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Irradiation effects on MoSi and PdSi amorphous alloys by means of positron annihilation
Nuclear Instruments and Methods 199 (1982) 393-395 North-Holland Publishing C o m p a n y
393
I R R A D I A T I O N EFFECTS O N M o - S i A N D P d - S i A M O R P H O U S ALLOYS BY M E A N S OF POSITRON ANNIHILATION
Fumitake ITOH ~, Toshiharu F U K U N A G A ~, Masayuki HASEGAWA 2 and Kenji SUZUKI 1 I The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai 980, Japan 2 The Oarai Branch, The Research Institute for Iron, Steel and Other Metals, Tohoku University, lbaraki 311-13, Japan
Fast neutron irradiation effects on M o - S i amorphous alloy and Pd-Si glassy alloy were studied by means of positron annihilation and X-ray diffraction measurements. Changes of positron annihilation characteristics in these amorphous alloys due to the neutron irradiation are discussed in correlation with structural changes of the atomic arrangement in these amorphous alloys and it is concluded that positrons are trapped in opening spaces inherent to the amorphous structure in these alloys.
1. Introduction
It has been suggested that positron annihilation in amorphous metal-metalloid alloys can be characterized by a shallow trapping in the vacancy-like sites which exist in amorphous alloys [1-3]. This implies that the structural change of amorphous alloys can be reflected in the positron annihilation characteristics. From this point of view, it would be of interest to study the irradiation effect of amorphous alloys by means of a positron annihilation method in connection with the structural change because the irradiation causes the change in the local atomic arrangement of amorphous alloys [4]. In this paper, we report fast neutron irradiation effects on the positron annihilation characteristics as well as the atomic arrangement for M045Si55 amorphous alloy (a-M045Si55) and Pd soSi 20 glassy alloy (g-PdsoSi 20).
2. Experimental
Mo45Si55 amorphous alloy in a form of 20 m m × 20 m m × 0 . 1 - 0 . 5 mm was prepared by a high-rate dc sputter deposition technique [5] and Pds0Si20 glassy alloy was prepared by rapid quenching from melts under an Ar atmosphere of 100 Torr into a form of long ribbons 30 #m thick and 1 mm wide. Parts of these samples were irradiated by fast neutrons up to a fluence of 9 × 1018 n / c m 2 ( > 1 MeV) in the Japan Material Testing Reactor. The sample temperature was kept
below 150°C during the neutron irradiation. The angular correlation curves of 2y-positron annihilation were observed using a conventional long-split type apparatus and the isotope of 1 Ci 64Cu as a positron emitter [6]. The positron lifetime measurements were made using a spectrometer consisting of the fast-slow coincidence circuit with the time resolution of 300 ps in fwhm of 6°Co prompt curve. The isotope used in the lifetime measurements was 22Na(10-50 /~Ci) sandwiched by a couple of thin Mylar foils. X-ray diffraction measurements were carried out using a conventional 0-20 diffractometer in a reflection mode. The radiation used was MoK~ X-ray emitted from an intense rotating anode.
3. Results
The effect of neutron irradiation on the angular correlation curve N(O) for a-M045Si55 is shown in fig. 1 together with that for Mo crystal. The N(O) for a-M045Si55 becomes narrower on irradiation but the change is very small compared with that for pure Mo crystal which was irradiated under the same conditions as in the present study. A similar tendency has been found for a-Mo6sSi32
[4]. Fig. 2 shows the N(O) for g-Pd80Si20 before and after the neutron irradiation. A small narrowing of the N(O) by the neutron irradiation can be seen. In table 1, we show positron lifetime data for a-Mo45Siss and g-PdsoSi20 alloy. The positron
0167-5087/82/0000-0000/$02.75 © 1982 North-Holland
vii. RADIATION EFFECTS
394
IC~ ltoh et a L /
Irradiation ef/~'cts on a m o r p h o u s alloys"
Table 1 Positron lifetime data for a-Mo45Si55 and g-Pd~oSi2o before and after the neutron irradiation. The data for a-M%sSi32 [4] are also included
a-Mo45Si55 • o
~-'lk.
\.°"~.
Mo (after S.SHIGA)
\ oL. \ ~",
I~J
as p r e p a r e d irradiated by neutron
........
< cc
pre )ared
as
by neutron
rradiated
i.z LU 0 (J Z
\
°' o
c.)
Sample
Before (ps)
After (ps)
a-Mo6~ Si 32 a-Mo45 Si 35 g-PdsoSi20
189.4 ~ 1.7 197.7 + 4.3 160.4+: 1.3
196.4 ~ 2.1 194.9 ~ 3.5 150.3 ~ 1.9
,
\.
• \.s
\° . . . . .
~
. ~ . 2..2,.:.~:~,.~.
10 ANGLE ( m r a d )
15
Fig. 1. Angular correlation curves for Mo45Si55 amorphous alloy before and after the neutron irradiation. The data for pure Mo crystal were taken from ref. 7. All the curves are normalized to give the same peak height.
lifetime spectrum was well analyzed by a three component analysis, two of which were source components of about 400 ps and 2000 ps. It is noteworthy that the positron lifetime for a-M068Si 32 increases due to the neutron irradiation in accordance with the narrowing of the angular
I
correlation curve [4], while the positron lifetimes for a-Mo45Si55 and g-PdsoSi20 decrease though their angular correlation curves become sharper as shown in figs. 1 and 2. Fig. 3 shows the radial distribution function ( R D F ) which is the Fourier transform of the structure factor S(Q) truncated at Q m a x - 14,~ ]. It can be found in fig. 3(A) that the structure in a-Mo45Si55 is modified by the neutron irradiation in such a way that three subpeaks in the first peak of the R D F for as prepared a-Mo45Si55 shift toward new higher values of atomic distances, while the R D F curve becomes more blurred beyond
60
~
T
1
(A) Mo455i55 amorphous as prepared ............ irradiated by
40
I
r
/ /
PdsoSi2o, g l a s s y
•
20 ~ 3k
..........
"~
as p r e p a r e d irradiated
u_ Q 0 --
(B) Pd80Si20 glassy
(]) z
- -
40
0
I
5
i
10 0 ( mrad )
15
Fig. 2. Angular correlation curves for PdsoSi20 glassy alloy before and after the neutron irradiation. Both curves are normalized to give an equal area under the peak.
o
0
/ /
as prepared
........... irradiated by
1
2
3
4
5
6
R(A) Fig. 3. The radial distribution function (RDF) for (A) Mo4~Sis5 amorphous alloy and (B) PduoSi2o glassy alloy before and after the neutron irradiation.
F. hoh et al. / Irradiation effects on amorphous alloys
R = 4,~. It has been observed that these features become more clear for a-Mo45Si55 irradiated by electrons (2.5 MeV) at liquid N 2 temperature, which will be published elsewhere. Therefore, these observations suggest that some kind of short-range order in a-Mo45Si55 is enhanced by the neutron irradiation, while the long-range periodic-like order is decreased. This fact is similar to the structural change observed for neutron irradiated a-Mo68Si32 [41. On the contrary, the structural change for gPdsoSi32 by the neutron irradiation is small compared with that for a-Mo45Si55 as shown in fig. 3(B). The short-range order in the atomic arrangement suffers no appreciable change and the blurring of the R D F at larger atomic distances is smaller than that found for a-Mo45Si55. This difference of structural change between a-Mo45si55 and g-PdsoSi20 m a y be due to differences in the preparation m e t h o d of the a m o r p h o u s alloy; that is to say, the sample prepared by the sputter deposition technique m a y have a more irregular and defective structure than that prepared by the melt quenching technique. Therefore, a-Mo45Si55 is rather more susceptible to structural modification as a result of neutron irradiation than is g- Pd 80Si 20.
4. Discussion
It should be emphasized that the angular correlation curves for a-Mo45Si55 and g-PdsoSi20 become narrower as a result of the neutron irradiation (figs. 1 and 2) in spite of the decrease in the positron lifetime for these a m o r p h o u s alloys (table 1). This fact seems apparently inconsistent given that there exists only one positron state,
395
because the p o s i t r o n - e l e c t r o n pair yielding a narrower angular correlation curve could give a longer lifetime component. Therefore, this implies that various kinds of positron state exist in these a m o r p h o u s materials. According to the shallow trapping picture in a m o r p h o u s materials [1-3], positrons are likely to sample electrons in various sizes of opening spaces inherent to the a m o r p h o u s structure with appropriate weighting factors. Therefore, the shortening of the positron lifetime and the narrowing of the angular correlation curve may be mainly due to the change in the size distribution of the opening spaces capable of positron trapping. The relation between the positron annihilation characteristics and the structure before and after the neutron irradiation is not yet clear. In order to verify this conjecture, a full analysis of the atomic arrangement is needed. Further studies on these lines are in progress.
References
[1] K. Suzuki and F. Itoh, Proc. 5th Int. Conf. Positron annihilation, eds., R. Hasiguti and K. Fujiwara (The Japan Institute of Metals, 1979) p. 861. [2] W. Triftsh~user and G. K0gel, Proc. Conf. Metallic glasses: science and technology, Budapest, vol. l (1980) p. 347. [3] G. K0gel, J. Winter and W. Triftsh~tuser, Proc. Conf. Metallic glasses: science and technology, Budapest, vol. 1, (1980) p. 311. [4] F. Itoh, S. Ikeda, M. Ikebe, M. Hasegawa, T. Honda, T. Fukunaga, H. Fujimori and K. Suzuki, Proc. 4th Int. Conf. Rapidly quenched metals, eds., T. Masumoto and K. Suzuki (The Japan Institute of Metals, 1982) p. 763. [5] S. Ikeda, H. Fujimori and K. Suzuki, Proc. 4th Int. Conf. Rapidly quenched metals, eds., T. Masumoto and K. Suzuki (The Japan Institute of Metals, 1982) p. 1253. [6] F. Itoh, J. Phys. Soc. Jpn 41 (1976) 824. [7] S. Shiga, Master Dissertation, Tohoku University (1979).
VII. RADIATION EFFECTS
393
I R R A D I A T I O N EFFECTS O N M o - S i A N D P d - S i A M O R P H O U S ALLOYS BY M E A N S OF POSITRON ANNIHILATION
Fumitake ITOH ~, Toshiharu F U K U N A G A ~, Masayuki HASEGAWA 2 and Kenji SUZUKI 1 I The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai 980, Japan 2 The Oarai Branch, The Research Institute for Iron, Steel and Other Metals, Tohoku University, lbaraki 311-13, Japan
Fast neutron irradiation effects on M o - S i amorphous alloy and Pd-Si glassy alloy were studied by means of positron annihilation and X-ray diffraction measurements. Changes of positron annihilation characteristics in these amorphous alloys due to the neutron irradiation are discussed in correlation with structural changes of the atomic arrangement in these amorphous alloys and it is concluded that positrons are trapped in opening spaces inherent to the amorphous structure in these alloys.
1. Introduction
It has been suggested that positron annihilation in amorphous metal-metalloid alloys can be characterized by a shallow trapping in the vacancy-like sites which exist in amorphous alloys [1-3]. This implies that the structural change of amorphous alloys can be reflected in the positron annihilation characteristics. From this point of view, it would be of interest to study the irradiation effect of amorphous alloys by means of a positron annihilation method in connection with the structural change because the irradiation causes the change in the local atomic arrangement of amorphous alloys [4]. In this paper, we report fast neutron irradiation effects on the positron annihilation characteristics as well as the atomic arrangement for M045Si55 amorphous alloy (a-M045Si55) and Pd soSi 20 glassy alloy (g-PdsoSi 20).
2. Experimental
Mo45Si55 amorphous alloy in a form of 20 m m × 20 m m × 0 . 1 - 0 . 5 mm was prepared by a high-rate dc sputter deposition technique [5] and Pds0Si20 glassy alloy was prepared by rapid quenching from melts under an Ar atmosphere of 100 Torr into a form of long ribbons 30 #m thick and 1 mm wide. Parts of these samples were irradiated by fast neutrons up to a fluence of 9 × 1018 n / c m 2 ( > 1 MeV) in the Japan Material Testing Reactor. The sample temperature was kept
below 150°C during the neutron irradiation. The angular correlation curves of 2y-positron annihilation were observed using a conventional long-split type apparatus and the isotope of 1 Ci 64Cu as a positron emitter [6]. The positron lifetime measurements were made using a spectrometer consisting of the fast-slow coincidence circuit with the time resolution of 300 ps in fwhm of 6°Co prompt curve. The isotope used in the lifetime measurements was 22Na(10-50 /~Ci) sandwiched by a couple of thin Mylar foils. X-ray diffraction measurements were carried out using a conventional 0-20 diffractometer in a reflection mode. The radiation used was MoK~ X-ray emitted from an intense rotating anode.
3. Results
The effect of neutron irradiation on the angular correlation curve N(O) for a-M045Si55 is shown in fig. 1 together with that for Mo crystal. The N(O) for a-M045Si55 becomes narrower on irradiation but the change is very small compared with that for pure Mo crystal which was irradiated under the same conditions as in the present study. A similar tendency has been found for a-Mo6sSi32
[4]. Fig. 2 shows the N(O) for g-Pd80Si20 before and after the neutron irradiation. A small narrowing of the N(O) by the neutron irradiation can be seen. In table 1, we show positron lifetime data for a-Mo45Siss and g-PdsoSi20 alloy. The positron
0167-5087/82/0000-0000/$02.75 © 1982 North-Holland
vii. RADIATION EFFECTS
394
IC~ ltoh et a L /
Irradiation ef/~'cts on a m o r p h o u s alloys"
Table 1 Positron lifetime data for a-Mo45Si55 and g-Pd~oSi2o before and after the neutron irradiation. The data for a-M%sSi32 [4] are also included
a-Mo45Si55 • o
~-'lk.
\.°"~.
Mo (after S.SHIGA)
\ oL. \ ~",
I~J
as p r e p a r e d irradiated by neutron
........
< cc
pre )ared
as
by neutron
rradiated
i.z LU 0 (J Z
\
°' o
c.)
Sample
Before (ps)
After (ps)
a-Mo6~ Si 32 a-Mo45 Si 35 g-PdsoSi20
189.4 ~ 1.7 197.7 + 4.3 160.4+: 1.3
196.4 ~ 2.1 194.9 ~ 3.5 150.3 ~ 1.9
,
\.
• \.s
\° . . . . .
~
. ~ . 2..2,.:.~:~,.~.
10 ANGLE ( m r a d )
15
Fig. 1. Angular correlation curves for Mo45Si55 amorphous alloy before and after the neutron irradiation. The data for pure Mo crystal were taken from ref. 7. All the curves are normalized to give the same peak height.
lifetime spectrum was well analyzed by a three component analysis, two of which were source components of about 400 ps and 2000 ps. It is noteworthy that the positron lifetime for a-M068Si 32 increases due to the neutron irradiation in accordance with the narrowing of the angular
I
correlation curve [4], while the positron lifetimes for a-Mo45Si55 and g-PdsoSi20 decrease though their angular correlation curves become sharper as shown in figs. 1 and 2. Fig. 3 shows the radial distribution function ( R D F ) which is the Fourier transform of the structure factor S(Q) truncated at Q m a x - 14,~ ]. It can be found in fig. 3(A) that the structure in a-Mo45Si55 is modified by the neutron irradiation in such a way that three subpeaks in the first peak of the R D F for as prepared a-Mo45Si55 shift toward new higher values of atomic distances, while the R D F curve becomes more blurred beyond
60
~
T
1
(A) Mo455i55 amorphous as prepared ............ irradiated by
40
I
r
/ /
PdsoSi2o, g l a s s y
•
20 ~ 3k
..........
"~
as p r e p a r e d irradiated
u_ Q 0 --
(B) Pd80Si20 glassy
(]) z
- -
40
0
I
5
i
10 0 ( mrad )
15
Fig. 2. Angular correlation curves for PdsoSi20 glassy alloy before and after the neutron irradiation. Both curves are normalized to give an equal area under the peak.
o
0
/ /
as prepared
........... irradiated by
1
2
3
4
5
6
R(A) Fig. 3. The radial distribution function (RDF) for (A) Mo4~Sis5 amorphous alloy and (B) PduoSi2o glassy alloy before and after the neutron irradiation.
F. hoh et al. / Irradiation effects on amorphous alloys
R = 4,~. It has been observed that these features become more clear for a-Mo45Si55 irradiated by electrons (2.5 MeV) at liquid N 2 temperature, which will be published elsewhere. Therefore, these observations suggest that some kind of short-range order in a-Mo45Si55 is enhanced by the neutron irradiation, while the long-range periodic-like order is decreased. This fact is similar to the structural change observed for neutron irradiated a-Mo68Si32 [41. On the contrary, the structural change for gPdsoSi32 by the neutron irradiation is small compared with that for a-Mo45Si55 as shown in fig. 3(B). The short-range order in the atomic arrangement suffers no appreciable change and the blurring of the R D F at larger atomic distances is smaller than that found for a-Mo45Si55. This difference of structural change between a-Mo45si55 and g-PdsoSi20 m a y be due to differences in the preparation m e t h o d of the a m o r p h o u s alloy; that is to say, the sample prepared by the sputter deposition technique m a y have a more irregular and defective structure than that prepared by the melt quenching technique. Therefore, a-Mo45Si55 is rather more susceptible to structural modification as a result of neutron irradiation than is g- Pd 80Si 20.
4. Discussion
It should be emphasized that the angular correlation curves for a-Mo45Si55 and g-PdsoSi20 become narrower as a result of the neutron irradiation (figs. 1 and 2) in spite of the decrease in the positron lifetime for these a m o r p h o u s alloys (table 1). This fact seems apparently inconsistent given that there exists only one positron state,
395
because the p o s i t r o n - e l e c t r o n pair yielding a narrower angular correlation curve could give a longer lifetime component. Therefore, this implies that various kinds of positron state exist in these a m o r p h o u s materials. According to the shallow trapping picture in a m o r p h o u s materials [1-3], positrons are likely to sample electrons in various sizes of opening spaces inherent to the a m o r p h o u s structure with appropriate weighting factors. Therefore, the shortening of the positron lifetime and the narrowing of the angular correlation curve may be mainly due to the change in the size distribution of the opening spaces capable of positron trapping. The relation between the positron annihilation characteristics and the structure before and after the neutron irradiation is not yet clear. In order to verify this conjecture, a full analysis of the atomic arrangement is needed. Further studies on these lines are in progress.
References
[1] K. Suzuki and F. Itoh, Proc. 5th Int. Conf. Positron annihilation, eds., R. Hasiguti and K. Fujiwara (The Japan Institute of Metals, 1979) p. 861. [2] W. Triftsh~user and G. K0gel, Proc. Conf. Metallic glasses: science and technology, Budapest, vol. l (1980) p. 347. [3] G. K0gel, J. Winter and W. Triftsh~tuser, Proc. Conf. Metallic glasses: science and technology, Budapest, vol. 1, (1980) p. 311. [4] F. Itoh, S. Ikeda, M. Ikebe, M. Hasegawa, T. Honda, T. Fukunaga, H. Fujimori and K. Suzuki, Proc. 4th Int. Conf. Rapidly quenched metals, eds., T. Masumoto and K. Suzuki (The Japan Institute of Metals, 1982) p. 763. [5] S. Ikeda, H. Fujimori and K. Suzuki, Proc. 4th Int. Conf. Rapidly quenched metals, eds., T. Masumoto and K. Suzuki (The Japan Institute of Metals, 1982) p. 1253. [6] F. Itoh, J. Phys. Soc. Jpn 41 (1976) 824. [7] S. Shiga, Master Dissertation, Tohoku University (1979).
VII. RADIATION EFFECTS