(6.7)--Superplasticity in electrodeposi机械工程材料机械工程材料.pdf

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1、Superplasticity in electrodeposited nanocrystalline nickelM.J.N.V.Prasad*,A.H.ChokshiDepartment of Materials Engineering,Indian Institute of Science,Bangalore 560 012,IndiaReceived 31 May 2010;received in revised form 17 June 2010;accepted 17 June 2010AbstractElectrodeposited nanocrystalline Ni film

2、s were processed with different levels of S,to evaluate the role of S on superplasticity.All thematerials exhibited high strain rate superplasticity at a relatively low temperature of 777 K.Microstructural characterization revealedthat the S was converted to a Ni3S2phase which melts at 908 K;no S co

3、uld be detected at grain boundaries.There was no consistentvariation in ductility with S content.Superplasticity was associated with a strain rate sensitivity of?0.8 and an inverse grain size expo-nent of?1,both of which are unusual observations in superplastic flow of metals.Based on the detailed e

4、xperiments and analysis,it isconcluded that superplasticity in nano-Ni is related to an interface controlled diffusion creep process,and it is not related to the presenceof S at grain boundaries or a liquid phase at grain boundaries.?2010 Acta Materialia Inc.Published by Elsevier Ltd.All rights rese

5、rved.Keywords:Nanocrystalline;Nickel;Electrodeposition;Sulfur;Superplasticity1.IntroductionThe ability of some fine-grained polycrystalline materialsto exhibit large elongations to failure of 500%,termedsuperplasticity,is of interest from both a scientific and atechnological viewpoint;the phenomenon

6、 is being exploitedto form components with complex shapes 1.The mechan-ical characteristics of superplastic materials are generallyrepresented as follows:_ e ADGbkTbd?prG?n1where _ e is the strain rate,A is a dimensionless constant,D isthe diffusion coefficient,G is the shear modulus,b is themagnitu

7、de of the Burgers vector,k is Boltzmanns constant,T is the absolute temperature,d is the grain size,r is the im-posed stress,and p and n are constants termed the inversegrain size exponent and stress exponent,respectively.Thediffusion coefficient can be expressed as D=D0exp(?Q/RT),where D0is the pre

8、-exponential term,Q is the appro-priate activation energy,and R is the gas constant.In super-plastic materials,the high temperature mechanical data arefrequently plotted as stress vs.strain rate on a logarithmicscale and expressed as r B_ em,where B is a constant,andm is termed the strain rate sensi

9、tivity;comparison of thisexpression with Eq.(1)indicates that m=1/n.Superplastic-ity is usually associated with m 0.3 or n 3,as such con-ditions retard flow localization.Grain boundary sliding(GBS)is the dominant straincontributing process in superplasticity,although the preciserate controlling defo

10、rmation mechanism is not yet clearlyestablished.In metals,the standard approach assumes thatGBS is controlled by dislocation nucleation from triplejunctions,movement of intragranular dislocations andtheir annihilation at other boundaries 15.In ceramics,GBS is frequently attributed to interface contr

11、olled diffu-sion creep 69.Superplasticity in metals requires a fine grain size0.4Tm,whereTmis the absolute melting temperature.These two contra-dictory requirements have led to microstructural design ofsuperplastic metals based on either microduplex alloys orquasi-single-phase alloys,with changes in

12、 chemistry andcrystal structure or fine second-phase particles limiting1359-6454/$36.00?2010 Acta Materialia Inc.Published by Elsevier Ltd.All rights reserved.doi:10.1016/j.actamat.2010.06.047*Corresponding author.Tel.:+91 80 2293 2684;fax:+91 80 2360 0472.E-mail address:prasadmaterials.iisc.ernet.i

13、n(M.J.N.V.Prasad) online at Acta Materialia 58(2010)57245736grain growth,respectively.In addition,superplasticity isgenerally not observed under conventional quasi-staticstrain rates in pure metals because of rapid grain growthat elevated testing temperatures,although large deforma-tion in Cu shaped

14、-charge liners deforming at ultrahighstrain rates of?106s?1(and short times)has been attrib-uted to superplasticity 10.Conventional superplasticity in a microduplex Ti6Al4 V and a quasi-single-phase 7475 Al alloy,with grain sizesof?10 lm,involve strain rates of?10?4s?1and testingtemperatures close t

15、o?0.9Tm.Clearly,from an energy-sav-ings and economics point of view,it is desirable to developsuperplasticity at lower temperatures and higher strainrates.Eq.(1)suggests that a decrease in grain size can leadto either an increase in strain rate at a constant tempera-ture or a decrease in temperature

16、 at a constant strain rate.This approach has been used successfully to develop highstrain rate superplasticity(strain rates 10?2s?1)or lowtemperature superplasticity 1,11,12;it has generally notbeen possible to obtain high strain rate superplasticity atlow temperatures.Higashi et al.13 attributed hi

17、gh strainrate superplasticity in Al alloys and composites to the pres-ence of a liquid phase at grain boundaries.Nanocrystalline materials can be processed using a top-down approach such as severe plastic deformation(SPD)or a bottom-up approach such as electrodeposition 14.There has been substantial

18、 research and development activ-ity in SPD,and Kawasaki and Langdon 15 have recentlyreviewed all the available data on superplasticity in suchultrafine-grained metals.In contrast to SPD,there hasnot been much activity related to superplasticity in materi-als processed by electrodeposition.Nanocrysta

19、lline Ni has become a model material forstudying deformation in electrodeposited metals becauseof the general ease in producing materials with grain sizesdown to?20 nm.In spite of the early suggestions of dif-fusion creep at room temperature in nanometals 16,aswell as the possibility of superplastic

20、ity 17,experimentson nano-Ni at room temperature have shown very lim-ited ductility of 10%18,19.The possibility of bimodalgrain size distributions leading to enhanced ductility havealso not been borne out in nano-Ni 19,20.Experimentsat higher temperatures have yielded significant ductility21,and sup

21、erplasticity has been reported in some stud-ies on nano-Ni and composites 2230.The experimentaldata on superplasticity in nano-Ni are summarized inTable 1 in terms of the materials,temperature,strain rateand stress ranges,m,initial grain size,gauge lengths andmaximum ductility.It is clear that,while

22、 there are severalreports of large elongations to failure,there is verylimited information available on the mechanical charac-teristics of the materials,with no data on the values ofp or Q.Furthermore,superplasticity has been attributedvariously to GBS enhanced by S at grain boundaries23,GBS assiste

23、d by twinning 26,29 and GBS pro-moted by a semi-liquid nickel sulfide phase at grainboundaries 25.In most of the earlier studies,there has been no attemptto control or modify the level of S in the nano-Ni sheets,sothat the influence of S content on superplasticity is notknown.Furthermore,it is well

24、known that Mn additionto steels leads to scavenging of S to form MnS particles,thereby mitigating the deleterious role of S in embrittle-ment 31.Clearly,if S at grain boundaries is a cause forsuperplasticity in nano-Ni,development of suitable nanoNi alloys with Mn can provide some useful data;thereh

25、as been only one study to examine deformation in electro-deposited nano NiMn 32.There are also very limitedTable 1Previous experimental observations of superplasticity in nano-Ni.MaterialT(K)Strain rate(s?1)Stress range(MPa)mGrain sizeGauge length(mm)Maximumelongation(%)ReferencesL0(nm)LT(lm)Lf(lm)N

26、i6238331?10?3703600.250.5035500.5121895McFadden et al.22Ni6837238?10?42?10?20.500.5565210550Chan et al.24Ni7238?10?45?10?360702.510380Zhang et al.25Ni22.6 at.%Co7238231?10?31?10?220500.50201210279Wang et al.26Ni0.5 wt.%SiC6437038?10?410162001210571Chan et al.27Ni1 wt.%SiC6837238?10?45?10?2401210836C

27、han et al.28Ni5 wt.%Si3N46737533?10?32?10?210600.50500.71210635Chan et al.29Ni3 wt.%ZrO27238?10?42?10?290451.810605Zhang et al.25NiCoSi3N46738231?10?31?10?2201400.4520110692Wang and Chan30L0is the grain size in as-deposited condition,LTgrain size at test temperature,and Lfgrain size near fracture ti

28、p.M.J.N.V.Prasad,A.H.Chokshi/Acta Materialia 58(2010)572457365725data available on texture evolution during superplasticityin such materials.The present study was undertaken with the specificobjectives of evaluating the mechanical and microstruc-tural characteristics of superplasticity in electrodep

29、ositednano-Ni to identify potential rate controlling mechanisms,with a specific emphasis on clarifying the role of S in suchbehavior.2.Experimental materials and procedurePulsed electrodeposition under galvanostatic conditionwas used to produce nano-Ni with different sulfur contentand a nano NiMn al

30、loy.Pulsing with an on-time of 5 msand an off-time of 40 ms was applied at a current density of430 mA cm?2.A modified Watts bath was used as an elec-trolyte for deposition.Nano-Ni with different S contentwas obtained by varying the quantity of saccharin(0.2,2and 20 g L?1)added to the electrolyte.Man

31、ganese chloride(MnCl2)(7 g L?1)was added as a source for Mn in thedeposit.Nano-Ni foils obtained from a commercial source(Integran Technologies Inc.,Canada)produced by electro-deposition were also used in the present study(nano-Nic).The nominal thickness of the as-deposited foils was?150 lm.The sulf

32、ur and carbon content in the foils was obtainedby combustion analyzer(LECO)using the ASTM E1019-02 standard test method.The manganese content in theas-deposited nano-NiMn alloy was determined by analkalimetric method.X-ray diffraction(XRD)measure-ments were performed using Cu Ka radiation at 40 kVan

33、d 30 mA in a Panalytical Xpert-pro XRD machine.Grain sizes and microstrains of foils in as-deposited condi-tion were determined by the WarrenAverbach(WA)method using the(1 1 1)(2 2 2)peaks pair.Tensile tests were performed at constant crossheadspeed using specimens with a gauge length of 5 mm and ag

34、auge width of 2 mm,cut from foils by electric dischargemachining(EDM).To remove the EDM affected zone,all tensile specimens were electropolished,prior to testing,in an electrolyte of 90%ethanol+10%perchloric acid ata voltage of 20 V and temperature of?260 K.Tensile testswere performed in air at temp

35、eratures in the range 573777 K and at initial strain rates in the range 3?10?37?10?1s?1.Strain rate jump tests were conducted todetermine the strain rate sensitivity(m)during deforma-tion.To evaluate the inverse grain size exponent(p),nano-Ni samples with three different initial grain sizes weredefo

36、rmed up to an elongation of 35%;in these tests,therewas no significant difference in grain size before and afteran elongation of 35%.Bulk X-ray texture of the as-deposited and deformedsamples was measured using a Bruker Discover-8 XRDmachine.The evolution of grain size in deformed samplesat the grip

37、 and near the fracture tip was characterized bysecondary electron microscopy(SEM,SIRION field emis-sion gun).Selected samples were examined by transmissionelectron microscopy(TEM,Tecnai-F30 field emissionTEM,)for characterizing microstructure in as-depositedand deformed conditions.TEM samples were p

38、reparedby ion milling at 5 kV in a precision ion polishing system.Quantitative elemental microstructural characterizationwas performed in scanning transmission electron micros-copy(STEM)mode with probe size?3 nm;a limited highresolution study was also conducted to examine the possi-bility of a thin

39、grain boundary phase.A linear intercepttechnique was used to measure the grain size from electronmicrographs,and the mean linear intercept grain size isgiven as L.3.Experimental resultsTable 2 summarizes the data on the S,C and Mn contentof the alloys studied,together with the grain sizes obtainedby

40、 XRD;additional information is also provided here,anddiscussed later,on the grain size at the test temperaturebefore and after deformation,the flow stresses at a strainof 10%and the ductility for samples tested at 777 K and astrain rate of 3?10?2s?1.The subscripts C,LS,MS andHS refer to the commerci

41、al material and nano-Ni withlow,medium and high sulfur content.It is clear that allmaterialshadarelativelylowcarboncontentof200 ppm,andtheScontentvariedfrom300to1900 ppm;the NiMn alloy contained 0.28 wt.%Mn.Theas-deposited grain sizes were 20 nm for commercial nano-Ni and?10 nm for nano-Ni with thre

42、e sulfur contentsand the NiMn alloy;XRD revealed a microstrain of?0.3%in the as-deposited condition for all films.The grainsizes measured from TEM dark-field images were consistentwith XRD measurements.Fig.1a and b shows bright fieldand dark-field images of the as-deposited nano-NiMS.Thegrain size d

43、istribution of this material is log-normalTable 2Chemistry and microstructures of various nano-Ni samples.MaterialS(ppm)C(ppm)Grain size in as-depositedcondition(L0)(nm)Grain size at777 K(LT)(nm)Grain size nearfracture tip(Lf)(nm)Stress at a strain of10%(r10%)(MPa)Elongation tofailure(ef)(%)Nano-Nic

44、31021019680 202250 140252520Nano-NiLS70010013500 2019510Nano-NiMS950800%to2520%.The nano-NiMSformed the base composition usedfor most of the experiments,and the results obtained onthis composition are used to describe the deformation char-acteristics of nano-Ni.Fig.2a shows the tensile behavior of n

45、ano-Ni as a func-tion of test temperature at an initial strain rate of3?10?3s?1in terms of the variation with elongation inthe nominal true stress.It is clear that there is a transitionfrom high stress and low ductility at T 6 625 K to low stressand large ductility at T P 679 K.The large elongations

46、 tofailure(ef)and low stresses at high temperature are charac-teristic of superplastic materials.The effect of initial strainrate on tensile deformation of nano-NiMSat 777 K is shownin Fig.2b:the strength increased with increasing strain rate.Therearethreeimportantobservationstonote.Thematerialexhib

47、its very large elongations to failure under some condi-tions,with a maximum elongation to failure of?1880%at777 K and 7?10?2s?1.An abrupt change in the flow curveisobservedatanelongationof?1500%;thisreflectsthelargeelongationatwhichthespecimenwasbeginningtocomeoutof the furnace.Serrations are clearl

48、y observed in specimenstested to large elongations at high strain rates.The photo-graphs of an undeformed specimen with a gauge length of5 mm and a specimen deformed to an elongation to failureof 1880%at an initial strain rate of 7?10?2s?1are shownin Fig.2c;the fairly uniform flow in the fractured s

49、ampleis consistent with observations of flow in superplasticmaterials.Fig.3 illustrates the high temperature deformation char-acteristics of nano-NiMSas a function of initial strain rateat 725 and 777 K.The lower part of this figure shows thevariation in elongation to failure with initial strain rat

50、e,whereas the upper part shows on a logarithmic scale thevariation with strain rate in true stress at a strain of 10%.Since the slope of logarithmic plot between stress andstrain rate gives strain rate sensitivity m(Eq.(1),it is clearthat the nano-NiMSexhibited a transition in strain rate sen-sitivi