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Selective radiation damage in the long period ordered Au4Zn alloy
Ultramicroscopy 00 (1977) 2 (1977) 385-387 © North-Holland Publishing Company
385
SELECTIVE RADIATION DAMAGE IN THE LONG PERIOD ORDERED Au4Zn ALLOY * G. Van TENDELOO, J. Van LANDUYT and S. AMELINCKX ** Ri/ksuniversitair Centrum Antwerpen, Middelheimlacn 1, B-2020 Antwerp, Belgium Received 27 May 1977
In the course of a high-voltage electron microscopic study of ordering phenomena in the gold zinc alloy Au4Zn a very striking radiation damage phenomenon was observed. After annealing at 480°C during one day the alloy was quenched to room temperature, further annealed at 300°C during one day and finally at 220 ° during 3 days. The resulting microstructure is shown in fig. 1a. It consists of a domain pattern each domain exhibiting in turn a long periodic antiphase boundary (LPAPB) structure. Half of the domains exhibit a one dimensional LPAPB structure; the other half a two-dimensional LPAPB structure. Fig. 2 shows schematically the alternation of domains of one- and two-dimensional LPAPB structures. The one-dimensional LPAPB structure has tetragonal symmetry, whereas the two-dimensional LPAPB structure causes an orthorhombic lattice deformation. This remarkable microstructure apparently minimizes the strains resulting from the lattice deformations due to ordering. When specimens exhibiting such a microstructure are examined at an accelerating voltage of 1000 kV radiation damage becomes visible after a few seconds of irradiation with a beam intensity of 14 A[cmZ. The damage consists of areas of black dot.~; sometimes also small loops are discernable. However the striking feature is that upon irradiation along the (010) direction, damage is only observed in the areas having the one-dimensional LPAPB structure and moreover preferentially in the vicinity of the domain boundaries (fig. lb). We initially assumed that
* Work performed under the auspices of the association RUCA-SCK. ** Also at SCK-CEN, 2400 Mol (Belgium).
Fig. 1 (a). "Checker board" domain structure as observed in Au4Zn before irradiation. (b) A similar area as in figure la after ixadiation with 1000 kV electrons along the (010)* directions. Alternating damaged and undamaged regions are produced.
386
G. van Tendeloo et al. / Radiation damage in the Au4Zn alloy t
Fig. 2. Schematic representation of the domain structu're in Au4Zn. The heavy lines indicate the domain walls, while fine lines inside the domains represent antiphase boundaries iri a one-dimensional or a two.dimensional LPAPB structure.
this was a pure contrast effect. However a systematic study under a variety of contrast conditions has shown unambiguously that the difference is o f structural origin i.e. in the t w o - d i m e n s i o n a l LPAPB almost no defect agglomerates are present as opposed to the onedimensional areas. Dark field images in superlattice Spots and dark field images in basic spots b o t h reveal the damage; whereas the first reveal both replacement and displacement damage, the second essentially map the displacement damage. High resolution observations imaging individual APB's in the LPAPB reveal two different types of damage. _The lattice fringes are deformed in the vicinity and within the black spots. Moreover short white "streaks" are produced parallel to the lattice fringes in the o n e - d i m e n s i o n a l LPAPB structure as indicated by means of the circles in fig. 3; they are in fact due to a small local increase of the fringe spacing.
Fig. 3. High resolution micrograph of a Au4Zn specimen which has been irradiated at 1000 kV. Radiation damage is mainly 6bserved here in the one-dimenSional LPAPB structure. Only exceptionally damage is observed in the two-dimensional area.
G. van Tendeloo et al. / Radiation damage in the Au4Zn alloy
The interpretation of these somewhat puzzling resuits is not yet clear. However, it does seem obvious that one has to accept a uniform production of point defects over the irradiated area of the foil; it is difficult in fact to imagine preferential production of primary knock-ons. The observed differences in behaviour between one- and t w o - d i m e n s i o n a l LPAPB structures must therefore be sought in the differences in migration and agglomeration of the radiation produced
387
defects. The presence of non-conservative APB's in the t w o - d i m e n s i o n a l LPAPB could play an important role. The preferential production of visible damage along the domain walls suggests that lattice effects such as focussing collisions and channelling, i.e., correlated collisions in general, seem to play a role. A detailed study of the ordering phenomena and of the radiation damage in Au4Zn will be published soon.
385
SELECTIVE RADIATION DAMAGE IN THE LONG PERIOD ORDERED Au4Zn ALLOY * G. Van TENDELOO, J. Van LANDUYT and S. AMELINCKX ** Ri/ksuniversitair Centrum Antwerpen, Middelheimlacn 1, B-2020 Antwerp, Belgium Received 27 May 1977
In the course of a high-voltage electron microscopic study of ordering phenomena in the gold zinc alloy Au4Zn a very striking radiation damage phenomenon was observed. After annealing at 480°C during one day the alloy was quenched to room temperature, further annealed at 300°C during one day and finally at 220 ° during 3 days. The resulting microstructure is shown in fig. 1a. It consists of a domain pattern each domain exhibiting in turn a long periodic antiphase boundary (LPAPB) structure. Half of the domains exhibit a one dimensional LPAPB structure; the other half a two-dimensional LPAPB structure. Fig. 2 shows schematically the alternation of domains of one- and two-dimensional LPAPB structures. The one-dimensional LPAPB structure has tetragonal symmetry, whereas the two-dimensional LPAPB structure causes an orthorhombic lattice deformation. This remarkable microstructure apparently minimizes the strains resulting from the lattice deformations due to ordering. When specimens exhibiting such a microstructure are examined at an accelerating voltage of 1000 kV radiation damage becomes visible after a few seconds of irradiation with a beam intensity of 14 A[cmZ. The damage consists of areas of black dot.~; sometimes also small loops are discernable. However the striking feature is that upon irradiation along the (010) direction, damage is only observed in the areas having the one-dimensional LPAPB structure and moreover preferentially in the vicinity of the domain boundaries (fig. lb). We initially assumed that
* Work performed under the auspices of the association RUCA-SCK. ** Also at SCK-CEN, 2400 Mol (Belgium).
Fig. 1 (a). "Checker board" domain structure as observed in Au4Zn before irradiation. (b) A similar area as in figure la after ixadiation with 1000 kV electrons along the (010)* directions. Alternating damaged and undamaged regions are produced.
386
G. van Tendeloo et al. / Radiation damage in the Au4Zn alloy t
Fig. 2. Schematic representation of the domain structu're in Au4Zn. The heavy lines indicate the domain walls, while fine lines inside the domains represent antiphase boundaries iri a one-dimensional or a two.dimensional LPAPB structure.
this was a pure contrast effect. However a systematic study under a variety of contrast conditions has shown unambiguously that the difference is o f structural origin i.e. in the t w o - d i m e n s i o n a l LPAPB almost no defect agglomerates are present as opposed to the onedimensional areas. Dark field images in superlattice Spots and dark field images in basic spots b o t h reveal the damage; whereas the first reveal both replacement and displacement damage, the second essentially map the displacement damage. High resolution observations imaging individual APB's in the LPAPB reveal two different types of damage. _The lattice fringes are deformed in the vicinity and within the black spots. Moreover short white "streaks" are produced parallel to the lattice fringes in the o n e - d i m e n s i o n a l LPAPB structure as indicated by means of the circles in fig. 3; they are in fact due to a small local increase of the fringe spacing.
Fig. 3. High resolution micrograph of a Au4Zn specimen which has been irradiated at 1000 kV. Radiation damage is mainly 6bserved here in the one-dimenSional LPAPB structure. Only exceptionally damage is observed in the two-dimensional area.
G. van Tendeloo et al. / Radiation damage in the Au4Zn alloy
The interpretation of these somewhat puzzling resuits is not yet clear. However, it does seem obvious that one has to accept a uniform production of point defects over the irradiated area of the foil; it is difficult in fact to imagine preferential production of primary knock-ons. The observed differences in behaviour between one- and t w o - d i m e n s i o n a l LPAPB structures must therefore be sought in the differences in migration and agglomeration of the radiation produced
387
defects. The presence of non-conservative APB's in the t w o - d i m e n s i o n a l LPAPB could play an important role. The preferential production of visible damage along the domain walls suggests that lattice effects such as focussing collisions and channelling, i.e., correlated collisions in general, seem to play a role. A detailed study of the ordering phenomena and of the radiation damage in Au4Zn will be published soon.