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Scripta Materialia 65 (2011) 727–730

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Effect of irradiation on the microstructure and mechanicalbehavior of nanocrystalline nickel

Garima Sharma,a,⇑ Apu Sarkar,a Jalaj Varshney,b U. Ramamurty,c Ajay Kumar,d

S.K. Guptad and J.K. Chakravarttya

aMechanical Metallurgy Division, Bhabha Atomic Research Centre, Mumbai 400085, IndiabMaterial Processing Division, Bhabha Atomic Research Centre, Mumbai 400085, India

cDepartment of Materials Engineering, Indian Institute of Sciences, Bangalore 560012, IndiadNuclear Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

Received 30 May 2011; revised 30 June 2011; accepted 8 July 2011Available online 14 July 2011

The effect of 4.0 MeV proton irradiation on the microstructure and mechanical properties of nanocrystalline (nc) nickel wasinvestigated. The irradiation damage induced in the sample was of the order of 0.004 dpa. Transmission electron microscopy of irra-diated samples indicated the presence of dislocation loops within the grains. An increase in hardness and strain-rate sensitivity (m) ofnc-Ni with irradiation was noted. The rate-controlling deformation mechanism in irradiated nc-Ni was identified to be interaction ofdislocations with irradiation-induced defects.� 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Keywords: Nickel; Nanostructure; Nanocrystalline materials; Nanoindentation; Irradiation

Nanocrystalline (nc) materials exhibit interestingmechanical, physical and chemical properties comparedto their coarse-grained (CG) counterparts [1]. This isdue to the relatively high volume fraction of grainboundary (GB) area in the former. Since the materialat GBs is relatively “more open”, it can act as a sinkfor irradiation-induced point defects. Therefore, thehigh density of GBs in nc materials can be expected toresult in more tolerance towards radiation-inducedswelling [2–5]. In fact, it has been shown that irradiationcan enhance the plasticity of amorphous alloys, whichhave disordered atomic arrangements similar to theGB regions in crystalline metals [6]. However, a detailedunderstanding of the irradiation-induced microstruc-tural modifications and the resultant changes in themechanical behavior of nc materials is far from com-plete, with only few studies being available in the litera-ture. Nita et al. [7] have reported the hardening of nc-Niafter irradiation with 590 MeV protons and 840 keV Niions. In the present study, the effect of irradiation on themicrostructure and on dynamic parameters such asstrain-rate sensitivity (m) and activation volume (V*)for nc-Ni are examined with the aim of understanding

1359-6462/$ - see front matter � 2011 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2011.07.021

⇑Corresponding author. E-mail: garimas@yahoo.com

the rate-controlling deformation mechanisms in nc-materials after irradiation.

nc-Ni foils of about 60 lm thickness were preparedby electrodeposition using a stainless steel substrate.The substrate was polished mechanically with differentgrades of SiC abrasive paper and finally diamond pol-ished to a 7 lm finish prior to deposition. The platingbath consisted of NiSO4, NiCl2 and Na3C6H5O7 withpH adjusted by the addition of dilute H2SO4. The depos-ited nc-Ni was removed from the substrate in the formof foil and characterized using X-ray diffraction(XRD) and transmission electron microscopy (TEM).Wavelength-dispersive spectrometry (WDS) analysiswas carried out on Cameca SX 600 with a beam sizeof 1 lm. WDS results showed 99.98% pure Ni. XRDwas carried out by using Cu Ka radiation (1.5406 A)and Si was used as an external standard for correctingthe broadening associated with the instrument. TEMsamples were prepared by jet polishing 3 mm discs using20% perchloric acid and 80% methanol at 20 V and�45 �C. TEM observations were conducted with aJEOL 2010 microscope operated at 200 kV using con-ventional bright-field and dark-field imaging. The Nifoils were irradiated with a proton beam of energy4 MeV using the Folded Tandem Ion Accelerator

sevier Ltd. All rights reserved.

Figure 2. Bright-field TEM image of (a) unirradiated nc-Ni with insetshowing selected-area diffraction pattern and grain-size distributionplot; (b) irradiated nc-Ni with inset showing grain-size distributionplot.

728 G. Sharma et al. / Scripta Materialia 65 (2011) 727–730

(FOTIA) facility at the Bhabha Atomic Research Cen-tre, Mumbai. The nc-Ni samples were irradiated upto a maximum fluence of 0.15 � 1018 protons cm�2 atroom temperature. The damage level obtained in nc-Ni foils was of the order of 0.004 dpa (displacementsper atom). In addition, bulk CG Ni was also irradiatedunder the same conditions so as to compare hardness ofnc-Ni with bulk Ni after irradiation. The samples weretested before and after irradiation using nanoindenta-tion with a Berkovich diamond indenter (UNHT,CSM, Switzerland) to investigate the effect of irradiationon mechanical properties. The P (load) vs. h (penetra-tion depth) measurements were carried out at 2000,4000 and 8000 lN load. Testing the change in strain ratewas done by changing the loading rate from 2500 to25,000 lN min�1, at a max. load of 4000 lN. Threeexperiments were conducted for each condition andaverage value of hardness was used to determine themechanical parameters.

Figure 1 shows the XRD pattern of the electrodepos-ited nc-Ni, with the major peaks corresponding to(1 1 1), (2 0 0), (2 2 0) and (3 1 1) reflections. The ratioof the major peak intensities indicates the grains are ran-domly oriented. The average grain size was estimated tobe �62 nm from the diffraction line width based onScherrer’s relation. Detailed dark-field TEM observa-tions also confirmed an average grain size of about60 nm. Figure 2a shows a bright-field TEM image withthe selected-area diffraction pattern and the grain sizedistribution (deduced from TEM) as insets. Note thatno grain refiners were employed in the present studyas the presence of grain refiners could possibly have asecondary effect on the grains and GBs during irradia-tion of nc-Ni.

The samples after irradiation were also investigatedby TEM in detail, but no significant change in the grainsize was noticed (see inset in Fig. 2b). During irradia-tion, high-energy particles knock atoms from their lat-tice sites, leading to collision cascades which last forsome picoseconds and generate point defects such as va-cancy–interstitial Frenkel pairs [3–5]. The high densityof GBs in nc-material can act as a sink for these pointdefects and thus counter swelling in the material. How-ever, it is well established that the annihilation of pointdefects is mostly dominated by vacancy–interstitialinteractions within a grain as compared to GBs. In fact,irradiation results can vary as these depend strongly onthe energy and the nature of particles used to irradiatethe material—proton, heavy ion, neutrons, etc. Albeet al. [3] used molecular dynamics simulation andshowed that ion irradiation in nc-Ni could lead to grain

20 40 60 80 100

200

400

600

(311)(220)

(222)

(200)

Inte

nsity

(arb

. uni

ts)

Two Theta (degrees)

(111)

Figure 1. XRD pattern of electrodeposited nc-Ni.

growth if the thermal spike volume is larger than thegrain size or overlaps the GB area. In the present study,we did not observe any change in grain size after irradi-ation. However, an increased dislocation density anddislocation loops were observed within the grains afterirradiation as shown in Figure 2b (marked by arrow).This could be due to the presence of vacancy-clustersformed during irradiation. Nita et al. [7] have reportedthat irradiation-induced microstructure in nc-Ni con-sists of a large density of stacking fault tetrahedra(SFT). SFT could not be observed in the present study,which may be due to different experimental conditions,e.g. proton beam energy, total damage in sample andgrain size of starting material used.

Figure 3a shows the representative load, P, vs. inden-tation depth, h, curves obtained on irradiated and unirra-diated nc-Ni and bulk CG-Ni samples. These P–h curveswere analyzed by employing the Oliver–Pharr method [8]to extract the hardness of the sample. The hardness ofirradiated nc-Ni as well as bulk CG-Ni was found to in-crease as compared to unirradiated samples. Insets inFigure 3a show the effect of the maximum applied load,Pmax, on load displacement curves on the irradiated nc-Ni sample. The hardness was found to be almost con-stant with the increase in Pmax from 2000 to 8000 lNin both irradiated and unirradiated samples. The in-crease in hardness of CG-Ni was found to be much morethan that observed in nc-Ni after irradiation. These re-sults indicated that under the same irradiation condi-tions, the hardening in CG-Ni was much more than intheir nc counterparts. The reason for this could be due

Figure 3. (a) Load–displacement curve for irradiated and unirradiatednc- and CG-Ni, with inset showing the effect of load on the hardness innc-Ni irradiated sample; (b) hardness vs. strain-rate plot for irradiated(marked as (a)) and unirradiated (marked as (b)) nc-Ni with insetshowing load–depth curves at different strain rates for irradiated nc-Ni.

4.6 4.8 5.0 5.210

20

30

40

50 Un-irradiated Ni

Act

ivat

ion

Volu

me

( V*/b

3 )

Hardness (GPa)

Irradiated Ni

Figure 4. Plot showing the variation of activation volume (V*) vs.hardness (H) for irradiated and unirradiated nc-Ni.

G. Sharma et al. / Scripta Materialia 65 (2011) 727–730 729

to the fact that the GBs present in high density in nc-Niact as a sink for some of the irradiation-induced defects.These results were found to be in agreement with thoseproposed in Ref. [5]. The mechanical behavior in CG-Ni after irradiation has already been reported in detail[9,10]. Figure 3b shows the effect of strain rate on hard-ness of both irradiated and unirradiated nc-Ni samples.The strain rate was varied by changing the applied load-ing rate from 2500 to 25,000 lN min�1, all at a constantpeak load of 4000 lN. As expected, the hardness wasfound to increase with the increased loading rate. Thestrain-rate sensitivity, m, was determined from the slopeof these plots. The unirradiated nc-Ni yields a value ofm � 0.041, which is in agreement with values reportedfor nc-Ni in the literature [11,12]. Note that amongstthe face-centered cubic (fcc) metals, nc forms are report-edly more strain-rate sensitive (typically by an order ofmagnitude) than their CG counterparts. This has beenattributed to the enhanced GB-mediated process in theformer [13]. After irradiation, nc-Ni showed a further in-crease in m value to 0.052 (Fig. 3b). The increase in defectdensity within the grains could be the reason for the high-er m values in irradiated nc-Ni [14]. The activation vol-ume, V*, was determined by using the followingrelation [15]:

V � ¼ 3ffiffiffi3p

KTD ln _eDH

� �; ð1Þ

where K is the Boltzmann constant and T is the absolutetemperature. Note that the activation volume estimatedby using Eq. (1) is an apparent one rather than true oneas one-third of the hardness obtained from indentationtests was used as representative of the yield stress incompression for calculating V*. The activation volumeis a measure of the volume of interaction of the disloca-tion with the obstacle and provides a good indication of

the type of mechanism responsible for this interaction.Figure 4 shows a correlation of the trends in activationvolume with indentation hardness/stress. As activationvolume, which is a structural parameter, changes, thisplot could be used to identify the deformation kineticsas a function of stress [16]. The activation volume ofirradiated nc-Ni did not follow the same trend as unirra-diated nc-Ni and showed an offset or a major shift in V*

towards higher hardness values as compared to unirra-diated nc-Ni. Since, in nc-Ni, there was no change incomposition or grain size after irradiation, the presenceof offset was attributed to the presence of irradiation-in-duced defects in the matrix. Wang et al. [11] have usedthe stress-relaxation technique to estimate V* and re-ported it to be around 10b3 for nc-Ni with a grain sizeof 30 nm. The activation volumes measured in thisstudy, as well as that reported by Wang et al. [11], aremuch lower than those obtained for CG fcc metals,which are usually in the range 100–1000b3. As shownin Figure 4, the activation volume decreased more rap-idly with an increase in hardness for unirradiated nc-Ni as compared to irradiated Ni. A sharp decrease inV* with increasing hardness (stress) in unirradiated nc-Ni indicated that dislocation density increases duringthe course of indentation/deformation with the increasein stress. This resulted in an increase in dislocation–dis-location interaction and thus a decrease in their interac-tion volume, and hence V*. In irradiated specimens, themain obstacles to gliding dislocation are not only dislo-cations generated during deformation but also radia-tion-induced defects which include vacancy–interstitialclusters in addition to dislocation–dislocation loops.As shown in Figure 4, V* remained more or less con-stant with increasing hardness, indicating that the inter-action of dislocations with irradiation-induced defectswas the dominant rate-controlling mechanism as thedensity of irradiation-induced defects did not changeduring the deformation process. Thus, the increasednumber of defects after irradiation and their interactionwith gliding dislocations resulted in a decrease in V*.

In summary, the present study showed the effect ofirradiation on the microstructure and mechanical prop-erties of nc-Ni. Irradiation resulted in hardening of nc-Ni. In addition, irradiated nc-Ni showed an increase instrain-rate sensitivity compared with unirradiated nc-Ni. The rate-controlling deformation mechanism forunirradiated nc-Ni was found to be dislocation–disloca-tion interaction, whereas in irradiated nc-Ni, the interac-tion of radiation-induced defects with dislocations wasthe dominant deformation mechanism. In fact, in

730 G. Sharma et al. / Scripta Materialia 65 (2011) 727–730

nc-materials, irradiation behavior strongly depends onthe grain size and GBs of the starting material as wellas the energy of the radiation source. For the develop-ment of new materials with optimized nanostructures,more dedicated efforts are required to understand theirradiation behavior, degradation of mechanical proper-ties, thermal stability and creep behavior during irra-diation before promoting these materials for nuclearapplications.

The authors would like to thank Dr. A.K. Suri,Director, Material Group, BARC for the encourage-ment provided in carrying out this work.

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