Journal of Non-Crystalline Solids 50 (1982) 71-78 71 North-Holland Publishing Company
L O W T E M P E R A T U R E E L E C T R O N I R R A D I A T I O N O F A M O R P H O U S A N D C R Y S T A L L I N E F e - B A L L O Y S
A. A U D O U A R D , J. B A L O G H *, J. D U R A L ** a n d J.C. J O U S S E T
Centre d'Etudes Nucl~aires de Saclay, lnstitul National des Sciences et Techniques Nuclbaires, 91191 Gif sur Yvette Cedex, France
Received 4 March 1981
Revised manuscript received 20 December 1981
Three amorphous Fe-B based alloys and crystalline Fe3B alloy were irradiated at a temperature of 21 K with 2.5 MeV electrons. The irradiation induced increase of electrical resistivity (Ap) was measured during irradiation. The damage produced was analysed with the conventional production curve A/~ = d(Ap)/d(~pt) versus A O where ~0t is the electron dose. After irradiation, isochronal annealings were performed up to room temperature. The behaviour of the four alloys was found to be similar both during irradiation and annealing.
For all the alloys: the production curve is linear at high doses; and the induced increase of electrical resistivity Ap anneals out at low temperature following different stages. These results are interpreted in terms of the creation of well defined defects.
These defects have the same nature in amorphous and crystalline alloys; i.e. probably of vacancy type.
1. I n t r o d u c t i o n
T h e k n o w l e d g e of the s t r u c t u r e of a m o r p h o u s alloys has a f u n d a m e n t a l role in o u r u n d e r s t a n d i n g of their physical properties. Several s t r u c t u r a l m o d e l s such as m i c r o c r y s t a l l i n e [ 1 ], dense r a n d o m p a c k i n g [2], t e t r a g o n a l close p a c k i n g [3], lattice defect [4] a n d q u a s i c r y s t a l l i n e [5] m o d e l s have b e e n proposed. T h e p i c t u r e is still far from b e i n g clear.
A l t h o u g h there is n o long r a n g e p e r i o d i c i t y i n a m o r p h o u s materials, the existence of a short r a n g e order has b e e n suggested i n m a n y e x p e r i m e n t s [6] so it is possible to i m a g i n e a defect as a d e v i a t i o n from the short r a n g e order.
I r r a d i a t i o n is the usual way to p r o d u c e defects. It is k n o w n [8] that the low t e m p e r a t u r e i r r a d i a t i o n of a m o r p h o u s alloys leads to d a m a g e which has b e e n i n t e r p r e t e d in terms of i n d u c e d s t r u c t u r a l defects. T h e c o m p a r i s o n b e t w e e n the i n d u c e d d a m a g e in a m o r p h o u s alloys a n d their crystalline c o u n t e r p a r t s c a n
* Present address: Central Research Institute for Physics, H-1525 Budapest, PO Box 49, Hungary.
** Centre d'Etudes Nucl+aires de Fontenay aux Roses, Section d'Etudes des Solides Irradi+s, BP no. 6, 92260 Fontenay aux Roses, France.
0 0 2 2 - 3 0 9 3 / 8 2 / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 7 5 © 1982 N o r t h - H o l l a n d
72 A. Audouard et al. / Low temperature electron irradiation of F e - B alloys
give more information on the nature of the induced damage in amorphous alloys and, perhaps, indirectly help improve our knowledge of the amorphous structure.
In previous publications, low temperature irradiation experiments on metal-metalloid glasses with 235U fission fragments [7,8], I°B fission fragments [8], a particles [9], ions [10] and electrons [8] have been reported.
In this paper, we study the effect of 2.5 MeV electrons on both amorphous and crystalline F e - B alloys.
2 . E x p e r i m e n t a l p r o c e d u r e s
The samples were irradiated in the V I N K A C facility of the Fontenay aux Roses Van de Graaff accelerator at a temperature of 21 K [11]. The instanta- neous flux was 4 × 1014 e c m -2 s -1.
Four alloys were studied:
(1) and (2) a-FesoB20 and a-Fe78Mo2B20 amorphous alloys as provided by Allied Chemical Co.
(3) a-Fe75B25 amorphous alloy elaborated at CRIP in Budapest by rapid quenching from the melt.
(4) c-Fe 3 B crystalline alloy obtained from the crystallisation of the a-Fe75 B25 alloy.
The crystallisation of the amorphous samples was performed in a calorime- ter at 680 K with a heating duration of about 20 min. The samples were then checked by MiSssbauer spectroscopy in order to verify that the stable Fe2B phase had not yet appeared.
Several samples from each alloy were irradiated:
- six c-F%B samples: Po = (30 -+ 7) ~f~ cm; R R R -- 1.50 -+ 0.15;
- three a-Fe75B25 samples: P0 = (115 -+ 10) #~2 cm; R R R = 1.04 -+ 0.01 ;
- three a-FresoB2o samples: Po = (150 + 15) #~2 cm; R R R = 1.04 -+ 0.01;
- three a-Fev8MozB2o samples: P0 = (150 + 15) ~fl cm; R R R -- 1.04 -+ 0.01.
P0 is the initial resistivity measured at 20 K and R R R is the ratio between the room temperature resistivity and Po. The uncertainty of Po is mainly due to the uncertainty of the shape factor.
The sample holder was mounted with three samples for each irradiation experiment. The electron flux was heterogeneous so the samples were not irradiated with the same dose. As we have only measured the average flux, we only get the electron dose for each sample within an estimated uncertainty of about 20%.
3 . R e s u l t s
3.1. Damage production
Fig. 1 shows four typical production curves (Ap, ~ot) for one sample of each type. Ap is the irradiation induced increase of the electrical resistivity in ~f~
A. Audouard et al. / Low temperature electron irradiation of F e - B alloys 73
" i Q . , e l
81 ~ a
/
2
o ~ ~ - ~ T a ~ . . .
O ¸
o
s~.,~1~
(e.cm-2~
Fig. 1. Ap(21 K) versus ~t damage production curves for each type of alloy: a-Fe75BEs, c-FesB, a-FeaoB20 and a-FevsMo2B20.
cm, ~t is the electron dose in e c m -2. All the samples exhibit the same behaviour: a saturation effect is observable o n each curve as a deviation f r o m linearity. In order to analyse the saturation process, the conventional variation of A/5 versus Ap was plotted. Aib = d ( A p ) / d ( q ~ t ) is expressed in #fl c m 3 e-1.
Figs. 2 a n d 3 refer to a-Fes0 [120 and c-Fe 3 B respectively. All the a m o r p h o u s alloys exhibit a similar b e h a v i o u r so, for the sake of clarity, we have only plotted one of the a-Fea0B20 (A/5, Ap) curves in fig. 2.
It is possible to distinguish two parts in the curves for high a n d low doses respectively:
- at high doses, the curve can be assimilated to a straight line;
- at low doses, Ap decreases very rapidly versus Ap.
0.6
E0.4
~0.,~
o ' ~ '
.~
&P(p~cm)
Fig. 2. Ah =d(Ap)/dq~t versus Ap damage production rate for a-FesoB2o alloy. The full line is the best double exponential fit (see section 3.1.2).
74 A. A udouard et al. / Low temperature electron irradiation of F e - B alloys
3.1.1. High doses
The linear dependence of A~ upon Ap often occurs in the case of the low temperature irradiation of crystalline metals. In this case, the damage rate is expressed as [12]:
A ~ = OdiO d - - 200~td Aio , ( 1 )
Where o a is the displacement cross section; Pd is the resistivity of the irradia- tion induced defect and v 0 is the recombination volume. In this expression it is assumed that h p = CPa, C being the defect concentration. Eq. (1) neglects the subthreshold effects and the overlapping of the recombination volume but it is a rather good approximation for our curves. It supposes that irradiation induces the creation of well defined defects. So, it is not surprising that the c-Fe3B (Ab, Ap) curve agrees well with this equation. In the case of amorphous alloy irradiation, because of the unknown nature of the damage, it is difficult a priori to know if the model applies and if the parameters have a physical meaning. Nevertheless as the amorphous and the crystalline alloys exhibit very similar behaviour under irradiation, we have analysed all the curves in the same way, assuming that well defined defects are present in the irradiated amorphous samples (see discussion).
Table 1 gives both the increase in saturation resistivity Aps = P d / 2 V 0 and the VoO d values. The amount of the uncertainties are given for both parameters.
They arise from the uncertainties both in the flux determination which is estimated to be 20~ and in the resistivity measurements. The first results of threshold energy determination experiments, in progress for the same alloys, indicate that the displacement cross sections have the same value in both the crystalline and amorphous samples. It can then be seen from table 1 that:
(v0) crystal ~ 1.5 to 3 (v0) amorphous, (Pa) crystal ~ 10 (pd) amorphous.
12
O
a~ x
0
c - F e 3 B
, , , ,
Ap (p~cm I Fig. 3. Same as fig. 2 for c-Fe3B alloy.
A. A udouard et al. / Low temperature electron irradiation of F e - B alloys Table 1
Fitting parameters of eq. (1) for each type of alloy
75
rood(10 -20 cm 2 e-1) mps = pd/2Vo (tt~2 cm)
a- Feso B2o 0.60 "+" 0.20 4.0 ± 1.5
a-FeTs Mo2 B2o 0.35-+0.10 2.8-+ 1.1
a-Fev5 B25 0.45±0.15 2.2-+ 1.0
C-Fe3B 0.95±0.10 10 -+3
3.1.2. Low doses
Aib decreases strongly at the beginning of irradiation. Several interpretations may be suggested:
(i) the creation of irradiation induced migrating defects: similarly to the crystalline metal case where if irradiation is conducted at a high enough temperature the motion of interstitials is possible. This situation can be described by the "unsaturable trap model" [13]. This model assumes that, during the irradiation of a "pure" metal, there is competition between the creation of defects (interstitials and vacancies) and the annihilation of mobile defects (interstitials) either on vacancies or on unsaturable traps such as impurities, dislocations, grain boundaries, etc. A 4.2 K irradiation, now in progress, indicates that some induced defects are indeed mobile below 20 K.
(ii) Two types of defects are Created during the irradiation; one of them being responsible for the strong decrease of A/~ at low dose, the other being responsible for the A/~ high dose linear decreasing. In this case, an analysis in terms of recombination volumes can be made for the whole dose range as was used for the high dose region. Let od, p~ and p,~ be the displacement cross section the recombination volume and the resistivity of a defect, respectively, for the defect responsible of the Aib low dose rapid decrease. We get [12]:
dC . . . . od(1 -- 2voC ) (2)
d~0t and
dC' dcpt = °'d(1 - 2v~C'), (3)
C and C' being the concentrations of the defects. The resistivity increase can then be expressed as:
Ap = pd C + p~C'. (4)
With relations (2) and (3), it follows that:
[1
' 'Ap ~ -exp(-2VoOdq~t)] + 2v---~o , - e x p - 2VoOdq~t ), (5) A~b = o d Pd exp( -- 2 VoOd~t) + o d O~ exp( -- 2 v~od~pt). (6)
76 A. Audouard et a L / Low temperature electron irradiation of F e - B alloys
With such an analysis, one gets the double exponential behaviour of Ap and A/~
with respect to the dose indicated by the relations (5) and (6). As there are six parameters for the fit and only four relations it is impossible to draw out significant values. Anyway, we have qualitatively verified that this double exponential fit works very well as can be seen in figs. 2 and 3 where the continuous line is the theoretical fit.
3.2. Annealing behaviour
The results of ten minutes isochronal annealings are shown in fig. 4 as the variation of A R / A R o versus T for c-Fe3B and a-Fe7sB25 and in fig. 5 as the variation of (d/dT)(AR/Ro) versus T. AR 0 is the irradiation induced increase of the electrical resistance and AR the remaining increase measured at 20 K after the annealing at the temperature T.
As all the samples from the same alloy exhibit exactly the same behaviour, we have only plotted the results for one sample of each alloy. The error in ( d / d T ) ( A R / A R o ) arises mainly from the error in annealing temperatures.
The relative uncertainty can be estimated to be less than 0.1.
The c-Fe3B alloy exhibits a sharp recovery stage around 90 K. The amorphous samples exhibit a broad stage in the range from 50 to 100 K.
From these results, we can note the following points:
- the annealing behaviour is quite similar for all the amorphous samples;
- the low temperature stage lies in the same temperature range in amorphous and crystalline samples; - this low temperature stage is sharper and better defined in crystalline than in amorphous samples.
|
.81 .6]
.z
| '
.6 .4 .2
0 600
• g O 0 000 o
0 O 0 i~) 0
Oo 00000 O 0
• • o o o ° o o
0 ° ° 0 o Oo
300T~K)400
f - 4 - 2 ,,,'l~ 0
10o 2 0 o !0o 2 0 0 " :300 " 4 0 0
T(K)
Fig. 4. Annealing curves for c-Fe3B(-O- ) and a-Fe75B25 (-O-) alloys.
Fig. 5. Derivative annealing curves for each type of alloy: c-Fe3B (-O-); a-FeTsB25 (-O-):
a-FeTsMO2B2o (-II-) and a-FesoB2o (-V-).
A. Audouard et al. / Low temperature electron irradiation of F e - B alloys 77
4. Discussion and Conclusion The most striking results are:
(i) the irradiation damage can be analysed in the same way in amorphous and crystalline metallic alloys, with the classical saturation eq. (1).
(ii) The damage anneals out at low temperature.
(iii) At least one defined annealing stage exists in the temperature range from 50 to 100 K in the case of amorphous alloys and around 90 K in the case of the c-Fe3B alloy.
(iv) The behaviour of the three amorphous alloys is similar both during irradiation (table 1) and annealing (fig. 5).
These results, particularly the analogous behaviour of both the crystalline and amorphous alloys under irradiation and during annealing, show that t h e 21 K electron irradiation has induced the creation of well defined stable defects in the amorphous structure. They are probably vacancies in the short range order. It is clear that this situation is different from what it was assumed by Spaepen [14] for the higher temperature region: a vacancy immediately collapses and is spread over the amorphous structure. On the other hand, the possibility of the annihilation of such defects has already been suggested by Chen [15] in the case of the room temperature electron irradiation of a-PdCuSi alloy.
The irradiation induced creation of such defects is responsible for the increase of Ap. The saturation phenomenon can consequently be analysed exactly as in the case of crystalline alloys.
The annealing stages are due to the migration of atoms towards the close neighbour vacancies which leads to the annihilation of these defects. As, even at 20 K, different energetically equivalent configurations probably exist in amorphous metallic alloys, it is probable that a vacancy type defect does not have a well defined migration energy but rather a spectrum of migration energies which leads to a broadening of the annealing stages. Let us remark that the low temperature annealing stage occurs around 90 K for crystalline and amorphous alloys, at about the same temperature at which, in iron, the immobile vacancies are annihilated by the migration of interstitials [ 15].
The question still remains whether or not interstitials exist in amorphous materials. Our experiments do not give any indication on this question.
Enough free volume probably exists to allow the local relaxation of such an interstitial. In this case, irradiation would also lead to chemical disorder.
Others experiments are necessary to answer this important question. On the other hand, the existence of an irradiation induced defect, mobile at 21 K, is possible according to the beginning of the (At~, Ap) curves. The preliminary results of a 4.2 K electron irradiation indicate that his is actually the case.
The authors are very grateful to Dr. C. Janot of the University of Nancy I and to Dr. D. Lesueur of the CEN Fontenay aux Roses for many helpful discussions. Many thanks are also due to M.A. Lovas and M.K. Zambo-Balla
78 A. ,4udouard et al. / Low temperature electron irradiation of Ire- B alloys
o f C R I P B u d a p e s t f o r t h e i r e x p e r i m e n t a l h e l p in m a k i n g a n d c h a r a c t e r i s i n g t h e s a m p l e s .
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