Deformation-induced thermally activated grain growth in nanocrystalline nickel
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ata ann Ins
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force for grain growth provided by the ner grain sizes.
mation modes such as indentation, scratch testing, uni-axial testing, high pressure torsion (HPT), rolling,
cryogenic temperatures than at room temperature,suggesting that stress rather than temperature is an
mon observation during superplastic deformation, with
mation induced grain growth in nanocrystalline materialsat or near room temperature involves much smaller
available only in small volumes. Apart from the complexstress states, the observation of an increase in hardnesswith decreasing depth (termed the indentation size eect,ISE) can complicate comparison between indentationstudies using diering loads, and also a comparisonbetween strengths obtained from hardness and uniaxialtests. The size eect on hardness has been observed in
Corresponding author. Present address: School of Engineering,Brown University, Providence, RI 02912, USA.; e-mail addresses:email@example.com; Prasad_Manepalli@brown.edu
Available online at www.sciencedirect.com
Scripta Materialia 67 (2012) 13313creep and superplasticity. The Supplementary Table S1lists such experimental data chronologically in somenanometals and alloys. There are several signicantpoints to note from Table S1. The nal grain sizereported varies with deformation mode, with uniaxialdeformation leading to less grain growth. Deformationenhanced grain growth has been reported under bothcryo- as well as elevated temperature creep conditions.Zhang et al.  reported greater grain growth in inertgas condensation nano-Cu during indentation under
strains and higher stresses. However, it is interesting tonote that there are models for deformation in nanometals that are very similar to those developed for superplas-tic ow in metals with typical grain sizes of 1 to 10 lm. The mechanism for deformation enhanced graingrowth in nanometals is not clear, although there havebeen proposals based on stress-driven grain boundarymigration, grain rotation and coalescence.
Hardness tests have provided a convenient means ofevaluating the mechanical characteristics of nanometalsThere have been several reports on the thermal stabilityof nanocrystalline materials, but the data are not alwaysconsistent. Thus, for example, nano-Pd  and nano-Cu are prone to grain growth even at room temperaturewhereas nano-Ni is relatively stable up to T < 473 K .
Grain growth accompanying deformation has beennoted in nanocrystalline materials under dierent defor-
the nal grain sizes increasing drastically by a factor ofup to 8 during superplastic ow . Such grain growthhas been attributed frequently to grain boundary sliding,with the disturbance of triple junctions providing anenhancement in the driving force for grain growth .In contrast to large-strain superplastic ow in conven-tional alloys at high temperatures and low stresses, defor-Deformation-induced thermin nanocrys
Department of Materials Engineering, India
Received 27 February 2012; revisedAvailable on
Grain growth during indentation at low temperatures has bathermal in nanometals. Indentation experiments on electrodgrowth decreases with an increase in temperature, suggesting s 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rig
Keywords: Nanocrystalline nickel; Indentation; Grain growth; Focused
Nanocrystalline materials have been a focus ofextensive research over the past two decades due to theirmany superior properties. However, the stability ofthese ultrane grain sizes is a matter of scientic andtechnological concern, due to their increased driving1359-6462/$ - see front matter 2012 Acta Materialia Inc. Published by Elhttp://dx.doi.org/10.1016/j.scriptamat.2012.03.041lly activated grain growthlline nickel
d A.H. Chokshi
titute of Science, Bangalore 560 012, India
arch 2012; accepted 29 March 2012April 2012
taken to imply that grain growth is largely stress induced andted nano-Ni indicate clearly that the load required for grainly that concurrent grain growth is thermally activated.eserved.
important factor in the observed grain growth. In con-trast, Gurao and Suwas  observed grain growth innano-Ni during rolling at room temperature, but notin a cryo-rolled sample.
Deformation enhanced grain growth is a very com-
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GND even under conditions where there is no signi-cant intragranular dislocation activity.
Figure 2 illustrates the eect of indentation load onmicrostructural evolution underneath the indents per-formed at room temperature (300 K). There was signif-icant grain growth underneath the indents, especially atloads >3 N. Some large grains tended to have an elon-gated shape initially; the possibility of low angle grainboundaries within such grains could not be ascertaineddue to the inability to use electron backscattered detec-tor in the present experimental setup. The extent ofgrain growth increased with increasing load and wasclose to that of depth of penetration. Moreover, therewas saturation in coarsest grains size at 1 lm. The in-creased eective grain size during indentation, accordingto the HallPetch relationship, can lead to reduction inhardness with increasing indentation loads.
Figure 3ac shows the FIB micrographs of indents
/ Scriboth single crystals and polycrystals. The strain gradientplasticity (SGP) model uses the concept of geometricallynecessary dislocations (GND) to rationalize the obser-vations :
whereH is the hardness of material,H0 is the hardness re-lated only to the statistically stored dislocations, h is thedepth of penetration, and h* is the characteristic lengththat indicates depth dependence of the hardness. It hasbeen noted that this approach cannot be used to explainthe ISE observed in ceramics, where dislocation activity isnegligible . There are limited data on eect of grainsize on ISE in polycrystalline materials. Manika andManiks  reported no ISE in ne-grained polycrystal-line materials, where indentation size exceeds the grainsize. In contrast, Mirshams and Parakala  and Mirs-hams and Pothapragada  reported ISE in nanocrys-talline Ni with a grain size of 20 nm, and attributed itto indenter geometry and dislocation-based mechanismsassisted by grain boundary sliding, without any micro-structural observations. The present investigation wasundertaken with the specic objective of studying the ef-fect of indentation load and test temperature on micro-structural evolution in nanocrystalline Ni underneaththe indents.
The present study utilized pulsed electrodepositednanocrystalline Ni with an initial grain size of 20 nm ob-tained commercially (Integran Technologies, Canada);the nano-Ni contained 300 ppm sulfur and 200 ppmcarbon. The as-received material exhibited a narrowlog-normal distribution of grain sizes, measured usingtransmission electron microscopy. Microindentationexperiments were performed on nano-Ni samples an-nealed for 0.5 h at 473 K, with a grain size of 30 nm. Tests involved three dierent temperatures, 300,373 and 428 K, and loads ranging from 1 to 12 N; thedepths of indentations ranged from 2.5 to 11 lm. Micro-indentation experiments were conducted using Vickersindenter made of zircon in a CSM hot-stage depth sens-ing hardness tester. The loading and unloading timeswere kept constant at 1 min, with a zero dwell time atall loads. Selected indents were trenched up to a depthof 30 lm, using focused ion beam milling (FEI-FIB)operating at 30 kV. The indentation cross-sections,milled at a current of 11.5 nA and polished at 2.7 nA,were tilted to 45 for microstructural characterizationunderneath the indents. Selected isothermally annealednano-Ni samples were also examined in a eld-emissionscanning electron microscope (FEI-SIRION).
Figure 1a shows the variation in microhardness as afunction of load at three dierent temperatures. Thereare three signicant points to be noted. First, the hard-ness decreased initially with increasing load and thenreached steady state beyond a certain load. Second,the load for onset of steady state decreased with increas-ing test temperature. Third, for a given load the hard-ness decreased with increasing test temperature. Sincethe trend noted is similar to ISE, the data were plotted
134 M. J. N. V. Prasad, A. H. Chokshiin the form represented by Eq. (1), Figure 1b. The slopeof the linear t in such a plot gives (H 20h
), whereas theintercept at 1/h = 0 gives H 20. The H0 value decreasedslightly with increasing test temperature from 4.2, 4.0and 3.8 GPa at 300, 373 and 428 K, respectively; notethat the H0 value at room temperature was signicantlylower than the value of 6 to 7 GPa reported for nano-Ni by Mirshams and Parakala . The depth depen-dence of the hardness h* also decreased with increasingtest temperature from 2.6, 1.2 and 0.7 lm at 300, 373and 428 K. This rst report of a reduction in H0 andh* with an increase in temperature can be rationalizedin terms of enhanced recovery and a reduction in statis-tically stored dislocation density due to a lower owstress.
Although the experimental data in Figure 1b followexpectations of a model for strain gradient plasticity, itis known that there is no signicant intragranular dislo-cation storage in nano-Ni . The retention of equi-axed grains after plastic deformation strongly supportsthe occurrence of grain boundary sliding and rotation.Such a process will lead to regions of material overlapand holes, so that the concept of geometrically necessarydislocations can be extended to internal strains resultingfrom the maintenance of grain contiguity. Conse-quently, it may be possible to invoke the concept of
Figure 1. (a) Variation in microhardness for nano-Ni as a function ofindentation load, and (b) indentation size eect plot of H2 vs. 1/h atthree dierent test temperatures.
pta Materialia 67 (2012) 133136obtained at 1 N for three dierent temperatures of300, 373 and 428 K respectively. Grain growth under-
Figure 2. Microstructural evolution underneath indents at 300 K and
M. J. N. V. Prasad, A. H. Chokshi / Scrineath the indents increased with increasing test temper-ature; note that these test temperatures were less thanthe pre-annealing temperature of 473 K. In comparison,earlier data on static annealing revealed that more than144 h at 473 K  and 0.5 h at 573 K (Fig. 3d) werenecessary for nano-Ni to have a similar microstructureto that observed during indentation at 12 N for 300 Kand 1 N for 428 K. Clearly, the indentation processhas signicantly enhanced grain growth, even at highertemperatures.
As shown in the supplementary table, there have beenmany experimental observations of grain growth innanocrystalline metals, with many results being attrib-uted to stress-assisted grain growth. It is interesting tonote that there was no grain growth in compressionexperiments on nano-Ni involving a maximum stressof 1.9 GPa ; this stress leads to a hardness of5.7 GPa with a Tabor factor of 3. Figure 1 shows that
loads of (a) 1, (b) 3, (c) 6, and (d) 12 N.the hardness values at room temperature range from5.9 GPa at 1 N to 4.7 GPa at 12 N, so that indenta-tion did not involve higher stresses compared to com-
Figure 3. FIB micrographs of indents performed at 1 N at tempera-tures of (a) 300, (b) 373, and (c) 428 K; (d) secondary electronmicrograph of nano-Ni annealed for 0.5 h at 573 K.pression testing. The lack of signicant grain growthduring uniaxial deformation in nano-Ni  suggeststhat strain gradient plasticity and the complex stressstate are important factors in the observed signicantgrain growth in nano-Ni during indentation. Extensivegrain growth has been reported at 77 K and room tem-perature in nano-Cu during both uniaxial compressiontesting and indentation testing; this contrasts with thelack of grain growth during uniaxial compression testingin nano-Ni, although indentation leads to signicantgrain growth. The dierences in behavior of nano-Cuand nano-Ni may be related to variations in impuritycontent as well as stacking fault energies, and this de-serves further attention.
Since FIB milling may lead to Ga ion induced dam-age during sample preparation [18,19], microstructuralexamination was carried out on both sides of the FIBtrench to evaluate this eect. The lack of grain growthon the undeformed surface of the trench suggests thatthe current observations of grain growth are not re-lated to FIB milling. Moreover, signicant changes inmicrostructure were noted underneath the indents ob-tained for the same indentation load (1 N) at dierenttemperatures (Fig. 3a-c), with the same FIB millingtime.
In comparison to earlier studies (Table S1), the size ofthe large grains in the present study was very high(1 lm), possibly due to the higher loads used. Thereis a possibility of grain growth even at low loads, witha grain size below the FIB channeling resolution limit.Since Vickers indenters (with a zero radius of curvaturefor tip) induce geometrically self-similar indentations atany load, the stress and strain levels are not expected tovary with load. The experimental observations suggestthat grain growth initiates near the tip of the indenterand then progress out to the periphery of the elastic/plastic zone.
It is known that plastic deformation in polycrystals is aheterogeneous process. Thus, for a higher load, there is alarger volume undergoing deformation, and it is possiblethat there is a critical deformation/strain energy level at agiven location to enable grain growth. Although the de-tails of the process are not clear, the experimental obser-vations of an inverse relationship between temperatureand load suggest strongly that grain growth is a deforma-tion-induced thermally activated process.
This work was supported by the Department ofScience and Technology (DST), India.
Supplementary data associated with this article canbe found, in the online version, at http://dx.doi.org/10.1016/j.scriptamat.2012.03.041.
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