deformation-induced thermally activated grain growth in nanocrystalline nickel

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  • ata ann Ins

    27 Mline 6

    eenepositronghts r

    ion b

    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:prasad@materials.iisc.ernet.in; 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. [4] 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[8] that are very similar to those developed for superplas-tic ow in metals with typical grain sizes of 1 to 10 lm[9]. 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 [1] and nano-Cu[2] are prone to grain growth even at room temperaturewhereas nano-Ni is relatively stable up to T < 473 K [3].

    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 [6]. 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 [7].In contrast to large-strain superplastic ow in conven-tional alloys at high temperatures and low stresses, defor-Deformation-induced thermin nanocrys

    M.J.N.V. Prasad

    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.

    eam

    important factor in the observed grain growth. In con-trast, Gurao and Suwas [5] observed grain growth innano-Ni during rolling at room temperature, but notin a cryo-rolled sample.

    Deformation enhanced grain growth is a very com-

    6

    www.elsevier.com/locate/scriptamatsevier Ltd. All rights reserved.

  • 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 [10]:

    HH 0

    1 h

    h

    r1

    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 [11]. There are limited data on eect of grainsize on ISE in polycrystalline materials. Manika andManiks [12] reported no ISE in ne-grained polycrystal-line materials, where indentation size exceeds the grainsize. In contrast, Mirshams and Parakala [13] and Mirs-hams and Pothapragada [14] 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[15]. 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 [13]. 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 [16]. 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 possi

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