IN SITU HIGH VOLTAGE ELECTRON MICROSCOPE STUDIES OF ION- AND ELECTRON-BEAM INDuCED MODIFICATION OF MATERIALS*
P. R. Okamoto, N. Q. Lam and A. Taylor Materials Science and Technology Division
Argonne National Laboratory Argonne, Illinois 60439
CONF-8511159—1 DE86 005577
November 1985
The submitted manuscript has been authored bv a contractor of the U. S. Government under contract No. W-3M09-ENG-38.
Accordingly, the U. S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so, tor U. S. Government purposes.
DISCLAIMER
This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsi- bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Refer- ence herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom- mendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
Submitted to the International Symposium on Behavior of Lattice Imperfections in Materials - In s i t u Experiments with HVEM; November 18-20, 1985 Osaka
J Hp3Q • *
1 IF MB DUIUUI 6
IN SITU HIGH VOLTAGE ELECTRON MICROSCOPE STUDIES OF ION- AND ELECTRON-BEAM INDUCED MODIFICATION OF MATERIALS*
P . R. Okamoto, N. Q. Lam and A. Taylor M a t e r i a l s Science and Technology D i v i s i o n
Argonne N a t i o n a l L a b o r a t o r y Argonne, I l l i n o i s 60439
November 1985
The submitted manuscript has been authored bv "a contractor of the U. S- Government under contract No. W-31-109-ENG-38.
Accordingly, the U. S. Government retains a nonexclusive, royalty-free license to publish or reproduce the published form of this contribution, or allow others to do so. for U. S. Government purposes.
Submitted to the I n t e r n a t i o n a l Symposium on Behavior of L a t t i c e Imperfections in Materials - In s i t u Experiments with UVEM; November 18-20, 1985, Osaka, Japan.
*Work supported by the U. S. Department of Energy, BES-Materials Sciences,
under Contract W-31-109-Eng-38.
"HTSITU'HIGH* VOLTAGE ELECTRON MICROSCOPE " STUDIES OF ION- AND' ELECTRON-3EAM""
INDUCED MODIFICATION OF MATERIALS*
P . R. Okamato, N. Q. Lam and A. Taylor
M a t e r i a l s Science and Technology D i v i s i o n , Argonne N a t i o n a l L a b o r a t o r y , 9700 C.
Cass A v e . , Argonne, I l l i n o i s 60439, USA
j In situ high-voltage electron microscope (HVEM) studies have shown that the highly focused electron beams normally employed
; for Irradiation purposes in the HVEM can cause easily measurable J composition changes in the irradiated volume of thin alloy films.
[ The kinetics of this "beam-induced" composition change has been i investigated and found to exhibit a strong dependence not only on [ temperature and peak electron flux, but also on the beam
• diameter. The dependence on beam diameter has far reaching
; implications for HVEM studies of radiation effects in alloys, and j for microchemlcal analysis techniques such as EDX and EELS.
'I. INTRODUCTION
j
i During the p a s t d e c a d e , the HVEM has been widely used to s i m u l a t e neutron damage in metals and a l l o y s . In order to reproduce in short times many of the h i g h dose e f f e c t s observed in r e a c t o r - i r r a d i a t e d a l l o y s , HVEM i r r a d i a t i o n e x p e r i - ments r o u t i n e l y employ h i g h l y focused e l e . t r o n beams. Although such beams t y p i - c a l l y have a Gaussian f l u x d i s t r i b u t i o n , and hence g e n e r a t e d i s p l a c e m e n t - r a t e p r o f i l e s t h a t are r a d i a l l y nonuniform, the importance of r a d i a l d i s p l a c e m e n t - r a t e g r a d i e n t s as d r i v i n g f o r c e s for s o l u t e s e g r e g a t i o n and i t consequences for in s i t u HVEM s t u d i e s of r a d i a t i o n e f f e c t s In a l l o y s hsve only r e c e n t l y been r e c o g - a i z e d [ 1 , 2 , 3 1 . A more d e t a i l e d study of t h e s e p i e g a t i o n e f f e c t and i t s depen- dence on beam c h a r a c t e r i s t i c s are r e p o r t e d h e r e . The r e s u l t s show t h a t focused e l e c t r o n beams can cause e a s i l y measurable composition changes in t h e I r r a d i a t e d volume of t h i n film a l l o y s during bombardment in the HVEM. These beam-induced composition changes pose p r e v i o u s l y unrecognized problems for k i n e t i c s t u d i e s of radiation—induced phenomena t h a t are s e n s i t i v e to a l l o y composition, and may become a s e r i o u s problem a t higher v o l t a g e s for microchemical a n a l y s i s techniques such as EELS and EDX.
I I . EXPERIMENTAL PROCEDURES AND RESULTS
Electron i r r a d i a t i o n s of Ni binary a l l o y s were c a r r i e d out at 1 MeV using the Kratos-AEI EM7 HVEM at the Argonne N a t i o n a l Laboratory E l e c t r o n Microscopy C e n t e r . A d e t a i l e d d e s c r i p t i o n of the e L e c t r o n dosimetry system use to c h a r a c - t e r i z e the beam may be found in ref 4 . A Faraday cup located above t h e specimen p o s i t i o n was used to measure t o t a l beam c u r r e n t s , I,., and a movable Karaday cup l o c a t e d in the viewing chamber was used for beam p r o f i l i n g and for measuring the peak e l e c t r o n f l u x , IQ. Typical beams employed i n t h i s study had a Gaussian r l u x d i s t r i b u t i o n defined, by I ( r ) = I0e x p [ - ( r / ro) ~ ] . The Gaussian parameter r_ .
*rn ! „ wa
defined by was taken as a measure of beam radius.
*Work supported by the U. S. Department of Energy, BES-Materials Sciences, under Contract W-31-lO9-F.ng-3S.
Approximately 843! of the total beam current is included in the effective beam diameter Dp = 2ro-
A particular striking example illustrating the effect of radial displace- ment-rate gradients is shown in Fig. 1. This series of dark-field images show beam-induced changes occurring in an initially uniform dispersion of y'-l^Al precipitates in a Ni-12.7 at.% Al alloy. The irradiation was carried out using focused beam of 1 MeV electrons. The effective beam diameter DQ shown by the circle at time zero is 0.64 gm, and the peak electron flux was 5x10 cm s Fig. 1 clearly shows that a well-defined circular zone of Al enrichment develops during irradiation. Precipitates within this zone grow to impingraent and eventually coalesce to form a single large particle of NijAl. A precipitate-free zone (pfz) forms around the particle. The outer diameter of the pfz is approxi- mately 3D and includes over 99% of the total beam.
Fig. 1. Dprk-field micrographs showing changes occurring in the precipitate microstructure in a Ni-12.7 at.% Al during irradiation at 700°C with 1 MeV electrons. The effective beam is D = 0.64 urn.
The microstructural changes occurring in Fig. 1 can be easily understood in terms of the point-defect concentration gradients produced by the irradiation.
As shown schematically in Fig. 2, the radial component of the point-defect concentration profile Cd(r,z) will resemble the beam intensity profile. Point- defects flowing out of the irradiated zone during irradiation will induce a net flux of solute atoms in the same or opposite direction as the defect flow. As a result, the solute concentration profile CB(r,z) will resemble the point-defect flux divergence profile, V^c^. The similarity arises because in the diffusion equation, 3CS/3t ~ "^J3, the induced solute t'lux J, is proportional to the defect flux, and hence to '/C^. Therefore, v*Crt is a measure of the local rate of solute acuumulation or deplption. When the solute and point-defect fluxes are in opposite directions as iq the case of Ni-Al alloys, then solute enrichment will occur in regions where T^Cj is negative. As shown in Fig 2, such regions occurs where the defect concentrat;on profile (or the beam intensity profile) is concave downward. For a Gaussian beam, the diameter D of this inner zone of solute enrichment Is twice the standard deviation of the beam intensity protile, i.e..
Dp = °o/"'2. tn Fig. 1. the inner zone or Al enrichment is clearly evident and is defined hy Lhe large particle of NijAl. The precipitate-tree zone surrounding
"the particle defines the region'where the" beam intensity profile is concave upward, i . e . , where r C j is positive. In this regiononly a depletion of Al can occur.
Fig. 2. Schematic of the point _defect_concentration profiles in
the_axial and radial directions in_
a thin film irradiated by Gaussian beam. The solid (dashed) lines 'are for thick (thin) f o i l .
Cd( O , r )
Fig. 2 also shows that due to the proximity of the surfaces as defect sinks, there will be an axial component to the segregation process. The axial flow of [defects towards the surfaces will induce a corresponding flow of solute atoms 'either towards or away from the surface. In the case of Ni-Al where the fluxes jof Al atoms and point defects are in opposite directions, the flow of defects to jthe surfaces will result in Al enrichment at the center of the foil. Much
clearer evidence of the axial segregation is presented later.
The change in alloy composition induced by highly focused electron beams can become sufficiently large to induce homogeneous precipitation of a second phase in a normally undersaturated solid solution. This is demonstrated in Fig. i for a Ni-10 at.Z Al alloy irradiated at 700°C, well above its solvus temperature.
The irradiation conditions are identical to that used in Fig. 1. Precipitates of the Ni-jAl phase first form in the center of the beam after about 500 s.
Precipitation is confined to a central circular zone of diameter D which grows radially outward with time. The zone shrinks in diameter during annealing with the beam off, indicating that the precipitates are thermodynamically unstable.
In Fig. 4 the diameter of the precipitate containing zone, D is shown plotted as a function of both irradiation and annealing times. The measurements show that during Irradiation radial growth of the zone stops abruptly at a diameter slightly smaller than the effective beam diameter. As seen in Fig. . . This behavior is expected since Al enrichment should not occur at radial
distances greater than une standard deviation of the beam intensity profile, i .e., 3bout 0.7DQ.
Fig. 2 also shows that the mean diffusion lengths involved in the radial segregation process ts the distance between the center ot the beam, where 3 minimum, and the point where it attains its absolute maximum value. This is simpLy the distance between regions of maximum enrichment and maximum deplerion.
IRRADIATION' 700*C Jo = 5x1o"e/cm'/.
. 790 S t »6
ANNEALING. 700*C
Fig. 3. (Top) Bright field micrographs showing the formation and growth of coherent Ni3Al precipitates in a Ni-10 at.% Al alloy irradiated at 700°C with
•1 MeV electrons. The effective beam diameter shown by the circle at time zero is i 0 -63 ym. (Bottom) annealing sequence showing precipitates are thermally iunstable.
and for a Gaussian beam this radial distance is approximately equal to the effective beam radius rQ. Hence, at a fixed temperature, and for a fixed peak 'electron flux, the time required to produce a given change in alloy composition
!at the center of the beam, should increase in proportion to the square of the ibeam diameter. In order to demonstrate this experimentally, the incubation time '.required to initiate precipitation of the Ni-jAl phase in the center of the
!irradiated zone in a Ni-10 at.% Al alloy was measured as a function of beam I diameter.
I Irradiations were carried out at 600 and 700°C, using fully focused beams (with beam parameters adjusted to maintain a constant peak electron flux of 5x10
cm ~ s for each beam diameter employed. Figure 5 shows precipitation sequences at 70Q°C induced by focused beams of various diameters. Measured incubation times are shown plotted as a function of beam diameter in Fig. 6. Although those measured at 700°C clearly exhibit the expected parabolic dependence on beam diameter, the incubation times measured at 600°C do not. Howevet, the results do not mean that radial segregation effects are not important at the lower
temperature. It clearly shows, however, that at 600°C the axial component of the segregation process dominates over the radial component during the very early stages of irradiation. The axial component, although very weak compared to the radial component, becomes important at bOO°C because at this temperature the solubility limit of Al in Ni is only 0.2 at .7. higher than the average alloy composition of 10 at./C ki. Consequently, only the weaker axial component ot the segregation process is needed to increase the Al concentration at the center or the foil beyond the solubility limit. The mean diffusion lengths for the axial segregation process is less than half the foil thickness, i.e., less than a tew tenths of a micron, and hence is much less than the smallest beam radius
employed. Consequently, at bOO°C precipitation is triggered by axial segregation effects long before radial segregation erfeccs begin to dominate the kinetics.
0.8 —
I.
0.2
50 100 TIME (min)
150
:Fig. 4 . Plot showing the growth of p r e c i p i t a t e containing zone during i r r a d i a t i o n
;and shrinkage during a n n e a l i n g .
iSince the maximum segregation rate in the a x i a l d i r e c t i o n depends only on the
;peak electron f l u x , which was held c o n s t a n t , the measured incubation times a t '600°C should be independent of beam diameter as i s observed. At 700°C, the 'incubation times become i n f i n i t e l y long for beam diameters l a r g e r than about
;4 Mm. This corresponds to uniform i r r a d i a t i o n conditions where only a x i a l segregation e f f e c t s can occur. C l e a r l y , at 700°C the a x i a l component cannot by i t s e l f induce p r e c i p i t a t i o n of the M.3AI phase in a Ni-10 at.% Al a l l o y . This observation i s in good agreement with recent model c a l c u l a t i o n s of the effect 1 2 ] .
I i l . DISCUSSION AND CONCLUSIONS
The present work demonstrates that r a d i a l gradients in the atomic
displacement-rate p r o f i l e generated by highly focused e l e c t r o n beams can indue;
large composition changes in thin film a l l o y s during i r r a d i a t i o n in the HVEM.
The k i n e t i c s of the composition change depend not only on temperature, but a l s o on the s p a t i a l c h a r a c t e r i s t i c s o£ the beam i n t e n s i t y p r o f i l e . The dependence on beam size and shape poses a number of previously unrecognized problems tor in s i t u HVEH studies of radiation-induced phenomena s e n s i t i v e to alloy composition, in p a r t i c u l a r , in the i n t e r p r e t a t i o n oi d o s e - r a t e e f f e c t s . In p r a c t i c e , the d o s e - r a t e dependence of a phenomenon i s determined by measuring changes in some property as a function of the peak e l e c t r o n f l u x . However, i t i s d i f f i c u l t to obtain large changes in peak electron flux without also changing the beam diameter. Consequently, changes in k i n e t i c s r e s u l t i n g solely rrom changes in peak electron flux cannot be separated form d i s p l a c e m e n t - r a t e g r a d i e n t e f f e c t s .
N i - 1 0 a l . 7 . A I T = 7 0 0 " C Jo =5.0X10"e/cm>/i
960 S 2220 S
i Fig. 5. Shows beam-induced precipitation sequences in a Ni-10 at.% Al alloy.
!Effective beam diameter are shown by the c i r c l e s at time zero.
j In fact, i t is doubtful whether the ordinary concept of a dose-rate dependence can retain i t s usual physical significance when highly focused beams are employed .to study composition—sensitive phenomena under conditions where displacement-rate gradients dominate the k i n e t i c s . As shown in Fig. 6, these conditions exist for beam diameters typically employed for HVEM irradiation experiments.
Most quantitative studies of radiation-enhanced coarsening have been carried out in the HVEM using highly focused beams. The Ni-12.7 at.% Al alloy shown in Fig. 1 has frequently been used as a c l a s s i c model system for such studies. The present work clearly demonstrates that segregation processes driven by
displacement-rate gradients will overwhelm simple radiation-enhanced coarsening processes. Highly focused beams are also commonly employed for in situ HVEM studies of radiation—induced segregation at grain boundaries. The results are generally interpreted in terms of composition changes induced by defects
diffusing to the grain boundary. However, when highly focused beams are centered on the boundary, the direction of the r a d i a l defect fluxes generated by the beam will not only oppose the defect flux toward the boundary, but will also be larger in magnitude. Consequently, the observed segregation may appear to contradict the expected behavior.
2 -
Fig. 6. Plot of measured incuba- tion times for beam-induced precipitation in a Ni-10 at.%
Al alloy as a function of beam
:diameter.
UJ
o
1 1
N i - 1 0 0 1 % Al 1-MeV Jo = 5 x
-
—
OS
1
ELECTRONS IOl 9/cm2-s
/
c
n
i
i I
/
/700-C
Jo
1 1 -
_
1
i
Do (/J-
I T h e i m p l i c a t i o n s o f the p r e s e n t w o r k e x t e n d b e y o n d r a d i a t i o n e f f e c t s p e r se. Figure 6 shows the composition changes driven by radial displacement
gradients occur at increasingly rapid rates as the beam diameter is reduced. The
;time required to increase the Al concentration in a Ni-10 at.% Al alloy by about 2 at.% is only a few hundred seconds as the diameter approaches 0.5 urn. These times are typical of those used to acquire EDX and EELS spectra in modern analytical electron microscopes. Since the current trend in microchemical analysis is to employ higher accelerating voltages, higher beam currents and smaller probe sizes, displacement-rate gradient effects are likely to become an important problem for EDX and EELS. Due to the extremely small probe size employed by these technqiues, significant changes during analysis may occur in many materials even at room temperature.
ACKNOWLEDGEMENTS
The authors gratefulloy acknowledge the expert assistance of B. Kestel and A. Philippldes. Special thanks are extended to L. E. Rehn and R. S. Averback tor stimulating discussions and helpful comments.
-REFERENCES
,~O~T. tiiroga, P. R. Okainoto and~H. Wiedersich, Radiation Ef f . Lett. 68 163 (1983).
!2) N. Q. Lam and P. R. Okamoto in Effects of Radiation on Materials: Twelfth Symposium, edited by F. k. Garner (ASTM, Philadelphia, 1984, in press).
3) N. Q. Lam, G. K. Leaf and M. Mlnkoff, J. Nucl. Mater. 118, 248 (1983).
4) A. Taylor and E. A. Ryan, IEEE Trans. Nucl. Sci. 30(2), 1263 (1983).
