Orientation-dependent grain growth in a bulk nanocrystalline alloy during the uniaxialcompressive deformationG. J. Fan, Y. D. Wang, L. F. Fu, H. Choo, P. K. Liaw, Y. Ren, and N. D. Browning Citation: Applied Physics Letters 88, 171914 (2006); doi: 10.1063/1.2200589 View online: http://dx.doi.org/10.1063/1.2200589 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/88/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Dislocation density evolution during high pressure torsion of a nanocrystalline Ni–Fe alloy Appl. Phys. Lett. 94, 091911 (2009); 10.1063/1.3095852 Mechanism of grain growth during severe plastic deformation of a nanocrystalline Ni–Fe alloy Appl. Phys. Lett. 94, 011908 (2009); 10.1063/1.3065025 Deformation-induced grain rotation and growth in nanocrystalline Ni Appl. Phys. Lett. 92, 011903 (2008); 10.1063/1.2828699 Uniaxial tensile plastic deformation of a bulk nanocrystalline alloy studied by a high-energy x-ray diffractiontechnique Appl. Phys. Lett. 89, 101918 (2006); 10.1063/1.2348783 Embrittlement in a bulk nanocrystalline alloy induced by room-temperature aging Appl. Phys. Lett. 89, 061919 (2006); 10.1063/1.2336595

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Orientation-dependent grain growth in a bulk nanocrystalline alloyduring the uniaxial compressive deformation

G. J. Fana�

Department of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee 37996

Y. D. WangDepartment of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee 37996and School of Materials and Metallurgy, Northeastern University, Shenyang 110004, China

L. F. FuDepartment of Chemical Engineering and Materials Science, University of California,Davis, California 95616

H. ChooDepartment of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee 37996and Metals and Ceramic Division, Oak Ridge National Laboratory, Oak, Ridge, Tennessee 37831

P. K. LiawDepartment of Materials Science and Engineering, The University of Tennessee, Knoxville, Tennessee 37996

Y. RenExperimental Facilities Division, Advanced Phonon Source, Argonne National Laboratory,Argonne, Illinois 60439

N. D. BrowningNational Center for Electron Microscopy, Lawrence Berkeley National Laboratory,Berkeley, California 94720

�Received 14 February 2006; accepted 4 April 2006; published online 28 April 2006�

The microstructural evolution during the uniaxial compression of an as-deposited bulknanocrystalline �nc� Ni–Fe �average grain size d�23 nm� at ambient temperature was investigatedby the high-energy x-ray diffraction �HEXRD� and the transmission-electron microscopy �TEM�.HEXRD measurements indicated that the grain growth occurred in the nc Ni–Fe alloy during theuniaxial compression tests and that the grain growth shows orientation dependence, i.e., the grainspreferentially grow perpendicular to the loading direction. This preferred grain growth was furtherconfirmed by the TEM observations, indicating that the grains were elongated after the compressiveplastic deformation. © 2006 American Institute of Physics. �DOI: 10.1063/1.2200589�

The mechanisms by which nanocrystalline �nc� metalsand alloys with grain sizes d less than 100 nm deform plas-tically have been extensively investigated using various ex-perimental techniques as well as the molecular-dynamicssimulations.1 Particular attentions have been paid to the me-chanical behavior of nc metals and alloys with d smaller than30 nm, where unusual deformation behavior2–12 may emergedue to the increase in the volume fraction of the grain bound-aries �GBs� and GB-mediated plasticity.

More recently, it was reported that the microstructuresof the nc metals and alloys are unstable under variousstress conditions �i.e., indentation, tension, high-pressuretorsion�,13–19 i.e., simultaneous grain growth occurred in thenc metals and alloys during the plastic deformation. Thegrain growth was observed during the indentation of nc Cu ata cryogenic temperature13,15 and recently during the tensiletests of the bulk nc Ni–Fe �Ref. 18� and nc Co–P �Ref. 19�alloys, which exhibits a high thermal stability against thegrain growth. These results suggested that the grain growthduring the plastic deformation of nc metals and alloys waslikely to be induced by the stress instead of the thermal ac-

tivation process, leading to the GB migration for the mini-mization of the GB areas.

In this letter, we employed a high-energy x-ray diffrac-tion �HEXRD� technique to monitor the microstructural evo-lutions in a bulk nc Ni–Fe alloy during the compression tests.The bulk nc Ni–Fe alloy used in this study has three dimen-sions in the range of millimeters, which allows the determi-nation of the compressive mechanical properties. We reportthat the uniaxial compressive deformation leads to an inho-mogeneous grain growth, i.e., the grains grow preferentiallyperpendicular to the loading direction.

The bulk nc Ni–18 wt %Fe �wt %: weight percent� al-loy sheets were produced using a pulsed-electrodepositiontechnique. The as-deposited alloy is a single phase with theface-centered cubic �fcc� structure. The alloy sheets have di-mensions of 70�70�3 mm3. The rectangular specimenswith dimensions of 3�3�5 mm3 were machined from theas-deposited sheets and were polished for the uniaxial com-pression tests. Optical microscopy and scanning-electron mi-croscopy examinations revealed that the as-deposited plate isfree of artifacts, such as the porosity and second-particleinclusions.

The ex situ HEXRD measurements were carried out onthe beam line 11-ID-C at Advanced Photon Source �APS�,Argonne National Laboratory, USA. An x-ray beam with ana�Electronic mail: gfan@utk.edu

APPLIED PHYSICS LETTERS 88, 171914 �2006�

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energy of 115 keV and the beam size of 50�50 �m2, fo-cused by a monochromator with a Si �113� single crystal,was used to measure diffraction patterns �Debye-Scherrerrings�. The patterns were collected by a two-dimensional de-tector �Mar345� in a transmission geometry over a plane en-compassing the loading direction and the transverse direc-tion. Diffraction peaks are fitted by a Cauchy peak shape,and the physical peak shape is obtained by the deconvolutionof the measured peaks from the instrumental peak shape�calibrated by Si powders�. The Williamson-Hall �WH� inte-gral breadth method is used to determine the grain size d andmicrostrain �M from the �111� and �222� reflections,20,21 i.e.,�i=1/d+2�MSi, where Si=2 sin��i� /� is the length of thediffraction vector for the diffraction peak, �i=�i

0 cos��i� /�is the integral breadth of the diffraction peak in the reciprocalspace, �i is the Bragg angle, � is the wavelength, and �i

0 isthe integral breadth of �i for the diffraction peak.

Transmission-electron microscopy �TEM� and high-resolution TEM �HRTEM� observations were carried out be-fore and after the compressive plastic deformation using aSchottky field-emission gun FEI Tecnai F20 UT microscopeoperating at 200 kV with a spatial resolution of 0.14 nm. TheTEM and HRTEM thin-foil specimens were prepared by theconventional twin-jet electropolishing technique using a25 vol % nitric acid+75 vol % methanol solution at −30 °Cand 10 V.

The electrodeposited nc materials often exhibit a colum-nar grain structure, i.e., the grains grow preferentially alongthe deposition direction, leading to a larger grain size alongthe deposition direction than the perpendicular to the depo-sition direction. Figure 1 shows a bright-field TEM imageand the grain-size distribution statistics of the as-deposited

bulk nc Ni–Fe alloy viewed perpendicular to the depositiondirection. The grains exhibit an equiaxed shape. The grainsizes have relatively narrow distributions with an averagevalue of about 23 nm.

The engineering stress-strain curve during the compres-sion test at a strain rate �̇ of 10−3 s−1 is shown in Fig. 2. Forcomparison, the corresponding engineering stress-straincurve during the tensile test is also included. The inset in Fig.2 displays the loading direction �Y axis� and the depositiondirection �Z axis�. The alloy exhibits a combination of thehigh strength and ductility with a maximum strength of about2.5 GPa and a compressive strain to failure of 11.6%. On theother hand, the bulk nc Ni–Fe alloy has an ultimate tensilestrength of about 2 GPa and a tensile elongation to failure of6.5%. Therefore, the bulk nc alloy shows a strong strengthasymmetry. The compressive strength is higher than the ten-sile strength, which has been reported previously in other ncmaterials.22–24

Figures 3�a� and 3�b� show the grain size and micros-train changes after the compression tests. The grain size andthe microstrain determined from Fig. 3�b� are listed in TableI. For the as-deposited sample, the grain sizes and the mi-crostrain are uniform in the XY plane perpendicular to thedeposition direction, i.e., dX=dY =21 nm and �M

X=�MY

=0.17%. The grain sizes determined from the HEXRD are ina good agreement with the TEM observations for the as-deposited specimen �about 23 nm�. However, the grain sizealong the Z axis �64 nm� is about two times larger than thatobserved in the XY plane, showing a characteristic grainstructure typically observed in the as-deposited state. Afterdeformed to �=11.6% at room temperature, the grain sizesalong the X, Y, and Z axes all increased. Moreover, the grainsize along the X axis increased more significantly �from

FIG. 2. The engineering stress-strain curve of the bulk nc Ni–Fe alloyduring the compression tests at a strain rate �̇ of 10−3 s−1. For comparison,the corresponding engineering stress-strain curve during the tensile tests atthe same strain rate is also included. The inset is a schematic of a rectangu-lar nc compression test specimen. The compressive load was applied alongthe Y direction.

FIG. 3. �Color online� The �111� peak �a� of the high-energy x-ray diffrac-tion patterns of the bulk nc Ni–Fe alloy in the as-deposited state �denotedAS� and after the compressive deformation �denoted DE� measured alongthe X, Y, and Z axes �Fig. 2�. The grain size and the microstrain wereevaluated from the plots of the integral breadth of the diffraction peak in thereciprocal space, �i, as a function of the length of the diffraction vector Si

for the �111� and �222� diffraction peaks in �b�.

FIG. 1. The bright-field TEM image of the as-deposited bulk nc Ni–Fe alloy�a� and the grain-size distribution statistics �b�. The image is from the XYplane. Z is the deposition direction.

TABLE I. The average grain size d and the mean microstrain �M along theX, Y, and Z axes �Fig. 2� before and after the room temperature compressivedeformation of the bulk nc Ni–Fe alloy at a strain rate �̇ of 10−3 s−1.

dX dY dZ �MX �M

Y �MZ

As-deposited 21 nm 21 nm 64 nm 0.17% 0.17% 0.5%Deformed ��=11.6% � 42 nm 28 nm 78 nm 0.34% 0.19% 0.41%

Percentage changes dueto deformation 100% 33% 22% 100% 12% −18%

171914-2 Fan et al. Appl. Phys. Lett. 88, 171914 �2006�

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21 nm in the as-deposited state to 42 nm� compared withthose along the Y and Z axes, indicating that the grain growthis directionally inhomogeneous during the compression tests.A significant increase in the microstrain along the X axis isalso observed.

The inhomogeneous grain growth was further confirmedby the TEM observations. Figure 4 displays the bright-fieldTEM images of the XY plane of the bulk nc Ni–Fe alloy afterthe compressive deformation at �̇=10−3 s−1 to �=7% �a� andto �=11.6% �b�. It was observed that the equiaxed grains inthe as-deposited state �Fig. 1�a�� grow into the elongatedgrains after the compression tests and that the grain growthbecomes more significant with increasing � from 7% to11.6%. The long edge of the elongated grains after �=11.6% is in the range of 2–130 nm, while the short edge ofthe elongated grains is in the range of 2–170 nm. The graingrowth was also noticed during the tensile plastic deforma-tion of the bulk nc Ni–Fe alloy.18 In contrast to the elongatedgrains after the compressive deformation, the grain remainsequiaxed after the tensile tests, suggesting that stress-inducedgrain growth in the nc materials depends on the stress con-ditions. Finally, it should be pointed out that the grain sizesestimated from the TEM observations are larger than thosemeasured by HEXRD, which measures the subgrain sizes.Therefore, the elongated grains observed by TEM may bedue to the subgrain agglomerations caused by the grain rota-tion, as evidenced by a texture development during thestress-induced grain growth in the bulk nc Ni–Fe alloy.18

In summary, an orientation-dependent grain growth wasreported in a bulk nc Ni–Fe alloy during the compressiveplastic deformation at room temperature. The grains prefer-entially grow perpendicular to the loading direction, leadingto the elongated grains. Our results indicate that the stress-induced grain growth in the nc metals and alloys is associ-ated with the stress conditions.

This work was supported by the National Science Foun-dation �NSF� International Materials Institutes �IMI� Pro-

gram �DMR-0231320�. The microscopy was performed atthe National Center for Electron Microscopy, Lawrence Ber-keley National Laboratory, supported by the Director, theOffice of Science, of the U.S. Department of Energy �DOE�under Contract No. DE-AC03-76SF00098 and by Grant No.DE-FG02-03ER46057. One of the authors �Y.W.� was alsosupported by the National Natural Science Foundation ofChina �Grant No. 50471026�. The use of the Advanced Pho-ton Source was supported by the U.S. Department of Energy,Office of Science, Office of Basic Energy Science, underContract No. W-31-109-ENG-38.

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FIG. 4. The bright-field TEM images of the bulk nc Ni–Fe alloy after thecompressive deformation at �̇=10−3 s−1 to �=7% �a� and to �=11.6% �b�,respectively. The images of the XY plane indicate that the grains grow pref-erentially along a certain direction, as marked by the arrows in �a� and �b�.

171914-3 Fan et al. Appl. Phys. Lett. 88, 171914 �2006�

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