HYDROGEN IN Fe80B20 AND Fe40Ni40P14B6 AMORPHOUS ALLOYS

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HYDROGEN IN Fe80B20 AND Fe40Ni40P14B6 AMORPHOUS ALLOYS

P. Moser, Chi-Mei Mô, M. Moumene

To cite this version:

P. Moser, Chi-Mei Mô, M. Moumene. HYDROGEN IN Fe80B20 AND Fe40Ni40P14B6 AMORPHOUS ALLOYS. Journal de Physique Colloques, 1983, 44 (C9), pp.C9-91-C9-97. �10.1051/jphyscol:1983908�.

�jpa-00223330�

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JOURNAL DE PHYSIQUE

Colloque C9, supplement a u n012, Tome 44, d6cembre 1983 page C9-91

HYDROGEN I N F e 8 0 B 2 0 AND F e 4 0 N i 4 0 P 1 4 B 6 N I O R P H O U S A L L O Y S

P. Moser, Chi-Mei MG* and M. Moumene

Centre d

'

~ t u d e s NucZdaires de GrenobZe, Ddpartement de Recherche Fondamenta Ze, S e c t i o n de Physique du SoZide, 8 5 X, 38041 Grenoble Cedes, France

* ~ n s t i t u t e o f S o l i d S t a t e Physics, Acadernica Sinica, Hefei, China

Resume : L ' e f f e t d e l l h y d r o g & n e e t / o u d e l a d e f o r m a t i o n s u r l e f r o t t e m e n t i n t e r n e , l e t r a i n a g e magnCtique e t l e temps de v i e d e s p o s i t o n s d a n s l e s a l l i a g e s amorphes e s t e t u d i e d e 4

B

400 K . On o b s e r v e v e r s 200 K un p i c de f r o t t e m e n t i n t e r n e e t v e r s 150 K l a r e l a x a t i o n m a g n e t i q u e c o r r e s p o n d a n t e . Apr&s r e c u i t

B

340 K , u n e a u g m e n t a t i o n t r a n s i t o i r e d e l ' a m p l i t u d e e s t o b s e r - vCe. C e c i i n d i q i r e r a i t un mecanisme d ' i n t e r a c t i o n : "nuage d r h y d r o g & n e

/

q u a s i d i p o l e de d i s l o c a t i o n s " .

A b s t r a c t : The e f f e c t of hydrogen a n d / o r d e f o r m a t i o n on i n t e r n a l f r i c t i o n , m a g n e t i c a f t e r e f f e c t , a n d p o s i t r o n l i f e t i m e i n amorphous a l l o y s i s i n v e s t i - g a t e d between 4 and 400 K . I n hydrogen c h a r g e d and c o l d worked + hydrogen c h a r g e d s p e c i m e n s , a n i n t e r n a l f r i c t i o n peak i s f o u n d a r o u n d 200 K and a c o r r e s p o n d i n g m a g n e t i c a f t e r e f f e c t a r o u n d 150 K . A f t e r a g i n g a t 340 K , t r a n s i e n t peak enhancements a r e o b s e r v e d . An e x p l a n a t i o n i n t e r m s of

" h y d r o g e n - d i s l o c a t i o n d i p o l e l i k e " i n t e r a c t i o n i s g i v e n .

Amorphous a l l o y s c o n t a i n i n g hydrogen p r e s e n t a n i n t e r n a l f r i c t i o n peak l o c a t e d a t low t e m p e r a t u r e [ I - 3 1

.

B e r r y i n t e r p r e t e d t h i s peak i n t e r m s of a n a n a l o g of Snoek mechanism

[ I ] .

I n o r d e r t o v e r i f y t h i s h y p o t h e s i s , hydrogen i s i n t r o d u c e d i n

Fe40Ni40P14B6, FegoB20 amorphous a l l o y s and a l s o i n c r y s t a l l i n e F e . A m a g n e t i c a f t e r e f f e c t zone c o r r e s p o n d s t o t h e i n t e r n a l f r i c t i o n i n b o t h c a s e s . T h e e f f e c t of a c o l d work i s i n v e s t i g a t e d . P o s i t r o n l i f e t i m e measurements a r e a l s o performed. These d a t a a r e b e t t e r e x p l a i n e d i n t h e h y p o t h e s i s of a n i n t e r a c t i o n between hydrogen c l o u d s and e x t e n d e d d e f e c t s .

I1

-

EXPERIMENTAL TECHNIQUES

2 . 1 . Spe_cigeg-pggpa_ra_tigg : Fe40Ni40P14B20 a r e o b t a i n e d from A l l i e d Chem. Co ; Fe80B20 i s p r e p a r e d i n CEN-G by F . Vanoni. Cold work i s made e i t h e r a t room tempera- t u r e o r a t 77 K by p r e s s i n g t h e specimen between two f i l e s ( t h i s g i v e s r e g u l a r and r e p r o d u c i b l e m i c r o f o l d s ) . Hydrogen i s e l e c t r o l y t i c a l l y i n t r o d u c e d u s i n g a 0.1N SO4H2 s o l u t i o n .

2 . 2 . $ t e g n a l f r r j c t j g p and m a g n e t i z - g f t e r e f f : I n t e r n a l f r i c t i o n measurements a r e made i n a 1 Hz, i n v e r t e d pendulum w i t h a 100 Ce a x i a l m a g n e t i c f i e l d . The moun- t i n g of specimen i s d i f f i c u l t and d o e s n o t a l l o y h e a v y c o l d work o r heavy hydrogen c h a r g i n g . ( I n t h e c a s e o f Magnetic A f t e r E f f e c t o r P o s i t r o n L i f e t i m e , i t i s more e a s y ) . Magnetic p e r m e a b i l i t y i s measured i n low f i e l d ( 5 mOe, 70 H z ) 1 and 150 s e c a f t e r specimen d e m a g n e t i z a t i o n . On F i g s . 3 t o 4 , t h e r e l a x a t i o n s t r e n g t h of t h e m a g n e t i c a f t e r e f f e c t Ay = ( 1 / ~ ) 1

-

( 1 /u) 150 i s p l o t t e d a s a f u n c t i o n of t h e tempe- r a t u r e .

2.3. P o s i t r o n l i f e t i m e s a r e measured a t 7 7

---

K u s i n g a 2 2 ~ a p o s i t r o n s o u r c e and a s t a r t s t o p ^{- /} c h r o n o m e t e r . The l i f e t i m e of a p o s i t r o n c o r r e s p o n d s t o t h e s l o w i n g down a f t e r i m p l a n t a t i o n i n t h e specimen (" I p s = 10-12sec) and t o t h e d i f f u s i o n i n t h e l a t t i c e (" l o o p s ) u n t i l a n n i h i l a t i o n which o c c u r s when a n e l e c t r o n h i t s a p o s i t r o n . I t i s o b t a i n e d by m e a s u r i n g t h e t i m e e l a p s e d between t h e e m i s s i o n of t h e y r a y of 1.28 MeV ( b i r t h of t h e e') and t h e two

Y

r a y s of 0.51 1 MeV ( d e a t h ) . I f l i s ? o c a t i o n s

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983908

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C9-92 JOURNAL DE PHYSIQUE

o r v a c a n c i e s a r e p r e s e n t i n t h e s p e c i m e n , t h e y t r a p s t r o n g l y t h e p o s i t r o n s and t h e l i f e t i m e i s d r a s t i c a l l y i n c r e a s e d b e c a u s e t h e e l e c t r o n i c d e n s i t y i s much l o w e r i n a v a c a n c y t y p e d e f e c t t h a n i n t k b u l k [4]

.

I11 - EXPERIMENTAL RESULTS

3.1. I n t e r n a l f r i c t i o n (More d e t a i l s w i l l b e g i v e n i n [5] )

I n h y d r o g e n a t e d and FegoB20, t h e p e a k s p r e v i o u s l y o b s e r v e d by B e r r y a r e found a r o u n d 200 K w i t h a n a c t i v a t i o n e n e r g y of 0 . 5 t 0 . 2 eV. I n Fe40Ni40P14Bfj a n n e a l i n g s a r e made and g i v e v e r y complex r e s u l t s . The peak a m p l i t u d e d e c r e a s e s a f - t e r a n n e a l i n g a t room t e m p e r a t u r e ( F i g . I ) , i n c r e a s e s when a n n e a l i n g a r e made i n t h e r a n g e 350 - 400 K and d e c r e a s e s r a p i d l y above 400 K . The peak t e m p e r a t u r e s h i f t s r e g u l a r l y towards h i g h t e m p e r a t u r e . As a f u n c t i o n of t h e c h a r g i n g t i m e , t h e peak h e i g h t and w i d t h i n c r e a s e a n d t h e peak t e m p e r a t u r e d e c r e a s e s .

E m b r i t l e m e n t l i m i t s t h e p o s s i b i l i t y of s t u d y , e s p e c i a l l y f o r FegoB20.

I I I I

100 200 300

TEMPERATURE

-

(it1

F i g . 1 Hydrogen i n t e r n a l f r i c t i o n peak i n Fe40Ni40P14B6 a s r e c e i v e d s p e c i m e n s . Char- g i n g c o n d i t i o n s 80 min. 0 . 1 A i n 0 . 1 H2SO4. Run 1 : 77 + 340K (-) Run 2 a f t e r a g i n g 17 h o u r s a t 300 K (---)

.

3.2. Z g n e t i c A f t e r E f f e c t

I n b o t h a s r e c e i v e d a l l o y s , a w e l l d e f i n e d r e l a x a t i o n i s o b s e r v e d a t 4 i( and v a n i - s h e s a b o v e 14 K . T h i s r e l a x a t i o n i s s u p p r e s s e d by a n n e a l i n g below c r y s t a l l i z a t i o n o r by hydrogen c h a r g i n g and i s s t r o n g l y enhanced by c o l d work ( F i g . 2 ) . I n t h e r a n g e 14

-

³⁰⁰K , t h e s p e c t r u m i s f e a t u r e l e s s w i t h t h e e x c e p t i o n of a weak, non a c t i v a t e d r e l a x a t i o n . Above 300 K , t h e w e l l known m a g n e t i c a f t e r e f f e c t zone i s o b s e r v e d [13].

In a s r e c e i v e d and t h e n hydrogen c h a r g e d s p e c i m e n s a d e e p m a g n e t i c r e l a x a t i o n i s found i n b o t h a l l o y s ( F i g . 3 ) . The a n n e a l i n g i s p r e s e n t e d on F i g . 4a and 4 b . A t r a n s i e n t enhancement (weak i n F e 4 0 N i ~ ~ p ~ ~ B 6 , s t r o n g i n F e g o B 2 ~ ) a p p e a r s c l e a r l y . The same e x p e r i m e n t i s r e p e a t e d on specimen p r e v i o u s l y c o l d worked. I n b o t h s p e c i - mens t h e a m p l i t u d e of t h e m a g n e t i c r e l a x a t i o n s t r e n g t h i s m u l t i p l i e d by t e n and t h e enhancement i s a l s o o b s e r v e d ( F i g . 4c a n d d )

3 . 3 . P o s i t r o n l i f e t i m e measurements

I n FegoBpo t h e p o s i t r o n l i f e t i m e i s measured i n t h e f o l l o w i n g c o n d i t i o n s : a s r e c e i - ved (1 52ps), w e l l a n n e a l e d (1 50ps), room t e m p e r a t u r e c o l d worked (152ps), h y d r o g e n a t e d

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( 1 5 1 ~ s ) . In f a c t , b e c a u s e t h e p r e c i s i - o n i s t I p s , t h e o b s e r v e d d i f f e r e n c e s a r e n o t s i g n i f i c a n t . I n Fe40Ni40P14B6 we o b t a i n r e s p e c t i v e l y 157, 155, 156 and 156ps i n s i m i l a r c o n d i t i o n s ; c o l d worked t h e n h y d r o g e n a t e d specimens g i v e 157ps.

I n a p r e v i o u s work, we showed t h a t i n b o t h amorphous a l l o y s , a 77 K e l e c t r o n i r r a - d i a t i o n c r e a t e s m e t a s t a b l e m e t a l l i c s i n g l e v a c a n c i e s [6]. We a t t e m p t t o c r e a t e s i m i - l a r v a c a n c i e s by a 77 K c o l d work i n Fe40Ni40P14Bg b u t n o s i g n i f i c a n t l i f e t i m e va- r i a t i o n i s o b s e r v e d , e i t h e r a f t e r c o l d work o r a f t e r s u b s e q u e n t a n n e a l i n g . T h i s r e - s u l t shows t h a t n e i t h e r v a c a n c y n o r c r a k s a r e g e n e r a t e d by a 77 K c o l d work.

AS a c o m p a r i s o n , t h e v a l u e s found i n c r y s t a l l i n e Fe and N i a r e r e s p e c t i v e l y :

well

a n n e a l e d 113 and 155ps, e + t r a p p e d i n s i n g l e v a c a n c i e s 170 and 1 6 2 p s , e + t r a p p e d i n d i s l o c a t i o n s 155 and 153ps* ( t h e s e v a l u e s remain c o n s t a n t from 5 % t o 98 % r e - d u c t i o n i n a r e a ) . No s i g n i f i c a n t e f f e c t o f hydrogen i s found i n c o l d worked s p e c i - men (154 and 153ps r e s p e c t i v e l y ) .

F i g . 2 Magnetic r e l a x a t i o n s t r e n g t h measured a t 4 K a s a f u n c t i o n of t h e d e f o r m a t i o n r a t e .

A

i s t h e d i f f e r e n c e between t h e r e l u c t i v i t y measured 1 and 900 s e c a f t e r d e m a g n e t i z a t i o n . P i s t h e p r e s s u r e e x e r t e d on t h e f i l e s f o r o b t a i n i n g t h e m i c r o f o l d t e x t u r e ( P r e s s u r e s h i g h e r t h a n 5 000 kg.cm-2 t r a n s f o r m t h e specimen i n d u s t ) .

I V - DISCUSSION

The hydrogen peak was e x p l a i n e d by B e r r y i n t e r m s of a Snoek t y p e mechanism : hydrogen atoms a r e l o c a t e d a t i n t e r s t i t i a l s i t e s of low symmetry i n 1 . e r i t e d from t h e B e r n a l s t r u c t u r e p r o t o t y p e [7] and t o o s m a l l t o accomodate B o r P atoms. These s i t e s form a n i n t e r c o n n e c t e d network of i n t e r s t i t i a l - s i t e s of d i f f e r e n t o r i e n t a t i o n s , s o t h a t t h e s t r a i n d i p o l e r e o r i e n t a t i o n o c c u r s a t each hydrogen atom jump [ I , 27.

Our I . F . e x p e r i m e n t s a r e n o t i n c o n t r a d i c t i o n w i t h t h e B e r r y ' s i n t e r p r e t a t i o n b u t o u r MAE r e s u l t s , a l l o w i n g a l a r g e r r a n g e of e x p l o r a t i o n s u g g e s t a n o t h e r m o d e l h a - v i n g some a n a l o g y w i t h t h e "hydrogen Cold Work Peak" i n c r y s t a l l i n e i r o n 18, 97:

*

I n a d d i t i o n a l o n g component due t o v a c a n c y c l u s t e r s formed by d i s l o c a t i o n c r o s s i n g i s a l s o f o u n d .

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JOURNAL DE PHYSIQUE

RELAXATION S T R E N G T H I m Oe/G) PERMEABILITY ( G / O r )

1 0 0 2 0 0

T E M P E R A T U R E

(K)

F i g . 3 Magnetic a f t e r e f f e c t and m a g n e t i c r e l a x a t i o n s t r e n g t h measured d u r i n g a l i n e a r i n c r e a s e of t h e t e m p e r a t u r e

a ) a s r e c e i v e d h y d r o g e n a t e d FegoBz0 ( 0 .I S04H2 N ; 150 min ; 0 . 0 3 A cm-2) b ) a s r e c e i v e d h y d r o g e n a t e d Fe40N140P14B6 (0.1 S04H2 N ; 180 min ; 0.1 A cm-2)

4 . 1 . The enhancement o b s e r v e d a f t e r a g i n g ( F i g . 4a - 4b) o r a f t e r c o l d work + a g i n g ( F i g . 4c - 4 d ) s u g g e s t s t h a t some e x t e n d e d d e f e c t s p i n s i m u l t a n e o u s l y Bloch W a l l s and hydrogen a t o m s . These d e f e c t s e x i s t a l r e a d y i n s u f f i c i e n t c o n c e n t r a t i o n i n a s r e c e i v e d s p e c i m e n s , b u t s i m i l a r a d d i t i o n a l d e f e c t s a r e i n t r o d u c e d by c o l d work.

4 . 2 . The l i f e t i m e e x p e r i m e n t s show t h a t p o s i t r o n s a r e t r a p p e d i n some empty s p a c e s which a r e " s m a l l v a c a n c i e s " , t h e s i z e of which b e i n g 0.7 m e t a l l i c a t o m i c volume

[6, 131. But p o s i t r o n t e c h n i q u e s a r e u n a b l e t o d i f f e r e n c i a t e a d i s t r i b u t i o n of i s o l a t e d "small v a c a n c i e s " o r a o n e d i m e n s i o n a l a g g r e g a t e o f s m a l l v a c a n c i e s c o r r e s - ponding t o rows o f some m i s s i n g atoms ( b e c a u s e t h e e l e c t r o n i c d e n s i t i e s i n t h e c e n - t e r of b o t h t y p e o f d e f e c t s a r e p r a c t i c a l l y t h e s a m e ) . The h y p o t h e s i s of " i s o l a t e d s m a l l v a c a n c i e s " l e a d s t o d i f f i c u l t i e s : t h e y a r e much b i g g e r t h a n empty B e r n a l H o l e s (max. s i z e 0.35 R[13]) and v e r y s t a b l e i n c o n t r a d i c t i o n w i t h t h e t h e o r e t i c a l c a l c u l a t i o n s [ 1 4 ] . I n a d d i t i o n , t h e s i z e o f 0 . 7 R a p p e a r s t o b e i n d e p e n d e n t on quenching r a t e 1151

,

c o l d work o r hydrogen c h a r g i n g . T h i s i s i n f a v o u r of t h e hypo- t h e s i s of a "row of m i s s i n g atoms" :

4 . 3 . The s e c o n d h y p o t h e s i s was f o r m u l a t e d p r e v i o u s l y by Kronmuller f o r e x p l a i n i n g t h e low m a g n e t i c p e r m e a b i l i t y of F e 4 0 N i ~ ~ P ~ 4 B g compared t o p e r m a l l o y a l l o y s . These d e f e c t s a r e i n t r o d u c e d by m e c h a n i c a l d e f o r m a t i o n s o c c u r i n g d u r i n g t h e m e l t s p i n n i n g : a model of a row of m i s s i n g atoms imbeded i n a r e l a x e d amorphous s t r u c t u r e ( s e e

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TEMPERATURE (Kl

Fig. 4 Evolution of the magnetic relaxation strength after annealing at 3 0 0 K (1) 3 5 0 K ( 2 ) 4 0 0 K ( 3 ) ; a) and b ) hydrogenated alloys ; c) and d) 1 5 0 0 kg cm-2 cold worked then hydrogenated alloys.

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C9-96 J O U R N A L DE PHYSIQUE

F i g . 12 i n r e f . [16]) f i t s w e l l w i t h m a g n e t i c and n e u t r o n s c a t t e r i n g d a t a [16,17].

The name of t h i s d e f e c t i s " d i s l o c a t i o n d i p o l e l i k e " ( d d l ) , i t i n d u c e s s t r e s s s o u r c e s and i t s c o n c e n t r a t i o n i s 1017 and 1015 mW2 r e s p e c t i v e l y i n a s r e c e i v e d and w e l l a n n e a l e d amorphous s t a t e . T h i s means t h a t i n any c a s e s , amorphous specimens b e h a v e s l i k e s t r o n g l y c o l d worked c r y s t a l l i n e i r o n [18].

4 . 4 . I n c o l d worked and h y d r o g e n a t e d c r y s t a l l i n e i r o n 18 - 111, t r a n s i e n t enhancement of t h e "hydrogen peak" i s f o u n d . I n c o l d worked i r o n , a micromovement i s ob-

s e r v e d a t 4K (KinK m i g r a t i o n s ) ; i n a d d i t i o n t h e l i f e t i m e of p o s i t r o n t r a p p e d i n e d g e d i s l o c a t i o n components i s i n d e p e n d e n t on t h e d e g r e e of c o l d work ( s a t u r a t e d p o s i t r o n t r a p p i n g s t a t e ) . The v a l u e , 155ps, i s s l i g h t l y s m a l l e r t h a n when p o s i t r o n s a r e t r a p p e d i n i s o l a t e d v a c a n c i e s ( 1 7 0 p s ) . Hydrogen c h a r g i n g does n o t c h a n g e s i g n i - f i c a n t l y t h i s v a l u e b e c a u s e i t i s l o c a l i z e d i n c l o u d s s u r r o u n d i n g t h e d i s l o c a t i o n . I n a d d i t i o n , t h e "Hydrogen Snoek peak" d o e s n o t e x i s t i n i r o n

LIZ].

4 . 5 . Tne g r e a t a n a l o g y between t h e c r y s t a l l i n e i r o n and t h e amorphous a l l o y s s u g g e s t s a s i m i l a r mechanism i n b o t h c a s e s . We d e v e l o p now a t e n t a t i v e model : The " d i s l o c a t i o n d i p o l e l i k e " [16] e x i s t s i n h i b h c o n c e n t r a t i o n i n a s r e c e i v e d spe- c i m e n s . They t r a p p o s i t r o n s a s p r e v i o u s l y s u g g e s t e d i n [20 - 211. They a r e t i p l y e d by a d d i t i o n a l c o l d work. They a r e n o t m o b i l e below 400 K, e x c e p t e d a movement a t a n a t o m i c s c a l e p r e d i c t e d i n [19] g i v i n g r i s e t o t h e 4 K r e l a x a t i o n ; ( b e t - ween 400 K and t h e c r y s t a l l i z a t i o n t e m p e r a t u r e , t h e d d l c o l l a p s e , p r o d u c i n g a l o c a l c o l l e c t i v e a t o m i c movement i n d u c i n g TSRO).

When hydrogen i s i n t r o d u c e d , t h e d d l a r e d e c o r a t e d , a s d i s l o c a t i o n s i n i r o n . The t r a n s i e n t s t a t e c o r r e s p o n d s t o s u c c e s s i v e c o n f i g u r a t i o n s of i n c r e a s i n g s t a b i l i t y . A p o s s i b l e e x p l a n a t i o n of t h e m a g n e t i c r e l a x a t i o n i s t h e f o l l o w i n g : t h e hydrogen d e c o r a t e d d d l a r e e n e r g e t i c a l l y f a v o u r i z e d o r n o t , depending on t h e p o s i t i o n of t h e d i p o l e s

2

o r

out

a Bloch Wall. A d e m a g n e t i z a t i o n c r e a t i n g a new d i s t r i b u - t i o n of t h e BLoch Wall changes t h e t o t a l e n e r g y , a n d t h e r e l a x a t i o n comes from t h e l o n g r a n g e m i g r a t i o n of hydrogen f r o m u n f a v o u r i z e d toward f a v o u r i z e d d d l . I n t h i s model t h e d d l a r e n o t m o b i l e ; t h e e n e r g y of 0 . 5 eV a p p e a r s a s a

sum

o f m i g r a t i o n a l and b i n d i n g e n e r g y and t h e h y d r o g e n d i f f u s i o n o c c u r s a s i n c r y s t a l l i n e i r o n w i t h a

s m a l l m i g r a n i o n a l e n e r g y and a r e l a t i v e l y h i g h hydrogen c l o u d - d i s l o c a t i o n b i n d i n g e n e r g y . T h i s model i s p r o b a b l y good f o r a l l Fe b a s e d amorphous a l l o y s .

V - CONCLUSIONS

I n two amorphous a l l o y s we o b s e r v e

- a m a g n e t i c r e l a x a t i o n a t 4 K which i n c r e a s e s w i t h c o l d work

- a m a g n e t i c "hydrogen peak" which b e h a v e l i k e t h e "hydrogen c o l d work peak" i n c r y s t a l l i n e i r o n .

A t e n t a t i v e e x p l a n a t i o n i n t e r m s of " d i s l o c a t i o n d i p o l e l i k e " i n t e r a c t i n g w i t h hy- d r o g e n c l o u d s i s p r o p o s e d . (We k e e p i n mind t h a t o t h e r e x p l a n a t i o n s a r e p o s s i b l e : f o r example a Snoek mechanism w i t h two t y p e s of Bloch W a l l s . C o n c l u s i v e d e t e r m i n a - t i o n s a r e d i f f i c u l t b e c a u s e of t h e s h i f t of t h e peak d u r i n g a g i n g ) .

Acknowledgements

The a u t h o r s a c k n o w l e d g e M. Z. Kabsch f o r e x c e l l e n t t e c h n i c a l a s s i s t a n c e .

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