Role of Dislocation Loops in the Development of Vacancy Voids

201

ROLE OF DISLOCATION LOOPS IN THE DEVELOPMENT OF VACANCY VOIDS

V.F.Reutov Joint Institute for Nuclear Research (JINR), Flerov Laboratory of Nuclear Reaction (FLNR), 141980 Dubna, Moscow region, Russia, e-mail: [email protected]

This paper presents the results of a transmission electron microscopy (TEM)-investigation of the effect of post-irradiation annealing on the structure of molybdenum and nickel doped with helium atoms that was irradiated with high-energy a-particles and fission neutrons. It is shown that: • The number of interstitial atoms in dislocation loops and the number of vacancies in helium bubbles increase as annealing temperature and time increase. • The growth of helium bubbles on the grain boundary is mainly observed on the side of the grain body where there are dislocation loops. • Post-irradiation annealing of dislocation loops formed by 50-MeV a-particles and fission neutrons at 60°C results in the growth of vacancy voids. Thus, high temperature post-irradiation annealing or irradiation at high temperature (more than 0.3Tme«) constitutes the conditions under which dislocation loops are powerful sources of vacancies. Consequently, if in the crystal structure of the irradiated material there are MemVn groups, vacancy oversaturation caused by the thermal emission of vacancies by radiation- induced dislocation loops may be the main driving force for the nucleation and growth of vacancy voids.

Introduction Results and discussion Radiation swelling, one of the last discovered Molybdenum and nickel were chosen as the radiation effects, has been known since 1967 [1] material to be investigated. The irradiation with when it was found that irradiation of a material neutrons was performed using the WWR reactor; makes its volume (geometrical dimensions) grow. the irradiation with 50 MeV a-particles was carried The effect is accounted for by there appearing and out on a cyclotron at a temperature of about 60°C. developing vacancy voids (W) in the irradiated The postirradiation isothermal and isochronous material. This unexpected phenomenon, which annealing was performed in a 5x103 Pa vacuum in results in the strenth of reactor building materials the temperature range from 100 to 1100°C. becoming catastrophically low, has attracted If the critical conditions for dislocation loops to considerable interest. coalesce during their sliding and crawling, which Unfortunately, up to date, theories of swelling result from the elastic interaction between them, are are unable to quantitatively predict the value of not met, then dislocation loops may grow by radiation swelling for a wide range of materials and emitting vacancies. For those conditions to be irradiation conditions. realized, dislocations must be prevented from According to up-to-date concepts, the transferring due to sliding and crawling. nucleation and growth of W in irradiated material Therefore, dislocation loops can be seen has oversaturation with vacancies as a driving force. through a TEM to emit vacancies if the following The oversaturation is assumed to result from the fact requirements are met: that the ways vacancies and interstitial atoms interact 1. The sliding and crawling of dislocation loops must with dislocations are different, dislocations showing be eliminated or decreased. preference to interstitial atoms [2]. 2. The annealing temperatures must be high; However the asumption that dislocations show sufficiently large loops must be dealt with. This will preferance to interstitial atoms has not been allow one to decrease the error in measuring the confirmed experimentally. The parameter of variations in the diameter of dislocation loops and, preference is difficult to calculate. It depends on the consequently, the error in measuring the number of elastic fields of discharge channels , the degree of the interstitial atoms in them. The dimentions of assimilation of point defects, the barriers to the loops must not be larger than the thickness of the absorption of point defects on the surface of TEM-object (100 nm), their density in the specimen discharge channels. Therefore, there a need to must be sufficient from the statistical standpoint. search for other sources of oversaturation of the 3. In the specimen there must be powerful crystal lattice with vacancies. In our view, it is discharge channel to capture the vacancies emitted reasonable to suppose that the dislocations and by dislocation loops in order to prevent them from dislocation loops of interstitial and vacancy types being captured by the neighbouring loops. produced during irradiation may be considered as Otherwise, the expected increase of interstitial such sources. atoms in loops will not be observed. This paper presents the results of a TEM- Those requirements are likely to be met in investigation of the effect of postirradiation specimens doped with helium atoms. In such annealing on the structure of molybdenum and specimens the appropriate annealing results in nickel were doped with helium atoms and also was dislocation loops of the interstitial type and helium irradiated with high-energy a-particles and fission bubbles being formed. The dislocation loops serve neutrons. as sources of vacancies and the bubbles as discharge channels for them.

4-H Me^cdynapoOHoa Komfiepemtwi «B3auModeucmeue uy\ymenuii c meepdbuu mejioM», 3-5 OKtnxdpn 2001 e., MUHCK, Eenapycb 4-th International Conference interaction of Radiation with Solids», October 3-5, 2001, Minsk, Belarus 202

Indeed, in Mo specimens doped with helium up The specimens were isothermally annealed at to 4.5x10"2 at% and annealed at 1000° C, temperatures of 1000 and 1100°C for 1, 5, 10, 50 dislocation loops and helium bubbles are formed and 100 hours. whose density and dimentions comply with the As shown in Fig. 1, the longer the annealing requirements of paragraphs 2, 3. time is, the larger the size of dislocation loops are The presence of helium bubbles on dislocation and the lower their density is. The TEM analysis loops prevent them from conservative motion, and carried out showed that dislocation loops, which are the high density of helium bubbles in the space mainly of the interstitial type, are situated in planes between dislocation loops provides a lot of efficient of the "1111J- type and have Burgers's vector of the and thermally stable discharge channels for 1/2a<111>type. vacancies. •VT -» --- «.;

Fig.1. Development of dislocation loops in Mo doped with helium (4.5x10"2at.%) during postradiation annealing at 1000°C, the annealing time being 1 hr (a), 10 hrs (b), 50 hrs (c)

On this basis, the number of interstitial atoms about 3eV, which is close to the energy required for in dislocation loops was calculated as a function of producing vacancies in molybdenum (3.3 eV [3]). annealing time at 1000 and 1100°C (Fig. 2). Fig. 2 In another series of experiments in which the shows that the number of interstitial atoms in peculiarities of the growth of helium bubbles near dislocation loops is a linear function of annealing grain boundaries in helium-doped Ni were studied, time. As seen, the number of interstitial atoms data were obtained that testify that interstitial increases at much larger rate at 1100°C than at dislocation loops are quite powerful sources of 1000°C. Moreover, it is significant that the increases vacancies. in the number of interstitial atoms in loops closely Near grain boundaries in Ni specimens, zones correlate with the increases in the number of were created that were depleted in dislocation vacancies in helium bubbles. loops, and that contained helium bubbles (Fig. 3a). On further annealing, as interstitial dislocation loops a in the grain body keep on growing, the diameter of helium pores is seen to increase not near grain boundaries (which are commonly accepted powerful £. «- ha S sources of vacancies) but in the opposite region, which borders on the area of the grain body where o 1000°C there is high density of interstitial dislocation loops 1 (Fig. 3b,c). We think that in this case the flow of vacancies from the area of the grain body where there are dislocation loops prevails over the flow of vacancies from the grain boundaries. E Thus, all the above-stated experimental B a results, in our opinion, strongly testify that interstitial J s and, of course, vacancy dislocation loops are Q powerful sources of vacancies and, therefore, may tin so be responsible for the oversaturation of the crystal annealing time, h lattice with vacancies at appropriate temperatures. And, if there are centres for them to condense on, particularly HemVn, this may result in the nucleation Fig.2. The number of interstitial atoms in dislocation and growth of vacancy voids. So, what is of loops and the number of vacancies in helium fundamental importance, the process concerned bubbles in Mo as a function of postradiation can be accounted for without using the hypothesis annealing time at 1000 and 1100°C. of so-called preference It is known that owing to interstitial atoms being highly mobile at ordinary temperature at which Thus, these results testify that interstitial loops irradiation is carried out, interstitial atoms in grow by emitting vacancies. Moreover, estimates of aggregations are in the form dislocation loops. As a the activation energy for loop growth give the value

Hox KOH(pepenuun «B3awMOdeucmetie tajtyiemm c meepdbiM menoM», 3-5 OKtnn6px 2001 z., MUHCK, Bejiapycb 4-th International Conference ((Interaction of Radiation with Solids», October 3-5, 2001, Minsk, Belarus 203

Fig.3. Dislocation loops in Ni doped with helium up to 2x102 at.% and annealed at 600°C for 1 hr.(a). Development of helium bubbles in Ni (2x10'2 at.% He) as a result of annealing at 800°C: b- in region A lying near the grain boundary, which is poor in dislocation loops (see Fig.3a); c- in the grain body (region B, see Fig. 3a). result, during postradiation annealing, they are in no The postirradiation annealing was performed in way involved in the evolitjon of dislocation loops and a 5x103 Pa vacuum in the temperature range from much less in the formation of vacancy voids. 100 to 1100°C, the step being 100°C. This and above-stated results suggest that Immediately after irradiation of specimens with vacancy porosity may be formed in specimens neutrons and a-particles there appears a irradiated at low (room) temperatures and then morphologically identical imperfect structure in the annealed. form of vacancy clusters/loops and interstitial For this purpose, Mo (99.97%) in the form of dislocation loops (Fig. 4a,b). The average density of 100 mkm foil was irradiated with neutrons in a defects and their size distribution also proved to be nuclear reactor at about 60°C and with 50 MeV a- similar. Defects of size less than 5 nm were mainly particles, ensuring shooting through, up to of the vacancy type whereas dislocation loops of 7x10° larger size were of the interstitial type.

Fig.4. Imperfect structure in Mo on irradiation at 60°C, a - with fission neutrons (7x10'2 dpa), b - with 50 MeV a-particles (7x102 dpa). Vacancy voids in Mo: c - irradiated with neutrons and annealed at 900°C, d - irradiated with a-particles and annealed at 900°C

The imperfect structure begin evolving at owing to the backward scattering of the bombarding 300°C: defects of size about 2 nm disappear. The a-particles in the sample. Therefore, there being marked evolution of the loop structure occurs at more helium atoms in Mo specimens irradiated with temperatures higher than 500 C. In the temperature high-energy a-particles as compared to those range from 800 to 1100°C voids are initiated and irradiated with fission neutrons seems to account for begin to grow (Fig. 4c,d). The way in which the the larger density and sizes of voids observed in the characteristics of voids depend on annealing case of irradiation with a-particles. temperature, that is, that the volume of voids Thus, the above results not only point to the increases at 900°C, and that they are practically possibility of the growth of vacancy voids during annealed at 1100°C, points to the fact that they are annealing of the imperfect structure formed under of a vacancy nature. irradiation at low temperature but also give strong Comparison of the increase in the number of evidence of emission of vacancies from dislocation interstitial atoms in dislocation loops and the loops, which grow at the appropriate temperatures. increase in the number of vacancies in voids indicates that interstitial dislocation loops are the Conclusions sources of vacancies for voids to grow. The presented experimental results show that: HemVn complexes seem to be the centres at • the number of interstitial atoms in dislocation which voids are nicleated. In the case of neutron loops and the number of vacancies in helium irradiation, helium accumulates as a result of (n,u) pores increase as annealing temperature and reactions. Under irradiation with a-particles helium time increase; accumulates both owing to the corresponding channels of nuclear reactions of the (a,xa) type and

4-H MeJtcdyhapodHaa KOHtpepemaa uBiaiiMOdeucmeue itvtyveHUU c meepdtaAi me/taM», 3-5 ommopH 2001 J., MUHCK, Benapycb 4-th International Conference «Interaction of Radiation with Solids». October 3-5, 2001, Minsk, Belarus 204

• the growth of helium bubbles on the grain thermal emission of vacancies by dislocation loops boundary is mainly observed on the side of the of radiation origin may be the main driving force for grain body where there are dislocation loops; the nucleation and growth of vacancy voids. • the postirradiation annealing of the structure with dislocation loops which was formed by References irradiating with 50 MeV a- particles and fission 1. Cawthorne C.Fulton E. Voids in irradiated stainless neutrons at 60°C results in the growth of steel. //Nature. 1967.216.N.5115.p.575-576. vacancy voids. 2. Hishinuma A.,.Mansur L.K. Critical radius for bias- The foregoing gives grounds to state that high driven swelling - a further analysis and its application temperature annealing or irradiation at high tobimodal cavity size distributions. // J. Nucl. Mat. temperature (more than 0.3T n) constitutes the 1983. 118.N1. p.91-99. me 3. Schwirtlich I.A,.Schulz H. Quanching and recovery conditions under which dislocation loops are experiment on Mo. // Phil. Mag. 1980. 42. N5. p.601- powerful sources of vacancies. Therefore, on 611. condition that in the crystal structure of the irradiated material there are groups of the HemVn type, vacancy oversaturation accounted for by

4-H Me3icd)mapodHaH Kompepemiux «B3auModeucmeue iBJiynemtu c meepdbiM menoM», 3-5 oKmstdpn 2001 z., MUHCK, Eempycb 4-th International Conference ((Interaction of Radiation with Solids», October 3-5, 2001, Minsk, Belarus