Radiation-induced segregation in binary and ternary alloys

Journal of Nuclear Materials 83 (1979) 2-23 0 North-Holland Publishing Company

RADIATION-INDUCED SEGREGATION IN BINARY AND TERNARY ALLOYS * P.R. OKAMOTO AND L.E. REHN Argonne National Laboratory, Argonne,

IL 60439,

USA

Received 1 March 1979

A review is given of our current knowledge of radiation-induced segregation of major and minor elements in simple binary and ternary alloys as derived from experimental techniques such as Auger electron spectroscopy, secondary-ion mass spectroscopy, ion-backscattering, infrared emissivity measurements and transmission electron microscopy. Measurements of the temperature, dose and dose-rate dependences as well as of the effects of such materials variables as solute solubility, solute misfit and initial solute concentration have proved particularly valuable in understanding the mechanisms of segregation. The interpretation of these data in terms of current theoretical models which link solute segregation behavior to defect-solute binding interactions and/or to the relative diffusion rates of solute and solvent atoms the interstitial and vacancy migration mechanisms has, in general, been fairly successful and has provided considerable insight into the highly interrelated phenomena of solute-defect trapping, solute segregation, phase stability and void swelling. Specific examples in selected fee, bee and hcp alloy systems are discussed with particular emphasis given to the effects of radiationinduced segregation on the phase stability of single-phase and two-phase binary alloys and simple Fe-Cr-Ni alloys.

1. Introduction

Questions have been raised about the technological impact of segregation effects on the void swelling problem and other irradiation-induced property changes in the fuel cladding and core structural alloys of fast breeder reactors. Segregation effects are also likely to be of serious concern for the first wall of a fusion reactor because of similar radiation damage problems. Experimental studies on nickel binary alloys [9,24] and austenitic stainless steels [2,19,20] indicate that solute additions such as Si or Be, which are particularly effective in inhibiting void formation in these alloys, also exhibit strong tendencies to segregate to surfaces and other defect sinks during irradiation. The depletion of these elements from the matrix may lead to a loss of swelling resistance, especially if segregation leads to second phase precipitation. On the positive side, the selective enrichment of low-2 elements like Be or Si at the surfaces of first wall components in a fusion reactor may help solve the problem of plasma contamination by high-2 elements either by forming self-regenerating low-2 coatings, or by stabilizing deposited low-2 coatings against radiation-enhanced interdiffusion which may limit the lifetime of such coatings [ 131.

During the past five years, a great deal of experimental [l-28] and theoretical [29-411 research has been directed toward a basic understanding of radiation-induced segregation in alloys. The current interest in this phenomenon was motivated by the discovery in 1973 [l] that defect-flux driven segregation processes can alter the phase stability of alloys under irradiation. Initially demonstrated in an electron and ion bombarded Fe- 18Cr-8Ni1Si stainless steel [ 1,2], the segregation of alloying elements toward or away from external surfaces, grain boundaries, dislocations, voids and other types of defect sinks has since become a commonly observed effect in a wide variety of alloys after elevated temperature irradiations with neutrons or charged-particles [3-281. Frequently, the local compositional changes are large enough to induce second phase precipitation in normally stable, under-saturated solid solutions [4-141, and to alter precipitate morphology and distributions in multiphase alloys [15-171. * Work supported by the US Department of Energy. 2

P.R. Okamoto, L.E. Rehn f Radiation-induced segregation in binary and ternary alloys

Considerable progress has been made during the past five years in the experimental characterization and theoretical understanding of the segregation phenomenon and its dependence on irradiation and material variables. A major contribution to our current understanding of the phenomenon was made by Johnson and Lam [29-3 11. They developed a phenomenological model for dilute alloys which includes solute transport via free defect fluxes as well as via the migration of vacancy-solute and interstitial-solute complexes. In a series of recent papers, the Johnson-Lam approach has been extended to include radiation-induced precipitation in undersaturated solid solutions [32], the effect of depth dependent damage rates on solute redistribution in ion bombarded alloys [33], and to explore the relationship between defect trapping, solute segregation, radiation-induced precipitation and void growth [34-371. Radiation-induced segregation in concentrated alloys has been treated theoretically by Manning [39], Marwick [40] and Wiedersich et al. [41]. The latter authors have made calculations for concentrated alloys similar to those of Johnson and Lam for dilute alloys. The present paper reviews recent experimental observations of radiation-induced segregation and its effect on phase stability. We will focus mainly on binary nickel based alloys, since systematic studies of the segregation phenomenon have been largely confined to these systems. In addition, some observations of segregation in irradiated Fe-Cr-Ni based austenitic alloys will also be discussed. We begin in section 2 with a discussion of basic segregation mechanisms causing solute enrichment or depletion at sinks. Experimental observations illustrating the effects of material and irradiation variables on the magnitude and direction of segregation are presented in section 3 for binary alloys and in section 4 for Fe-Cr-Ni alloys. The effect of segregation on the phase stability of alloys is examined in section 5. The concluding section summarizes the experimental observations and identifies areas needing further study.

2. Segregation mechanisms Current theoretical models link solute segregation in irradiated alloys to the formation of mobile

3

defect-solute complexes and/or to inverse Kirkendall effects arising from differences in the diffusion rates of free solute and solvent atoms when migrating via a vacancy or interstitial mechanism. 2.1. Inverse Kirkendall effects As is well known from Kirkendall type experiments, when the components of an alloy diffuse via vacancies at different rates, a composition gradient can induce a net flux of vacancies across a lattice or “marker” plane even though the vacancy distribution is initially uniform. During irradiation, the inverse situation arises near sinks where gradients in the vacancy and interstitial concentrations can induce a net flux of solute and solvent atoms across a “marker” plane in an initially homogeneous ahoy. This inverse Kirkendall effect is illustrated in fig. la for the case of a vacancy gradient near a sink in a binary alloy composed of elements A and B. The vacancy gradient generates a vacancy flux, JV, toward the sink which induces an atom flux (JL + Ji) of equal magnitude in the opposite direction, where JIand Ji are the fluxes of A and B atoms, respectively. Since JL and Jk transport A and B atoms in amounts proportional to their local atomic fractions, C, and Cn, and to their partial diffusion coefficients, Ok and Di, it is easy to see that the alloy composition around the sink does not change when 01 = Di. However, if 01 f Di, the flux away from the sink of the faster diffusing component will be proportionately greater then its concentration in the alloy. Therefore, the inverse Kirkendall effect induced by a vacancy flux will always cause depletion at the sink of the faster diffusing component. An inverse Kirkendall effect can also be induced by an interstitial gradient. However, because the interstitial flux and the associated atom fluxes, Ja and JL, move in the same direction (fig. lb), any difference in the partial diffusion coefficients of the A and B atoms via an interstitial mechanism, i.e. Di # DL, will result in the preferential transport of the faster diffusing component toward the sink. Therefore, depending on the relative magnitudes of the ratios DlfDi and D~fD~, the two inverse Kirkendall effects may aid or oppose each other in causing solute enrichment or depletion near a sink. For a concentrated binary alloy Wiedersich et al.

P.R. Okamoto, L.E. Rehn /Radiation-induced

4

segregation in binary and ternary alloys

SINK c a

a

I-

X-

n SINK

Fig. 1. Schematic illustration of Inverse Kirkendall Effects induced by (a) vacancy flux, (b) interstitial flux.

xFig. 2. Effect of partial diffusion coefficients on the depth distribution of A atoms.

(4 l), derived the following relation between the steady-state concentration gradient for the A component and the vacancy concentration gradient: vc,

=

where Df and Df are the partial diffusion coefficients of an interstitial diffusing via A and B atom exchanges, respectively. DiRR and DbRR are the total radiationenhanced diffusion coefficients for the A and B atoms and 01is a thermodynamic factor which deviates from unity for non-ideal solutions. The two cases of interest predicted by eq. (1) are illustrated in fig. 2. Depletion of the A component occurs at the sink when DlfD; > D?JDb, i.e., when the preferential transport of A atoms via vacancies outweighs that via interstitials. Conversely, enrichment of A atoms at a sink occurs when DL/Di < Di/Db.

The two inverse Kirkendall effects will cause maximum enrichment of the A component when the A atoms diffuse exclusively via an interstitial mechanism and the B atoms via a vacancy mechanism. 2.2. Defect -Solute complexes In addition to the inverse Kirkendall effects, solute segregation to sinks can also occur if solute atoms interact with vacancies or self-interstitials to form mobile defect-solute complexes. The complexes are especially important for segregation in dilute alloys and have been extensively treated by Johnson and Lam [293 11. If the complexes undergo protracted random walks before thermally dissociating, they may be regarded as distinct entities diffusing down their own

P.R. Okamoto, L.E. Rehn /Radiation-induced segregation in binary and ternary alloys

concentration gradients. In initially homogeneous alloys, the complexes flow toward sinks and, therefore, tend to cause solute enrichment neir a defect sink. in general, the contribution of defect-solute complexes to segregation depends on their binding and migration energies. As pointed out by Okamoto et al. 1341, selection resulting from the fo~ation and migration of interstitial-solute complexes goes through a maximum when the migration energy of the complex becomes approximately equal to the vacancy migration energy, Thus especially strong solute enrichment via complexes is expected for solutes that trap interst~ti~s at least up to temperatures where vacancies become mobile. This segregation mechanism is expected to be especially important for undersize * solutes since both experimental and theoretical studies [42-451 indicate that undersize solutes trap interstitials much more strongly then do oversize solutes. Mechanisms for the long range transport of undersize solutes in the form of ( 100) mixed dumbbells have been proposed by Johnson and Lam [29] and Dederichs et al. [43], Since defect-solute interactions are less effective at high temperatu~s, d’iffmion of defect-sofute complexes can dominate at low temperatures, while the inverse Kirkendall effects may dominate at high temperatures. In the absence of interstitial-solute interactions, solute enrichment at sinks can occur at low temperatures via vacancy-solute complexes, and solute depletion at high temperatures due to the vacancy-induced inverse Kirkendall effect. Calculations by Wiedersich et al. [36 ] for the case of dilute nickel binary alloys indicate that this reversal will occur for vacancy-solute binding energies in the range 0. I-0.2 eV. For lower binding energies only solute depletion occurs due to the inverse Kirkendall effect; above -0.2 eV segregation is dominated by the diffusion of vacancy-solute complexes.

3. Segregation in binary ahys In this section the effects of material and irradiation variables on the radiation-induced segregation * Undersize solutes are those which reduce the mean atomic volume of the alloy 1461.

5

near the surfaces of ion bombarded alloys are examined. While defect fluxes induce compositional changes in the vicinity of any sink, the external surface is by far the most accessible sink for quantitative studies of segregation. The one dimensional nature of the compositional gradient near the surface is readily amenable to depth profiling by conventional techniques such as Auger electron spectroscopy (AES), secondary-ion mass spectroscopy (SIMS) and ion-beam analysis. 3.1. Solute size effects As discussed in section 2, appreciable differences in the sizes of solute and solvent atoms are expected to result in an enrichment of undersize and a depletion of oversize solutes near sinks during irradiation. This effect, first noted by Okamoto and Wiedersich f2 ] in irradiated Fe-18Cr-8Ni-lSi, has been confirmed in dilute and concentrated nickel binary alloys containing the oversize solutes Al, Ti, MO [9,15,17,21, 221, Cr, Mn [21,23] and Au [25] and the undersize solutes Be [4,24] and Si [S-14,16,17,22]. Fig. 3 shows AES measurements of the depth distribution of the solutes Ti, MO, Al and Si obtained by Rehn et al. [9] from a series of 1 at% nickel binary alloys after irradiation with 3.5 -MeV Nif ions. The three oversize solutes (Ti, MO and Al) exhibit a depleted zone close to the irradiated surface followed by a region of solute enrichment. The undersize Si exhibits an enriched zone at the surface followed by a depleted zone nearly 300 run in width. The constant Si/Ni peak-to-peak ratio in the surface-enriched zone corresponds to -25 at% Si as determined by AES analysis of samples of known Si concentration, indicating the formation of a surface film of the NisSi phase. The radiation-educed segregation of Be and Au has been investigated in dilute nickel alloys irradiated with 3.2-MeV Ni+ ions by ion beam analysis techniques. Pronko et al. [24] used the low energy nuclear reactions, 9Be(p,d) *Be and ‘Be(pp) 6Li, to depth profile Be in a Ni-0.7 at% Be alloy, while the Rutherford backscattering technique was used by Jack [25] to profile Au in ion bombarded Ni-1 at% Au. The results show that the undersize Be, like Si, migrates toward the surface during irradiation, The oversize Au, like Al, Ti and MO,migrates away from the surface. Fig. 4 shows typical energy spectra obtained from

6

P.R. Okamoto. L.E. Rehn /Radiation-induced

I

_0

(energy). This reflects the rapid decreasing cross section resulting from the deceleration of the 300-KeV incident protons by electronic stopping. In the irradiated sample, segregation of Be toward the surface can be deduced from the strong peak corresponding to the near surface region and the dip in the count rate of particles originating in the subsurface region. The enrichment of undersize and depletion of oversize solutes near irradiated surfaces have been confirmed by Piller and Marwick [21,22] in more dilute nickel alloys irradiated with 75-KeV Ni+ ions at a peak dose rate of 4.5 X lo-* dpa/s. Depth profiles obtained by SIMS are shown in fig. 5b and 6b for Si and Al, respectively. The Si depth profile in fig. 5b shows a depleted zone at a depth of -20 nm and several enrichment peaks, one at the irradiated surface and others at -40 and -120 nm. The depleted zone and the surface enrichment peaks result from segregation processes driven by the point defect gradients generated on either side of the. damage peak where the defect production rate varies rapidly with depth [21,33]. Si, which is undersize in nickel, traps interstitials strongly as known from low temperature recovery data [45]. The depleted zone has been attri-

I Ni+ ON NICKEL ALLOYS

3.5~MeV

50

100

150

2oD

DEPTH hm) Fig. 3. AES chemical depth profiles of solute in a series of 1 at% nickel alloys after ion irradiation at the temperatures and doses indicated (Rehn et al. [9]).

irradiated (upper curve) and unirradiated specimens of Ni-0.7 at% Be [24]. That ly distributed in the unirradiated control evident in the smoothly decreasing yield

segregation in binary and ternary alloys

(lower curve) Be is uniformsample is with depth

ENERGY 20

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625°C - 23 dpo

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CHANNEL Fig. 4. Observed spectra for irradiated (upper curve) and nonirradiated (lower curve) specimens of Ni-0.7 at% Be (Pronko et al. 1241).

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P.R. Okamoto, L.E. Rehn /Radiation-induced segregation in binary and ternary alloys

buted to the migration of S&interstitial complexes out of the peak damage region, where the concentration of defects is highest, toward the surface and interior of the specimen where the resulting build up of Si causes the enrichment peaks. Lam et al. [33] have performed model calculations to simulate Si segregation in nickel under the irradiation conditions used in Piller and Marwick’s experiment. The calculated profiles shown in fig. 5a are based on Si migration in the form of (100) - mixed dumbbell interstitials. The spatially-dependent damage production rate used in the calculations is shown by the dashed curve and the initial solute con-

centration was taken to be 1OF3 (atomic fraction). Various binding energies for the interstitial-solute complex were employed. While all positive binding energies lead to solute enrichment at the surface and to a solute depleted zone in the vicinity of the damage peak no unique value was found to reproduce the double-peak feature behind the damage region. However binding energies of about 2 eV yielded profiles that fit the main enrichment peak -60 nm, while profiles for binding energies of -0.2 eV tit the deeper enrichment peak at -120 nm. These calculations sug gest that more than one type of Si-interstitial com-

0 0

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BOO IO00 I200 1400 1600 18002000 DEPTH (1,

Fig. 5. (a) Calculated depth distribution of a solute in nickel after 75-KeV Ni+ ion irradiated for c”o = 10m3 and various interstitial-solute binding energies. The depth dependent damage rate used is shown by the dashed curve. (b) SIMS measurement of the depth distribution of Si in nickel after irradiation with 7%KeV Ni+ ions (Lam et al. j33 J).

DEPTH

tft

Fig. 6. (a) Calculated depth distributions of a solute in nickel after 75-KeV Ni+ ion irradiation for various temperatures and for Cf = 10e3. Damage rate profile used is the same as that in fig. 5a. (b) SIMS measurement of the depth distribution of Al in nickel after irradiation with 75-KeV ions (Lam et al. 1331).

8

P.R. Okamoto, L.E. Rehn /Radiation-induced

plex could be involved in the segregation process ]331. Profiles for the Al in nickel case were also calculated (fig. 6a) {33]. Good agreement with the experimental prof?les was obtained assuming a weak Al-vacancy binding energy of 0.05 eV and no binding to interstitials. The main peak in the calculated Al profiles at -20 nm is due to the inverse Kirkendall effect, i.e., the preferential transport of Al toward the peak damage region by the flow of vacancies out of the peak damage region. At high temperatures all the oversize solutes (Ti, Al, MO,Cr, Mn, Au) in nickel are depleted near the irradiated surface, while the undersize solutes (Be,Si) are enriched. The observed segregation of these solutes cannot be attributed solely to a vacancy induced inverse Kirkenda~ effect. Fig. 7 shows the tracer impurity diffusion coefficients of these solutes in nickel as a function of inverse temperature. The self-diffusion coefficient for nickel is given by the dashed curve. With the possible exception of MO, fig. 7 shows that all the solutes are fast diffusers

lO4,Tt K) Fig. 7. Tracer impurity diffusion coefficients vs reciprocal absolute temperature for various solutes in nickel (Treheux et al. [60], Be data from Grigorev et al. [61]).

segregation in binary and ternary alloys

compared to nickel. If only the vacancy-induced inverse Kirkendall effect were responsible for causing segregation, the faster diffusing component should be depleted at the surface, as discussed in section 2. While this is observed for most of the oversize solutes, it is clearly not observed for the undersize solutes Be and Si, which in fact, have the highest thermal diffusion coefficients of all the solutes investigated. Solute enrichment at sinks has also been observed in irradiated V-Cr [ 181, Al-Si [27], Mg-Cd [8] and Ti-Al 1281 alloys. While once again no consistent correlation is found between the direction of segregation and thermal diffusion data, i.e., with the inverse Kirkendall effect for vacancies, the solutes in all these alloys are undersize [46]. Their behavior is, therefore, consistent with the view that undersize solutes segregate to sinks as a result of strong boding to interstitials [2]. 3.2. L&e dependence Only a few systematic investigations of the dosedependence of radiation-educed segregation have been made. Rehn et al. [IO] have measured Si depth profiles as a function of dose in a Ni-1 at% Si alloy after irradiation with 3.MeV Ni+ ions at nominal temperatures of 525 and 600°C. The profiles for the nominal 525°C irradiation are shown in fig. 8. The results show ~diation-educed segregation to be an extremely rapid process. The dose required at 535°C to attain a surface concentration of 10 at% Si, the solubility limit at the irradiation temperature, is only -0.05 dpa. The Si enriched zone at the surface continues to increase with dose both in spatial extent and in the amount of Si contained in the zone. Between l-2 dpa, the surface composition approaches a saturation level of -25 at% Si. This indicates that by -2 dpa a continuous film of the Ni3Si 7’ phase covers the surface. Since the Auger peak-to-peak ratios between 20 and 40 correspond to Si concentrations intermediate between the solubility limit and NiJ Si, the profiles for 0.16 and 1.O dpa indicate the existence of a two-phase region in the first 5 nm below the irradiated surface. This interpretation is consistent with TEM observations which show that NiJSi initially forms on the surface as small isolated domains having the shape of hillocks which extend into the

. 3

P.R. Okamoto, L.E. Rehn / Radicltion-inducedsegregation in binary and ternary alloys

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6Ot f\

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I

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6.5 0 2.8

l

I

“0

Ni-I

A 0.05

DEPTH

dpa AT 535’C

hm)

Fig. 8. Si/Ni peak-to-peak ratios as a function of depth from the irradiated surface for a series of Ni-1 at% Si alloys irradiated to different doses at a nominal temperature of 525°C. The right ordinate gives Si/Ni peak-to-peak ratios obtained from unirradiated Ni-Si alloys of known Si concentration (Rehn et al. [lo]).

interior [ 121. With increasing dose the domains of NisSi grow and impinge on other domains eventually forming a contiguous layer of antiphase domains. The Si profnes after irradiation of the same alloy at a nominal temperature of 600°C were found to be much flatter then those observed at the lower temperatures. The Si/Ni Auger ratio at the surface after irradiation to 13.7 dpa at 600°C indicated only partial coverage of the surface by NisSi precipitates. By contrast, complete surface coverage occurs by -2 dpa at the lower temperatures. The increase of -75’C in the irradiation temperature is apparently large enough to cause Si back diffusion to limit the amount of Si enrichment near the bombarded surface. Rehn et al. [18] have investigated the dose depen-

dence of Cr segregation in V-l 5 wt% Cr irradiated with 3.5-MeV 51 v’ ions at 650°C. A rapid increase in the surface composition also occurs in this bee ahoy with the surface composition saturating at -50 wt% Cr by -5 dpa. Unlike Ni-Si alloys, V and Cr form a continuous solid solution so that the saturation cannot be attributed to formation of a second phase as in the case of Ni-1 at% Si. Presumably, saturation in the V-l 5 wt% Cr alloy is due to the attainment of a steady-state compositional gradient where the back diffusion of Cr down the compositional gradient balances the defect-flux induced flow of Cr toward the surface. Although limited, the available dose dependent measurements indicate that a ten-fold increase in the

10

P.R. Okamoto, L.E. Rehn /Radiation-induced

solute concentration at the irradiated surface can occur by dose levels as low as 0.05 dpa. These results are substantiated by the theoretical studies of Johnson and Lam [29] and by the more recent studies of Lam et al. [33] of the kinetics of radiation-induced precipitation at ion-bombarded surfaces. Theoretical depth profiles based on Si diffusion in Ni-I at% Si in the form of Si-interstitial complexes predict that when the migration energy of the complex becomes comparable to the migration energy of the free vacancy, only a small fraction of a dpa is required to produce a surface enrichment of Si in excess of the solubility limit. Similar compositional changes can be expected to occur at grain boundaries after equivalent doses in neutron irradiated alloys. Such changes may seriously affect mechanical properties of an alloy, such as its fracture behavior, at dose levels far below the onset of significant void swelling. 3.3. Temperature dependence

Similar to void growth in metals, radiation-induced segregation is caused by the flow of vacancies and interstitials to sinks. Consequently both phenomena are expected to occur in approximately the same temperature range and to exhibit similar temperature dependences. Rehn et al. [9] investigated the dependence of Si segregation on irradiation temperature in a Ni-1 at% Si alloy bombarded with 3.5MeV Ni+ ions at a peak damage rate of -2.5 X 10v3 dpa/s. The Si depth profiles as measured by AES are shown in fig. 9. Little segregation occurs below -400°C and above N660°C. The Si/Ni Auger ratio of 0.033 in this figure corresponds to a Si concentration of 25 at%, indicating that at intermediate temperatures segregation of Si occurs at a sufficient rate to form a thin layer of the NisSi 7’phase. The film growth rate and, hence, the segregation rate can be seen to go through a maximum at -560°C. The observed temperature dependence of Si segregation is similar to the temperature dependence of void swelling in pure nickel as determined by Ryan [47] under similar irradiation conditions. However the temperature of maximum film growth rate occurs about -4O’C below the peak swelling temperature reported by Ryan. The formation and growth of NisSi surface films

segregation in binary and ternary alloys

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(i,

Fig. 9. Si/Ni peak-to-peak ratios versus depth from the irradiated surface for a series of Ni-1 at% Si alloys irradiated at various temperatures and doses. A Si/Ni ratio of 0.033 corresponds to 25 at% Si (Rehn et al. [9]).

cause changes in the infrared emittance of the surface which can be measured with an infrared pyrometer (IRP) as apparent changes in the surface temperature [13,16]. The IRP temperature changes are quite pronounced in more concentrated alloys. As an example, fig. 10 shows the IRP temperature changes in Ni-12.8 and Ni-12.7 at% Si alloys irradiated at differ-

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4201’ 1 ’ ’ ’ 0 IO 20

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Apparent surface temperatures as measured with an infrared pyrometer during irradiation of Ni-12.7 at% Si and Ni-12.8 at% Al. Arrows indicate times when irradiation were started or terminated (Potter et al. [ 16)).

P.R. Okamoto, L.E. Rehn /Radiation-induced segregation in binary and ternary alloys

ent temperatures with 3.5-MeV Ni+ ions at -3 X 10e3 dpa/s [ 161. Apart from beam heating effects when the beam is turned on or off, the IRP temperatures of the Ni-Al samples remain constant during irradiation. However, as a result of the formation of Nia Si surface films, the IRP temperatures of the Ni-Si samples increase throughout the irradiation period. As shown by Potter et al. [16], the initial slope of the IRP temperature curves can be used to estimate the initial film growth rate as a function of temperature. The data for the Ni-12.7 at% Si alloy, shown in fig. 11, indicate that the film growth rate is low below -400°C and above -7OO”C, and goes through a maximum at approximately 550°C. The temperature dependence of radiation-induced segregation in Ni-12.7 at% Si is therefore very similar

3.5+&i

Ni+ ON Nil127

a/a Si

to that of Ni-1 at% Si (see fig. 9) when irradiated under similar conditions. The kinetics of film growth rates in initially undersaturated solid solutions under heavy-ion bombardment has been treated theoretically by Lam et al. [33]. In their model the solute enrichment at the surface is allowed to increase with irradiation time up to the solubihty limit, whereupon the excess solute is assumed to form the precipitate phase. The precipitate thickness can be calculated from the amount of solute removed from the matrix and the solute concentration in the precipitating phase. Sample calculations of the growth of the Ni3Si y’phase on the surface of a Ni-1 at% Si alloy irradiated with 3-MeV Ni+ ions at a peak dose rate of 10v3 dpa/s are shown in fig. 12. The ordinate on the left gives the fraction of solute depleted from the matrix as a function of temperature for various irradiation

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T FC) Fig. Il. Initial slopes of apparent surface temperature curves versus irradiation temperature for Ni-I 2.7 at% Si irradiated at -3 X lo-’ dpa/s (Potter et al. [16]).

Fig. 12. Calculated film thickness as a function of temperature at various irradiation times for a 1 at% nickel binary alloy. The solubility limit is 10 at% and the solute concentration in the film was taken as 25 at%. The left hand ordinate gives the spatially averaged decrease in the solute concentration in the matrix.

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P.R. Okamoto, L.E. Rehn /Radiation-induced

times. In these calculations, Si is assumed to migrate as Si-interstitial complexes. The Si-interstitial binding energy was taken as H&, = 1.88 eV and the migration energy of complex as, h$& = If,” = 1.28 eV. For this set of parameters the film growth rate goes through a maximum near 625°C. While this value is slightly higher than the experimental observations, the curves are in good qualitative agreement with the film growth rate behavior of Ni-1 at% Si and Ni-12.7 at% Si portrayed in figs. 9 and 11. The sharp cut-off in the film thickness at -700°C shown in fig. 12 is due to the absence of precipitation, not of segregation. Above -700°C segregation stilt persists but is insufficient to cause the solute concentration at the surface to exceed the solubility limit. The temperature dependence of segregation shown in figs. 9-12 is readily understood qualitatively. At low temperatures segregation is limited because the low vacancy mobility results in high excess vacancy concentrations and high defect recombination rates. As a result, the fluxes of defects and solute-interstitial complexes to the surface are reduced. At high temperatures, the large thermal vacancy concentration leads to high solute diffusion rates and high defect recombination rates; the former reduces segregation by fast back diffusion of solute and the latter, by reducing the defect fluxes to sinks. Therefore, the maximum segregation rate occurs at intermediate temperatures where the defect recombination rate is a minimum, i.e. near the peak swelling temperature of the alloy. Marwick et al. [23] have recently investigated the effects of temperature on the segregation behavior of the oversize solutes Al, Ti, Mn and Cr in a dilute nickel alloy &radiated with 75-KeV Ni+ ions. The segregation of Al, Mn and Cr solute atoms was found to go through a maximum at -5OO’C (see fig. 6b) and very little segregation occured below -300°C and above 600°C. Ti on the other hand, behaved differently. Ti enrichment at the surface occured at temperatures below -400°C while above this temperature Ti depletion was observed. The segregation behavior exhibited by Ti in nickel is consistent with the calculations of Lam et al. [33] and Wiedersich et al. [36] describing segregation of solutes that interact mainly with vacancies. Calculations for a solute-vacancy binding energy of 0.1 eV give a cross-over temperature very near the 400°C observed for Ti in nickel. Solute segregation below

segregation in binary and ternary alloys

the cross-over temperature is dominated by the diffusion of solute-vacancy complexes which results in solute enrichment at the irradiated surface, whereas above this temperature, solute depletion occurs due to the inverse Kirkendall effect via vacancies. The calculations of Lam et al. [33] also indicate that the temperature dependence of segregation near the surface and in the peak damage region in ion bombarded alloys is sensitive to the incident ion energy. Solute concentration depth profiles for dilute Ni alloys, using defect-solute binding energies representative of over- and undersize solutes such as Al and Si, were calculated for two damage rate distributions corresponding to 75-KeV and 3-MeV Ni+ ions. For a constant peak damage rate of 10e3 dpa/s, a decrease in the incident ion energy from 3-MeV to 75-KeV results in a decrease in the temperature for maximum segregation by as much as 250°C. Increasing the peak damage rate for the 75-KeV ions to 4.5 X 10e2 dpa/s reduces the difference in the peak segregation temperature relative to the 3-MeV ions to -150°C. The latter result is consistent with the recent observations of Marwick et al. [23] and Rehn et al. [9] for the temperature dependence of Si segregation in ion bombarded dilute Ni-Si alloys. The latter authors irradiated Ni-1 at% Si with 3.5 MeV Ni+ ions at a peak damage rate of 2.5 X lop3 dpa/s and found maximum Si segregation at -560°C (see fig. 9) while Marwick and co-workers, using 75-KeV Ni+ ions and a peak damage rate of 4.5 X low2 dpa/s, found maximum Si segregation near 400°C in Ni-0.33 at% Si. Marwick and co-workers [23] also reported that below -lOO”C, the oversize solutes Al, Ti, Mn and Cr become enriched at the irradiated surface and proposed that at temperatures where the vacancies are immobile, oversize solutes segregate to sinks by an interstitial mechanism. The low temperature segregation behavior of oversize solutes is not well understood. At present, the long range transport of oversized solutes by interstitials seems unlikely on theoretical grounds [43]. 3.4. Dose rate effects The magnitudes of the defect fluxes, which determine the degree of radiation-induced segregation near sinks, are temperature and dose rate dependent.

P.R. Okamoto, L.E. Rehn 1 Radiation-induced segregation in binary and ternary alloys

Since the minimum in the defect recombination rate shifts toward lower temperatures for lower damage rates, decreasing the defect production rate is expected to decrease the temperature where maximum segregation occurs. This expectation has been confirmed in Ni-Si alloys. Robrock et al. [12] have investigated the temperature dependence of Nis Si coatings induced on the surface of initially single-phase, Ni-6 at% Si specimens during irradiation at dose rates of 4.5 X 10m4 dpa/s and 2 X IOU2 dpa/s. The specimens were bombarded with 3.5-MeV Ni+ ions to a peak dose of 3 dpa. Fig. 13 shows typical morphologies of the NisSi phase on or just below the bombarded surfaces of samples irradiated at 4.5 X IOF dpa/s. At 42O”C, Nis Si precipitates are found on dislocation loops but not on the surface. At 53O”C,a continuous ftm of Nis Si completely covers the irradiated surface, while at 61O”C, only isolated islands of Nis Si are found at the surface. At 7 17”C, no evidence of Ni$i was found either at the surface or on internal sinks. At 2 X 10m2 dpa/s, similar morphologies were found but at correspondingly higher temperatures. At 48O’C no evidence of Nia Si precipitation was observed, while Nis Si precipitates decorating dislocation loops were found at 58O’C. A continuous surface film, as well as grain boundary films, were formed at 614”C, but at 725°C no precipitates were found in or on the sample. These observations, along with previous investigations of Ni-I at% Si and Ni-12.7 at% Si, indicate that the peak segregation temperature in Ni-Si alloys occurs at about 530, 560 and 615°C for peak dose rates of 4.5 X 10m4,3 X 10m3 and 2 X 10e2 dpa/s, respectively. As expected, the dose rate dependence of the peak segregation temperature is very similar to that for void swelling. The data of Ryan [47] and Schmidt et al. [48] show that when pure nickel is irradiated with Ni+ ions in the energy range of 3 MeV, the peak swelling temperatures corresponding to peak dose rates of 4 X 10M4,3 X 10d3 and 4 X 10m2 dpa/s are approximately 550,600 and 625”C, respectively. As noted earlier, the peak segregation temperatures lie slightly below the peak swelling temperatures. While this may be due in part to the lower dose rate at the surface where segregation is measured, a more plausible reason is that near the peak swelling temperature back diffusion of solute becomes a limiting factor for segre-

13

Fig. 13. Dark field images displaying -y’-Ni3Si phase at the SUI faces of Ni-6 at% Si samples irradiated to 3 dpa at 4.5 X IOdpa/s with 3.5-MeV Ni+ ions. (Top) 610°C; (middle) 530°C; (bottom) 420°C (Robrock et al. [12]).

gation but not for swelling [34]. The effect of dose rate on the temperature dependence of segregation in Ni-6 at% Si has also been studied during 3-MeV Ni+ ion bombardment by Rehn et al. [13] using the in-situ infrared pyrometry technique discussed earlier. Lowering the dose rate was found to decrease the growth rate of Ni3Si surface films during irradiation at temperatures above the peak segregation temperature, but to increase the film growth rate at temperatures below the peak.

4

P.R. Okamoto, LX. Rehn /Radiation-induced

14

These in-situ IRP results show that both the high and low temperature sides of the peak in the growth rate versus temperature curves shift toward lower temperatures as the dose rate is reduced. It was also found that films formed during irradiation completely dissolve when the dose rate is reduced to sufficiently low values, clearly demonstrating the nonequilibrium nature of the segregation phenomenon. The above observations confirm the predictions of the Johnson-Lam model summarized in the temperature-dose rate diagram in fig. 14, which shows the temperature range where significant segregation to surfaces is expected as a function of dose rate. For a given dose rate the temperature for maximum segregation lies approximately in the middle of the range. Although fig. 14 is based on calculations for a dilute alloy, the downward shift in the temperature range of segregation with decreasing dose rate is also predicted for concentrated alloys [41]. Calculations also predict that at steady

0.6

z

0.5

RAO~AT~~N-IND~CED SOLUTE SEGREGATION

: 0.4

t

0.3

0.2

0.1 lo+

lo-”

lo-4 K,

Fig. 14. Temperature-dose segregation.

lO-3

idpaM

rate diagram for radiation-induced

Id2

segregation in binary and ternary alloys

state, the maximum solute enrichment at the surface increases with decreasing dose rate [29,41].

4. Segregation in Fe-Cr-Ni

alloys

While radiation-induced segregation in binary alloys has been extensively studied, comparatively few studies have been made on Fe-Cr-Ni based alloys.. Nevertheless, a number of important observations now seem well established. In this section these results are reviewed and some new observations are discussed. 4.1. Solute size effects Okamoto and Wiedersich [2] were the first to demonstrate that radiation-produced defect fluxes can alter the distribution of major and minor elements near sinks in Fe-Cr-Ni alloys. Following earlier TEM observations of radiation-induced precipitation on voids during electron irradiation of an Fe-l 8Cr-8Ni-1 Si alloy [ 11, they bombarded samples of the same alloy at 600°C with 3.25-MeV Ni+ ions and used AES to measure the near surface composition. The Auger spectra indicated large increases in the concentrations of the undersize elements, Ni and Si, and a decrease in the concentration of the oversize Cr at the bombarded surface. The correlation between solute size and segregation behavior was noted, and attributed to a preferential transport of undersize solutes to sinks by interstitials. The enrichment of Ni and the depletion of Cr at the bombarded surface was confirmed by the AES measurements of Johnston et al. [19] in Fe-lSCr20Ni and Fe-l 5Cr-20Ni-0.7Si after irradiation with 4-MeV Ni+ ions at 675°C. Similar observations have recently been made by Sethi et al. [20] in 3-MeV Ni+ irradiated 316 stainless steel, Fe-18Cr8Ni-1Mo and Fe-18Cr-8Ni-1Si. These ion bombardment results are in agreement with the AES studies of Clausing and Bloom [49] on neutron irradiated 304 stainless steel which show that Ni is enriched and Cr is depleted at grain boundary fracture surfaces. Examples of some AES results of Sethi and co-workers, shown in figs. 15 to 17, also show that undersize Si behaves like Ni and oversize MO like Cr.

P.R. Okamoto, L.E. Rehn /Radiation-induced segregation in binary and ternary alloys

I

I 20

OO

I 40

I 60

I

I 80

mu

u

zu

4u

DEPTH hml

DEPTH

Fig. 15. AES chemical depth profiles of Fe-18Cr-8Ni with 3.5-MeV Ni+ ions (Sethi et al. [ 201).

90

I

I

I

and Fe-18Cr-1Mo

I

1

90

89

I

1

I

Ft-lUCrUNi-ISi ~3.oep;54oT

I

90

$

of an Fe-18Cr-8Ni-1Si

t

I

EO-

7

ip----t

60-

100

alloys bombarded at 600°C to a peak dose of 60 dpa

60r

Fig. 16. AES depth profile of constituents (Sethi et al. 1201).

w hid

I

I

I

Fe-l6Cr-UNi-ISi 2.4dm77O.C

-

m-,V-++-----‘”

-

60-

i

alloy bombarded with 3-MeV Ni+ ions at 3 X 1 I4 dpa/s

16

P.R. Okamoto, soy

I

LX. Rehn /Radiation-induced

I

I

segregation

in binary and ternary alloys

1

I

316SS HL t 11893 3.ldpo;440.C

90

-I

L

Oo

0 20

40

60

90

KPlti Ird

Fig. 1’7. AES depth profiles of constituents

1

1 loo

loo

DEPTH hnl

of a 316 stainless steel bombarded with 3-MeV Nif ions at 3

x

10h4 dpa/s (Sethi et al.

[ml).

These observations are consistent with the idea that the undersize solutes diffuse preferentially via an interstitial mechanism. However, as recently pointed out by Marwick [40], the available thermal diffusion data on Fe-Cr-Ni austenitic alloys [50,5 1 ] indicate that Cr diffuses faster then Ni. Thus the enrichment of Ni at the surface is also consistent with a vacancy-induced inverse Kirkendall effect. 4.2. Effect of minor elements The radiation-induced segregation of the major elements in Fe-Cr-Ni alloys appears to be sensitive to the presence of minor elements as seen in fig. 15. The addition of 1% MO to an Fe-l Wr-8Ni alloy significantly alters the composition depth profiles of the major alloying elements. The composition

gradients near the surface become significantly steeper resulting in a more pronounced surface enrichment of Ni and depletion of Cr. Similar effects on the concentration profiles of the major elements due to the addition of Si have been reported by Johnston et al. [19] for an ion bombarded Fe-15Cr-2ONi alloy. Although the changes are difficult to quantify, the effect of MO and Si on the major element profiles appears to be similar to a decrease in the irradiation temperature. This suggests an enhanced recombination effect which would be consistent with the recent observations of Gessel and Rowcliffe [52] which show that both Si and MO reduce void swelling in Fe-Cr-Ni alloys, presumably by trapping point defects. Venker and Ehrlich [53] have emphasized that minor elements can have a large effect on the diffu-

P.R. Okamoto, L.E. Rehn j Radiation-induced

sion coefficients of the major elements and, therefore, can affect void swelling as well as segregation. While some evidence exists consistent with this suggestion in the case of MO [.53,54], recent diffusion studies by Rothman [55] show that Si additions of 1.4% to the system Fe-l 5Cr-20Ni have only a small effect on the ratios DFe/DNi and Dc,/DNi, yet the same addition is known to drastically reduce void swelling in the alloy. 4.3. Temperature

dependence

Sethi et al. [20] have also studied the temperature dependence of segregation in 3 16 stainless steel and in Fe-18Cr-8Ni-1Si alloys irradiated at a peak dose rate of 3 X 10e4 dpa/s with 3-MeV Nit ions. AES composition profiles of the alloy components in samples irradiated to 3 dpa are shown in figs. 16 and 17. Maximum segregation occurs in the temperature range of 400-500°C. The available data is too limited to define the peak segregation temperature more precisely. However, it appears to lie in the same temperature range as the peak swelling temperature of 3 16 stainless steel which is between 450 and 500°C when irradiated with 46.5-MeV Ni+ ions at the comparable dose rate of 3.3 X 10m4 dpa/s [56]. Figs. 16 and 17 also indicate that at equivalent doses and temperature, less segregation of Si to the surface occurs in 3 16 stainless steel than in Fe-l 8Cr8Ni-1 Si as judged from the widths of the Si enriched zones as well as from the increases in the Si concentrations near the surfaces.

5. Effect on phase stability That irradiation can cause the appearance of precipitate phases which are not found in thermally aged alloys or appear out of place on the phase diagram has been known for some time. Although the physical origins of these phase changes are still undergoing active debate, it is suspected that many of these are caused by radiation-induced segregation. In addition to inducing precipitation of nonequilibrium phases, it is now apparent that radiation-induced segregation can induce changes in the spatial distribution and morphology of equilibrium phases. In this section these segregation effects on alloy stability are reviewed.

segregation in binary and ternary alloys

17

5.1. Single-phase alloys

As shown in previous sections, precipitation in single-phase alloys can occur on external surfaces, grain boundaries, dislocations and voids if, as a result of radiation-induced segregation, the local solute enrichment at the sinks exceeds the solubility limit. Unambiguous evidence of radiation-induced precipitation on sinks has been observed in undersaturated solid solutions of Ni-Si [5-141, Ni-Be [4] and Mg-Cd [8]. Precipitation in regions away from sinks may occur if segregation leads to solute depletion at sinks and, thus, to solute enrichment in the matrix. However, as illustrated in fig. 3, enrichment of oversize solutes in regions away from sinks is generally small so that precipitation in the matrix is expected only when the homogeneous composition of the alloy is close to the solubility limit at the irradiation temperature. Because of the temperature and dose rate dependences of segregation, the stability of an undersaturated solid solution is also dependent on these variables. These effects have been explored theoretically by Lam and co-workers [32]. They show that in systems such as Ni-Si and Ni-Be, an undersaturated solid solution can become unstable in a temperature range that is dependent on the temperature dependence of radiation-induced segregation, on the temperature dependence of the solubility limit, as well as on the dose-rate, dose, and solute concentration. In particular, their calculations predict that at high temperatures a solid solution which is stable at low dose rates can become unstable and decompose if the dose rate is increased. A solid solution which is unstable at a given dose rate and temperature can become stable if the temperature is increased. These predictions can be readily understood in terms of the shifts in the temperature range for segregation to lower temperatures with decreasing dose rate as shown in fig. 14. These high temperature predictions have been verified by Barbu and Martin [6] who systematically investigated the effects of temperature, dose, doserate and initial solute concentration on the stability of undersaturated Ni-Si alloys. The dose rate dependence of the film growth rate of the Nig Si y’ phase in Ni-6 at% Si determined by Robrock et al. [ 121, and by Rehn et al. [ 131 are also in accord with the predictions of Lam and co-workers [32].

18

P.R. Okamoto, L.E. Rehn /Radiation-induced

5.2. Phase redistribution in two-phase alloys In two-phase alloys radiation-induced segregation can lead to pronounced changes in the morphology and spatial distribution of precipitates. For alloys in which solute enrichment occurs at sinks, the solute concentration in regions away from sinks may fall below the solubility limit. As a result, enhanced precipitation can occur at sinks at the expense of precipitate dissolution in the matrix. Hence, spatially uniform precipitate distributions frequently observed in y’ forming alloys after thermal aging treatments may not be observed after irradiation at the same temperature. Clear evidence of this type of phase instability has been observed by Potter et al. [16] in Ni-12.7 at% Si after irradiation with 3.5MeV Nif ions. An example is shown in fig. 18. The figure on the left is a y’ dark field TEM image of the near surface region of a sample which was thermally aged without irradiation at 67O’C for 72 min. Under these aging conditions the y’-Nia Si precipitate distribution remains uniform up to the external surface. The figure on the right is a y’

Fig. 18. Dark field images showing without irradiation for 72 minutes (Potter et al. [ 161).

segregation in binary and ternary alloys

dark field image of the near surface region of a sample that had been irradiated at 670°C for 72 min. The segregation of Si to the irradiated surface results in the formation of a y’ surface film at the expense of the y’ particles which normally occur in the matrix. A similar redistribution of y’ particles occurs in the vicinity of dislocation loops [ 161. During irradiation the enrichment of Si at the loops leads to nucleation and growth of y’ precipitates on the loops and to the dissolution of y’ particles in the matrix. Eventually all the y’ phase in the sample interior is converted into coatings on dislocation loops and other defect sinks. The inverse situation can arise in two-phase alloys when segregation leads to solute depletion at sinks and, thus, to solute enrichment in the regions between sinks. In this case, represented by Ni-12.8 at% Al, y’-Ni, Al particles dissolve in the vicinity of the sink while coarsening of the y’ particles is enhanced in regions between sinks [ 1.51. When samples are irradf ated in the form of thin foils, Al migration away from the surfaces leads to precipitate-free zones near the top and bottom surfaces and to a localization of the y’ phase into a thin layer in the middle

the y’-Ni$i phase in the near surface regions of Ni-12.7 at 670°C; (right) after irradiation at the same temperature

at% Si alloys. (left) After thermal aging and time with 3.5-MeV Ni+ ions

P.R. Okamoto, L.E. Rehn /Radiation-induced segregation in binary and ternary alloys

--

-..

19

____~----.-

Fig. 19. TEM images showing 7’-Ni#U precipitate redistribution in Ni-12.8 at% Al alloys irradiated at 550°C with 3.5-MeV Ni+ ions. (left) Bright field image of dislocations after 1.8 dpa; (middle) dark field image of y’ particle distribution after 1.8 dpa; (right) 7’ distribution after 8 dpa (Potter et al. [ 151).

of the foil. In bulk samples, the effect leads to precipitate-free zones surrounding internal sinks such as dislocation loops and voids. This is illustrated in fig. 19, where the figure on the left shows the dislocation loop structure in a Ni-12.8 a% Al sample bombarded at 550°C with 3.5MeV Ni+ ions to 1.8 dpa. The middle figure is a dark field image displaying only the 7’ precipitates in the same sample. Each spherically shaped precipitate-free zone contains a loop or void at its center and, with further irradiation, the loops and voids grow as do their associated precipitate free zones. The figure on the right shows that by 8 dpa, the precipitate free zones occupy almost half the sample volume with the y’ precipitates packed into the remaining volume between the loops. Eventually the zones begin to intersect and renucleation of y’ particles occurs between sinks, resulting in a more uniform y’ distribution. Radiation-induced precipitation has also been reported at sinks in other two-phase binary alloys. Farrell et al. [27] have seen Si-coated voids in Al after irradiation in a thermal reactor where the Si is created as a transmutation product. Precipitation of y’-Ti3Al on voids, surfaces and grain boundaries has been reported by Erck et al. [28] in Ti-14.4 a% Al irradiated with 3-MeV Ni+ ions at 600°C. Si and Al are undersize solutes in these two-phase alloys, and the observed enrichment of these solutes at sinks is consistent with the behavior of undersize solutes in nickel binary alloys.

5.3. Fe- Cz-Ni alloys At present irradiation-induced phase changes in Fe-Cr-Ni austenitic alloys are not well understood. Interpretations are complicated not only by the inherent thermodynamic metastability of austenitic alloys, but also by the absence of reliable equilibrium phase diagrams. However, by altering the alloy composition at sinks and in the matrix, radiation-induced segrega-

tion effects almost certainly play a role in inducing many of the irradiation-induced phases seen in these systems. In the following we cite some experimental observations of irradiation-induced phase changes in Fe-Cr-Ni austenitic alloys in which segregation effects appear to play a major role. In attempting to rationalize, irradiation-induced phase changes in ternary alloys in terms of segregation effects, it is useful to view radiation-induced segregation as effectively translating local regions of the sample to different parts of the phase diagram. This point is illustrated schematically in fig. 20 for the Fe-Cr-Ni phase diagram, which shows various “diffusion” paths representing possible radiationinduced compositional changes for several different alloys. Precipitation is expected in any local region of a sample whose path crosses a phase boundary such as those labeled 1 and 2, for alloys of compositions A and B. This illustrates the importance of knowing the segregation behavior of the various alloy components in order to predict which phases

P.R. Okamoto, L.E. Rehn /Radiation-induced

20

/ / Fe

PERCENT NICKEL

Ni

Fig. 20. Schematic illustration of radiation-induced segregation effects in Fe-Cr-Ni alloys. Arrows indicate diffusion paths representing various possible compositional changes. Paths 1 or 2 lead to irradiation-induced phase changes in alloys A and B.

are likely to appear during irradiation. The Auger chemical depth profiles described in the previous section have shown that Ni and Cr segregate toward and away from sinks, respectively, in

segregation in binary and ternary alloys

irradiated Fe-Cr-Ni alloys. For example, fig. 15 shows that the Ni concentration is nearly doubled at the surface of an Fe-18Cr-8Ni alloy irradiated at600”C to 60 dpa with 3.5MeV Ni+ ions. The Cr concentration is reduced to nearly one-fourth of the initial level. Therefore, during irradiation the alloy composition near sinks moves from its initial position at A in fig. 20, deeper into the y-austenite field as indicated by path 3. The matrix composition in regions away from sinks moves in the opposite direction toward the austenite-ferrite phase boundary as indicated by path 1. Thus, we can expect that in alloys like Fe-18Cr-8Ni, irradiation will tend to enhance the stability of austenite around sinks at the expense of an increased tendency to form ferrite in the matrix. Evidence of this type of phase instability in 3.5MeV Ni+ irradiated Fe-18Cr-8Ni alloys can be seen in fig. 2 1 which shows the damage microstructure at the peak damage depth in a sample bombarded at 600°C to 60 dpa. The left micrograph is a bright field image showing voids in an initially austenitic grain that had transformed to ferrite during irradiation. The right-hand figure is a dark field image of the same area taken with an austenitic reflection, and shows that the only remaining austenitic phase in the grain

Fig. 21. Electron micrographs of a transformed grain in an Fe-18Cr-8Ni alloy irradiated at 600°C to 60 dpa. (left) Bright field image of void microstructure; (right) dark field image of the same area displaying retained austenite as precipitate shells around irradiation-induced voids.

P.R. Okamoto, L.E. Rehn /Radiation-induced segregation in binary and ternary alloys

occurs as coatings on voids. Similar observations in neutron irradiated 304L stainless steel have been reported by Porter [57]. The enrichment of Ni and Si at internal sinks in 3 16 stainless steel appears to be a common observation. Lee et al. [58] have shown that during neutron irradiation of a modified 3 16 stainless steel, segregation of Ni and Si to dislocations results in precipitation of the G-phase which is structurally similar to the fee M2sC6 carbide. The enrichment of Si in the vicinity of Frank loops in the same alloy has been measured by Kenik [26] by energy dispersive X-ray techniques. Brager and Garner [59] have recently made an extensive study of the chemistry of radiation-induced precipitates in neutron irradiated AISI type 3 16 stainless steel, and have shown that Ni and Si are concentrated in practically all irradiation-induced phases. They observed not only the Nia Si y’ phase, but also Ni and Si enriched M2aC6 carbides. These irradiationinduced carbides have nonstandard c~st~lograp~c orientations with the matrix. In contrast to those which form thermally in the absence of irradiation, the nonstandard carbides have only one set of {I 11) planes parallel to one of the four {111) planes of the matrix. Brager and Garner suggest that these carbides precipitate on Frank loops, as a result of Ni and Si segregation to the loops during irradiation. These authors also cite evidence which indicates that MO in solution retards the formation of the NisSi phase. This would be consistent with the AES measurements of Sethi and co-workers f20], which show that less Si segregation to the surface occurs in 3 16 stainless steel then in Fe- 18Cr-8Ni-1 Si; which does not contain MO.

6. S~rna~

and conclusions

Experimental observations have established that radiation-induced segregation occurs in the same temperature range as does void swelling, the peak segregation temperature lies near the peak swelling temperature, and that the peak segregation temperature shifts to lower temperatures with decreasing dose-rate. In nickel-based binary alloys, oversize solutes such as Ti, MO,Al, Cr and Mn segregate away from defect sinks while undersize solutes such as Si and Be, segregate toward sinks. The composi-

21

tional changes which result from segregation of undersize solutes in solid solutions are fr~uently large enough to induce precipitation of non-equilibrium phases near sinks such as grain boundaries, external surfaces, dislocations and voids. In two-phase alloys, radiation not only enhances coarsening.processes but also alters, by segregation, the morphology and spatial dist~bution of precipitates. In Ni+-ion irradiated Ni-12.7 at% Si, the NiaSi y’-phase forms and grows on interstitial loops and other defect sinks at the expense of pre-existing, coherent y’ particles. In Ni-12.8 at% Al alloys, y’ particles dissolve in the vicinity of interstitial loops resulting in precipitate free zones which grow in size with the loops. y’ particles grow preferentially in the sink free regions between dislocation loops. Similar redistribution of y’ precipitates occurs near the external surfaces of irradiated alloys. In Fe-Cr-Ni base alloys, the radiation-induced segregation behavior of the major and minor alloy ing elements is consistent with the size-effect correlation. Ni is an undersize solute and is enriched in the near surface region, while Cr is an oversize solute in these alloys and is depleted. Similarly for the minor alloying additions, undersize Si is enriched and oversize MOis depleted at the surface. The segregation behavior of the major elements tends to enhance the stability of austenite in regions close to sinks while enhancing the stability of ferrite in the matrix. Both Si and MOsignificantly affect the segregation behavior of the major alloy components. The experimental observations in binary alloys are consistent with the predictions of current theoretical models which link radiation-induced solute segregation to solute-defect binding and/or to differences in the diffusion rates of alloying components via vacancy and interstitial mechanisms. However, our limited knowledge of solute-defect binding energies, the migration energies and migration mechanisms of solute-defect complexes, as well as the lack of information on the diffusivities of alloying elements via interstitial mecha~s~, preclude quantitative predictions of the segregation behavior of real alloys. Although undersize and oversize solutes generally segregate toward and away from sinks, respectively, a possible exception to the rule brought out at this conference [62] indicates that the current interpretation of the size-effect correlation may be over-

22

P.R. Okamoto, L.E. Rehn /Radiation-induced

simplified. Clearly much more systematic experimental work on defect properties in dilute and concentrated alloys will be required in order to assess critically the existing theories and to determine which fundamental properties of alloys control the segregation behavior of solutes. Despite gaps in our existing knowledge of the phenomenon, it is clear that radiation-induced segregation plays an important role in altering the stability of alloys and must be taken into account in any assessment of compositional effects on irradiation-induced property changes in alloys.

Acknowledgments The authors thank H. Wiedersich, N.Q. Lam, K.-H. Robrock, D.I. Potter and V.J. Sethi for fruitful discussions, comments on the manuscript and permission to use unpublished results of their work.

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segregation in binary and ternary alloys

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