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DIFFUSION AND AGGLOMERATION

OF HELIUM IN STAINLESS STEEL

Ganeev G.Z., Turkebaev T.E.

Institute o f Nuclear Physics, Almaty, Kazakhstan

ABSTRACT

Diffusion of helium and formation of helium bubbles in stainless steel in condition of atomic displacement are studied theoretically using standard rate equations [1], Besides this bubble coalescence is assumed to result when collisions occur between bubbles as they migrate by surface diffusion through the solid [2], The dissociative mechanism via self-interstitial He replacement is assumed to control helium diffusion and bubble formation. The theoretically analysis is based on the diatomic nucleation model where two helium atoms are assumed to compose a stable nucleus. For nucleation of interstitial loops, two atoms are assumed to compose a stable nucleaus. In the present work is assumed that coalescence follows bubble collisions resulting from random migration of bubbles in Chandrasekhar’s approximation.

INTRODUCTION

The introduction of helium into metals by ion implantation and nuclear transmutation can result in a serious loss of ductility, which is of great concern for performance of metals in nuclear power plants, high-flux spallation neutron sources and tritium storage.

In the present work, we study a model for helium diffusion and bubble formation in stainless steel implanted with energetic helium ions in the intermediate temperature range. The temperature dependence of helium diffusivity and bubble structures is our main concern. The model is simplified by ignoring the effects of grain boundaries. The migration and coalescence of bubbles is not ignored. While formation of bubbles at dislocations has to be considered in the model.

MODEL DESCRIPTION AND RATE EQUATIONS

The theoretical analysis is based on the diatomic nucleation model where two helium atoms are assumed to compose a stable nucleus. In this case bubble formation is controlled by the diffusion of helium. The usual vacancy mechanism is not likely operative because of the immobility of vacancies at ambient temperature. In the model we consider another possible mechanism for radiation-enhanced helium diffusion, the so-called "dissociative mechanism" by which He atoms dissociate from the vacancies to interstitial lattice sites as a result of recombination of Frenkel pairs. The dissociative mechanism via the self-interstitial/He replacement is expected to be operative in the intermediate temperature range, since self­ interstitials and He atoms at interstitial sites have considerable mobility even at temperatures far below room temperature. It is suggested that mobile only complex containing two //e-atoms (surface diffusion mechanism). Equilibrium complex of two or four //e-atoms is pore.

In the following rate equations, kinetic evolution of self-interstitials, vacancies, interstitial loops and //e-vacancy clusters (bubbles) are described according to [1], Formation of bubbles on dislocations is described according to [3], The meaning and values of the symbols, parameters and constants (in stainless steel) are described in Table 1.

For point defects and interstitial loops,

- 71 = K{\ - vsCj - vsCv) / Q - Awr (D, + Dv )C,CV - 2 ,0 ,0 , (p d + A ) at

- Aw- 0 ,0 ,0 ],e - 4xZf 0 ,0 , (rbC b + r dbC db ) - %w,l),C}

( 1)

(2)

1/ 1

— }L = K ( \ - v sCl - v sCv) / Q - Awr(D, + Dv)C,CV - Z VDVCV(pd + p x) - at

- 47irvHe(Dv +D iHe)CvC iHe - 4xZbv DvCv(rbCb +~rîcdb )-% w lDlC}

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Table 1 Symbols, parameters and constants used in the calculation

Symbols Meaning Values used in calculation Unit

rv Radius of recombination of Frenkel pairs 3.5xl0‘10 m

n Trapping radius between self-interstitials 3.5xl0‘10 m

rHe Trapping radius between interstitial He atoms 3.5xl0‘10 m

r- Trapping radius between a self-interstitial and a substitutional He atom

3.5xl0‘10 m

rHe Trapping radius between a vacancy and a 3.5xl0‘10 m

interstitial He atom

'İle Trapping radius between a interstitial He atom and a substitutional He atom

3.5xl0‘10 m

Di Diffusivity of self-interstitials 10‘5e x p ( - E ^ /k bT) m2/s

Dv Difiusivity of vacancies 5xl0‘5e x p ( - E ; / k bT) m2/s

D ‘He Diffusivity of interstitial He atoms ~D, m2/s

DHe Diffusivity of helium on dislocations » D e^IJIIe m2/s

E™ Energy of migration of interstitials 0.15xl.6xl0‘19 eV

Fm Energy of migration of vacancies 1.3 eV

KHe Average number of He within a bubble nv Average number of vacancy within a bubble

PA Density of pre-implantation dislocations 1011 m’2

P\ Density of interstitial loops 27iriC1 m’2

Z, Dislocation bias factor for interstitials 1.1

Zv Dislocation bias factor for vacancies 1

^He Dislocation bias factor for helium 1.1

z b

Bubble bias factor for interstitials exp(-(PHe- 2 Y/ r b)Q /k bT)

z h

Bubble bias factor for vacancies 1

7 b

LHe Bubble bias factor for He atoms 1

GHe Generation rate of helium in matrix Ö (N ° Ö O O appm/s

/ x d

GHe Generation rate of helium on dislocations

O ^ C\ 0 lyi O

4-s'1

K Atomic displacement rate dpa/s

a Lattice constant 3.5xl0‘10 m

b Burger's vector 2.5xlO‘10 m

bv Van der Waal's constant of helium 1.59xl0‘29 m3

n

Metal atomic volume 1.2x10‘29 m3

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Vs Spontaneous recombination volume 100Q m3

y Surface energy 2.0 J/m

M Shear module 100 GPa

h Boltzmann's constant 1.38xl0‘23 J/K

T Temperature 300-900 K

Ds Surface’s diffusivity of Fe-atoms 0.58 exp(-Q /R T ) m2/s

Q Energy activation 142 kJ/mol

R Gas const. 8.31 J/mol

dC‘He

dt - Gjje / Q + Amt.I)lC lC s!!e ~ zffeD'HeC'He(p d + p x) ~ 4mJje (Dv + D lHe)CvC'He

-(3)

- 4xZ bHeD lHeC lHe (rbCb + r h Cf, ) - 4xrHeD lHeC lHeC sHe - %m-lHeD lHeC lHe

dCs (4)

J h’ = 4mJje (Dv + D ‘He )CvC‘He - Am-'- I)t C- C sHe - Aw^eD lHeC lHeC sHe dt (^-Z- = 8 m-jDjCf dt 1 1 1 (5) dr. dt'1- = (ZiDiCi - Z vDvCv)b~1 Q (6)

where Cj,Cv,Ci,C'He,C ^e are concentration of self-interstitials, vacancies, interstitial loops. He atoms at interstitial and substitutional sites, respectively. is the mean radius of interstitial loops. Spontaneous recombination of Frenkel pairs is included in the first terms in Eqs. (1) and (2). The fourth term in Eq. (1) and second term in Eq. (4) represent the self-interstitial///e replacement mechanism.

For formation of bubbles in the matrix,

dC2 j dt dH ± = t nD2r2f l 1 , 4x - r l - Dj Cj ( Cj + C , ) - 2 - ^ - J 2j , J 2j = A7tD 2 {h + r2 j ) ' ^ 2 j > D2 = 3 / 2 - n 4l3Ds /(xrlj), (V) (8) (9) ( 10)

where Q, is the number density of bubbles in matrix, Cnj- concentration of complex of n He- atom, r„j - equilibrium rad. of complex, D2 - dif.coeff. for surface diff Migration [2], Ds - surface diffusion coefficient.

d n , dt - 4x Z bHerbD ‘HeCj i f j + Cs )+ 2 J2j , (11) h~ ( ) (12) : AmZbv rb[DvCv - Z;. !),( , J+ nv2 ■ J 2 j, dnv dt

where « y ,n vare the average number of He atoms and vacancies, respectively contained in a

(4)

bubble, nV2 - number vacancies in two //e-atoms complex. The growth of bubbles is dependent on the inner gas pressure PHe, which is described by Van der Waal's gas law

(13)

Pffe = HHekbP {- u r \ .

Bubbles are expected to grow by absorbing vacancies (i.e., bias-driven growth) when there is a considerable concentration of freely migrating vacancies, while they are expected to grow by pushing out some matrix atom planes when the inner gas pressure exceeds the critical value of

mechanical stability [pHe - 2 / / rb)> pb/ rt(/r is the shear module of matrix),

( d r b ) u > = b /4 .(14)

For nucleation of bubbles at dislocations, dislocations produced before ion implantation (for example by cold working) or during implantation (by production of dislocation loops) are expected to supply equally favorable sites for bubble nucleation. The former are constant in

density (denoted as pd)while the latter increase (denoted as pi) during He ion implantation. (15) d (\ dt = f z - J d -%DlC } l l 2s - \ 6 D lC, n s dC dt— = 8D,C,2 / L (16) — = 8DtCx / / 5 + 4 rbD C , d j J J (17) where Cı, C2 -liner’s concentrations (nT1) of //e-atoms and bubbles on dislocation, rij - the average number of He atoms, contained in a bubble, nv - the average number of vacancies give equation (12),

J ^ Z jDjCj, <l8>

Df = Cj /(Cy + Cs]Dj (eff.diff.coeff), (19)

f z - Q / ( p d + pi ) / b (corr.factor), (20)

ls = (l - 2rbC§)/C2 (distance between pore’s surface). (21) RESULTS AND DISCUSSION

The time dependence of concentrations and number of vacancies and //e-atoms in pore is

shown in fig. 1-4 (T=873 K, £=10‘6apa/s, A '^lappm /s). The correct analysis in pore’s nucleation on dislocation show significant differences with results [1],

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Fig.3. Number vacancies in pore on Fig.4. Number vacancies in pore in

dislocation versus time. matrix versus time.

REFERENCES

1. C.H.Zhang, K.Q.Chen, Z.Y.Zhu. Diffusion and agglomeration of helium in stainless steel in the temperature range from RT to 600°C. // Nucl.Instr.Meth., B169 (2000), pp.64-71.

2. E.E. Gruber. Calculated size distributions for gas bubble in solids. // J.Appl.Phys., v.38(l), 1967, pp.243-250.

3. B.N.Singh, T. Leffers at al. Nucleation of helium bubbles on dislocations network. //J.Nucl.Mater., v. 125(1984), pp.287-297.

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