Void formation in solution-treated aisi 316 and 321 stainless steels under 46.5 mev ni6+ irradiation

Journal of Nuclear Materials 60 (1976) 0 North-Holland Publishing Company

89-106

VOID FORMATION IN SOLUTION-TREATED UNDER 46.5 MeV Nib+ IRRADIATION

AISI 316 AND 321 STAINLESS STEELS

J .A. HUDSON Metallurgy Division, AERE Harweli, Didcot, Oxon, UK

Received

27 October

1975

The swelling and radiation damage structure developed in solution-treated 3 16 and 321 stainless steels bombarded by 46.5 MeV Ni6+ ions in the Variable Energy Cyclotron (VEC) have been determined. Foils were pre-injected with lo-’ a/a He at room temperature and subseqtfently bombarded by Ni6’ Ions in the temperature range 450-750°C at a damage rate of 1-3 x 10-a dpa per second to doses up to 300 dpa and specimens from the foils were examined by transmission electron microscopy. The data obtained were compared with data from other experiments aimed at simulating the fast-neutron irradiation of 316 and 321 steels, in particular previous work with 20 MeV C2’ ions and with data on fast-reactor bombarded material. The swelling rates in Ni-ion bombarded specimens were about a factor two less than those in C.-ion bombarded specimens and in good agreement with swelling rates in 5 MeV Ni’- and neutron-bombarded material. The peak swelling temperature after a dose of 40 dpa was 650°C in 316 steel and 625°C in 321 steel where the swelling was about 5.8% and 4.6% respectively.

Le gonflement ct la structure des dommages par irradiation developpes dans des aciers inoxydables 316 et 321 trait& a haute temperature pour mise en solution solide et bombard& par des ions Ni6’ de 46,5 MeV dans un cyclotronaenergie variable (VW) ont etd d6terminds. Des feuilles furent au prdalable soumises‘a l’implantation de lo-’ a/a Hea la temperature ambiante puis bombardees par des ions Ni6+ dans l’intervalle de temperatures de 450 i 750°C i un taux d’cndommagcment de 1 a 3 X 10-j dpa par seconde jusqu’i atteindre des doses de 300 dpa. Les echantillons furent ensuite examines par microscopic electronique par transmission. Les donndes obtenues furent compardes i celles d’autres experiences realiskes en vue de simuler l’irradiation par des neutrons rapides d’aciers inoxydables 316 et 321. En particulier ces observations furent comparees a celles d’un travail antdrieur sur des aciers inoxydables bombard& par dcs ions C2+ de 20 MeV et a des don&es obtenues sur des materiaux bombard& dans un reacteur rapide. Lcs vitesses de gonflement des echantillons bombard&s par des ions Ni etaient inferieures d’un facteur 2 environ i celles d’ichantillons bombard& par des ions C et en bon accord avec les vitesses de gonflement observees dans ces matdriaux bombard& par des ions N? de 5 MeV et par des neutrons. La temperature de gonflement maximum apres une dose de 40 dpa dtait de 650°C dans l’acier 316 et de 625°C dam l’acier 321, aciers dont le gonflement dtait rcspectivement dgal i environ 5,8% et 4,6% respectivement.

Das Schwellen und die durch Strahlenschadigung entstandene Struktur wurden in den losunggegliihten rostfreien Stahlen 316 und 321 untersucht, die im Zyklotron variabler Energie mit 46,s MeV Ni6’ -1onen beschossen worden waren. Die Folien waren zuvor mit lop5 a/a He bei Raumtemperatur und darauf mit Ni6’ -1onen zwischen 4.50 und 750°C bei einer Schadigungsrate zwischen 1 und 3 X 10W3 Verlagerungen pro Atom und Sekunde und einer Dosis von 300 Verlagerungen pro Atom beschossen worden. Die Folienabschnitte wurden transmissionselektronenmikroskopisch untersucht. Die Ergcbnisse werden mit denen anderer Experimente verglichen, die auf die Simulation einer Bestrahlung der Stable 3 16 und 321 mit schnellen Neutronen abgezielt haben; insbesondere mit einer friiheren Arbeit mit 20 MeV C2’-1onen und mit Ergebnissen an Material, das im schnellen Reaktor bestrahlt worden war. Die Schwellrate der mit Ni-Ionen beschossenen Proben liegt urn den Faktor zwei niedriger als die der mit C-lonen beschossenen Proben. Die iibereinstimmung mit der Schwellrate dcs mit 5 MeV Nit-Ionen und mit Neutronen bestrahlten Materials ist gut. Die Temperatur des maximalen Schwellens bei einer Dosis von 40 Verlagerungen pro Atom liegt fur Stahl 316 bei 650°C und fur Stahl 321 bei 625°C; bier betragt das Schwellen etwa 5,8% bzw. 4,6%.

89

I. Introduction Much work has been done in recent years on the simulation of fast-neutron induced void formation by ion bombardment [l-4]. Early work at Harwell was based on 20 MeV C2+ irradiations in the Variable Energy Cyclotron (VEC) [l]. Once beams of high energy metal ions became available work was concentrated in this area since it was considered desirable to avoid, if possible, the implantation of carbon into the bombarded materials, many of which were austenitic stainless steels and other fast reactor structural materials containing controlled amobnts of alloying elements, including carbon. Moreover it was considered that in terms of the recoil spectra of bombarded atoms, high energy Ni-ion bombardment was a better simulation of the fast-neutron bombardment case than was high-energy C ion bombardment [5]. The beam most commonly used has been of 46.5 MeV Ni6+ ions which have a range in stainless steel of about 5 pm compared with 7 pm of the 20 MeV C2+ ions, still sufficient for surface effects to be neglected at the peak of the damage distribution. The calibration of the damage intensity produced by the Ni-ion beam as a function of depth in pure nickel and the subsequent development of a programme for a rocking target holder, which causes the peak damage region to be uniformly spread over 2 pm, has been described previously [6]. The calibration was based on the assumption that void-swelling produced at a certain damage rate in pure nickel is dependent solely on dose and not on the recoil spectrum characteristic of the bombarding species. Similar swellings had been observed in neutron, proton, carbon and electron-bombarded nickel, which supported this view. Subsequent calculations [5] have also predicted values of the peak damage intensities for 46.5 MeV Ni6+ ion bombardment very near to the calibrated ones. Despite the insensitivity of swelling and void concentration to recoil spectrum in nickel and other pure metals the same is not true in alloys. Preliminary work on Nib+-bombarded 321 steel bombarded near the peak swelling temperature did indicate similar results to those from C2+ bombardment experiments but subsequent experiments consistently revealed lower swelling values. Thus many of the early experiments with 20 MeV C*+ were repeated on identical starting materials using 46.5 MeV Ni6+ ions. In this report the

results are presented, compared with results ot‘othel simulation experiments and discussed in terms of‘ current ideas on the behaviour of collision cascades produced at elevated temperatures.

2. Experimental The materials used in the experiment were FirthVickers 555 austenitic stainless steel (cast VR 906 A), having a specification falling within the AISI 316 range and En58B stainless steel(Samue1 Fox and Co., cast 57590) which is within the AISI 32 1 specification. The composition of these two alloys are given in table 1 below. Material from the same casts was used in previous work involving 20 MeV C2+ irradiations [7]. Specimens of each material were cold-rolled to a final thickness of about 80 pm. Foils, 2 cm X 1 cm, were cut from the rolled strip and solution-treated in vacua for 40 min at 105O’C after which they were uniformly filled with helium to a concentration of 10e5 a/a by room temperature (Yparticle bombardment on the VEC, as described by Worth [8]. Prior to irradiation with Ni6+ ions all foils were copperplated on one side to a thickness of about 200 pm to increase thermal contact with the target block and to increase their mechanical strength. Irradiations with 46.5 MeV Ni6+ ions to the required doses at the required temperature were performed using the rocking target holder to produce a uniform peak damage region from about 2 to 4.5 pm below the bombarded surface [9]. The total area of foil bombarded by the Ni ion beam was 1.6 cm2, usually in separate halves so that specimens of each steel could be irradiated simultaneously at one temperature. In the case of unrocked Ni6+ irradiations the experimental calibration gave a peak damage intensity of 12 dpa for an ion dose of 1 X 10’ 6 ions cmp2 and in order to spread this peak over 2.5 pm a total dose of about 4 times this value was necessary. The peak damage intensity calculated by Marwick [5] using the TRN (Torrens, Robinson and Norgett) standard and a displacement energy of 40 eV with the E-DEP-1 Computer code of Manning and Mueller [lo] was 10.5 dpa for a dose of 1 X 1016 ions cm-* and so there is now good agreement between experiment and theory in terms of the numbers of displaced atoms produced by high energy

J.A. Hudson / Void formation in stainless steels

Table 1 Chemical

composition

Element

C

Si

Mn

Cr

Ni

MO

B

11.8

2.5

0.0018

of materials

91

used

316

wt%

0.03

0.3

1.1

17.5

321

wt%

0.05

0.42

1.76

18.09

Ti

P

9.57

0.33

Fe Bal

0.025

0.03

Bal

in the range 450-750°C are shown in figs. 1 and 2. The 16 dpa swelling values for 316 steel at 525°C and for 321 steel at 600°C were interpolations from the dose dependence determinations described in section 3.2. The 40 dpa swelling data of figs. 1 and 2 are compared with previous data from C2+ bombarded steels [7] in figs. 3 and 4. In fig. 3 the swelling versus temperature profiles for 316 steel are compared. The Ni ion irradiations produced less swelling at all temperatures and the temperature of peak swelling was increased from 600°C to 650°C where 6% swelling developed rather than the 12% which developed under C ion irradiation. The swelling versus temperature profiles for 321 steels are compared in fig. 4 but here the C ion data refers to a dose of 60 dpa. Again the peak swelling temperature is higher for Ni ion bombardment at about 650°C compared with 625°C for C ion bombardment. Below 525°C no voids have been identified in Ni6+ bombarded foils whereas in 321 steel 3.5% swelling has been reported for a specimen bombarded to 60 dpa at 450°C by C2+ ions at a com-

3. Results 3.1. Swelling versus temperature The swellings in the solution treated 3 16 and 32 1 steels bombarded to 16 and 40 dpa at temperatures I

S

0.023

heavy ions. With the 2-4 E.IAbeams of Ni6+ ions available, the peak damage rate was in the range l-3 X lop3 dpa per set in the present experiment. After irradiation, the copper plating was dissolved and specimens were extracted from the peak damage region of the foils by vibratory and electrolytic polishing as described previously [ 111. For a particular dose at a particular temperature three specimens suitable for electron microscopy were obtained for each stainless steel. These specimens were examined in transmission at 100 kV in a Philips EM300 microscope or at 1000 kV in the Harwell AEI Em7 microscope. In all cases suitable micrographs were taken to permit a determination of the void size and concentration and the dislocation line and loop densities.

1

N

I

I

I

I

.o-

0

.o0

2.0-

_-

0 450

500

/X-550

IRRADIATION

Fig. 1. The variation

of swelling

with temperature

_

__--x,

-x-

I\ 1

1

600 TEMPERATURE

650 (“C)

in st 316 steel bombarded

‘\\+ 700

I

to 16 and 40 dpa.

92

6.0

5.0

z c 0 I

3.0-

40dpa

0

: w z r\

z.o4.0-

0

n

ecx-N I.0 0’

/ 400

450

550

IRRADIATION

Fig. 2. The variation

,

x, ,

600

bS0

0,’

500

Ibdpa .\

If

TEMPERATURE

of swelling with temperature

__

,

700

750

.N&

000

(‘C)

in st 321 steel bombarded

parable damage rate. The void concentrations in Ni and C ion bombarded steels after 40 and 60 dpa are shown together plotted against 1/kT in fig. 5 where it can be seen that no straight line Arrhenius relation will fit the data from either steel indicating that simple

,

to 16 and 40 dpa.

homogeneous nucleation is not the dominant mechanism of void nucleation. In the high and low temperature regions the C2+ data show higher void concentrations but between about 525 and 600°C the values lie close together, within experimental error,

/

/o

_ \ \

/

0. ‘\

3 0

z

a

z

1

20

MeVC

i

0

2+ d I’ I

, I (

= w

‘\

I

I

6r L L6

5 MeV

N, 6.

2

0 ;

1

i LOO

I 450

500

550 IRRADIATION

Fig. 3. Comparison

of swelling versus temperature

600

650

TEMPERATURE

profiles

700

750

/ 800

i°C 1

in C’+and

Ni6+ bombarded

316 steel.

J.A. Hudson / Void formation

20 HeV

lo-

C2'1

'60 dpa' 3 -

93

in stainless steels

b

: P

.9-

z i tJ

:

Z6

I

0

1

I

LOO

L50

,

I 500

550 IRRADIATION

Fig. 4.

Comparison

of swelling

versus temperature

and in this region the variation with temperature is relatively small. The reason for the large difference in the C ion data for 316 steel at 500 and 525°C has not been resolved. The dislocation densities determined in specimens bombarded to 40 dpa are shown in fig. 6, again plotted against l/kT. The total dislocation density after 40 dpa is seen to fall by about an order of magnitude from 2 X 10” cm cmV3 to 2 X lQl” cm cmP3 between 450 and 75O’C. Dislocation densities have been reported for only few C2+ bombarded specimens but the values are all in the same range as those plotted in fig. 6 and there was no obvious difference in the dislocation distributions in specimens bombarded by the different species. After 16 dpa the dislocation densities were 50-I 00% of the 40 dpa values falling from 2 X 10” cm crne3 at 450°C to 1.5 X lOlo cm cmm3 at 750°C. The structure in both steels bombarded to 16 dpa at temperatures below 7OO’C consisted of faulted and unfaulted loops together with a network formed from the interaction of the largest of the unfaulted loops. Details of the loop distributions after low dose irradiations are given in subsection 3.2 below. No loops were observed in specimens bombarded at 700 or 75O’C.

600

TEMPERATURE

profiles

800

750

700

650

('CI

in C*+and

Ni6+ bombarded

IRRADIATION

321 steel.

TEMPERATURE(?)

625 600

560550 1,

525

500

I

I

L50 Id

c 316 N,6+

El l

LOdpa

0

321

A

3l6 C2,

LOdpa

A

321

60dpa

. A

.

0

0

OR

0

1

10131

11

1

I

I

I

I

12

13

Ii

15

16

& (ev-‘1 Fig. 5. Comparison bombarded steels.

of void concentrations

in C*+and

Ni6+

I

94

J.A. Hudson / Void formation in stainkss steels

3.2. Dose dependencies of swelling

in both cases are displaced to higher doses by about a factor of two. The curves are no longer parallel towards the higher doses studied, above about 60 dpa, where the swelling rates in the C ion bombarded material decrease at both temperatures. The more marked decrease is at 525°C where the swelling has almost saturated at 10-l 5%. In the Ni ion bombarded material the swelling rate decreases at about 100 dpa (- 10% swelling) where the two curves cross but there is no clear indication of saturation and 16% swelling has built up by 300 dpa. The chief difference in the void distributions in the C and Ni ion bombarded 3 16 steel is in the concentrations as indicated in figs. 9 and 10. In both cases the concentration of visible voids goes through a maximum with dose at about 20 dpa for C bombardment and at about 40 dpa for Ni bombardment, but in the Ni ion bombardment case the initial rise to the maximum is only a factor 2 to about 2 X 1015 cme3 whereas in the C ion case the rise is an order of magnitude to about 2 X 1016 cmd3. Taylor et al have also observed a higher void concentration in 316 steel bombarded by 5 MeV C ions than in the

As an additional comparison with data from C2+ ion irradiations, dose dependencies of swelling in 316 and 321 steels at two temperatures were determined. The swelling in 3 16 steel bombarded by Ni6+ ions at 525’C is shown in fig. 7 in which the original C ion data is included and fig. 8 shows the equivalent data for 321 steel bombarded at 600°C. Several specimens of 3 16 steel have also been bombarded by Ni6+ ions at 600°C to doses up to 120 dpa and the swelling values, shown in fig. 8, lie very close to the curve through the 321 swelling values: The variations of void size and concentration for the C2+ and Ni6+ irradiations are shown in figs. 9 to 12. The large (- 30%) swelling after 180 dpa at 600°C was also investigated at 1000 kV and a micrograph taken in the Em7 microscope is shown in fig. 13. The swelling curves for the Ni6+ bombarded steels shown in the logarithmic plots of figs. 7 and 8 are approximately parallel to the original curves for the C2+ bombarded material over most of the dose range and

IRRAOIATION

TEMPERATURE 600

(‘C)

SSO,

525

500

450

0

8

0

10101

I

I

I

I

I2

I3

14

IS

+ Fig. 6. Dislocation

density

WISLIS irradiation

1 I6

I

(ev-1)

temperature

in 316 and 321 steels bombarded

to 40 dpa.

J.A. Hudson / Void formntion in stainless steels

the same at about 10% swelling it seems unlikely that a true saturation effect would occur at higher doses under C ion bombardment. As in the 316 steel bombarded at 525°C the void concentration in Ni ion bombarded 321 steel passes through a maximum with dose at about 40 dpa as shown in fig. 11. Similar behaviour was not observed in the C ion bombarded specimens, as shown in fig. 12, where at all doses studied the visible void concentration was falling so that any maximum occurs below 5 dpa. The variations of dislocation density with dose in 316 steel at 525°C and 321 steel at 600°C are shown in figs. 14 and 15 respectively. Previous data for C-ion bombarded 3 16 are also included in fig. 14 where it can be seen that in both this and the N&ion bombarded material the dislocation density rises to about 101’ cm cmm3 and saturates with dose. In the C ion case this saturation occurs after about 20 dpa whereas in the Ni ion case it occurs after about 40 dpa. In the Ni ion bombarded 321 steel at 600°C the dislocation density also rises to about 1Ol1 cm cm-3 after about 40 dpa as shown in fig. 1.5. No detailed figures are available for the C-ion bombarded material but

lo-

g

z

lo-

d

-

5

-

Ol-

0.01:

r

10

95

1

1

111111~

I

10

100

LJ

1000

DOSEldwi Fig. 7. Swelling versus dose in 316 steel bombarded by Cz+ and Nf’+ ions at 5 25 ‘C. same material bombarded by 5 MeV Ni ions [ 121. The void distributions in the high energy C and Ni ion bombarded material become similar after high doses i.e. 700-800 A diameter voids at a concentration of about S X 1014 cmP3 after about 300 dpa, due to agglomeration in the former case. Once this has occurred it is likely that the original swelling rates would again develop at higher doses in the Ni and C ion bombarded material. In the case of the 321 steel bombarded at 600°C the void concentration along the first half of the dose curve is again higher in the C ion bombarded material at 3-4 X 1 O-l5 cmP3 compared with about 1.5 X 1O-l4 in the Ni ion bombarded material. At higher doses the values become similar and towards the highest doses studied the void concentration is marginally greater in the Ni ion bombarded specimens. Since the void and dislocation distributions in the 321 steel bombarded by C or Ni ions at 600°C are almost

loom Fig. 8. Swelling versus dose in 321 steel bombarded by C2+ and Ni6+ ions at 6OO’C.

.I..4 Hudson / Void.formatioa irr stainless steels

96

I I

x ’

I

2

j12 i t; 1L

I

ilO 18

IX

= 200

;

416 g

1

300 c

5 w

-p

1

!

c1

0

IX

/

”

4,6

100

-+!, ; A

10

10

Fig. 9. Variation of void size and concentration

the same range of values as those shown in fig. 15 for the Ni ion bombarded specimens has been reported. At the lower doses studied individual dislocation loops were identified and details of their mean size and con-

Fig. 10. Variation of void size and concentration with dose in 316 steel bombarded by 46.5 MeV Ni6’ at 525°C.

100

1000

with dose in 316 steel bombarded by 20 MeV C*+at 525°C.

centration are given in table 2. It is hoped to irradiate further specimens in the dose range 1 - 10 dpa at different temperatures to obtain a more comprehensive survey of the loop populations developed under Ni-

Fig. 11. Variation of void size and concentration with dose in 321 steel bombarded by 20 MeV C*+ at 6OO’C.

J.A. Hudson / Void formation

in stainless steels

91

3.3. The effect of helium concentration in 321 steel

Fig. 12. Variation of void size and concentration with dose in 321 steel bombarded by 46.5 MeV Ni6+at 600°C.

ion irradiation. No analysis of these particular loops has been done but similar loops in low dose C-ion bombarded material and in other austenitic materials bombarded by Ni ions have been identified as interstitial in nature [13].

Fig. 13. Electron

micrograph

In addition to the studies of temperature and dose effects on void growth in 316 and 321 steels so far described additional experiments were done with 321 steel containing different concentrations of pre-injetted helium. These experiments were a preliminary to an investigation of the possible effects of different sized inert gases on the scale of void nucleation [ 141. Foils containing 0 -+ 1OW4a/a helium were bombarded to 40 dpa at 600°C with Ni ions and the damage structure investigated as described in section 2. The results are given below in table 3 and examples of the void distributions in the specimens containing different amounts of pre-injected helium are shown in the micrographs of fig. 16. The specimens with no implanted helium contained a very inhomogeneous void distribution, as indicated in table 3, presumably because of the inhomogeneous distribution of innate void nucleating gases such as oxygen. Above 10e6 a/a He the void concentration increases approximately as the square root of the He content and about about lop5 a/a He the swelling falls as the void concentration increases. Similar be-

taken at 1 MV of 30% swelling in 321 steel bombarded

to 180 dpa by 46.5 MeV Ni6+at

600°C.

J.A. Hudson / Void formation in stainless steels

98

1 I

I

I

1

N16+data T ’ I

1olOl 0 I

I

I

I

I

I

I

I

I

I

I

20

LO

60

80

100

120

140

160

180

200

220

of dislocation

I

I

I

density

I 2LO

with dose in 316 steel bombarded

I

I

I

260

280

Id pal

DOSE

Fig. 14. Variation

data

c2*

I

I

I

I

at 525°C.

I

L

l-

10101

I

I

I

I

I

I

I

I

I

0

20

LO

60

80

100

120

1LO

160

180

DOSE

Fig. 15. Variation

of dislocation

density

(dpa)

with dose in 321 steel bombarded

at 600°C.

200

300

99

J.A. Hudson / Void formation in stainless steels Table 2 Dislocation

loop distributions

Irradiation temperature

Dose (dpa)

(“C)

at low doses Mean loop diameter

Loop concentration

(A)

(cm

-3

)

Loop dislocation density -3 (cm cm )

Total dislocation density -3 (cm cm )

2 x 1o’O

(a) 316 Steel

525

5

260

3

x lOI

1.9 x 1o’O

IO

355

4

x lOI

4.5 x 1o’O

5 x 1o’O

20

315

2.3 x 10”

2.2 x 1o’O

6 x 10”

560

16

417

1.6 x 10”

2.1 x 1o’O

9 x 1o’O

650

16

1100

4.5 x 1o13

2

6 x 10”

10

500

1

1.6 x 10”

x log

(b) 321 Steel 600

haviour has been found in several materials bombarded by 1 MeV electrons in the High Voltage Microscope [ 151. The implications of this swelling variation with helium content are discussed in section 4. The dislocation density was measured to be 1 X 10” cm cmP3

x 1o15

4 x 1o’O

in all the specimens bombarded to 40 dpa at 600°C despite the order of magnitude increase in void concentration in going from zero to 1OP4 a/a pre-injected He. 3.4. Precipitate behaviour The main precipitation characteristic of C-ion bombarded stainless steels was the occurrence of M2,C6 within the grains and such precipitation was very marked ( 5 1% volume fraction in some cases) in specimens irradiated to doses in excess of about 100 dpa. In the present Ni-ion bombardment experiments only few examples of M2,C6 precipitation have been identified even after the largest doses studied. In some of the 321 steel specimens bombarded to doses above Table 3 Swelling in 321 steel containing to 40 dpa at 600°C Pre-injected helium concentration

0-10K4

a/a He bombarded

Void concentration

Mean void diameter

(cm

)

(,co

1.5-5

x 1014

420-490

l-3

1.3

x 1ol5

400

4.5

-3

Swelling (%I

(a/a) 0 lo+ Fig. 16. Electron micrographs ing 0 - lo4 a/a He bombarded

of voids in 321 steel containto 40 dpa at 600°C.

1o-5

3.1

x

10”

285

4.4

1o-4

5

x 10”

216

2.1

20 dpa a fine scale precipitation of TiC has been identified and other, as yet unidentified precipitates have also been observed in specimens irradiated at temperatures above 600°C. These include rod-like precipitates similar in appearance to those observed in other irradiated stabilised stainless steels 116, 17,231.

4. Comparison of results with other swelling data 4.1. Other simulation work Stainless steels including AISI 3 16 and 321 have been studied for void formation in several laboratories and with several irradiation regimes. Simulation studies have included electron, proton and heavy ion irradiations and in the case of 316 substantial swelling data from reactor irradiations is now available. Detailed data comparisons are not possible, however, because of the different conditions obtaining in the different experiments and there is considerable evidence now that minor alloying changes can signifi-

cantly alter the swelling characteristics of steels failing within the AISI 316 category. The aim of the present experiments was to compare the swelling behaviour of 46.5 MeV Ni6+ bombarded steels with the behaviour of identical material previously bombarded by 20 MeV C2+ ions. However, there are obvious points of similarity or differences with other results and these are described in subsequent sections and discussed with reference to the collective results shown in figs. 17 and 18. The main characteristics of the different simulation experiments referred to are given in table 4 below. 4.1.1. 4and5MeVNi+ irradiations Data on ST 316 and 321 stainless steels bombarded by 5 MeV Ni+ ions at a damage rate of l--2 X IQ -* dpa per set [ 18 (a,b)] and on ST 316 by 4 MeV Ni+ ions at 525°C at a damage rate of 5 X lo- 3 dpa per set [ 191 have been presented by Johnston et al. and by McDonald and Taylor respectively. In comparing this data with the 46.5 MeV Ni6+ data allowance must be made for the displacement energy of 32 eV used

Fig. 17. Swelling versus dose in 316 bombarded

by different

species at 500~-525°C.

1 Void

Hudson

J.A.

,forma

101

tion in stainlesssteck

w

N

1 0

0 0

3

‘i‘ & k-2

3

‘0

m ‘0

\o I

2

E: \o I

x k-2

8 d

“;0 w

x v,

0

0

2 I

00 vl

P

aa -0-x -Jo 3

Q)

J. A. Hudson / Void .formation in stainless stwls

102

at 625°C in 321 steel and the present data for irradiations at 600°C falls between the two. For example at 625°C the reported swelling is equivalent to about 30% at 200 dpa, 1% at 40 dpa and 0.1% at about 20 dpa in 316 steel. In 321 steel at 625°C 0.1% had built up by about 6 dpa as in the 46.5 MeV Ni6+ bombarded material. It is not surprising that similar swellings should be seen in the 321 steel after irradiations at the different dose rates around this temperature since this is in the flat portion of the swelling profile as shown in fig. 2 whereas in 3 16 steel this is not the case as shown in fig. 1. In fig. 18 the swelling in ST 321 steel containing 1 - 2 X 10-S a/a He under 5 and 46.5 MeV Ni ion bombardment is compared, the lower energy irradiation being at 625°C and the other at 600°C

o

lMeVe-

316 no He

b

lMeVH*

316+5x10-6He

t 20MeVC2+321+10-5

He

d L6~5MeVNi6'321/316+10~5He

Fig. 18. Swelling versusdose in 316 and 321 steels bombarded by different species at 600°C.

by the above authors since the experimental calibration used in the Ni6+ work more nearly corresponded to a displacement energy of 40 eV as discussed in section 1. The general characteristics of swelling in 316 and 321 are similar in all the Ni-ion bombardment work. Johnston determined the peak swelling temperature in 316 containing 20 ppm He to be at 625-650°C with a swelling of 4-S% at about 50 dpa in good agreement with the present work. His determination of l-2% at 750°C and about 2% at 575°C is also in good agreement although his 0.1% at 525°C is less than in the present work. This discrepancy can be explained by the order of magnitude increase in dose rate in the 5 MeV Ni case. In the 4 MeV Ni+ case the swelling after this dose was 0.5-l% as in the present experiments at the same dose rate. The void size and concentration were similar in 3 16 steel bombarded by 5 MeV and 46.5 MeV ions in the temperature range 575-650°C but outside these limits larger voids were observed in the present work. As in the 46.5 MeV Ni6+ work the void concentration passed through a maximum with dose in the 5 MeV Nii- work, at about 50 dpa rather than 40 dpa. Johnston et al. performed dose dependency tests at 625°C and 575°C in 316 and

4.1.2. 1 Me V pro ton irradiations Solution treated 316 steel preinjected with 5 X low6 a/a He has been bombarded by 1 MeV protons at damage rates in the range lop4 ~ low3 dpa per set by Keefer et al. at 400, 500 and 600°C [20]. The quoted doses were based on a Kinchin and Pease calculation so that some correction would be necessary for a true comparison with present data. Uncorrected results for the 500°C and 6OO’C irradiations are plotted in figs. 17 and 18 respectively. The chief characteristics of both results are that the swelling rate is similar to that found for 1 MeV electron irradiations and at 500°C a very low incubation dose for visible void formation, also similar to the electron case, was observed. At 500°C the swelling was 0.1% at a quoted 0.5 dpa and increased as (dose)1.3 to 20% at a quoted 50 dpa. The void concentration increased continuously from about 2 X 1015 cmp3 to 5 X 1015 -3. At 600°C the swelling was proportional to Fise)2.9 varying from 0.1% after 5 dpa to about 20% after 20 dpa, the void concentration rising from 2 X 1014 cm-3 to about 2 X 1015 cmm3. At 4OO”C, where no voids were observed in other ion bombardment studies the swelling rose from 0.8% after about 2 dpa to about 4% after 50 dpa. 4.1.3. I Me V electron irradiations Void growth in 3 16 steel under 1 MeV electron bombardment at dose rates of l-2 X lop3 dpa per set has been investigated by Garner and Thomas [21] and by Makin et al. [ 151 and in 321 steel by Buswell et al. [22]. Calculation of damage rates in this regime

J.A. Hudson / Void formation in stainless steels

are based on measured displacement threshold energies which are in the range 20-25 eV for stainless steels. Many of the experiments have been performed on low carbon varieties of 316 [21] and usually in the absence of implanted helium and so again detailed comparisons with the present work are not possible. The data for 316 from the same cast as that used in the present work is plotted in figs. 17 and 18. Voids are first observed after relatively low doses under electron irradiation and significant swelling develops at doses where voids have not been resolved in ion bombarded material. For example, 2-3% swelling was seen in the 3 16 steel after 10 dpa over a wide range of temperatures [ 151 and in 32 1 steel bombarded at 500°C more than 10% swelling developed after only 1 dpa [22]. The most recent determinati.9.n of the temperature profile of the average swelling m 316, without He, bombarded in the Harwell 1 MV electron microscope shows the swelling rising steadily from lo-15% at 500°C to 25-30% at 650°C after 30 dpa [15]. The void concentrations in the unimplanted 316 were similar to those shown in fig. 5 for Ni ion bombarded material containing lop5 a/a He and mean void diameters of up to 2000 A have been observed in electron bombarded specimens. 4.2. Comparison with reactor data There are now considerable reactor data related to swelling in many different casts of 316 steel but effectively no such data on 321 steel. Much of the 316

E

0.6

c

/

\

\ 0 400

I

450

I 500

I 550

IRRADIATION

I

600

-I

I

_I

650

TEMPERATURE

700 (“C)

Fig. 19. Swelling versus temperature in 3 16 steel bombarded to 16 dpa by (a) neutrons and (b) 46.5 MeV N?+ions.

103

data is from fuel pin diameter changes alone but in some cases immersion density and electron microscopy measurements have been made. The stress state of the fuel pins during irradiation is often not known with any certainty and the temperature of irradiation can usually only be estimated to within 50°C of the true temperature. In the simulation experiments the stress state of the region of void growth is again not known but the temperature is usually known and controlled to within a few degrees. As in simulation experiments, the influence of alloy composition on the swelling characteristics of 3 16 steel is apparent from reactor data and so it is preferable to choose materials of similar compositions for any comparison of data from the two sources. Undoubtedly the best comparison for the present Ni-ion bombardment data is with the results of Barton et al. [23] who neutronirradiated unstressed specimens from the same cast of 316 steel in an instrumented rig in DFR. Data were obtained at neutron doses equivalent to 6 and 16 dpa over the temperature range 430-740°C at temperatures known to within 20°C; indeed the 16 dpa simulation data were determined for just this comparison. The helium concentration built up as a result of transmutation reactions during the 16 dpa irradiations was calculated to be 4-5 X lo@ a/a. The swelling in the two cases at 16 dpa is plotted in fig. 19. At 6 dpa the swellings in the neutron case were 0.12% at 43O”C, 0.09% at 506”C, 0.05% at 525°C and zero at higher temperatures. This sort of incubation dose is just what is observed in the heavy ion simulation work, in contrast to the proton and electron irradiation cases. The 16 dpa data do not allow a complete comparison since it is not known whether the swelling at this dose increases or decreases below 470°C. One can say that at 16 dpa the peak swelling temperature is shifted upwards by at least 160°C on increasing the dose rate from 10e6 to 2 X 10e3 dpa per sec. The other high dose fuel pin data are beginning to indicate a peak swelling temperature nearer 600°C in reactor in which case the temperature shift is about 50°C compared with the data of fig. 1. The void concentration at the peak swelling temperature in the neutron case was 2.6 X 1014 cmW3, the same as in the Ni-ion bombarded material at 65O”C, near the Peak swelling temperature. Void concentrations in the material neutron-bombarded at other temperatures were similarly shifted by 150-2OO’C with respect to

J. A. Hudson

104

/ Void formlion

those in the Ni-ion bombarded material. No obvious change of void concentration with dose was observed from 6 to 16 dpa in the temperature range 430-530°C. The total dislocation density at the peak swelling temperature in the neutron bombarded specimens was 4 X lOlo cm cme3 after 6 dpa and 9 X IO9 cm cm -3 after 16 dpa compared with 6 X 1Ol” cm cm -3 after 16 dpa in the Ni ion bombarded specimens. The dislocation loop structure after 6 dpa at 430°C was the same as that in the Ni ion bombarded steel after 4-10 dpa at 525”C, 300 a diameter loops in a concentration of about 4 X 1015 cm-3. Thus there is evidence of a temperature shift ofapeak swelling, void concentration and dislocation structure in going from neutron to heavy ion bombardment and reasonable agreement in the magnitude of swelling at 6 and 16 dpa. American data on swelling and damage structures in neutron-irradiated 3 16 steel has been reviewed by Brager and Straalsund [ 161 and a comparison of heavy ion and neutron irradiation simulation data has been made by Johnston et al. [18]. In general the data from 5 MeV Ni+ ion bombardments at 625°C showed good agreement with respect to incubation dose, swelling rate and void concentration with data from neutron bombardment at 480-540°C [24] where the swelling rose from 0.1% after 1-2 X IO** fast neutrons cmM2 to about 10% after about 8 X lo** n cm-- *.

if1 stainless stwls

fore voids are identified in the electron microscope is about a factor of two greater and the subsequent swelling rate a factor two less in the Ni ion bombardment case. The so called incubation doses required under proton bombardment are less than those for C-ion bombardment and for electron bombardment even lower incubation doses and higher swelling rates have been observed as described in section 4. Although the full interpretation of temperature shift effects between the lo@ dpa per set damage rate of a fast reactor and the lop3 - 10e2 dpa per set damage rates of the simulation experiments is far from complete, there are now enough data from reactor experiments to show that incubation doses and swelling rates under neutron irradiation are almost the same as those found under Ni ion bombardment. In contrast to the behaviour of alloys like stainless steel, pure nickel containing pre-injected helium shows similar swelling behaviour under ion, electron [25] and neutron irradiation. Thus swelling in the pure material seems to be dependent on the total number of displaced atoms whereas in the alloys different behaviour is found under different damaging regimes. Electrons produce near threshold collisions and the damage is in the form of isolated Frenkel pairs whereas heavy ions towards the end of their tracks produce damage almost exclusively in collision cascades. Marwick [5] has calculated the fraction of the elastic energy lost in cascades of 5 keV and above for various regimes and his results are reproduced in table 5. In terms of swelling rate and incubation dose 3 16 and 321 steels behave according to the form of the damage rather than simply the total number of displaced atoms. A further clue to this behaviour was obtained in experiments where pre-thinned specimens were simultaneously irradiated by low energy heavy ions and observed by transmission electron microscopy [26]. Black spot defects and small vacancy loops were found to have a significant lifetime in several

5. Discussion The results presented in section 3 of the swelling behaviour of solution treated 316 and 321 steels under 46.5 MeV Ni6+ bombardment show characteristic differences from previous results of 20 MeV C*+ bombardments and characteristic similarities with results of 4 and 5 MeV Ni+ irradiations. The number of displacements per atom received by specimens be-

Table 5

The proportion

of displaced

atoms

from recoils of energy

greater

than 5 keV (Marwick

1975)

Regime

1 MeV e-

5 MeV H+

20 MeV c*+

46.5 MeV Ni6+

5 MeV Ni+

Fast reactor spectrum

Fraction of damage in cascades > 5 keV

0

0.42

0.54

0.19

0.79

0.99

J.A. Hudson / Void formation in stainless steels

alloys, including 3 16 and 321 steel, investigated in the void formation temperature range whereas such effects were less obvious in pure nickel. Vacancy loops formed from the collapse of collision cascades in alloys may thus be the reason for the different swelling behaviour under different irradiation regimes and for the correlation of observed swelling rates with the calculated results given in table 5. The reasons for the different behaviour of collision cascades in stainless steels and pure nickel are not wholly understood but the much lower stacking fault energy (- 20 ergs cm-*) in steels compared to nickel (- 300 ergs cmF2) ]27] will certainly contribute to the stability of faulted vacancy loops formed from the collapse of collision cascades. Bullough et al. have now included this effect of the formationpf vacancy loops with a significant lifetime in the rate theory of void growth 1281. Good agreement with experiment is obtained if the bias factor of dislocations for interstitials in a particular case is taken as the value (usually greater than 5%) derived from void growth experiments in the High Voltage Electron Microscope. The chief effect of the transient vacancy loops is to greatly reduce the point defect concentration and thereby reduce void growth rates. This does not mean that fewer vacancies and interstitials are initially produced per incident ion than previously calculated and the effect of the extra interstitials introduced by the ion bombardment (about 1% per 60 dpa as described in section 2) is not significant in reducing void growth. In fact the effect is even less than previously supposed in view of the higher interstitial bias factor now thought to operate. The dose dependencies of swelling in 316 steel bombarded at 525’C and in 321 steel bombarded at 600°C shown in figs. 7 and 8 illustrate different behaviour towards possible saturation of swelling between the C-ion and the N&ion bombarded material. An explanation of this different behaviour based on a theory of vacancy emission from sources such as void surfaces stimulated by sub-threshold collision events, has been suggested by Nelson 1291. On this theory the different approaches to saturation are due to the different void concentrations produced by the C and Ni ion bombardments. As suggested in subsection 3.2, once void coalition has occurred giving similar distributions in both specimens, the difference in sweiling rates should return to its original value of

105

about 2, defined by the different recoil spectra of the C2+ and Ni6+ ions. The results show that high-energy Ni bombardment is a good simulation of the reactor case as previously concluded by Johnston et al. for 316 and 304 steels [ 181 and also predicts unacceptably high swellings in solution-treated 3 16 and 32 1 steels. Thus a maximum swelling of more than 20% can be anticipated in material bombarded to doses above about 100 dpa (equivalent to about 2 X 1 023 n cmW2 E > 0.1 MeV in a fast reactor). Since swelling in 321 steel is dependent on helium content below 10s6 a/a and above about 10M4 a/a, it will be necessary to inject the gas in several stages in future experiments to give a more exact simulation of the neutron case particularly with respect to incubation doses.

6. Conclusions 1) The incubation dose for the observation of voids is approximately doubled and the subsequent swelling rate approximately halved in solution treated 316 and 321 steels in going from 20 MeV C2+ to 46.5 MeV Ni6+ bombardments. 2) No saturation of swelling occurs under Ni-ion bombardment. Although a reduction in swelling rate occurs in 316 steel bombarded at 525°C after about 100 dpa, the swelling continues to increase reaching 15-20% after 300 dpa. No significant reduction occurs in the swelling rate of 316 and 321 steel bombarded at 6OO*C where about 30% swelling builds up during 180 dpa. 3) Void concentration in 316 steel bombarded at 525°C and in 321 steel bombarded at 600°C goes through a maximum after about 40 dpa. Similar behaviour occurred in 3 I6 steel bombarded by 20 MeV C2+ ions at 525’C, where the maximum occurred after 20 dpa at about 2 X lo16 crn3, an order of magnitude higher than that in the Ni6+-bombarded material. 4) In specimens bombarded to 40 dpa with Ni6+ ions at a damage rate of 2 X 10v3 dpa per set voids are formed in the temperature range 450-750°C in the case of 316 steel and in the range 500-700°C in the case of 321 steel. In both materials the irradiationinduced dislocation density varies from about

106

J.A. Hudson / Void .formation in stainless steels

2 X 10” cm cm-3 at 450°C to about 2 X lOlo cm cm-3 at 75O’C. 5) The temperature of maximum swelling in 316 steel is 625°C after 16 dpa and about 650°C after 40 dpa. In 321 steel this temperature is 600°C after 16 dpa and 625°C after 40 dpa. Under 20 MeV C*+ irradiation the peak swelling temperatures are 600°C in 3 16 steel after 40 dpa and 625°C in 321 steel after 60 dpa. 6) Values of incubation dose for visible void formation and swelling rate are similar in 46.5 MeV Ni6+ ion, 5 MeV Ni+ ion and fast neutron irradiated steels. In going from a damage rate ofabout 1O@ dpa per set in the neutron bombardment case to a rate of about 2 X 10d3 dpa per set in the ion bombardment case the temperature of peak swelling, the void concentration and dislocation density are shifted to higher temperatures by at least 150°C at low doses. 7) The differences in swelling behaviour in specimens bombarded under different irradiation regimes are consistent with the differences in recoil spectra in the different regimes. Where a significant fraction of the displaced atoms are in collision cascades, vacancy loop formation can lead to larger incubation periods and lower swelling rates than when most atoms are displaced as individual Frenkel pairs.

131 141

[51 [61 [71 [81 I91 [lOI [Ill 1121 1131 I141 [I51 [I61 [I71 [I81

[I91 1201 WI 1221

Acknowledgements Thanks are due to Mr. J.H. Worth and the staff of the VEC for carrying out the irradiations and to Drs. R.S. Nelson, R. Bullough, D.R. Harries and D.J. Mazey for useful discussions and to Mr. S.J. Ashby and Mrs. S. Francis for experimental assistance.

References

~31

(241 1251 [*61 ~71 WI [*91

[ 11 Proc. Conf. on Voids Formed by Irradiation Materials,

L21 Proc. Conf. on Radiation

BNES (Reading

1971).

of Reactor

Induced Voids in Metals, (Albany 1971), AEC Symposium Series 26. Effects of Radiation on Substructure and Mech. Prop. of Metals and Alloys, ASTM STP 529 (1973). Proc. Consultant Symposium on The Physics of lrradiation Produced Voids (Harwell Sept. 1974). AERE Report R7934 (1975). A.D. Marwick, J. Nucl. Mat. 55 (1975) 259. J.A. Hudson, S. Francis, D.J. Mazey and R.S. Nelson, pp. 326-333 in ref. [3]. R.S. Nelson, J.A. Hudson, D.J. Mazey, G.P. Walters and T.M. Williams, pp. 430-445 in ref. [2]. J.H. Worth, AERE Reporl R5704 (1968). J.H. Worth, P.A. Clark and J.A. Hudson, J. BNES, Oct. (1971). I. Manning and G.P. Mueller, Computer Physics Comm. 7 (1974) 85. D.J. Mazey, J.A. Hudson, S. Francis and J.L. Whitton, J. of Microscopy 96 (1972) 77. A. Taylor, Private Communication. D.J. Mazey, J. Nucl. Mat. 35 (1970) 60. R.S. Nelson and J .A. Hudson, AERE Report R7991, (1975). M.J. Makin and G.P. Walters in ref. [4]. H.R. Brager and J.L. Straalsund, J. Nucl. Mat. 46 (1973) 134. E.E. Bloom and J.O. Stiegler, 360-380 in ref. [3]. W.G. Johnston, J.H. Rosolowski, A.M. Turkalo and T. Lauritzen, (a) ~213-227 in ref. [3], (b) General Electric Research and Development Report 72CRD 269, (1972). S.G. McDonald and A. Taylor, pp. 228-240 in ref. [3]. D.W. Keefer, A.G. Pard and D. Kramer, pp. 244-257 inref. [3]. F.A. Garner and L.E. Thomas, pp. 303-323 in ref. [3]. J.T. Buswell, S.B. Fisher, J.E. Harbottle, D.I.R. Norris and K.R. Williams, pp. 533-549 in ref. [2]. P.J. Barton, B.L. Eyre and D.A. Stow, Proc. Conf. on Irradiation Behaviour of Fuel Cladding and Core Component Materials, BNES/KGDA Karlsruhe, Dec. (1974). W.K. Appleby and U.E. Wolff, pp. 122-136 inref. [3]. F.W. Minter and J.A. Hudson, unpublished. D.J. Mazey, unpublished. P.C.J. Gallagher, Met. Trans. 1 (1970) 2429. R. Bullough, B.L. Eyre ,and K. Krishan, AERE Report R7952 and Proc. Roy. Sot. (1975), to be published. R.S. Nelson, J. Nucl. Mat. 57 (1975) 77.