 

apparent activation energy for densification and not the true activation energy for mass transport 

to occur. An increase in the apparent activation beyond that for grain boundary diffusion 

411 kJ/mol - Activation 

Energy for Grain Boundary 

Diffusion

Additional Apparent 

Activation Energy 

Caused by Grain 

Growth
9

indicates that a significant portion of the energy is being used to change the structure of the 

material without contributing to the densification process. Therefore, the deviation from the 

activation energy for grain boundary diffusion could be correlated to the efficiency of the 

sintering process for providing a maximum density with minimal energy losses to 

microstructural changes such as grain growth. While this technique is a very powerful 

characterization tool, the scope is limited to the temperatures where densification rates are high 

and can be misleading if a significant amount of coarsening occurs during the final stages of 

sintering.

Table 1. Measured activation energies for self diffusion based on three 

mass-transport mechanisms in pure tungsten (20, 28).

Diffusion Type Activation Energy

(kJ/mol)

Surface diffusion 251–287

Grain boundary diffusion 377–460

Volume diffusion 502–586

The average activation energies described by figure 5 show a strong correlation to the grain size 

measured by electron microscopy. In many cases, the composition can play a vital role in the 

sintering behavior of materials. The addition of small dispersiods can inhibit the grain growth of 

a material by effectively reducing the mobility of particle boundaries. It is duly noted that the 

apparent activation energy for densification is dependent on the concentration of dispersiods in 

the sample. The relations between composition, activation energy, and the resulting grain size of 

a sintered sample are shown in figure 6. 

Grain Pinner and Activation 

Energy Relationship

0

200

400

600

800

1000

0.0000 0.2000 0.4000 0.6000 0.8000

Grain Pinner Concentration (Wt%)

Activation Energy (kJ/mol)

Grain Size and Activation Energy 

Relationship

0

200

400

600

800

1000

0 200 400 600

Grain Size (nm)

Activation Energy (kJ/mol)

Figure 6. Plot of the measured apparent activation energy as a function of grain pinner concentration 

and grain size. 
10

The plots in figure 6 demonstrate that the final grain size that results from sintering is dependent 

on the quantity of grain pinner present in the sample. The measured activation energy is 

significantly higher than that for grain boundary diffusion for the milled nanocrystalline tungsten 

with the smallest amount of grain pinner. This deviation describes the instability of the system 

that results from nanocrystalline grains and high lattice strains. Without any barriers to grain 

boundary mobility, the energy of the system is relaxed by significant grain growth that occurs 

independently from the densification process. It is only after the addition of the grain pinning 

material that the apparent activation energy decreases to the values expected for grain boundary 

diffusion. These barriers allow the coarsening to occur only in conjunction with densification. 

This allows for the production of an ideally dense and ultrafine-grained structure. The master 

sintering curves for the HS, MS, and LS samples are shown in figure 7.

Master Sintering Curves

35%

40%

45%

50%

55%

60%

65%

70%

75%

80%

85%

90%

95%

-150 -100 -50 0

Ln(θ(T,t))

Percent of Theoretical 

Density

Figure 7. Master sintering curves for tungsten powders under different milling conditions. Each master 

sintering curve represents the dilatometry results from three different heating rates (3, 10, and 

25 °C/min.)

The master sintering curves clearly show that low stability powders can achieve moderate 

densities with minimal thermal work; however, the slope of the density curve decreases 

gradually for the LS while the HS material remains fairly constant. This phenomenon is 

observed as a result of the reduction in the driving force for densification that occurs with grain 

growth. While the MS sample does show a moderately higher final density compared to the HS 

and LS samples, this is likely a result of the higher packing density in the green compact. 

Master Sintering Curves

.0208x + 2.4464

R

2

 = 0.9482

y = 0.0232x + 2.273

R

2

 = 0.9429

y = 0.0534x + 2.7245

R

2

 = 0.9799

5%

0%

5%

0%

5%

0%

5%

0%

5%

0%

5%

0%

5%

-100 -80 -60 -40 -20 0

Ln(θ(T,t))

High Stability

Moderate Stability

Low Stability

Linear (Low Stability)

Linear (Moderate Stability)

Linear (High Stability)
11

Similar densities were achieved for samples undergoing the same temperature program in a 

standard hydrogen furnace. This is a result of the plateau that occurs during the final stages of 

sintering, where densification is minimal even for a significant amount of thermal energy input. 

4. Summary and Conclusions

The thermal stability and densification behavior of a material are critical to producing bulk 

materials that retain the nanocrystalline properties of the starting powders. Though these 

parameters are critical to the quality of the final sintered product, they are difficult to measure 

and are often optimized solely through iterative trial and error experimentation. The use of 

dilatometry and a combined stage sintering model can help to identify the influence of key 

processing parameters on the stability and densification behavior of materials. Furthermore, this 

analysis can identify the optimum thermal stability for minimizing grain growth without 

inhibiting the diffusion mechanisms that are required for densification. While the master 

sintering curve is excellent in predicting the sintering curves for a given set of sintering 

conditions, even minor changes to the powder processing and compaction can have significant 

influence on the densification parameters and can invalidate the general master sintering curve. 

Future development of this combined stage sintering model should work to define the influence 

of initial conditions on the entire sintering process. The resulting model would be an outstanding 

tool for understanding and predicting the sintering behavior of particulate systems. 
12

5. References

1. Fecht, H. J. et al. Nanocrystalline Metals Prepared by High-energy Ball Milling.

Metallurgical and Materials Transactions A 1990, 21 (9), 2333–2337.

2. Brinker, C. J.; Scherer, G. W. Sol-gel Science: The Physics and Chemistry of Sol-gel 

Processing; Academic Press Ltd, 1990.

3. Tong, L.; Reddy, R. G. Synthesis of Titanium Carbide Nano-powders by Thermal Plasma. 

Scripta Materialia 2005, 52 (12), 1253–1258.

4. Karagedov, G. R.; Lyakhov, N. Z. Preparation and Sintering of Nanosized [alpha]-Al2O3 

Powder. Nanostructured Materials 1999, 11 (5), 559–572.

5. Siegel, R. W. et al. Synthesis, Characterization and Properties of Nanophase TiO 2. Journal 

of Materials Research 1988, 3 (6), 1367–1372.

6. Mayo, M. J. Processing of Nanocrystalline Ceramics from Ultrafine Particles. International 

Materials Reviews 1996, 41 (3), 85–115.

7. Gladman, T. On the Theory of the Effect of Precipitate Particles on Grain Growth in Metals.

Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences

1966, 294 (1438), 298–309.

8. Cahn, J. W. The Impurity-drag Effect in Grain Boundary Motion. Acta Metallurgica 1962,

10 (9), 789–798.

9. Kirchheim, R. Grain Coarsening Inhibited by Solute Segregation. Acta Materialia 2002, 50

(2), 413–419.

10. VanLeeuwen, B. K. et al. Thermal Stability of Nanocrystalline Pd81Zr19. Acta Materialia,

in press.

11. Detor, A. J.; Schuh, C. A. Grain Boundary Segregation, Chemical Ordering and Stability of 

Nanocrystalline Alloys: Atomistic Computer Simulations in the Ni-W System. Acta 

Materialia 2007, 55 (12), 4221–4232.

12. Shvindlerman, L. S.; Gottstein, G. Efficiency of Drag Mechanisms for Inhibition of Grain 

Growth in Nanocrystalline Materials. Zeitschrift fur Metallkunde 2004, 95 (4), 239–241.

13. Chen, L. C.; Spaepen, F. Analysis of Calorimetric Measurements of Grain Growth. Journal 

of Applied Physics 1991, 69 (2), 679.

14. Liu, K. W.; Mücklich, F. Thermal Stability of Nano-RuAl Produced by Mechanical 

Alloying. Acta Materialia 2001, 49 (3), 395–403.
13

15. Natter, H. et al. Grain-growth Kinetics of Nanocrystalline Iron Studied In Situ by 

Synchrotron Real-time X-ray Diffraction. J. Phys. Chem. B 2000, 104 (11), 2467–2476.

16. Iordache, M. C. et al. Grain Growth Kinetics in Nanostructured Nickel. Nanostructured 

Materials 1999, 11 (8), 1343–1349.

17. Moelle, C. H.; Fecht, H. J. Thermal Stability of Nanocrystalline Iron Prepared by 

Mechanical Attrition. Nanostructured Materials 1995, 6 (1–4), 421–424.

18. Darling, K. A. et al. Grain-size Stabilization in Nanocrystalline FeZr Alloys. Scripta 

Materialia 2008, 59 (5), 530–533.

19. Perez, R. J.; Huang, B.; Lavernia, E. J. Thermal Stability of Nanocrystalline Fe-10 wt.% Al 

Produced by Cryogenic Mechanical Alloying. Nanostructured Materials 1996, 7 (5), 565–

572.

20. Lassner, E.; Schubert, W. D. Tungsten: Properties, Chemistry, Technology of the Element, 

Alloys, and Chemical Compounds; Kluwer Academic and Plenum Publishers: New York, 

1999.

21. German, R. M. Powder Metallurgy and Particulate Materials Processing; Metal Powder 

Industries Federation: Princeton, NJ, 2005, p. 293–294.

22. Su, H.; Johnson, D. L. Master Sintering Curve: A Practical Approach to Sintering. Journal 

of the American Ceramic Society 1996, 79 (12), 3211–3217.

23. Chen, I. W.; Wang, X. H. Sintering Dense Nanocrystalline Ceramics Without Final-stage 

Grain Growth. Nature 2000, 404 (6774), 168–171.

24. Gupta, T. K. Possible Correlation Between Density and Grain Size During Sintering.

Journal of the American Ceramic Society 1972, 55 (5), 276–277.

25. Rusin, R. P. Sintering of Nickel Powder With or Without Dispersed Alumina Particles. 

Ph.D. Dissertation. Northwestern University, Evanston, IL, June 1993. 

26. Wang, J.; Raj, R. Estimate of the Activation Energies for Boundary Diffusion from Ratecontrolled Sintering of Pure Alumina, and Alumina Doped with Zirconia or Titania. Journal 

of the American Ceramic Society 1990, 73 (5), 1172–1175.

27. Kevin, G. E.; Donald, T. E.; Christopher, B. D. Analysis of Nanocrystalline and 

Microcrystalline ZnO Sintering Using Master Sintering Curves. Journal of the American 

Ceramic Society 2006, 89 (6), 2003–2009.

28. Bettler, P. C.; Charbonnier, F. M. Activation Energy for the Surface Migration of Tungsten 

in the Presence of a High-Electric Field. Physical Review 1960, 119 (1), 85.
14

List of Symbols, Abbreviations, and Acronyms

HS high stability

LS low stability

MRD Materials Research Diffractometer

MS moderate stability

SEM scanning electron microscopy

TEM transmission electron microscopy

XRD x-ray diffraction
15

NO. OF

COPIES ORGANIZATION

1 ADMNSTR

ELEC DEFNS TECHL INFO CTR

ATTN DTIC OCP

8725 JOHN J KINGMAN RD STE 0944

FT BELVOIR VA 22060-6218

1 CD OFC OF THE SECY OF DEFNS

ATTN ODDRE (R&AT)

THE PENTAGON

WASHINGTON DC 20301-3080

1 US ARMY RSRCH DEV AND ENGRG 

CMND

ARMAMENT RSRCH DEV & ENGRG 

CTR 

ARMAMENT ENGRG & TECHNLGY 

CTR

ATTN AMSRD AAR AEF T J MATTS

BLDG 305

ABERDEEN PROVING GROUND MD 

21005-5001

1 PM TIMS, PROFILER (MMS-P) 

AN/TMQ-52

ATTN B GRIFFIES 

BUILDING 563

FT MONMOUTH NJ 07703

1 US ARMY INFO SYS ENGRG CMND

ATTN AMSEL IE TD A RIVERA

FT HUACHUCA AZ 85613-5300

1 COMMANDER

US ARMY RDECOM

ATTN AMSRD AMR 

W C MCCORKLE

5400 FOWLER RD

REDSTONE ARSENAL AL 35898-5000

1 US GOVERNMENT PRINT OFF

DEPOSITORY RECEIVING SECTION

ATTN MAIL STOP IDAD J TATE

732 NORTH CAPITOL ST NW

WASHINGTON DC 20402

3 US ARMY RESEARCH LAB

ATTN RDRL WMM F B BUTLER

ATTN RDRL WMM F H MAUPIN

ATTN RDRL WMM F B KLOTZ

ABERDEEN PROVING GROUND MD 

21005

NO. OF

COPIES ORGANIZATION

1 US ARMY RSRCH LAB

ATTN RDRL WMM F E KLIER

BLDG 4600

ABERDEEN PROVING GROUND MD 

21005

1 US ARMY RSRCH LAB

ATTN RDRL WMM F 

M GALLAGHER

ABERDEEN PROVING GROUND MD 

21005

1 US ARMY RSRCH LAB

ATTN RDRL CIM G T LANDFRIED

BLDG 4600

ABERDEEN PROVING GROUND MD 

21005-5066

3 US ARMY RSRCH LAB

ATTN IMNE ALC HRR 

MAIL & RECORDS MGMT

ATTN RDRL CIM L TECHL LIB

ATTN RDRL CIM P TECHL PUB

ADELPHI MD 20783-1197
16

INTENTIONALLY LEFT BLANK.