Influence of Grain Boundary Character and Annealing Time on Segregation in Commercially Pure ...

16 Pages·2016·22.36 MB·English

Hindawi Publishing Corporation
Journal of Materials
Volume 2016, Article ID 4597271, 15 pages
http://dx.doi.org/10.1155/2016/4597271
Research Article
Influence of Grain Boundary Character and Annealing
Time on Segregation in Commercially Pure Nickel
SheryL.Welsh,1 MonicaKapoor,2OliviaD.Underwood,3RichardL.Martens,4
GregoryB.Thompson,2 andJeffreyL.Evans1,5
1MaterialsScienceProgram,UniversityofAlabamainHuntsville,USA
2DepartmentofMetallurgical&MaterialsEngineering,TheUniversityofAlabama,Tuscaloosa,AL,USA
3MechanicsofMaterials,SandiaNationalLaboratories,Albuquerque,NM,USA
4TheCentralAnalyticalFacility,UniversityofAlabama,Tuscaloosa,AL,USA
5DepartmentofMechanicalandAerospaceEngineering,UniversityofAlabamainHuntsville,USA
CorrespondenceshouldbeaddressedtoSheryL.Welsh;[email protected]
Received18October2015;Revised2January2016;Accepted5January2016
AcademicEditor:BengtG.Svensson
Copyright©2016SheryL.Welshetal.ThisisanopenaccessarticledistributedundertheCreativeCommonsAttributionLicense,
whichpermitsunrestricteduse,distribution,andreproductioninanymedium,providedtheoriginalworkisproperlycited.
Commercially pure nickel (Ni) was thermomechanically processed to promote an increase in Σ3 special grain boundaries.
Engineering the character and chemistry of Σ3 grain boundaries in polycrystalline materials can help in improving physical,
chemical,andmechanicalpropertiesleadingtoimprovedperformance.Type-specificgrainboundaries(specialandrandom)were
characterizedusingelectronbackscatterdiffractionandthesegregationbehaviorofelementssuchasSi,Al,C,O,P,Cr,Mg,Mn,
B,andFe,attheatomiclevel,wasstudiedasafunctionofgrainboundarycharacterusingatomprobetomography.Theseresults
showedthattherandomgrainboundarieswereenrichedwithimpuritiestoincludemetaloxides,whileΣ3specialgrainboundaries
showedlittletonoimpuritiesatthegrainboundaries.Inaddition,theinfluenceofannealingtimeontheconcentrationofsegregants
onrandomgrainboundarieswasanalyzedandshowedclearevidenceofincreasedconcentrationofsegregantsasannealingtime
wasincreased.
1.Introduction Segregation, a local enrichment at the GB, can alter the
mechanical and physical properties including tensile
Nickel is the major alloying element in some of the most strength, ductility, and fracture toughness, electrical prop-
advanced alloys used today in applications ranging from erties, and so forth [5]. GB structure, as compared to the
nuclearpowerplantcomponentstohightemperatureaircraft bulkstructure,isverydifferentandexhibitsitsownunique
enginecomponents[1,2].Therefore,studyingthecharacter environment, structure, composition, and energy levels at
and chemistry of grain boundaries in commercially pure theatomiclevel[6].ThisGBstructureservestoeitherattract
nickel, known as Nickel 200, could help elucidate various or repel segregants [7]. The process of segregation leads to
fundamentalmicrostructure-propertyrelationshipsofthese a decrease in the energy of the system following the ther-
materials.Engineeringthecharacterandchemistryofgrain modynamic principles of Gibbs free energy as the GB tries
boundariesinpolycrystallinematerialscanhelpinimproving to attain the lowest energy possible [6, 8, 9]. In addition,
physical, chemical, and mechanical properties leading to preferential interactions also exist between the segregants
improvedperformance[3].However,mostoftheworkdone themselveswhichcanaffectsegregationbehavior[4].
on grain boundary (GB) segregation has been in the mod- Grain boundaries can be classified as low angle (angle
eling and computational arenas [4]. Limited studies exist, of misorientation <10–15∘), high angle, and coincident site
whichinvestigatetheGBcharacter’sinfluenceonsegregation lattice (CSL) [10]. CSL boundaries are characterized by
behaviorattheatomiclevelforNi[4]. misorientationsthatallowatomsfromneighboringlatticesto
2 JournalofMaterials
Table1:Specimenparameters.
∘
Specimen GBtype Strain(cold-rolled) Annealingtemperature(C) Annealingtime(hours)
1 RGB 3% 1000 30
1 SGB 3% 1000 30
2 RGB 3% 1000 1
coincidenotionally[11].Thefractionofatomsthatcoincide and RGBs followed by atom probe tomography(APT). The
at the boundary is called the density of coincidence sites goal of this research was to study GB segregation behavior
[11]. The reciprocal of this density of the coinciding sites is experimentallyattheatomiclevelusinglocalelectrodeatom
used to describe a CSL boundary and is designated as Σ probetomography(LEAP).
[12, 13]. For example, Σ3 boundary will have one-third of
theatomsineachofthe2grainscoincidingnotionally.Low 2.ExperimentalProcedures
Σ CSL boundaries, 3 ≤ Σ ≤ 29, are called special grain
boundaries(SGBs)andhighΣCSLboundarieswithΣ >29 CommerciallypureNi,3.12mmthicksheet,wasthermome-
are called random grain boundaries (RGBs) [13, 14]. Σ3, a chanicallyprocessedtodrivesegregation[3].Niwasstrained
specialboundary,hasthelargestnumberofcoincidinglattice (cold-rolled) to reduce the thickness by 3% followed by
∘
points which translates into a more ordered structure and annealingat1000 Cinairfor30hoursand1hourandwas
lower free volume [8] and GB energy when compared to a labeled as specimens 1 and 2, respectively. SGB and RGB
random grain boundary [8, 15]. This results in SGBs being wereextractedfromspecimen1andRGBwasextractedfrom
slowerdiffusionpathways[8,15]. specimen 2. Several attempts were made to extract SGBs
Altering the GB character distribution in a microstruc- from specimen 2 and prepare them for APT analysis but
ture is known as “GB Engineering” (GBE) [3, 15]. GBE theywereunsuccessful.Table1showsthethermomechanical
consists of single or multiple thermomechanical processing processingparametersforeachspecimen.
cyclesthatincludecombinationsofstrainingandannealing EBSDwasconductedonJEOLJSM-7000FFieldEmission
toalterthedistributionofthedifferenttypesofgrainbound- ScanningElectronMicroscope(FESEM).TheEBSDGBmaps
aries[3].Particularprocessingcombinationshavebeenfound wereobtainedwithastepsizeof2micronsat30kVfollowed
toproducehigherfractionsofSGBswhicharekeytoimprov- by analyses using the HKL Channel 5 software program
ingmaterialperformance[16–18].Ithasbeenshowninsev- where Brandon criterion was applied [24]. EBSD was used
eralstudiesthatwhenthefractionofspecialgrainboundaries to identify the grain boundaries based on their angles of
(Fsp) increased, the resistance to the following properties misorientation and provided a map of SGBs and RGBs as
increased:creep,cavitation,fracture,corrosion,intergranular shown on the micrographs of Figures 1(a), 1(e), 2(a), and
corrosion, grain boundary sliding, and cracking [8, 19–21]. 2(e). Based on CSL statistics, specimen 1 SGB was Σ3 grain
∘
Grainboundarycharacterdistribution(GBCD)characterizes boundarywithamisorientationangleofapproximately3.575
thegrainboundariesbyprovidingthefractionofeachtype asthenumberfractionofΣ3grainboundarieswas0.34,Σ9
ofboundaryfoundinthemicrostructure[10].Σ3,orspecial was0.022,andΣ27awas0.0038.Specimen1RGB,basedon
boundary, is the main contributor to GBCD optimization CSLstatistics,wasΣ47bwithamisorientationangleof57.125∘.
[10,14]. Specimen 2 RGB is Σ43c based on frequency percentage of
The above discussion suggests that it is possible to CLSboundariesgiveninthehistogramstatistics.
thermomechanicallyprocessNi-basedalloystoincreasethe FEI3DQuantawasusedtoliftoutthegrainboundaries
SGBswhichshouldresultinimprovedsegregationresistance usingatypicalbeamcurrentrangingfrom2.3nAto7nAand
toimpurities.MostGBstudieshavefocusedonmodelsand beam energies of 30kV and 5kV for milling and cleanup,
simulationsbecauseexperimentalstudiesonsegregationare respectively.EachGBwasapproximately20𝜇minlengthand
difficulttoconductduetothedifficultiesintherandomness produced10–12atomprobetips.TheSEMimagesofthetip
ofsamplepreparationandexperimentalprocedures,identi- preparation for specimen 1 are shown in Figures 1(b), 1(c),
fyingthetype-specificGBinsuchasmallvolumeofmaterial, 1(f),and1(g),andthoseforspecimen2areshowninFigures
isolatingitforextraction,preparingthesample,andstudying 2(b),2(c),2(f),and2(g).
it atomically [22]. In order to understand the controlling APT studies were conducted on an IMAGO (Cameca)
mechanisms,itisimperativetostudythemattheatomiclevel. LEAP3000X-Si.LEAPprovidestherequiredsensitivityand
Astudywasdesignedbytheauthorstothermomechan- resolution (spatial resolution of 0.1nm–0.5nm) to study
ically process Ni to promote the formation of SGBs and intergranular segregation at the atomic level [25]. An atom
then atomically investigate the concentration of segregants probefieldevaporatesindividualatomsbyapplyingstanding
in SGBs and RGBs. In addition, the effect of increased voltageto aneedle-shapedspecimen (<50nmradius)over-
annealingtimeon2RGBsandtheirsegregationbehaviorwas laid with short laser pulses that cause ionization [24]. The
studied.Weutilizedelectronbackscatterdiffraction(EBSD) emittedionshitapositionsensitivedetectorandthiscoupled
tocharacterizethegrainboundariesandafocusedionbeam with time-of-flight mass spectrometry provides the posi-
(FIB)forsite-specificlift-outprocedure[23]toisolateSGBs tions and chemical identities for a 3D atom-by-atom LEAP
JournalofMaterials 3
85nm 85
nm
m
n
0
9
225𝜇m 2𝜇m 4𝜇m
(a) (b) (c) (d)
100nm 100
nm
m
n
0
0
1
225𝜇m 2𝜇m 2𝜇m
(e) (f) (g) (h)
Figure1:(a)Specimen1EBSDGBmapsidentifyingtheSGB(inred)extractedforanalysis(locationofextractionhighlightedinyellow).(b-c)
Specimen1SGBSEMimagesoftheFIBpreparationoftheAPTtip.(d)Specimen1SGB3Datom-by-atomLEAPtomographyreconstruction
showingthecylindricalROIforanalysis(ionsshownareSionly;100%ofionsareshown).(e)Specimen1EBSDGBmapsidentifyingtheRGB
(inblue)extractedforanalysis(locationofextractionhighlightedinyellow).(f-g)Specimen1RGBSEMimagesoftheFIBpreparationofthe
APTtip.(h)Specimen1RGB3Datom-by-atomLEAPtomographyreconstructionshowingthecylindricalROIforanalysis(ionsshownare
Sionly;100%ofionsareshown).
tomographic reconstruction of the specimen at the atomic 3.ResultsandDiscussion
level [24, 25] shown in Figures 1(d), 1(h), 2(d), and 2(h).
Silicon and boron ions are simply chosen to best high- 3.1. GBC Influence on Segregation to SGBs and RGBs. Fig-
light the GB in the reconstructions. All 3D reconstructions ure3(a)displays3Datom-by-atomLEAPtomographyrecon-
presentedinthisstudyhaveapointsizeof1.5.APTstudies structions of each element identified using the IVAS 3.6.6
were conducted in laser mode at a pulse repetition rate analysissoftwareinspecimen1SGBandFigure3(b)displays
of 200kHz at a pulse energy of 0.20nJ under ultrahigh specimen1RGB.Itcanbenotedthat,qualitatively,theSGB
vacuummaintainingatemperatureof40Katthespecimen ismoredifficulttoidentifythantheRGBifthesegregation
tip utilizing a green (532nm) laser pulse. Data analyses itself is used as a visual aid. As is seen in the SGB, figures
were performed using IVAS 3.6.6 software program. The are more reflective of the grain boundary mismatch versus
GB locations provided in each data set of this study are actualsegregationtothegrainboundaryexceptinthecaseof
given in ranges as the GB is not always positioned in a Si.Figure3(b)displays3Datom-by-atomLEAPtomography
straight line. The cylindrical regions of interest (ROIs) for reconstructionsoftherandomGB,whereitshowsnearlyall
all specimens in this study were sized the same at ∼28nm elementssegregatedtotheGB.Itisalsoqualitativelyevident
diameterand∼70nmlength(𝑧-direction)withtheanalysis thatthereisagreaterconcentrationofsegregantsattheRGB.
in𝑧-direction. The special GB images in Figure3(a) include 100% of the
ROIsencompassingtheGBandthematrixoneitherside ions except for Ni which includes 0.1% of the ions and Al
wereanalyzedforthisstudyaswell.ArectangularprismROI which includes only 50% of the ions as the image becomes
was divided into 1nm slices and analyzed across the array toosaturated.TherandomGBimagesinFigure3(b)include
paralleltotheGB.Resultant1Dcompositionalprofilegraphs 100% of the ions except for Ni at 0.1%, Si at 50%, and Al at
weredevelopedfromthisdata. 50%.
4 JournalofMaterials
100nm 100
nm
m
n
0
0
1
225𝜇m 2𝜇m 2𝜇m
(a) (b) (c) (d)
90nm
90
n
m
m
n
0
9
1
250𝜇m
2𝜇m 1𝜇m
(e) (f) (g) (h)
Figure2:(a)Specimen1EBSDGBmapsidentifyingtheRGB(inblue)extractedforanalysis(locationofextractionhighlightedinyellow).
(b-c) Specimen 1 RGB SEM images of the FIB preparation of the APT tip. (d) Specimen 1 RGB 3D atom-by-atom LEAP tomography
reconstruction showing the cylindrical ROI for analysis (ions shown are Si only; 100% of ions shown). (e) Specimen 2 EBSD GB maps
identifyingtheRGB(inblue)extractedforanalysis(locationofextractionhighlightedinyellow).(f-g)Specimen2RGBSEMimagesof
theFIBpreparationoftheAPTtip.(h)Specimen2RGB3Datom-by-atomLEAPtomographyreconstructionshowingthecylindricalROI
foranalysis(ionsshownareBonly;100%ofionsshown).
Aquantitativecomparisonofthesegregationofallimpu- there is a 46% increase in the amount of silicon segregated
ritiesbetweentheRGBandSGBwasevaluatedbyidentify- to the RGB shown in Figure4(b) compared to the SGB in
ingaregionofinterest(ROI)demarcatedbythecylindrical Figure4(a).Figure4(b)displaysthe1Dconcentrationprofile
volumes that pass perpendicularly through each GB within for the RGB. The number of silicon atoms in each GB was
the 3D atom-by-atom LEAP tomography reconstructions nearlyidenticalwith3,590atomsintheSGBand3,684atoms
showninFigure1(d)sothatonlytheatomsintheROIswere intheRGB.TheconcentrationofSiintheSGBwas0.43at.%
analyzed.ThisROIencompassedtheGBandbulkregionof withastandarddeviationof0.039%,whiletheconcentration
thesamplesuchthattheconcentrationofanyatomsattheGB of Si in the RGB was 0.80at.% with a standard deviation
could be quantified with resultant 1D concentration profile of0.059%.Thereisnoenrichmentofanyotherelementsin
graphs.BulkcompositionsoftheROIsinbothgrainbound- the SGB. The RGB, however, provides clear evidence that
aries of specimen 1 are shown in Table2. 1D concentration allthesegregantsareenrichedattherandomGBlocatedat
profilegraphsofalltheelementsaredisplayedinFigures4(a) approximately45–46nm.
and4(b). ThissignificantsegregationofelementsattherandomGB
Figure4(a)displaysthe1Dconcentrationprofileforthe and the lack of segregation, or reduced segregation in the
SGBanditisevidentthattheonlyincreaseinconcentration case of Si, at the special GB provide evidence at the atomic
atthespecialGB,locatedat∼33nm,isinsilicon.Aluminum levelthatspecialgrainboundaries,specificallyΣ3,doinhibit
is shown as it has equal concentration to that of silicon. segregation of elements. The exception is silicon, although
Elemental oxygen and carbon were also analyzed and no the concentration is half of that found in the RGB, which
increase in concentration was found at the GB. However, correlates with studies on segregation enthalpies of various
JournalofMaterials 5
Special grain boundary
85nm 85
nm
Si C Al
m
n
0
9
19,137 atoms 638 atoms 25,147 atoms
SGB
Ni O
1.117×107atoms 1,115 atoms
(a)
Random grain boundary
100nm 100
nm
Si C P
m
n
0
0
1
31,894 atoms 5,464 atoms 1,437 atoms
Ni O Al
1.567×107atoms 992 atoms 36,917 atoms
Figure3:Continued.
6 JournalofMaterials
Cr Mg AlO
823 atoms 1,430 atoms 1,026 atoms
(b)
Figure3:(a)3Datom-by-atomLEAPreconstructionsofspecimen1SGB;pointsize=1.5andthepercentageofionsshownis100%except
forNi(0.1%)andAl(50%).(b)3Dreconstructionsofspecimen1RGB;pointsize=1.5andthepercentageofionsshownis100%exceptfor
Niat0.10%,Alat50%,andSiat50%.
Table2:RGBandSGBROIbulkcompositionsofspecimen1.
RGB SGB
Iontype Count(atomic) Ranged Iontype Count(atomic) Ranged
Al 4513.0 0.2273% Al 4649.0 0.2277%
Si 3684.0 0.1855% Si 3590.0 0.1758%
Ni 1975573.0 99.4936% Ni 2032126.0 99.5377%
O 65.0 0.0033% O 126.0 0.0062%
C 449.0 0.0226% C 120.0 0.0059%
P 232.0 0.0117%
AlO 86.0 0.0043%
Cr 51.0 0.0026%
Mg 122.0 0.0061%
elements[26].Comparedtocarbon(−40kJ/mol),siliconhas reducetheoverallenergyofasystem[29];however,itiscru-
segregation enthalpy of about −9kJ/mol at a given grain cialtorememberthatagrainboundaryitselfisadefectand
boundary orientation so silicon has been shown to readily willprovideapathwayforsomeamountofsegregation.This
segregateatbothrandomandspecialGBsbut,asdiscussed, segregation may be undetectable and may be in extremely
theSGBconcentrationinthisexperimentwasabouthalfof lowquantitiesbutitsrelativebehaviortoRGBsegregationis
thatintheRGB[27]. the subject of this study. Therefore, the minimum available
ThislowertendencyforsegregationattheSGBisdueto energyofΣ3boundaryprovideslittledrivingforcerequired
alowergrainboundaryenergywhichtranslatestolowerdif- for diffusion, when compared to a RGB, while the ordered
fusivityforΣ3boundary.GBenergyforΣ3isontheorderof structureresultsinaslowerdiffusionbecausevacanciesneed
<0.1J/m2 comparedtoahighanglerandomgrainboundary to move in a coordinated manner that preserves ordering
of >0.4J/m2 [8, 28]. This segregation resistance is because [8]. This describes the relationship between GB diffusion
Σ3,aspecialboundary,hasthelargestnumberofcoinciding coefficientsandGBenergies,wherelowenergygrainbound-
latticepoints,lowerGBinterfacialenergy,andhinderedGB arieshavelowerdiffusioncoefficients,whichtranslatestothe
migration[15]. lowersegregationamounts.
First,thelargenumberofcoincidinglatticepointstrans- Second, segregation resistance in the SGB is caused by
latesintoamoreorderedstructureandlowfreevolumesince the inhibited GB migration due to the large fraction of Σ3
oneoutofeverythreesitesbetweenthetwograinscoincides boundaries[14].TheabundanceofΣ3scanbeexplainedby
[8].Thisorderedstructurehashighboundarycohesionwith theirlowstackingfaultenergy(SFE).LowSFEmaterials,such
reduceddislocationsandvacanciessinceSGBsactasbarriers asNi,readilyformΣ3sduringthermomechanicalprocessing
to dislocations [20]. This high number of coinciding atoms exhibiting high fractions of these boundaries because low
also corresponds to a minimum GB interfacial energy and SFEmaterialshaveanabundanceofverywidestackingfaults
leadstoΣ3boundaryhavingaboutone-tenththeenergyof orinterruptionsinthestackingsequenceofthecrystal,and
any other CSL boundary [8]. The segregation resistance is thisinhibitsdislocationmovement(cross-slipandclimb)and
drivenbythislowerinterfaceenergyassegregationoccursto subsequently GB migration which is why Σ3s act as good
JournalofMaterials 7
Special grain boundary specimen1(30hr) Random grain boundary specimen1(30hr)
0.6 0.8
%) %) 0.6
at. 0.4 at.
n ( n (
o o
ati ati 0.4
ntr ntr
ce 0.2 ce
on on 0.2
C C
0 0
20 30 40 30 40 50 60
Distance (nm) Distance (nm)
Al% Al% C%
Si% Si% P%
(a) (b)
Random grain boundary specimen1(30hr)
0.1
%)
at.
n (
o
ati 0.05
r
nt
e
c
n
o
C
0
30 40 50 60
Distance (nm)
O% Cr%
AlO% Mg%
(c)
Figure4:(a)1DconcentrationprofilegraphsforthespecialGBofspecimen1.(b)1DconcentrationprofilegraphsforAl,Si,C,andPforthe
randomGBofspecimen1.(c)1DconcentrationprofilegraphsforO,AlO,Cr,andMgfortherandomGBofspecimen1.
barrierstodislocations[2,30,31].TheslowerGBmigration shown in Figure5(c). Again, the concentration at the RGB
alsonucleatesΣ3smorereadily.Thisabundanceinstacking isstill46%higherthanthatintheSGBprovidingevidence
faultsequatestoanabundanceofΣ3s. thatthespecialΣ3GBdoesinhibitsegregationattheGB.This
TofurtherinvestigatethesiliconenrichmentattheSGB, couldalsoindicatethatthesegregationenthalpyforSiinNiis
anotherROI(rectangularprism)of30nm×30nm×50nm enoughtoovercomethelowenergyandorderedstructureat
was extracted from this specimen. This procedure extracts thespecialGBtosomeextent.IthasbeenfoundinsomeNi
fewerionsandthereforeidentifieselementsthatarepresentin alloys that Si does have a higher segregation tendency than
verylowconcentration.Thesewouldnotnecessarilyshowup even B or C as shown in Figures 5(c) and 5(d) for Si and
intheentirespectrumbecauseofvariousbackgroundissues. C comparison [32]. The higher the energy state of a solute
ThisROIencompassedtheGBandthematrixoneither atominthebulk,thehigherthedrivingforceforsegregation,
side as shown in Figure5(a) and is highlighted in yellow which could mean that Si has lower bulk solubility leading
(only Si atoms are shown for visualization of the GB). The topropensitytosegregatemorereadilyinnickeleveninthe
GBislocatedat11–16nm,asshowninFigure5(b),duetothe presence of SGB [9]. GB Engineering and the control of Si
curvature of the boundary within the analyzed region. The segregationcouldleadtoadditionalimprovedperformance
rectangular prism ROI is divided into 1nm slices and was inmetalalloysasSialreadyreducestheeffectsofembrittle-
analyzedacrossthearrayparalleltotheGB.Theresultant1D ment [33]. Moreover, no typical embrittling elements, such
compositionalprofilegraphstillshowsaconcentrationofSi as C and P, were enriched at the SGB suggesting that this
at the GB approximately 54% higher than Si concentration isanothermechanismbywhichSGBscanimprovematerial
inthebulk10–15nmoutfromtheGBinbothdirectionsas performance.
8 JournalofMaterials
x
15 10 5 0 −5 −10 −15
−20
−10
0 y
10
20
(a) (b)
SGB slices of ROI SGB slices of ROI
400 20
350
16
300
ms) 250 ms) 12
o o
unt (at 125000 unt (at 8
o o
C C
100
4
50
0 0
0 5 10 15 20 25 30 0 5 10 15 20 25 30
Distance (nm) Distance (nm)
Al O
Si C
(c) (d)
Figure5:(a)Theatom-by-atom3DreconstructionwitharepresentationoftherectangularprismROIinyellow.(b)TheIVASimageofthe
one-nanometerslicesoftherectangularprism.(c)Theconcentrationinatomiccountsofthesegregants(AlandSi)oftheROIfortheSGB.
(d)Theconcentrationinatomiccountsofthesegregants(OandC)oftheROIfortheSGB.
This same analysis on the RGB was done for further Figure7 displays 3D atom-by-atom LEAP tomography
quantification and identification of other possible segre- reconstructions of each element identified in the RGB of
gants.Figures6(c)–6(e)showthatalltheimpurities,except specimen 2. Figure3(b) displays the same for the RGB of
CrandO,segregatetotheRGB.Inaddition,Mn,Nioxides, specimen1.
and Si oxides were identified and shown to be enriched A quantitative comparison of the segregation of all
as well. The lack of segregation of Cr could indicate that impurities was evaluated by identifying a region of interest
bulksolubilitiesorinterfacialenergiesbetweensegregantsare (ROI) demarcated by the cylindrical volumes that pass
in play. The carbon content can also affect the segregation perpendicularly through each GB within the 3D atom-by-
behaviorofelements(specificallytheirdiffusionbehaviorin atom LEAP tomography reconstructions shown in Figures
thebulk)andtheRGBdoescontainabout73%moreCthan 2(d)and2(h)sothatonlytheatomsintheROIswereana-
theSGBbut,attheselowconcentrations,theerrorsarelarge lyzed. The ROI encompassed the GB and bulk region of
[34]. the sample such that the concentration of any atoms at
theGBcouldbequantifiedwithresultant1Dconcentration
3.2. Influence of Increased Annealing Time on Segregation profilegraphs.ThecylindricalROIsforallspecimensinthis
of RGBs. The random grain boundaries from specimens 1 study were sized the same at ∼28nm diameter and ∼70nm
(annealedfor30hours)and2(annealedforonehour)were length (𝑧-direction) with the analysis in 𝑧-direction. Bulk
studiedbyevaluatingtheeffectofincreasedannealingtimes compositionsofthecylindricalROIsfromspecimens1and2
∘
at1000 Consegregationlevelsatrandomgrainboundaries. areshowninTable3.Subsequently,1Dconcentrationprofile
JournalofMaterials 9
y
15 10 5 0 −5 −10−15
−20
−10
0 z
10
20
(a) (b)
RGB slices of ROI RGB slices of ROI
600 30
500 25
ms) 400 ms) 20
o o
nt (at 300 nt (at 15
u u
o 200 o 10
C C
100 5
0 0
0 5 10 15 20 25 30 0 5 10 15 20 25 30
Distance (nm) Distance (nm)
Al C AlO SiO
Si P Mg NiO
(c) (d)
RGB slices of ROI
14
12
ms) 10
o
at 8
nt ( 6
u
o
C 4
2
0
0 5 10 15 20 25 30
Distance (nm)
O
Cr
Mn
(e)
Figure6:(a)Theatom-by-atom3DreconstructionwitharepresentationoftherectangularprismROIinyellow.(b)TheIVASimageofthe
one-nanometerslicesoftherectangularprism.(c)Theconcentrationinatomiccountsofthesegregants(Al,Si,C,andP)oftheROIforthe
RGB.(d)Theconcentrationinatomiccountsofthesegregants(AlO,Mg,SiO,andNiO)oftheROIfortheRGB.(e)Theconcentrationin
atomiccountsofthesegregants(O,Cr,andMn)oftheROIfortheRGB.
10 JournalofMaterials
Table3:RGBROIbulkcompositions(atomic)forspecimens1and2,respectively.
Specimen1 Specimen2
RGB RGB
Iontype Count(atomic) Ranged Iontype Count(atomic) Ranged
Al 4513 0.2273% Al 5263.0 0.2238%
Si 3684 0.1855% Si 3824.0 0.1626%
Ni 1975573 99.4936% Ni 2340822.0 99.5411%
O 65 0.0033% O 30.0 0.0013%
C 449 0.0226% C 403.0 0.0171%
P 232 0.0117% P 49.0 0.0021%
AlO 86 0.0043% B 1030.0 0.0438%
Cr 51 0.0026% Cr 89.0 0.0038%
Mg 122 0.0061% Mg 103.0 0.0044%
90 nm 90nm
B Si P O Al
m
n
0
9
1
6,972 atoms 52,450 atoms 815 atoms 836 atoms 77,915 atoms
Ni Mg Cr C
3.334×107atoms 2,363 atoms 4,875 atoms 6,526 atoms
Figure7:AtomprobetomographyreconstructionsoftheRGBinspecimen2(annealedforonehour);pointsize=1.5andthepercentageof
ionsshownis100%exceptforNi(0.03%),Si(15%),Cr(50%),B(50%),andAl(7%).
graphsofalltheelementsweredevelopedandaredisplayed Figures8(a),8(d),and8(e)providethe1Dconcentration
inFigure8. profilesofAl,Si,Mg,andCr,whereitcanbeseenthatthese
AsshowninFigure4(b)anddiscussedinSection3.1,the elements are not enriched at the random GB, whereas P, B,
RGBfromspecimen1,annealedfor30hours,exhibitsenrich- andCinFigures8(b)and8(c)areshowntobeenrichedat
mentofallsegregantsattheGBexceptCr.Specimen2RGB therandomGB.ThisobservedbehaviorforAl,Si,Mg,andCr
was only annealed for one hour and the 1D concentration indicates that complex grain boundary migration processes
profilegraphsareshowninFigure8withvarious𝑦-scalesto and/orsitecompetitionhaverolesinGBenrichmentatthe
bettershowtheconcentrationenrichmentorlackthereofat RGB.
therandomGB.ThedottedlineprovidestheGBlocationat Althoughthisphenomenonismorelikelyduetocomplex
31.2nm–32.4nmthroughthecylindricalROIwithanalysisin grainboundarymigrationprocesses,thisenrichmentcould
𝑧-direction. alsobeduetophysicalsitecompetition,whichwarrantssome