Low-dose neutron irradiation damage in FCC and BCC metals

Journal of Nuclear Materials 108 & 109 (1982) KM- 123

104

North-Holland

Publishing Company

LQW-DOSE NEUTRON IRRADIATION DAMAGE IN FCC AND BCC METALS C.A. ENGLISH Metallaqy Division, AERE Hamcl~ Oxon UK

Received 8 January 1982; accepted 1 February 1982

In this paper the progress made by the author and co-workers in trrmsmission electron microscope studies of the irradiation damage structure. induced in pure metals (Cu, MO and a-Fe) and dilute Cu-Ge alloys by low-dose fission neutron irradiation will be reviewed. Neutron doses (> 1 MeV) were generally lo*‘- IOX n/m* and irradiation temperatures were between 80” and 600°C. A common feature of the results in all the systems studied was that frequently the damage showed considerable spatial variation with the development of highly damaged regions around m-grown dislocations. The form of the damage between the dislocations depended sensitively on the metal, irradiation temperature and neutron dose. For example, in copper at elevated temperatures the damage was predominantly vacancy in nature, with a complex interdependence between stacking fault tetrahedra and voids. An extensive parallel study has been undertaken of cascade collapse in different metals as a function of material purity, irradiation temperature, and cascade parameters. The considerable insight these results have given

1. Intmduction Although there has been intense interest in the damage structures created by fast neutrons in metals,

there have been relatively few systematic studies of the damage created at very low doses in pure metals and simple binary alloys. It is the purpose of this paper to review the progress made by the author and co-workers in the study of the damage created by low-dose fission neutron irradiations in selected pure fee and bee metals. The main features will be presented of the neutron damage structures formed over a wide range of temperatures in pure copper, copper-germanium, molybdenum and a-iron. The detailed results of the transmission electron microscope studies by Muncie in pure copper, Shepherd on copper-germanium, Riley on molybdenum, and Robertson on a-iron may be found in references [l-4] respectively. A major feature of damage production by fast neutrons is that the majority of the displacements are created in cascades caused by energetic knock-ons of energy > 10 keV. Thus, an understanding of the damage created at low dose requires a detailed insight into the point defect behaviour in cascades. In parallel with the low-dose neutron studies experiments have been carried out on the same metals [S-II] using heavy ions with similar energies (< 100 keV) to the energetic knock-ons

0022-3 115/82/OCMIQ-0000/$02.75

0 1982 North-Holland

generated by the fission neutrons. The heavy ions create cascades within - 10 nm of the incident ion surface, and the vacancy loops formed from the collapse of the cascade centres may be studied using the transmission electron microscope. Thus one aspect of the presentation of neutron irradiation results will be to stress the relation between the vacancy loop component of the neutron damage structures and the results from the parallel studies on cascade collapse. A second aspect of the results which will be stressed is that in all the metals studied there were irradiation conditions where the distribution of damage was inhomogeneous and high and low damage regions developed. It will be emphasized throughout that there was frequently a complex interdependence between the various point defect sinks comprising the damage structure; this was particularly evident in the fee metals. In order that the last point becomes clear the results will be presented for each material in turn rather than in terms of the development of each particular sink, i.e. voids. Thus in sections 3-6 the results are presented for the fee metals copper (section 3), copper-germanium (section 4), and the bee metals molybdenum (section 5). and u-iron (section 6). In section 7 the principle results obtained in the different metals are discussed. Prior to this the essential details of the experimental procedure will be given in section 2.

C.A. English /Low-dose

neutron irradiation

aimage

2.3. Characterisation the fission neutrons

Table 1 Material purity Metal

Interstitial impurity levels

cu Cu-Ge alloys (typical levels only)

0.9-1.2 appm C, CO.9 appm 0 C, N, 0
MO

u-Fe

2. Experimental procedure 2. I. Specimen purity High-purity single crystals of copper, coppergermanium and molybdenum were grown with the orientations and purities given in table 1. The preparation of the polycrystalline a-iron is discussed in Robertson et al. [12] and the impurity levels of the highest purity material are given in table 1. 2.2. Fission-neutron irradiations The single crystal material was cut into 3 mm X 4 mm thick discs for irradiation while the a-iron was irradiated in the form of sheets (15 mm X 6 mm X 0.1 mm). All the irradiations were carried out in a hallow fuel element in the material testing reactor PLUTO at Harwell. The copper, copper-germanium and molybdenum were irradiated at half-power (11 MW) at reactor ambient (80°C) and at selected temperatures between 25O’C and 600°C; doses were between 1 X lo*’ n/m* and 2 X lo** n/m* (> 1 MeV). Polycrystalline a-iron was irradiated at full reactor power (22 MW) at reactor ambient to doses between 2 X 10” n/m* and 1.4 X 1O24 n/m* (> 1 MeV). The precise irradiation conditions are given in table 2.

105

in fee and bee metals

of the damage structures created by

In all cases the irradiated material, in the form of electro-polished foils, has been examined in 100 keV transmission electron microscopes. The irradiation-produced damage structures have been characterised by performing careful contrast experiments on suitable flat electron-transparent regions of the foils. In this manner the concentration, size, geometry and nature (interstitial or vacancy) of the point defect clusters which were above the visibility limit (- 1.5-2 nm) could he determined; where appropriate the dislocation lines and low-angle boundaries were also character&d. The procedures employed in the contrast analysis of the point defect clusters followed the methods established in references 24-26.

3. Pure copper The main feature of the damage structures obtained in pure copper [ 11, was that the homogeneous distribution of damage found at reactor ambient, 80°C changed at elevated temperatures (> 250°C) to an inhomogeneous distribution with the development of high and low damage regions. The two types of structure will be discussed separately; in section 3.1 the results at reactor ambient will be presented and in section 3.2 the results at elevated temperature. 3. I. Reactor ambient - 80°C At reactor ambient the damage consisted of an even distribution of dislocation loops having image widths of c 12.0 nm. A detailed analysis of the loop nature was carried out on specimens irradiated to 1.3 X lo*’ n/m*. The majority of the clusters were found to be Frank 100~s (b - a/3 (111)) with 8% showing significant’dis-

Table 2 Irradiation conditions Specimen details

Metal

cu Cu-Ge

MO a-Fe

alloys

Dose range n/m2

Single crystal {0 I 1)

1x102’-2x1022

Single crystal

1 x 102’-2x

(011)

Single crystals (01 I) and (137) Polycrystalline foils

(E > 1

MeV)

Temperature range (“C) 80,250-350

1022

80, 250-350

1x102’-IX1022

80, 250-600

1x1o22-1x1o24

80 only

106

neutron irradiation damage in fee and bee metals

CA. English /Low-dose

35r

/INTERSTITIAL

\

0

1

2

3

L

5

IMAGE WIDTH

6

7

6

9

Inm)

Fig. 1. Vacancy and interstitial loop size distributions in pure copper irradiated at 8O’C to a dose of 1.3X 10” n/m2 (E > 1 MeV). sociation towards stacking fault tetrahedra. A small population (- 8%) of perfect loops was found and these were on average - 20% larger than the Frank loops. The

majority of the loops were found to be vacancy in nature (- 87%) with the remainder being interstitial. The size distributions of both types of loops are presented in fig. 1, where it can be seen that the-larger loops are predominantly interstitial in nature. Measurements of the total loop concentration yielded a value of 6.23 X lO*r loops/m3, of these 5.43 X lo*’ loops/rr? were vacancy in nature. From the vacancy and interstitial size distributions the estimated concentrations of vacancies and interstitial point defects stored in clusters was 6.8 X 1021m-3 and 4.2 X 102’m-3 respectively. 3.2. Irradiations at elevated temperatures The main feature of the damage structures created at elevated temperatures was that the damage was no longer homogeneously distributed in the foil, with the formation of highly damaged regions containing tangled perfect dislocations, separated by relatively undamaged regions containing small (< 6 nm) dislocation loops and stacking fault tetrahedra, and/or voids. The development of these two regions will be described separately. 3.2.1. High damage regions At 250°C the separation of the damage into well-de-

fined high and low damage regions was particularly pronounced. At this temperature the high damage regions (HDR) consisted of tangles of dislocations; the dislocation line length in the irradiated foils was greatly increased over the line length in the unirradiated foils. At a dose of 1 X lo*’ n/m* the HDR consisted of loose tangles of dislocations but when the dose was increased to 2 X lo** n/m* these developed into dense tangles of jogged dislocations decorated by resolvable dislocation loops. The structure formed at 9.4 X lo*’ n/m* is illustrated in fig. 2(a)-(c). The region marked A in fig. 2(a) is shown at a higher magnification in fig. 2(b) where the dislocation loops adjacent to the dislocation line are clearly visible. These are clearly larger than the point defect clusters visible in the low damage regions, marked B (fig. 2(c)). The average and maximum loop sizes of the decorating loops increased by factors of two and three respectively when the dose increased from 3.1 X lo*’ to 2.1 X lo** n/m* (> 1 MeV) (see table 3) while the minimum loop size detected was -4.0 nm throughout this dose range. Of the loops whose geometry and nature could be determined the majority (- 90%) were interstitial in nature with the remaining 10% identified as vacancy. The majority of loops irrespective of nature, had b = a/2( 110) and they did not necessarily have the same Burgers vectors as the adjacent dislocation segment. At higher temperatures, 3OO’C and 350°C high damage regions were not frequently observed, indicating an increase in spacing. In the high damage regions which were observed the dislocations were relatively unjogged, loosely tangled and undecorated by loops. 3.2.2. Low damage regions The regions separating the high damage regions were found to contain small point defect clusters which were either stacking fault tetrahedra or dislocation loops and/or voids. The exact composition depended strongly on irradiation temperature and dose. The point defect cluster concentrations are shown in fig. 3 for the three different irradiation temperatures. The concentrations at 25O’C increased rapidly between 1 and 3 X lo*’ n/m* and then remained constant over the dose range studied. At 300°C the concentrations increased steadily while at 35O’C there was a rapid increase in concentration up to 1 X 10” n/m2, followed by a dramatic drop at 2 X lo** n/m* to below the detection limit. At 250°C and 300°C the great majority of the clusters were stacking fault tetrahedra, 90% and 98% respectively, with the remainder identified as Frank loops. At 350°C only stacking fault tetrahedra were observed. The size distributions of the stacking fault

C.A. English /Low-dose

neutron irradiation

damage in fee and bee metals

Fig. 2. Micrographs of pure copper irradiated at 250°C to a dose of 9.4X IO*’ n/m2 imaged in g =200, z = {01 1) at low magnification (a), and high magnification (b) (c). The micrographs show (a) the general form of the damage structure, (b) high damage region marked A in (a), and (c) low damage region maiked B in (a).

tetrahedra were essentially independent of dose and temperature with a mean diameter of 2.4 nm. Voids Were detected in all samples irradiated at

25OT and 3OOT but at 35OT only in samples irradiated to 2 X 10z2 n/d. The void volume fraction and concentration at different temperatures and doses are

108

CA.

English/Law-d~e

neutron irrodiotion domoge in ICC ond bee metois

Table 3 Decorating loop sizes in high damage regions in pure copper irradiated at 250°C Dose n/m’(>I

MeV)

3.1 x 102’ 9.4x 102’ 2. I x 102’

Avera@diameta d(nm)

d-

htwinwm-

11.9 14.6 20.2

16.7 33.3 56.3

(~1)

presente+ in fig. 4(a) and (b) and in table 4 the mean void volume, expressed in terms of (dl)“‘, where d, is the void diameter, is given. It can be seen that the dose dependence of void volume fraction was similar at both 250°C and 300°C. ‘Ihis is perhaps surprisiig in view of the fact that at the two temperatures there were differences in both the dose dependence of the void concentration and the form of the size disiributions. At 250°C the void concentration increased linearly with dose and the voids were contained in a single peaked. distribution; the void size increased with increasing dose. At 300°C the void concentration was lower than at 250°C and did not change significantly as the dose increased. The voids at 300°C were also larger than for the same irradiation dose at the lower temperature and a bimodal size distribution devekped with increasing

-

lon DOSE (n m-‘, E> 1 MeV

1

Fig. 3. Plot of the dose dependence of the point defect cluster concentration in pure copper irradiated at 250% 300°C.

35OT.

dose. The mean diameter of the voids in the lower size section - 3 nm, was not strongly dependent on dose, while that in tbe upper section &creased witb &reasing dose,- from 9.9 fun it 4.5 X.#“&/u+ to 15.9 nm at 2.5 x lo= II/&In foils irradiated at 3WC to a dose of 1.13 X 10U n/m2 a very high concentration of stacking fault tetrahedra was obsezvai. At -2.2 X 10” n/m’ thifi was repIaad by a population of voids which exhibited a bimodal size distribution. An important feature of the damage in the low damageregionsistbatattbebigbestdoses,-2X10U n/d, the voids represent the meet Important vacaacy sink; at 250°C and 3WC the concentrations of vacancies in voids exceeded that in stacking fault tctrakh by more than an order of magnitude. 3.3. Comparison with the results of cam&

stutucs

In a recent study English and Jenkins [8] have shown that the cascade region created in Cu,Au by neutron irradiation at reactor ambient in PLUTO may be rendered visible in the traas&&n electron microstqc. Further it was possible to measure the concentration of cascades with energy > 10 keV created during the irradiation: a value of 2.5 X 10” cascades/n? was determined for a dose of 102’ n/m2. Muncie (1) has shown that the casc&s generated with energy > 10 keV during ncutron irradiation of Cu will be similar to that in Cu,Au. The cascade density measurements in Cu, Au thus provide a means of de: termining the cascade collapse efficiency in the neutron-irradiated Cu. Thus if one assumes that only cascades with energy above 10 keV collaPse to give a visible loop then a cow of the vacancy loop numbers with the estimated cascade densities gi= that inCuO.3~0.1ofthetieswitbenergy>lOkeV collapse to produce a visible loop Further, a comparison of the calculated total number of vacancies created in the irradiation with the number retained in visible loops demonstrated that only 0.045 of those created survive in loop form. As only 7051,of the displacements in the irradiation are c&dated to be created in recoils of > 10 keV then 0.2 of the vacancies calculated to be created in those cascades which collapse survive. Experiments in Cu,Au have also shown that the individual cascade regions produced by tbe neutron irradiation are similar in size and shape to those produced by 30 keV Cu+ ions. A similar result might be expected for pure copper and it is thus mosi interesting to compare the efficiencies of cascade collapse and vacancy retention derived above with the results of

C.A. English /Low-dose

neutron irradiation

damage in fee and bee metals

109

10-S

I_ 1on DOSE

(r

Fig. 4. Plot of the dose dependence

DOSEW-?E>lMeV) in

pure copper irradiated at 250-35O’C (a) void volume fraction, (b) void concentration.

Stathopoulos [lo], who conducted a very thorough study of cascade collapse to vacancy loops in pure copper irradiated with 30 keV Cu+ ions. The size distribution of the vacany loops formed in the ion irradiation is shown in fig. 5. In contrast to the neutron case the size distribution is sharply peaked, with 42% of the loops having an image diameter in the range l-2 nm. The peaked distribution can be attributed to the mono-energetic ion beam as compared to the relatively broad range of primary knock-on energies which cccur during neutron irradiation. Stathopoulos [lo] found that 0.5 -C 0.02 of the 30 keV Cu+ cascades yield a visible loop and that 0.34(* 0.02) of the vacancies generated in the cascades which collapse survive in loop form; these are approximately twice the equivalent values for the neutron irradiation. A comparison with the ion results suggests that the fraction of cascades which collapse in the neutron-irradiated material is not radically different

to that found for 30 keV cascades. On average fewer vacancies are retained in loop form in the neutronirradiated material possibly as a result of the self-interstitials produced throughout the bulk causing vacancy loop shrinkage; in the ion-irradiated material the selfinterstitials are lost to the nearby surface. Merkle [27] in a study of proton and alpha particle-irradiated copper also found that not all the cascades collapsed to form visible loops. Indeed the data was consistent with a threshold energy for visible loop production of 10 keV and only one in ten of the cascades collapsed. On increasing the irradiation temperature to 25OOC from reactor ambient (80°C) the vacancy loop concentration decreased by two orders of magnitude. This observation can be explained on the basis of the simple model applied by English et al. [5] to vacancy loop shrinkage in 30 keV Cu’ ion-irradiated copper. In this model the vacancy lops shrink at elevated temperatures

Table 4 Mean void volume in pure copper; expressed as (d,-3 ) ‘Ia, at 25OT

25O’C and 3OO’C(d, cvoid

diameter)

300°C

Dose n/m2 ( > 1 MeV)

(&T)‘j3 (nm)

Dose n/m2 (11

1.0x 3.1 x 9.4x 2.1 x

2.8 4.1 5.3 5.0

8.1X1020 4.5x 102’ 1.2x 1022 2.5X lO22

102’ 102’ 102’ 1022

‘) Bimodal size distribution.

MeV)

( Jz)1/3 3.0 4.8 6.8 15.3

a) ‘) ‘) a)

(nm)

110

$

35-

ZI 2

30-

: 2

25-

,” i=

20-

%! z

1510 -

0

0

I

I

I

1

2

3 IMAGE

4

5

DIAMETER

6

7

6

9

I”-

d72

50

Inm)

150 iRRADlATl~N

250 TEMPERATURE

350 F

Fig. 5. Comparison of the size distribution of vacancy loops produced in pure copper by a dose of 1.3 X 102’ n/m2 (E> 1 MeV) and 2X lOI 30 keV CIA+ions/m’.

Fig. 6. Comparison of the observed decrease of vacancy.loop concentration in pure copper and the predictions of the thermal emission model (solid line) 1.3X 102’ n/m2 ( E sl MeV).

by the thermal emission of vacancies, the major driving force loop shrinkage is the elastic self-energy and stacking fault energy of &heloop. To estimate the number of loops surviving elevated temperature irradiation it is assumed that the cascades collapse at high temperatures in exactly the same manner as at reactor ambient. The vacancy loop density and size dist~butions at reactor ambient can’then be combined with the calculated loop lifetimes at the different irradiation temperatures to give the fraction in each size interval remaining in the foil after the specimen has cooled to room temperature. The results of such a calculation are shown in fig. 6 together with the observed Frank loop concentration. It can be

seen that satisfactory agreement is ,obtained, particularly when it is noted that in the bulk samples some vacancy loop shrinkage due to recombination with interstitials is expected. 3.4. Summary Before discussing the effect of alloying with germanium, the main features of the damage structures in pure wpper will be summarised. These are presented in table5. The separation of the damage at elevated temperatures into low and high damage regions resulted in the retention of the vacancy component in the low

Table 5 Summary of damage structure in pure copper 8OT

250°-35ooc

Damage homogeneously distributed.

Damage inhomogeneously distributed - high and low damage regions present.

Predominantly vacancy loops formed by the collapse of the vacancy rich cores of displacement cascades.

Vacancy clusters in the low damage regions; mainly voids and/or stacking faul’ tetrahedra. Reduced vacancy loop concentration.

The few interstitial loops observed larger than the vacancy loops.

Interstitials lost to high damage regions comprised of tangled dislocations decorate by dislocation loops (250°C); sub-boundaries formed at higher temperatures (300% and 35O’C).

Vacancy loop concentration and sizes consistent with the results of cascade studies.

CA.

English /Low-dose

neutron irrodiotion domoge in fee ond bee meiols

damage regions and the loss of the interstitial component to the loops and dislocations of the high damage regions. In the low damage region the reduced vacancy loop concentration at elevated temperatures is satisfactorily explained by the shrinkage of loops by thermal emission. There is a complex interdependence between the populations of stacking fault tetrahedra and voids which develop under irradiation. This is particularly true at 35O’C where a high concentration of stacking fault tetrahedra developed up lo - 1 X 10” n/m2 (E > 1 MeV); at 2 X 1O22n/m2 (E > 1 MeV) this was replaced by a low concentration of voids which had not been present in the foils irradiated to lower doses.

4. Copper-germanium alloys As described in section 2 single crystal specimens of the different alloy compositions Cu-O.Ol%, O.l%, 1% and 5%Ge were irradiated under nominally identical conditions of dose and temperature lo the pure copper discussed in section 3. In this section the main effects of alloying with germanium on the development of the damage structure will be given. Preliminary results have been presented by Shepherd et al. [ 131 and the detailed results are given in Shepherd [z]. The majority of the results were obtained from specimens irradiated to 1O22 n/m2, (Shepherd (21). As with pure ooppcr it is most convenient 10 separate the presentation into reactor ambient and elevated temperatures. 4.1.

Reactor

The

neous

ambient

(SOT)

damage in all specimens consisted of a hornMe_ distribution

of small (<

14 nm) dislocation

loops.

The results for 0.1% Ge and 1% Ge are given in table 6. The concentration of 3 X 1O22loops/m3 in Cu-O.l%Ge irradiated lo I X 1O22n/m2 was similar to that observed

Table 6 Loop concentration and nature alloys: 1 X 102’ n/m2 (> 1 MeV)

at reactor

I%Ge

ambient

Cu-Ge

Cu- I %Ge

Alloy

Cu-0.

Concentration loops (m-3) Mean loop diameter (nm) Nature

3x lo=

6.3X 1022

4.4

3.5

10% interstitial 90% vacancy

50% interstitial 50% vacancy

III

in pure copper; as in copper there was evidence that the largest loops were interstitial (- 10% of the total) and the rest (- 90%) vacancy. An increase in alloying content to 1% Ge resulted in a two-fold increase in loop - 50% of these were interstitial which concentration; implies that the vacancy loop concentration did not increase significantly. The interstitial loops were considerably smaller than in Cu-O.l%Ge. These results indi-. cate that the main effect of alloying is on the interstitial loops, the increase in germam ‘urn content to 1% does not appear lo have significantly influenced the vacancy loop population. ms is consistent with the results of Stathopoulos et al. [l l] who reported that 5% Ge was required lo increase the defect yield by 20% after 30 keV Cu+ irradiation; little effect could be expected for 1% Ge. 4.2. Elevated temperature irraa?ation In all alloys other than the 5% Ge alloy, the damage was again inhomogeneously distributed, with the d@opment of high damage and low damage regions. fie form and content of each region was sensitive to both irradiation temperature and composition. 4.2.1. Effect of alloying on the high damage regions X IO” n/m’ The high damage regions (HDR) were frequently observed in alloys with 6 1 at% germanium irradiated at 250 and 300°C. Examples of HDR regions at 250°C and Cu-O.Ol%Ge and Cu-l%Ge a1 300°C are presented in fig. 7. Typical dimensions for the spacing and lateral extent of the HDR are given in table7, together with decorating loop sizes and remarks on the form of these regions. At each of these temperatures the average spacing between regions was not markedly affected by alloying content. However, the distances separating HDR in alloys irradiated at 300°C. - 20~1, was significantly greater than at 250°C, 3-10 ~1. The relative absence of HDR in alloys irradiated at 350°C is consistent with the distance increasing still further with increasing irradiation temperature. The sizes and complexity of these regions decreased with increasing alloying content in specimens irradiated at both 25O’C and 300°C (see fig. 7 and table 6). In Cu-l%Ge no dislocation line segments were observed in HDR formed at 300°C and as in the alloy irradiated at 250°C the loops were spread over large distances (> 1.5 a). Decorating loop concentrations in the same alloys irradiated at 3OO’C tended to be lower than aftei irradiation at 250°C. while loop sizes increased on going lo 300°C. At both temperatures only interstitial loops

created at I

112

CA. English/Luwdose neutron irradiation &wage in fee and inx metals

Fig. 7. Dark-field micrographs of high damage regions formed in (a) Cu-0.01% Ge, (h) Cu-1% Ge at 3OWCby a dose of n/m’(E>l McV).Imagimgconditionsareg=200~tz={O11). were identified, and in general the loops in the WDR were considerably larger than the vacancy loops or stacking fault tetrahedra observed in the low damage regions. At all irradiation temperatures no high damage regions were observed in Cu-5%Ge. 4.2.2. Low &mage regions (LBR} The majority of the point defect clusters in the low

1 X 1O22

damage region were either stacking fault tetrahedra and/or voids; voids were not present at all temperatures and alloy concentrations. The dislocation loop concentration was very low, increasing the irradiation temperature from 80°C to 25O’C resulted in a decrease in the dislocation loop condensation of at least an order of magnitude. As with the high damage regions the form and content of the low damage regions depended sensi-

C.A. English /Low-dose

Table 7 Summary of data on high damage regions in Cu-Ge cu-0.01

neutron irradiation

Spacing between HDR (P)

25OOC 300°C

Typical

%Ge

Cu-0.1 XGe

Cu-l%Ge

Less tangling than with O.Ol%Ge at both 25O’C and 3OO’C.

No tangling. Dislocation lines and associated

loops at 25O’C. No dislocation lines observed at 300°C - collection of loops. 3-10 10-20

3-10 10-20

25ooc

-5-6

( p)

300°c

~6

c3

Typical dimensions of loops associated

250°C 300°c

-15

-13 -12

dimensions

113

in fee and bee metals

alloys: 1 X 10z2 n/m2 (> 1 MeV)

Dislocation tangles at 250°C and 300°C. Decoration of dislocation lines.

General remarks

abnage

2

3-10 =-20 1 Cl.5

-11 - 14.6

with dislocations (nm)

tively on both irradiation temperature and alloying content. In table 8 the planar defect cluster concentrations are presented as a function of alloying content and temperature together with the pure copper data. The dose in all cases was - 1 X 10z n/m’. The concentrations tended to decrease with increasing alloying content and increasing temperature, these trends were most pronounced at 250 and 3OO“C. At 350°C the situation is more complex with no systematic change in concentration, although it is far lower in the alloys than in pure copper. This is the temperature where in pure copper the high density of stacking fault tetrahedra was replaced by a population of voids when the dose was raised to 2 X 10” n/m*. No such change was observed in the copper-germanium

Table 8 Point defect cluster concentrations (1 X 1O22 n/m2 (>l MeV)) Alloy

250°C

Pure Cu Cu-0.01 %Ge Cu- 1%Ge Cu-S%Ge

3.4x 3.4x 1.7x 3.3x

102’ 102’ 102’ 1020

(me3)

in Cu-Ge

alloys

3tWC

350°C

1 x102’ 1 x102’ 3.5 x 1020 Not measured

8 X102’ 1.5x 10’9 4.8x 1020 Not measured

Fig. 8. Three-dimensional plot of the dependence of the void concentration in Cu-Ge alloys on composition and irradiation temperature. Neutron irradiation dose is 1 X 1O22 n/m2 (E > 1 MeV).

114

CA. English /Low-dose

Table 9 Mean void diameter (nm) in Cu-Ge

cu CU-0.01 %Ge Cu-0. l%Ge Cu-l%Ge

neutron irradiation damage in fee and bee metals

alloys at 250°C and 300°C

250°c

300°C

5.1 5.9 8.1 n’ No voids

4.9 a) 9.1*’ 11.5 b, No voids

a) Bimodal size distributions. b, Large voids with sixes characteristic

of second

14.1 No voids

peak ob-

served.

alloys. In pure copper the stacking fault tetrahedra size was independent of dose and temperature. In the alloys larger stacking fault tetrahedra were observed with the size depending on composition and temperatures; the average diameter 6, was in the range 3.4-4.2 nm, compared to d= 2.4 nm in pure copper. Although stacking fault tetrahedra were observed in all the alloy compositions studied, voids were only observed in Cu-O.l%Ge irradiated at 250, 300 and 350°C and in Cu-O.l%Ge irradiated at 250 and 3OO’C. No voids were observed in Cu- 1WGe at any irradiation temperature (250-35OOC). The measured void concentrations and mean sizes for a dose of - 1 X 1O22 n/m2 are presented in fig. 8 and table 9 respectively as a function of alloying content and irradiation temperature. Results from pure copper are included for comparison in both cases. Void concentrations decreased and void sizes increased with increasing alloying content and temperature. Bimodal void size distributions are again a feature of the results (table9). The maximum void volume fraction Au/u decreased at all temperatures with increasing alloying content with the highest swelling at 300°C in Cu-O.Ol%Ge (Au/u = 5.6 X 10T5);

Table 10 Summary of damage formed in Cu-Ge

alloys 1 X lO22 n/m2 (> 1

WC Damage homogeneously Vacancy content.

component

unaffected

by Ge

Interstitial-loop concentration increased and mean loop decreased with increasing Ge size.

Vacancy studies.

loop component

consistent

4.3. Summary The main features of the results, summarised in table 10 follow the pattern established in pure Cu. That is the damage changed from a homogeneous distribution at 80°C to an inhomogeneous distribution at elevated temperatures with the development of high and low damage regions. The composition of both these regions was surprisingly sensitive to Ge content with as little as 0.1% Ge causing marked changes in the visible damage. In the low damage regions small changes in Ge content had a greater effect on the void population than on the stacking fault tetrahedra. Bimodal void size distributions were observed at a lower temperature (25O’C) than in pure copper. There was no strong evidence in the alloys irradiated at 350°C for stacking fault tetrahedra concentration increasing up to a critical dose and then decreasing dramatically with the appearance of voids.

5. Molybdenum In this section the main results of Riley [3] will be presented. High purity (011) and (137) single crystal discs of molybdenum were irradiated to doses of up to 1 X 1O22n/ms at reactor ambient and at temperatures between 250°C and 600°C. Prior to irradiation the crystal had a very low dislocation density with dislocations only observed in very widely separated sub-boundaries. After irradiation at all temperatures and doses the distribution of visible damage was inhomogeneous with high damage regions associated with dislocation lines and regions containg a lower density of dislocation loops. Typical examples of this type of

MeV)

250-350°C Damage inhomogeneously

distributed relatively

although it is to be noted that in pure copper Au/u was lower at this temperature.

with cascade

distributed

Form of the HDR influenced by Ge content - sizes and complexity decreased with increasing Ge - not observed in 5% Ge. LDR contain voids, stacking fault tetrahedra and loops. Void concentrations decreased and void sixes increased with increasing Ge. Voids not observed in 1% and 5% Ge at any temperature or 0.1% at 35O’C.

CA. English /Low-dose

Fig. 9. Micrographs of the inhomogeneous reactor ambient gOT, (b) 350%.

distribution

neutron irradiation damage in jcc and bee metals

of the damage in molybdenum

irradiated to X IO*’ n/m2 (E>l

115

MeV)a It (4

116

C.A. English /Low-dose

neutron irradiation

structure are illustrated in figs. 9(a) and (b) for irradiations at reactor ambient and 350°C. 5.1. High damage regions

After irradiation the dislocation line length is greatly increased and dislocations are widely distributed. The complex tangles found in pure copper or the very dilute alloys were not observed in copper-germanium molybdenum. Usually isolated a/2( 1 I 1) dislocation line segments were observed with associated dislocation loops, a zone denuded of loops frequently surrounded the dislocation line-loop complex. The a( 100) dislocation segments formed by junction reactions were also found to be decorated. Occasionally in regions, where the dislocation line density was high, undecorated dislocation lines were observed. With increasing dose the extent of the association and the related denuded zone increased as did the loop density within the association. Typical dimensions of the high damage regions were -0.1 p after a dose of 1 X lo*’ n/m* increasing to - 0.25 f~after a dose of 1 X lo** n/m*. As the temperature increased the loop density within the association decreased, the loop size increased and the association extended further from the dislocation line. The form of the loop association was dependent on the character of the line dislocation. For an edge dislocation, loops were situated on one side of the dislocation line with the other side denuded. The loops fanned out from the dislocation line to form a crude wedge which is thought to be on the dilated side of the dislocation line. For a screw dislocation the loops were situated all around the dislocation core. A denuded zone was sometimes present between this region and the low damage region. Mixed character dislocation lines exhibited a combination of these features. All the dislocation loops analysed were found to be interstitial in nature and to have 6 = a/2( III). The

Table 11 Burgers vector analysis of loops in high damage regions in MO irradiated

at 250°C

to a dose of 5 X 102’ n/m2

Burgers vector

Percentage ‘)

a/2

(III)

38

a/2

(II

a/2

(lil)

a/2

(1

‘)

11

1)

;;w

ii)

60 loops analysed.

b, Dislocation

line segment Burgers vector.

(E > 1 MeV)

damage in /cc and bee metals

results of such an analysis are shown in tgble 11. The Burgers vector of the dislo&tian$ne did,not seem to influence the Burgers vectors ‘& the ‘loops, although evidence was found for the loss of glissile a/2(1 11) loops to the foil surface. Within the association of loops some grouping of loops with identical Burgers vectors was found. These groups extended spatially up to 4 pm and contained up to -40 loops. A 8x&l number of loops with Burgers vectors different to the majority were usually present. 52. Low damage regions The analysis of these regions is not as complete as in copper or copper-germanium. The damage consisted of a population of dislocation loops ranging in Sk! from =K5 nm to > 20 nm in diameter. The dislocation density and loop size increased with increasing dose, while with increasing temperature the loop density decreased and the average loop size increased. All loops were found to be a/2(111) and the large loops were predominantly interstitial in nature. Groupings of both small and large loops were observed at all temperatures but particularly at temperaAn example of this taken from a tures above -400°C. foil irradiated at 600°C to I i< lo** n/m* (ES 1 MeV),

is presented in fig. 10. The arrows in the figure indicate groups of loops which frequently share a common Burgers vectors. Clusters of up to six large loops were observed at the higher temperature while, at lower temperatures groups of up to three loops are present. As well as sharing a common Burgers vector they occasionally shared a common habit plane. 5.3. Comparison with cascade studies Unfortunately, data on the vacancy loop component in the low damage regions is not available at present. However, the temperature dependence of the vacancy loop concentration was studied in earlier work where molybdenum was irradiated to the higher doses of 6 X lO23 to 1 X 10M n/m* (> I MeV) [Id]. The quantitative results are given in fig. 11(a) and are compared in fig. 1I(b), to the results from 60 keV self-ion irradiation over the same temperature range. It can be seen that in both neutron-and self-ion-irra&ated MO the vacancy loop numbers decrease sharply above - 2OO“C w&i& is considerably less than the temperature at which v~cllllcy loop shrinkage occurs by thermal vacancy emission. Using the thermal stability model that fitted the copper results the perfect vacancy loop population in irradiated MO is predicted to remain constant up to 700-800%

n

I

I

I

I

1

ioo

200

300

100

500

~RRh~l~T~~

0

200 fRR~~i~TlON

TEWERATURE

*C

A -

60 keV Xe%ns

-

50 keV Mo’ims

300

400

rao

TEMPERATURE *C

Fig. 10. Dark-field micrographs of molybdenum irradiated tc 4X 102’ n/m2 (Es 1 MeV) at 600°C imaged at 2 = (011) with s =0 in (a) g =Zf, (b) g=2il.

Fig. I I, Plot of the temperature dependence in molybdenum of the vacancy loop concentration in (a) neutron irradiated to doses between 6X 102’ and 10” n/m*, (b) 60 keV MO+ and W+ ion irradiated with 5X IOn’ ions/m’.

In fact Loops in room temperature ion-irradiated No do not shrink during post-irradiation annealing at temperatures up to 400°C. It can be concluded therefore that the behaviour in Mo is quite different to that in copper and that an increasing fraction of the cascades do not cohapse above 200°C. It is interesting to note that this temperature corresponds approximately to Stage HI recovery temperature in irradiated MO which is widely

considered to be due to long range vacancy migration. Thus, the observations indicate that when vacancies are able to diffuse away from the cascades, the wncentration decreases to below the critical value required for collapse. This interpretation is supported further by the results obtained using heavier ions which cause a higher vacancy concentration in the cascade. Included in fig. I I(b) is the curve for MO irradiated with 60 keV

CA.

118

En&h/Low-dose

neutron irradiation

Xe+ ions which shows no decrease in vacancy loop numbers up to -400°C. A second aspect where molybdenum also differs from copper is that the results of self-ion studies in the two metals show that cascade collapse is less efficient in the bee metal. The defect yield, defined as the fraction of cascades which collapse to give visible loops, after room temperature irradiation of molybdenum with 60 keV ions is - 0.2 (corrected for loop loss to the surface). The results of Stathopoulos [lo] show that the yield in copper irradiated with a lower energy ion (30 keV) is higher at 0.5 rt 0.02.

6. a-Iron In this section we discuss the main features of the results in polycrystahine a-iron, which was irradiated at reactor ambient to higher doses (5 1.2 X 102’ n/m2) than the metals already discussed. The detailed results are presented in Robertson et al. [ 121and only the main features of the damage created in the high-purity iron will be discussed here. In the unirradiated iron an appreciable density of dislocation lines ( lOI lines/m2) and low-angle tilt (pure edge dislocations) and twist boundaries (pure screw dislocations) were present. In contrast to the other pure metals studied no visible damage was found in grains of any orientation irradiated to low doses: (5 1 X 1O23n/m2). At higher doses (*4X 1O23 n/m2) the damage consisted- primarily of dislocation loops associated with in-grown line dislocations and low-angle boundaries, see fig. 12. A low density of small dislocation loops (< 10 nm) was ob-

damage in /cc and bee metals

served between these regions at 1.2 X 102’ n/m2. The damage in the high damage regions was analysed in some detail and the main results arc summansed in table 12. At both doses the dislocation lines with ass&iated loops assumed a wavy appearance with some evidence of loop-line interaction. At lower doses (< 1 X 10z3 n/d) there was no evidence of dislocation climb. Botha(lOO)andu/2(lll)loopswerefout&andasin molybdenum, some grouping of loops of similar Burgers vector was observed. There Were two main effects of increasing the dose from 4 X 10” to 1.2 X,lO” n/m2 (E > 1 MeV). First, fmascnk*age ckv&ped around both large interstitial loops and dirlocatiosr lines, as illustrated in fig. 12(b). !Secot& an increased fraction of screw dislocations was obaavcd with afSociatod dislocation loops; this applied to both isolated screw dialocationsartdthoaewhicbcompriaedtwiatbm&arka.It proved itqpoasibk to datermine if the W damage wasintemtititslorvacancyinantureorcventoprove that it consisted solely of point defect clusters. Robertson et al. [ 121have also demonstrated that the formation of these high damage regions was sensitive to impurity levels. They did not develop in iron with *higher interstitial and substitutional impurity levels. 6.1. Compon’son with cascaa2 sttdies No evidence was found for vacancy loop production in cascades created in, neutron-irradiated a-iron. and this represents a basic difference in behaviour compared to copper, copper-germanium and molybdenum. This result is consistent with the cascade studies [7] where self-ion irradiation with energies between 40-240 keV

Table 12 Summary of the main features of the high damage structures in a-iron irradiated at 8O’C to a dose of 4X 1O22and 1 X 102’ n/m2 (> I MeV) I X IO” n/m2 (>

4X IO” n/m’ (> I MeV) Dislocation

I MeV)

sub-structure unchanged by irradiation

Both isolated dislocation low angle tilt boundaries

lines and dislocation lines in decorated by large interstitial

loops.

All isolated dislocation lines. all tilt and some twist boundaries decorated by large interstitial loops on one side and small clusters (< 3 nm) on the other side.

Dislocation loops (d (20 nm) with mixed Burgers vectors found within 60 nm of one side of dislocation line.

Loops larger than those found at 4X 102’ n/m2.

No difference observed in loop configurations edge and screw dislocations.

Some of the larger loops decorated by small clusters, like those found around dislocation lines.

around

Not all screw dislocations and no low angle boudaries were observed with associated loops.

twist

CA.

English /Low-dose

neutron irradiation

damage in fee and bee metals

Fig. 12. Micrographs of the damage created in a-iron after a dose of 1.2X 10m n/m* (E > 1 MeV) at 80°C imaged in g = 2( IO at at low magnification (a) and high magnification (b). The micrographs show (a) the general distribution of damage, (b) a high damage region around an in-grown dislocation. z = (011)

120

C.A. Engltih /L.ow-dose neutron irradiationdamage in

Table 13 Conditknu u&x whidt daawge buxnne~ is@y Material

bwest tempcr8tunc iiIhom~rdanuge distribution obaerwd

cu Cu-Ge alloys

2wc 2wc

fee and bee metals

distributi

Lowestdose inhomogeneous damage distribution observed n/m2 (> 1 MeV)

Ixslowion line length d irradiated as compared to irradiated material

MO

80°C .’ WC .’

HDR

1 x 102’ b’

Increased

Didocation taogk.

1x 102’b,

Increased

Dislocation tan&s at low CIe eontents. DishYcationline segments athighGecuntutts.

1 x 102’ b’ 4x 102’

increased

Diakxadon~~ts.

No

(5% Gc)

a-iron

CommeMon8lrwwed di&cation Iii in

change

lsolati disloeaciott lines and d&cations in low angk grain boundsries.

a) Low&Stteqmnwe mtKlied. b’ Lowest dose studiuk

produced no viftibk dnmage in both high- and low-putity matesi& Can&e cdlrp# did occur when the ion mass incfuaaed.

7. DW 7.1.

Introduction

The results presented in sections 3-6 have revealed a surprisingly complex pattern of behaviour. The damage structures created by low-dose fission neutrons are clearly strongly influenced by both irradiation and material variables. The most surprising aspect of the results is that in all the systems studied, with the exception of Cu-SIGe, there were irradiation conditions under which the damage was vy distrib uted and high and low damage regiona’&veIoped. This is an unexpected result particularly when contrasted with the development of a homogeneous distribution of interstitial loops in electron-irradiated pure metals [ 161. It is beyond the scope of this paper to embark on a detailed discussion of the results in each metal studied. Rather, the purpose of this section will be to compare the results in the different metals and to discuss the agreement found with the results of caaeada studies.

7.2.

Formation

of high and low damage regions

Although the content of the low damage regions differed from metal to metal there were two common

features in all the high damage regions observed. First, they were always located about dislocation lines and, thus, a necessary ‘precursor to the development of the inhomogeneous distribution of the damage is the presence of in-grown dislocations (or sub-boundaries). Setond, the dislocation lines were always decorated by dislocation loops, and there was little evidence of significant dislocation climb occuring once these loops had nucleated. This indicates a state is reached during the irradiation when the irradiation-produced point defects can no longer be absorbed on to the dislocation lines. The reason for this is diffmlt to establish, but it most probably involves impurities segregating to the dislocation lines and hit&ring&g nucleation and migration. It must be noted that this occurs in material with very low levels of int&&ial impurities. The basic form of the dislocation line component and the lowest temperature and dose at which HDR regions were observed are given for the four systems in table 13. Two main differences are apparent from the table. First, in copper, copper-germanium (< 5% Ge) and molybdenum, the in-grown dislocation must climb through the lattice to produce the observed increase of the dislocation line length after irradiation. The exact me&an& which causes the dislocation lines to cease climbing in these metals is not certain; it most probably involves impurity atoms segregating to the dislocation lines which makes jog nucleation and motion more difficult. In the Cu-Ge alloys the presence of Ge also clearly influences dislocation line motion and at high Ge contents dislocation tangles are not apparent. In

C.A. English /Low-dose

neutron irradiation

contrast, in a-iron no evidence was found for dislocations climbing through the lattice in the early stages of irradiation; presumably the higher interstitial impurity atom levels lead to pinning of the in-grown dislocation by impurity atmospheres. Second, the HDR were not formed in copper or the copper-germanium alloys until 250°C, presumably at 80°C the high density of loops, predominantly vacancy loops from collapsed cascades, act as the major sink for point defects and also pin the in-grown dislocations. In molybdenum at 80°C a factor allowing dislocation movement might be that a far smaller fraction of cascades will collapse to form vacancy loops and thus there are fewer sinks for point defects in the early stages of irradiation and also fewer pinning points opposing dislocation line motion. The separation of the damage at elevated temperatures into high and low damage regions has been reported previously in copper [ 17- 191. These studies have concentrated on the development of the higb damage regions and the results are broadly consistent with the present work. Although Jackson et al. [19] used transmission electron microscopy to examine the high damage regions they did not report details of the damage in the regions between the dislocation tangles. The form of the damage in the low damage regions is clearly also very sensitive to material and irradiation conditions. In the fee metals studied they contained predominantly vacancy clusters and there is clearly sufficient residual gas to stabilise the void nuclei. The formation and growth of stacking fault tetrahedra and voids must be linked as they represent alternate sinks for the excess vacancies resulting from the preferential loss of interstitials to the high damage regions. It is interesting that in Cu-Ge alloys no voids were observed in circumstances where high damage regions were not a prominant feature of the damage structure. In molybdenum there is insufficient data to establish whether or not the LDR contain predominantly vacancy clusters. The most interesting feature of the structures analysed was the observation of groupings of loops of like Burgers vector, clearly the precursor to the large scale rafting observed at higher doses [20]. Only a/2(1 11) loops were observed in molybdenum while both a( 100) and a/2( 111) loops were found in a-iron; see Robertson et al. [12] for a discussion of the significance of observing the energetically ,unfavourable a (100) loops. In a-iron, the lack of visible damage in the bulk even at the highest dose is extremely difficult to understand, particularly in view of the high damage efficiency observed by Kirk and Greenwood [21] after fission-neutron irradiation at 4 K. Further it contrasts

damage in fee and bee metals

121

with the work of Eyre [22] and Bryner [23] who observed that the onset of visible damage in less pure material generally occurred at lower doses than found here and that the damage consisted of a homogeneous distribution of dislocation loops. 7.3.

Vacancy

loop component

of the damage

A striking feature of the results on the vacancy loop component is the reasonably good match in behaviour between the self-ion and neutron irradiation results. This indicates that the close proximity of the incident surface in the ion-irradiated metals does not, in any fundamental way, change the processes occurring within the cascades, although it clearly exerts some influence by acting as a sink for interstitial point defects. The effect of both of these factors is illustrated by the results from Cu in which the number of vacancies surviving to form Frank loops is somewhat higher in the ion-irradiated specimens. Nevertheless heavy ion irradiation experiments, particularly using self-ions, can provide a useful guide to the likely way in which cascades collapse to vacancy loops in fission-neutron irradiation. The results of both the ion and neutron irradiations show that there are very considerable differences in cascade collapse in the three metals Cu, MO and a-Fe, both in terms of the low temperature collapse process and the effects of increasing irradiation temperature. In copper the results are consistent with a fairly efficient collapse process at all temperatures but that the vacancy loops formed at > 250°C shrink by the thermal emission of vacancies. In molybdenum cascade collapse is less efficient than in copper and, more importantly, the collapse process itself is temperature dependent. The fact that cascade collapse occurred at elevated temperatures with heavy ions suggests that the collapse process in molybdenum is dependent on the vacancy concentration in cascades. In o-iron collapse was not observed to occur in either the self-ion or neutron irradiations, but when the vacancy concentration in the cascade was increased by using heavier ions, vacancy loops were produced, suggesting as in molybdenum that the collapse process depends on vacancy concentrations. With regard to the barrier to collapse, Cu has a relatively low stacking fault energy and thus the barrier to Frank loop formation is correspondingly low and the tendency to cascade collapse is comparatively high under all of the conditions studied. The bee metals MO and a-iron, on the other hand, are believed to have very high stacking fault energies making the collapse process on to { 110) planes very difficult. For a further discussion of this aspect of the results see Eyre and English [15].

122

C.A. English /Low-dose

neutron irradiation damage in fee and bee metals

8. Condusions The study of the damage structures created by low-

dose fission neutron irradiation of Cu, Cu-Ge alloys, MO and a-iron has shown that: 1. In Cu, Cu-Ge and MO visible damage was observed at all temperatures after irradiation at the lowest dose employed (1 X 10” n/m2 (> 1 MeV)). 2. In a-iron irradiated at 80°C visible damage was not observed until a dose of 4 X lOI n/m2 (> 1 MeV). 3. In all the materials, with the exception of Cu-5% Ge, there are irradiation conditions under which the damage was inhomogeneously distributed, with the development of high and low damage regions. 4. In all cases the high damage regions consisted of the in-grown dislocations, decorated with interstitial dislocation loops. In Cu, Cu-Ge and MO the dislocation lines climbed during the early stages of the irradiation while in a-iron they did not. 5. In Cu and Cu-Ge the low damage regions consisted of vacancy loops, stacking fault tetrahedra and voids, and in MO of dislocation loops. In a-iron no visible damage was observed away from the dislocation lines. 6. The results on vacancy loop formation in the neutron-irradiated material are in good accord with the results previously obtained using self-ions with energies typical of the primary knock-ons generated in the neutron irradiation.

Acknowledgements The following have also been involved in these studies: Professor B.L. Eyre, Drs. M.L. Jenkins and M.H. Lorretto and their contribution is gratefully acknowledged.

References [l] J.M. Muncie, D. Phil. Thesis, University of Sussex (1979). 12) B.W.O. Shepherd, D. Phil. Thesis, University of Oxford (1981). [3] B. Riley, Ph. D. Thesis, University of Birmingham, to be submitted. [4] I.M. Robertson, D. Phil. Thesis, University of Oxford (1981). [S] C.A. English, J. Summers and B.L. Eyre, Phil. Mag. 34 ( 1976) 503. [6] C.A. English, B.L. Eyre, A.F. Bartlett and H.N.G. Wadley, Phil. Mag. 35 (1977) 533.

[7] M.L. Jenkins, (1978) 97.

C.A. English

and B.L. Eyre, Phil. Mag. 38

[8] C.A. English and M.L. Jenkins, J. Nucl. Mater. 96 (1981) 341. [9] CA. English, AERE Report, to be published. [IO] A. Stathopoulos, Phil. Mag. A44 (1981) 285. [ 11) A. Stathopoulos, CA. English, B.L. Eyre and P.B. Hirsch. Phil. Mag. 44 (1981) 309. [12] I.M. Robertson, C.A. English and M.L. Jenkins, these Proceedings, p. [13] B.W.O. Shepherd, M.L. Jenkins and C.A. English, Proc. European Congress on Electron Microscopy, The Hague, Vol. I, p. 349. [14] B.L. Eyre and C.A. English, Proc. Harwell Consultant Symposium, Physics of Irradiation Produced Voids (1974) p. 239. [I51 B.L. Eyre and CA. English, Proc. Intern. Conf. on Point Defects and Defect Interactions in Metals, Japan, to be published. 1161M. Kiritani, Proc. Conf. on Fundamental Aspects of Radiation Damage in Metals, Gatlinburg, USA (1975) p. 695. 1171 B.C. Larson and F.W. Young, J. Appl. Phys. 48 (1977) 880.

1181 P.J. Jackson,

K.E. Black, P.D.K. Nathanson and D.R. Spalding, Phil. Mag. 35 (1977) 509. ‘[191 P.J. Jackson, K. Kemm, J. Nevin and D.K. Spalding, Radiation Effects 35 (1978) 1. WI J. Bentley, B.L. Eyre and M.H. Loretto, Proc. Intern. Conf. on Radiation Effects and Tritium Technology for Fusion Reactors, eds. J. Watson and F.W. Wiffen, NTIS Conf. 750 989 (1976) I-297. 1211 M.A. Kirk and L.R. Greenwood, J. Nucl. Mater. 80 (1979) 159. WI B.L. Eyre, Phil. Mag. 7 (1962) 2107. ]231 J.S. Bryner, Acta Met. 14 (1960) 323. 1241 C.A. English, B.L. Eyre and S.M. Holmes, J. Phys. F (Metal Phys.) 10 (1980) 1065. [25] D.K. Saldin, A.Y. Stathopoulos and M.J. Whelan, Phil. Trans. Roy. Sot. (London) 292 (1979) 5 13. [26] D.M. Maher and B.L. Eyre, Phil. Mag. 23 (1971) 409. [27] K.K. Merkle, Phys. Status Solidi 18 (1966) 173.

Discussion K. Boning: Would it be possible or do you even plan to perform such TEM investigations after irradiation at lower temperatures (either with fast neutrons or with self-ions)? It would be interesting to see if a threshold dose for the observation of visible damage (loops) could be found. C. English: Although it would be interesting to perform low temperature irradiations, we do not at present have facilities to neutron irradiate TEM discs at low temperatures and to observe in the TEM without subsequent warming up. I agree it would be very informative to study the dependence of any threshold dose on irradiation temperature, as it would probably

C.A. Enghsh/Low&e

neutron irrudiation damage in fee and lice met&

yield data on the effect of both irradiation temperature and cascade overlap on the cascade collapse process, about which relatively little is known. H. Peisl: After irradiation of Cu at 8OT you have a homogeneous defect distribution, after irradiation at 2S0°C a highly inhomogen~us dist~bution. Is this created during the formation of a cascade or afterwards. Have you irradiated a sample at 80°C and than warmed up to 25OT?

123

C. English: The highly inhomogeneous distributions in Cu were created by the “redistribution” of the point defects outside the cascade region, see section 3.2 above! We have not performed any post-irradiation annealing experiments on neutron-irradiated copper samples The most probable effect of post-annealing the specimens irradiated at 80°C to 250°C would be the shrinkage of the vacancy loops by thermal emission of vacancies, see section 3.3 above.