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Submitted on 1 Jan 1984
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FIM/IAP/TEM STUDIES OF HYDROGEN IN METALS
S. Walck, J. Hren
To cite this version:
S. Walck, J. Hren. FIM/IAP/TEM STUDIES OF HYDROGEN IN METALS. Journal de Physique
Colloques, 1984, 45 (C9), pp.C9-355-C9-360. �10.1051/jphyscol:1984959�. �jpa-00224446�
FIM/IAP/TEM STUDIES OF HYDROGEN IN METALS
S.D. Walck and J.J. Hren
Department of Materials Science and Engineering, University of Florida, Gainesville^ FL Z2611, U.S.A.
Résume: Cet article est un compte rendu des premières étapes de nos travaux concernant la capture de l'hydrogène par les défauts du réseau métallique. Les imperfections de structure, ainsi que l'hydrogène, sont introduits par implantation ionique. La plupart des résultats tournent autour de la caractérisation des défauts structuraux par utilisation des techniques FIM et TEM. La faisabilité de
l'application de 1'IAP à l'étude de l'interaction hydrogène/défaut est aussi discutée.
Abstract: This paper is a report on the early stages of our work in the area of hydrogen trapping to defects in metals. Both the hydrogen and the defect structure are introduced by ion implantation. The results center on the characterization of the defect structure using both FIM and TEM techniques. The feasibility of applying the IAP to the study of hydrogen/defect interactions is also addressed.
Introduction
The interactions of hydrogen with defects such as vacancies, dislocations, grain boundaries, and phase boundaries are extremely important because of the insidious, often catastrophic results its presence brings to many metals. The trapping of hydrogen to these sites can be either beneficial by reducing the susceptibility for hydrogen embrittlement or deleterious and actually causing it.
In studying hydrogen in metals the experimentalist is faced with four problems: 1) getting it into the metal, 2) keeping it there for sufficiently long times, 3) to detect and analyze it and 4) differentiate it from background hydrogen.
In describing the proposed plan for applying the Field Ion Microscope/Imaging Atom Probe (FIM/IAP) to study hydrogen/deuterium, these points will be addressed along with the special problems inherent with using FIM techniques. Most of these have been pointed out by Spitznagel, Miller and Brenner [1].
Ideally, any technique for studying hydrogen trapping to defects with the FIM/IAP should be able to be applied to any type of trap. Indeed, Kellogg and Panitz [2] have already shown the IAP to be useful in observing qualitatively, the trapping of deuterium to a grain boundary in tungsten. The approach taken should account for all field effects, including field-induced stresses, volume dilations, adsorption, the pervasive background signal, the effect of hydrogen on field evaporation, and statistics. Because of the uncertainties on the amount of
interference these effects may have on the outcome of any investigation, it was decided to use a system which has been well studied by an alternative method.
Besenbacher et al. [3,4,5] has studied the trapping of deuterium to helium implanted bubbles by the use of Nuclear Reaction Analysis (NRA) and Rutherford Backscattering
(RBS) techniques in nickel, iron and 304 stainless steel.
In this method, thin foils of the metal under study are first implanted with helium to create bubbles and lattice damage at cryogenic temperatures. At approximately 80 K the vacancies created by the implanted species (both He and D) have sufficient trapping strength to bind the deuterium. Redistribution of the deuterium from weak sites to stronger He bubbles occurs as the temperature is raised. If there is not enough of the second trap site available for the deuterium, or the temperature is raised above the second release stage, bulk release of the Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984959
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d e u t e r i u m occurs. The amount o f d e u t e r i u m r e m a i n i n g i n t h e sample i s found b y u s i n g t h e 'D (3He,P) 'He r e a c t i o n . The r e s u l t s of t h e i r work g i v e a good s t a n d a r d on which t o base t h e v a l i d i t y o f FIM/IAP r e s u l t s i n t h e s t u d y o f hydrogen i n metals.
The a p p l i c a t i o n o f t h e s e t e c h n i q u e s t o d i f f e r e n t t y p e s of d e f e c t s i s t h e n p o s s i b l e . I o n i m p l a n t a t i o n o f f e r s a h i g h l y r e p r o d u c i b l e means of c h a r g i n g s p e c i e s w i t h v a r i o u s d i s t r i b u t i o n s by s i m p l y a d j u s t i n g t h e energy and f l u e n c e of t h e i o n s . I n t r o d u c t i o n a t c r y o g e n i c t e m p e r a t u r e s t r a p s t h e h i g h l y mobi 1 e d e u t e r i u m t o d e f e c t s i t e s . U s i n g t h e i s o t o p e of hydrogen a l l e v i a t e s t h e background p r o b l e m due t o t h e s m a l l r e 1 a t i v e abundance o f n a t u r a l l y o c c u r r i n g deuterium. The n e c e s s i t y o f i m p l a n t a t i o n o f c r y o g e n i c temperatures under LIHV c o n d i t i o n s f o r c e s a c h o i c e between i m p l a n t a t i o n ex s i t u f r o m t h e FIM/IAP w i t h a s u i t a b l e c r y o g e n i c t r a n s f e r d e v i c e and i n s i t u i m p l a n t a t i o n . Because o f t h e c o m p l e x i t y i n v o l v e d w i t h t h e f i r s t choice, c o u p l i n g o f a d i f f e r e n t i a l l y pumped i o n gun t o t h e FIM/IAP has been chosen.
However, t h i s approach s a c r i f i c e s t h e l o w d e u t e r i u m background l e v e l when d e u t e r i u m i s b e i n g used as t h e i m p l a n t i n g s p e c i e s and s p e c i a l p r e c a u t i o n s must be t a k e n t o a v o i d t h i s .
Experimental D e t a i l
N i c k e l has been chosen f o r t h e p r e l i m i n a r y work f o r s e v e r a l reasons. I t has b e t t e r c o r r o s i o n p r o p e r t i e s t h a n does i r o n , w h i c h i s i m p o r t a n t when t r a n s f e r r i n g between TEM and FIM. Most i m p o r t a n t l y i s t h e f a c t t h a t t h e r e l e a s e s t a g e o f d e u t e r i u m f r o m He bubbles i s above room t e m p e r a t u r e and t h a t o f i r o n i s below [4,51.
A l t h o u g h a r e d i s t r i b u t i o n o f t h e d e u t e r i u m w i l l o c c u r between t h e weaker t r a p s i t e s t o t h e s t r o n g e r He s i t e s , no b u l k r e l e a s e o f d e u t e r i u m s h o u l d occur. T h i s s h o u l d a l l o w t h e a n a l y s i s i n t h e FIM/IAP when samples a r e i m p l a n t e d ex s i t u .
The samples were p r e p a r e d f r o m Marz grade 0.25 mm d i a m e t e r N i w i r e o f quoted p u r i t y 99.99% f r o m M a t e r i a l Research Corp. The w i r e was annealed a t 1173 K f o r a p p r o x i m a t e l y 4 h o u r s i n He. E m i t t e r s were prepared b y e l e c t r o p o l i s h i n g i n 10% HC1 s o l u t i o n u s i n g
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4.5 vdc. The specimens were f i e l d evaporated t o y i e l d e m i t t e r s w i t h imaging v o l t a g e s between 7 and 1 5 kV p r i o r t o i m p l a n t a t i o n .The i o n gun i s a model 6-2-0 f r o m C o l u t r o n Research Corp. and i s d i f f e r e n t i a l l y pumped by a B a l z e r s 170 l / s turbopump. I t can be o p e r a t e d i n t h e range 10 eV t o 20 keV and can be used f o r b o t h gases and s o l i d charges. I t i n c o r p o r a t e s a v e l o c i t y f i l t e r p r o v i d i n g mass s e p a r a t i o n w i t h r e s o l u t i o n of a p p r o x i m a t e l y m/Am = 400. The gun can produce b o t h a focused c i r c u l a r s p o t o f about . 2 mm o r a l i n e image a t a d i s t a n c e of a p p r o x i m a t e l y 1 m. U s i n g a s p o t s i z e s l i g h t l y l a r g e r t h a n 2 mm a t t h e t a r g e t , t h e gun produces a beam c u r r e n t d e n s i t y of 15, 7 and 42 p ~ / c m ~ f o r Hef,
~ f ,
and D$, r e s p e c t i v e l y ; m a i n t a i n i n g t h e s e v a l u e s when t h e energy i s changed between 5 t o 10 keV. W i t h t h e s e values, t h e t i m e r e q u i r e d f o r a c t u a l i m p l a n t a t i o n t o a c h i e v e a f l u e n c e of 5 x 1015 He/cm2 i s 53, 114, 9.5 sec, r e s p e c t i v e l y . The i n i t i a l i m p l a n t a t i o n s were done o n l y w i t h He i n o r d e r t o o b t a i n i n f o r m a t i o n on t h e m i c r o s t r u c t u r e and compare i t t o t h e l i t e r a t u r e . Fluences o f
-
5 x 1015 ~e+/crn' were n o r m a l l y used.O b s e r v a t i o n s i n b o t h TEM and FIM were performed. E m i t t e r s were examined w i t h e l e c t r o n microscopy a t 200 keV u s i n g a JEOL 200 CX u t i l i z i n g t h r e e d i f f e r e n t t y p e s of h o l d e r s f o r t h i s purpose. The f i r s t i s a s i m p l e specimen h o l d e r f o r e m i t t e r s h a v i n g o n l y a s i n g l e a x i s o f r o t a t i o n about t h e e m i t t e r ' s a x i s . F i g u r e s l ( a ) and (b) a r e two TEM m i c r o g r a p h s t a k e n b e f o r e and a f t e r He+ i m p l a n t a t i o n t o ,a f l u e n c e o f a p p r o x i m a t e l y 1 x 1017 He/cm2. A m o d i f i e d JEOL d o u b l e - t i l t h o l d e r h a v i n g +27" t i l t p e r p e n d i c u l a r t o t h e e m i t t e r ' s a x i s , i n a d d i t i o n t o t h e r o t a t i o n a b o u t i t s a x i s , was b u i l t f o r improved TEM image a n a l y s i s . A t h i r d h o l d e r , has a s i n g l e a x i s of
r o t a t i o n a b o u t t h e e m i t t e r ' s a x i s , and i s capable o f h a v i n g an a p p l i e d v o l t a g e of up t o 5 kV and was used f o r f i e l d - e f f e c t o b s e r v a t i o n s . A p p l i c a t i o n o f 3 kV a t room temperature t o N i specimens i n t h e TEM produces an evaporated end f o r m o f
-
70 nm o r an imaging v o l t a g e o f a b o u t 1 5 kV. One a s - p o l i s h e d specimen w i t h a d i s l o c a t i o n and a d i s l o c a t i o n l o o p a b o u t 500 nm and 800 nm f r o m t h e e m i t t e r ' s apex was imaged b e f o r e and a f t e r a p p l i c a t i o n o f 3 kV i n s i t u . A d o u b l e - t i l t i n g experiment p r i o r t o t h e a p p l i c a t i o n of t h e f i e l d was performed on t h i s specimen t o d e t e r m i n e t h e Burgerswould change due t o f i e l d - i n d u c e d s t r e s s . E m i t t e r s were f i e l d evaporated a t 3 kV i n t h e TEM, i m p l a n t e d i n t h e C o l u t r o n i o n gun, and imaged i n t h e TEM b e f o r e and a f t e r a p p l i c a t i o n o f 2.5 kV w i t h t h e same beam c o n d i t i o n s .
r. 2
F i g u r e
i ,
a) ~ n i m p l a n t e d N i e m i t t e r , b ) same e m i t t e r i m p l a n t e d t o a p p r o x i m a t e l y1
x l o i 7 He+/cm2 w i t h 9.75 keV energy.The FIM/IAP employed i n t h i s s t u d y has a f i x e d 1 5 cm t i p - t o - s c r e e n d i s t a n c e w i t h f l a t channel p l a t e s . Time o f f l i g h t d a t a i s c o l l e c t e d w i t h a T e k t r o n i x 7912AD waveform d i g i t i z e r and a 4052A s e r i e s computer c o n t r o l l e r . The chamber i s pumped by a P e r k i n Elmer 60 l / s B o s t i v a c d e s i g n pump i n c o r p o r a t i n g an 80 l / s hydrogen i o n pump and a t i t a n i u m s u b l i m a t i o n pump. A d d i t i o n a l pumping i s p r o v i d e d by an A i r Products d i s p l e x He c l o s e d - c y c l e r e f r i g e r a t i o n u n i t used f o r c o o l i n g t h e sample. A
goniometer s t a g e a l l o w s t h e specimen t o be r o t a t e d 210" about t h e v e r t i c a l a x i s and a p p r o x i m a t e l y
+
30" about t h e second a x i s , Two s i x - i n c h p o r t s a r e l o c a t e d 150"a p a r t and t h e sample can be r o t a t e d t o f a c e e i t h e r . The IAP p o r t i s l o c a t e d on one of these. P r i o r t o t h e c o u p l i n g o f t h e i o n gun t o t h e o t h e r p o r t , a 7 cm FIM screen was i n p l a c e . F i g u r e 2 shows t h e C o l u t r o n i o n gun c o u p l e d t o t h e FIM/IAP.
F i g u r e 2. D i f f e r e n t i a l l y pumped C o l u t r o n i o n gun c o u p l e d t o FIM/IAP.
A f e a s i b i l i t y t e s t o f c o u p l i n g t h e i o n gun t o t h e chamber was perfofmed.
S e v e r a l s p e c t r a were c o l l e c t e d f r o m a N i specimen t o e s t a b l i s h t h e background hydrogen s i g n a l p r i o r t o t h e i n t r o d u c t i o n o f any d e u t e r i u m i n t o t h e system.
Deuterium was t h e n i m p l a n t e d i n t o t h e
side
o f t h e specimen h o l d e r , t h u s i n t r o d u c i n g d e u t e r i u m t o t h e background, b u t n o t t h e specimen i t s e l f .R e s u l t s and D i s c u s s i o n
-
- -
I n s p e c t i o n o f micrographs o f 9.75 keV o f He+ i m p l a n t e d N i e m i t t e r s show
q u a l i t a t i v e l y t h e same s t r u c t u r e as t h o s e found by Besenbacher e t a l . [4], and 5 keV
C9-35 8 JOURNAL DE PHYSIQUE
implantations by JB'ger e t a l . [8] and van Swygenhoven e t a l . [6,7]. FIM micrographs of evaporation sequence show q u a l i t a t i v e l y s i m i l a r s t r u c t u r e s t o those analyzed by Seidman [9]. Figure 3 ( a ) i s a FIM-Ne image of Ni p r i o r t o He+ implantation taken a t t h e 7 cm screen. Figure 3(b) is an image of t h e same t i p implanted with 9.75 keV He+
t o a fluence of 5
x
10" ~ e + / c m ' taken a t t h e 15 cm screen.Figure 3. a ) FIM-Ne image of Ni sample ( t i p t o screen i s 7 cm, 12.45 kV, 80 K ) , b) same sample implanted t o 5
x
1015Hef/cm2 a t 9.75 keV energy ( t i p t o screen i s 1 5 cm, 12.27 kV, 80 K).Figure 4 shows t h e r e s u l t s of t h e double t i l t i n g experiment t o determine t h e Burgers v e c t o r of t h e d i s l o c a t i o n . Unfortunately, not enough t i l t was a v a i l a b l e t o g e t t o a t h i r d < I l l > r e f l e c t i o n needed t o determine i t by using t h e g - b = 0
c r i t e r i a . For t h e r e f l e c t i o n s (111) and ( I l l ) , g - b = 0 f o r only t h e f i r s t . This leads t o b = 6[110] o r &[101]. Figures 5(a) and ( b ) a r e t h e r e s u l t s of t h e a p p l i c a t i o n of t h e f i e l d t o t h i s t i p . I t i s obvious t h a t f i e l d evaporation has occurred and i n f a c t approximately 300 nm has been removed. Upon c l o s e r inspection of Figure 5 ( b ) i t i s seen t h a t t h e d i s l o c a t i o n has disappeared and t h a t t h e r e a r e two p a r a l l e l s l i p t r a c e s v i s i b l e where i t has swept through.
Figure 4.
An
as-polished Ni e m i t t e r with d i s l g c a t i o n p r e s e n t . Dark-field images usi'ng a ) (111) r e f l e c t i o n , gab = 0 , and b) (111) r e f l e c t i o n , g - b f 0.evaporated. S l i p t r a c e s e v i d e n t i n b ) . D i s l o c a t i o n l o o p a t 800 nm was unaffected.
Several r e s u l t s have come o u t o f t h e use o f t h e f i e l d - e f f e c t TEM h o l d e r f o r t h e
~ e + i m p l a n t e d specimens. F i g u r e s 6 ( a ) and ( b ) show t h e r e s u l t o f a p p l i c a t i o n o f t h e f i e l d t o such a specimen. Even though t h e e m i t t e r s were f i e l d evaporated t o 3 kV p r i o r t o i m p l a n t a t i o n , some f i e l d e v a p o r a t i o n s t i l l o c c u r r e d when t h e v o l t a g e was b r o u g h t t o 2.5 kV. T h i s i s a l s o c o n s i s t e n t w i t h FIM o b s e r v a t i o n s t h a t e m i t t e r s p r e v i o u s l y imaged b e f o r e i m p l a n t a t i o n image a t s i g n i f i c a n t l y l o w e r v o l t a g e s when t r a n s f e r r e d back t o t h e FIM. Some s h a r p e n i n g o f t h e t i p has occurred, p r o b a b l y due t o o x i d a t i o n d u r i n g t r a n s f e r s and/or an i o n beam e f f e c t . Upon e x a m i n a t i o n of F i g u r e s 7 ( a ) and ( b ) by superimposing t h e n e g a t i v e s o f each, i t i s seen t h a t t h e d e f e c t s t r u c t u r e has n o t changed except f o r t h e f i e l d e v a p o r a t i o n w h i c h has occurred. Van Swygenhoven e t a l . [6,7] have observed He b u b b l e g r o w t h under 1 MeV e l e c t r o n i r r a d i a t i o n a t 300 K. Bubble g r o w t h was n o t observed under any o f t h e f o l l owing c o n d i t i o n s : 1 ) 200 keV e l e c t r o n i r r a d i a t i o n , 2) a p p l i c a t i o n o f a p p l i e d f i e l d i n t h e TEM w i t h o u t e l e c t r o n beam and 3 ) w i t h e l e c t r o n beam. There, i s s t i l l a q u e s t i o n as t o whether t h e h i g h e r s t r e s s l e v e l s d u r i n g FIM imaging w i l l a f f e c t t h e d e f e c t s t r u c t u r e . An a d d i t i o n a l experiment i s proposed i n w h i c h an i m p l a n t e d specimen i s imaged i n t h e TEM and i t s t i l t i n g c o n d i t i o n s r e c o r d e d p r e c i s e l y w i t h t h e use of t h e d o u b l e - t i l t h o l d e r and K i k u c h i l i n e i n d e x i n g , t r a n s f e r r e d t o t h e FIM and imaged, and t h e n r e t u r n e d t o t h e TEM and t i l t e d t o e x a c t l y t h e same o r i e n t a t i o n and imaged.
F i g u r e 6. a ) ~ e + i m p l a n t e d specimen b e f o r e a p p l i c a t i o n o f 2.5 kV t o e m i t t e r i n s i t u . b ) A f t e r a p p l i c a t i o n o f f i e l d , s l i g h t f i e l d e v a p o r a t i o n i s e v i d e n t , b u t t h e d e f e c t s t r u c t u r e i s u n a l t e r e d .
The i n t r o d u c t i o n o f d e u t e r i u m i n t o t h e FIM/IAP was done by i m p l a n t i n g i n t o t h e s i d e o f t h e specimen h o l d e r . Beam c o n d i t i o n s were D3 a t 40 pA/cm2 w i t h an approximate s p o t s i z e o f 2 mm i n d i a m e t e r f o r 4 minutes. The p r e s s u r e i n t h e gun and FIM/IAP p r i o r t o opening t h e v a l v e f o r i m p l a n t a t i o n was 6 x T and 5 x 1 0 - l o T,
r e s p e c t i v e l y . The p r e s s u r e i n t h e FIM/IAP r o s e t o 8 x 1 0 - l o T d u r i n g i m p l a n t a t i o n and remained c o n s t a n t u n t i l t h e v a l v ? was closed. The p r e s s u r e dropped back t o 5 x 1 0 - l o T w i t h i n 5 minutes. A s m a l l D2 peak appeared on a r e s i d u a l gas a n a l y z e r w i t h an approximate 10% i n c r e a s e on t h e Hz ( ~ f ) peak. The f i r s t IAP t i m e - o f - f l i g h t s p e c t r a o f t h e N i t t k e n a f t e r d e u t e r ~ u m was i n t r o d u c e d , showed a l a r g e
~f
peak, comparable t o t h e HI peak. Subsequent s p e c t r a showed somewhat s m a l l e r peaks. The D:s i g n a l can be e l i m i n a t e d by p u l s i n g b e f o r e t h e s p e c t r a i s c o l l e c t e d . F i g u r e 7 shows two s p e c t r a , one w i t h p u l s i n g o c c u r r i n g a t 2.5 sec i n t e r v a l s p r i o r t o c o l l e c t i o n o f t h e spectrum and t h e o t h e r w i t h o u t . The maximum t i m e i n t e r v a l between p u l s e s t o
C9-360 JOURNAL DE PHYSIQUE
m i n i m i z e t h e a d s o r p t i o n o f d e u t e r i u m and t h e r e b y m i n i m i z e t h e D] background w i l l be i n v e s t i g a t e d i n f u t u r e work. W i t h p r o p e r c o n t r o l experiments, a l o n g t h e l i n e s o f P a n i t z
[ l o ] ,
i t i s f e l t t h a t t h e i n s i t u i m p l a n t a t i o n s t u d i e s o f d e u t e r i u m a r e v i a b l e .I BE-> v 842586-56 N I # I 9 7 . 8 2 7/12/84
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I s .
8 8 B I 8 2 8 3 8 1 8 . 5 B.B 8 1 8 . 8 B.9 I S KVDC=S 64 X Y P = ~ 8 2 ' ' l P ~ l s e s TFILEI S I
F i g u r e 7. Two IAP TOF s p e c t r a o f N i (reduced by h a l f ) a f t e r d e u t e r i u m i n t r o d u c e d i n t o FIM/IAP. Top spectrum i s a r e s u l t o f p u l s i n g t w i c e w i t h 2.5 sec i n t e r v a l p r i o r t o a c q u i s i t i o n and b o t t o m w i t h none.
Acknowledgements
The a u t h o r s would l i k e t o t h a n k John A. P a n i t z f o r a l l o w i n g t h e use o f h i s f i e l d - e f f e c t TEM h o l d e r and t h e many f r u i t f u l d i s c u s s i o n s and t h e a i d he has
p r o v i d e d . We would l i k e t o thank t h e M a j o r A n a l y t i c a l I n s t r u m e n t a t i o n Center (MAIC) a t t h e U n i v e r s i t y o f F l o r i d a f o r t e c h n i c a l a s s i s t a n c e . T h i s work was funded by t h e U n i t e d S t a t e s Department o f Energy (DOE), C o n t r a c t #DE-A505-83ER45039 and supported i n p a r t b y t h e MAIC.
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