NASA Technical Reports Server (NTRS) 20050123899: Energetics of Oxygen Interstitials in Cr and V

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Enurgeties of Oxygen interstitials in Cr and V

Brian S. Goo and evan Copland’
Mateinls Division, NASA Glonn Research Center, Cleveland OH 44l;
‘Cage Western Reserve University, Cleveland OF

ABSTRACT

Dissalvecl oxygen in group ILA-VA (Nb, Ti, Za, ¥> hase allay is 2 fuanlarental problen,
affecting beta meslinnical properties and oxidation resistance, yet details of the phenomenon ate
poorly undershis, Tn these allows, oxygen is more arable dissolved im the mictl than a3 an
ooxide-campaund. In cemtrast, alloys based on Ni, Fo, Al and C exhibit almost no oxygen
salubittg. ‘To improve the performance es! Uh aad Ti bused ullays il iy Tecessary Wo cralersiind
‘he dtterences in oxygen solubility between Creve bso groupie metals. Aw fir ep we
considered the energetics of interstitial oxvnen in a-¥ and e-Cr. Both of these metabs heve a
HCC structure, yer the oxygen sclubility in V is much higher than that in Cr. We obtain total
energivs, densiliey ol states and population aualyses using the CASTEP plane wave
pseudiptential density lunclional earmpuler code. The differences inthe encrgotis and
eleccnnie structures ofthe tivo maveinls, articular the puri densities af stsles associa
‘with che imersitial oxygen, ae discussed.

INTRODUCTION

Th we of alloys in high temperature oxidizing atmospheses relies on the formation of 9
confinvous oxige ycron te surface, separa ic fom te environmen and lig the
Oxidation renotion ate, Useful oxide eommponnds (6... Ces, ALO, and SiO;) muse bave slow
oxygen transport Kinetics, hul this isnot the wal eriterionrequized, The oxide must forma on the
allos suface cud the alloy-sele interface must approach a condition of cquilibrinm, ‘be fists
2 function ef composition and trans kinetics, while he scandy puro thermedysamic and
_eques that the ally’ is saturated with oxygen. For Ni aa Te bund alls thin does met prosent
‘problem as oxygea is more stable as an Gnide and the concenzaicn reir For saturation és
below levels that affect mechanical properties. Ta contact, oxygen is more sable dissolve i
group DIA-VA (Nb,'It, 2. Y 9 based alloys: oxypen conventeations of 2 uy 40 at are eequined
Jo satecaion and thee levels ate estrimental to mcebanicl propetiss. ‘To maintain bulk

i+ peygen conceatation needs to rconsin below saturation eves. wich
results in aZually stat vondiion the alloy vale mtoracs whore oxide is reduced and oxygen
<ontiaualy diffuses into the alloy. This issue moods to he aetressed before suitable oxidation
bchavior dan bo obra tor these alloys,

The sayyen sturation limit js determined by the stability of che exide cumpound and the
suahility of dissalved oxygen in the allay, AS the oxide eomponni is typically fixed by the
Tequited transport kinetics the stability of dissolved cxygen isthe fimdarmental issue. ‘herckore,
2 would be interesting to gain a beuer understanding of the eitity af oxygen in these mstals
Neutron difftaction studies show that oayen atamns randealy aceupy gctahedral sites in BCC
‘Vanadium [1 ith similar behavior assurned for Cr. In this sak, the energetics of fntertitial

Inge 8 preprin ar reprnt ot a rapa manda tr osentaton at @
‘arene, Baeause cvangos tay ba mae beats nna]
bss hs fe mada ava nb mt the unt yh il
of ne ce or repro vlhau the poms oan
oxygen in pure a-¥ and pure «Cr ure considered, Both of those motals ace BCC. y=ttheit
oxygen stlubilities are very caftotent, sith that of V being nich higher tha that of Cr, We
busi oul energies, densities of stares and populstion analyses using the CASTOR plane-wave
preuleyctential density functional computer code, We investigate che energecics af bods hulk
interstitials, aed interstials ewar Cr and V (001) surfaes. ‘The differences in the cnergetics and
clecuonie structs of the 10 materils, purlieulurly the patil donsiti of states (POS)
associated with che interstitial uxyyen, mre discussed

RESULTS AND DISCUSSION

The calculaions described in thie paper were performed using the CASE ab initio
computer code [2]. CASTUP ix uplane-swive psewapotertil cede based on éeusity fumetiounl
Thoory [3]. The calcalztions wore cared cul Ling ultsasoM pseupotentaly §4], the
sgencralizod gradicat approximation (CIGA} tn the lnc! density fimetional of Pe-dew
Burke and Emzethof [5]. and Gawssinm smearing. wnd Pulay dousity mixing. Because ofthe
Jauge sizeof the supercells ued (an aeseribed below), the Brillouin zone was sampled using only
aastmill number of k-poiors, tepcally one or two, The caleulations priduve Catal energies,
levine sitisure information including baad strucuse ces wal and purtal densities of ses,
{uid populatioa unalyses. AHI caleulations were porformed without atomistic elaxation

1"

Two ses of calculations wore
pperfarmed. Tm fhe first set, Uae energios
oT interstitial oxygen Toeated om
octahedra. sites in bulls V and Cr were
computed. Un the second set, siTar
weleulations were cartied out on
inierstitials located ir che topmost
layers ef ¥ aral Cr cals with (DOL)
surfaces exposed

OS. electronsiov

2
4 The formation energy of an interstitial,
us camputed here is the difference in

ga energy be:ween n computational ell
containing the defect, and the sum
the snespies of the wefectefee cel an
my oxygen ston: in its reference state
Typically, the referense sate for
igure 1. Pasta! density states of bsygen is diatomic Oz ul tmospherie
atomic oxypen. pressure. [1 tis werk, however, we
choose atomic oxygen asthe eeerence
stats. At low oxyeco pressures where
boxides are nol slabl, atomic oxygen is
the comninant sup species, nad the
species tat sselves ithe meta

Energy, &¥
So usin maintain eansisleney with our olher véleulations, we vompule the energy pV alee
‘oxygen using CASIE®, Because CASTUD isa zerindic code, we perfor this calenlarion usu,
sa cubic latice having a flttiously large lattice canstant af 1@ angsteoms, and take the encrgy’
‘and density of states ta be represcrstive ef those of fae appropriac reference slute. The sinale-
rom total cuctgy camnputed in th's mnumcr is ~431,097 eV, The density of tates (ius shew in
Figore 1 ubave) exhilits vo peaks soerespeauding ky the boeing sane p level, with the p peak,
Jocated ul the Fermi eneegy (the energy sara athe PDOS plots) soi “he speak about 15 eV
ower. the separation ofthe peaks iz consistent with Llarree Fock calculstions ofthe Is<nd Ip
level sopacation in storie oxygen [6], indicating that our chosca referenrs state is reasonably
lv br thal eam figure,

The (urmation encegios of bulk getahedeal inwersttials were computed using 2x2x2 BCC periodic
supercells of V and Cr. witha single oxygen atom placed at an octehedral site, corresponding to
emoxygen conventration of 5.al%, Whose coergios were eempuled und compared with the sua
of che snargies of hs oxygen-free suporeells cd atomic oxygen, The “irmalion energies or re
Vand Cr interstitials were 29.622 and -14.42eV. respectively. While the niagniteves of these
energies depend on the erence state oxygen energy, the sigas indicare that octahedral
interstitials are sta2ke in both mnetals, aud ae siguificantly more stable in V ther. in Cr. Onyyen
partial dcusitics of siees forthe interstitials are shown in figures 2a aad 2b, In both melas, the
‘oxygen POS is structurally the stn; the p level shows spiting ne evidene in the atomic
‘oxygen POS, and the bonding levels une Tower with reeot ta the Hern energy foe Cetaan for
V. The apliting is lizoly duc t0 a change in local syramety for the octahedral interstitial
‘compared with the isolated atom reterence state. ‘The separation bocween s aad p levels is similar
tw that inthe -eference state. While the magaitudes of the formation cneryies “ney change aehen
lomnisie elexation iy incladed, we de not expect a rcordcring ofthe relive slakilliey of the W
re Cr intertlial,

s .
cy s 4
V
z 2 ke 4
3 i !
g ¢ gS 4
a 2 i. "
go | = gs i it
° o My
a a | a
cy
Energy, <¥ Energy, eV

Figure 2, Paral density a stues hulk oxygen inertial in 42) Vand (>) Cr.
In uddition a the slubilily of hulk rxygen intersiul, ofher energetic fsswes re important in the
consideration of the details of oxygen diffusion. In general, the energies of detects near surfsce
‘xi be different from those of defects in the bull. Ln addition, to understand the dynamics of
interstitial diflusion, tne nergy barriers slong likely diffvsion paths need to be computed.
Fimly, Ux presence of subsiitulionel alloying clements may have a significent effect on vxygen
diffusion. We addeess the fist ol'these issues eles; caleuluins ef diVusion arciers. and oI he
effects of substinudonal alloying e:ements, ae currently under way in our laboratory

‘We compute the relive energies aad F1DOS gar octalacaal oxy gen interstitials Located in fhe
fire Kaur ayers pI 28234 supercell serine sibs wth The (001) surface expetsed. Results we
shown in Tube 1; Tor hel: metals, he zero of energy is ksken ke be the energy a The rerstiial
ithe tise layer

‘Table &, Energies and parial charges of oxygen infoisital in V aud Cr

Lam RS ED
T Te ag SF “048
2} sue 34 “37

5 236 —T 63980

+ 255 039

For both metals the energy ofthe interstitial :n tho dest laysr is ‘ower than these of into-st.tals
in lasers further fem the surface, This suggests “hs at low concentrations, the interstitials wl
be primarily confined ta the Ciel layer. The energy cost lo ereeiy.aninlenstitis} in we sewamd
Gro.uh Jourth layers Is larger for boch mela's, und fy larger lx Cx sav: lor V, which is consistent
with the lack of selubility of ( in Cr. Itis algo warth nating tha: the energies of second-layer
intersticals for both mitals are substantially the sante as those af thitd- and tiurth--ayee
interstitials, indicating that effective bulk conditions are attained clos> to the surface. ‘This
onelusien is supported by une values of the oxygen partial charges. cbtcned via Mulliken
population snalysis [7] als shown in Table

‘Tho exg'geu POS for interstitial inthe firs and fowl layers ace shiowa In tiguees 3a-h and 4a
befor ¥ aud Cr, rexpectively. ‘The BDOS stmetures for all layers resemble tho: of the bull
‘nteratals, exepl thul the PDOS othe first-layorintorsttials is clover to the Fermi cusrgy by
approximately 3e¥, oynin indicating thn: bulk candétions exist relalively close 0 Are surlace,
5 s
Bos Bos i
g, Ey ii
7 ih oa

6 > in

4 a a a
@) (b)

Energy, eV

Figure 3, Partial demsily of states of oxygen intestcial in V, Layer 1. (9) anal layer 4 (6)

‘ .
i
2 5 4
z Boe —f.
ic 3 i
Ba Bos 4
a, g, lt
a g iH
go go tht
° > vi

ls an as aS oS SO

@ Energy, 0 tb Energy, 8

Figure 4, Partial dorsity of sates of oxygen interstitial in Cr, ger 1, (a) and layer 4 (0),
CONCLUSIONS

Ab initio culoulations of the cncrgctios of ootableal oxsgen intestats in BOC V and Cr are
teonsistent with the known psygen soliaility ofthese matetialsat low concentrations, Bulk
interstitials in both ¥ and Crare Four ua Fe slab%e, wilh Cay V inerstibe’eirg substantially
niore favorable energetically thao the Cr interstitial, Por iversitals nent (001) V and Cr
surlwvos, it's energetically Revorable for isolated interstitials to evist inthe frst layer ir “ayers
Teather frm the surfase, th cusrgies converge wnhin another layer ex co ta values higher thant,
Gre of surlave-layer interstitials, irliceiny that bulk concitions exist to witbiu a eoupfe lagers
of dee suwlice. The energy cost ol plicing an inkxsttial i the sccond layer, versus the frst
Jayor, i frgor for Cr than for V. ‘The magntudes of the energies invslved wil charge when
atomistic rslaxation is aneluded, and sush ealeulatianss ure npg

The enecgy protMe of inteestiia” energy with depuh is ota complete pictus of ths encugstic
factors thar limit diffusion iote these materials. lwp sf concer ise height othe sneTey
Dautic that cooctols the likelihood of an cxygen uta hopping la a neighboring oclahestal sive
Accum euleulaons uf this uric invelvo arolexation oF the ceighbring atnms adjacent te the
reaction path; sich ealeulations are prssently bomg catvied out

ACKNOWLEDGEMENT

Thea

whors gratefully acknowledge calightening discussions with Waluer Lurmbrech

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