ROTATION OF GRAINS OF METAL TIPS (ANNIHILATION OF GRAIN BOUNDARIES)

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ROTATION OF GRAINS OF METAL TIPS (ANNIHILATION OF GRAIN BOUNDARIES)

M. Drechsler, S. Ramdani

To cite this version:

M. Drechsler, S. Ramdani. ROTATION OF GRAINS OF METAL TIPS (ANNIHILATION OF GRAIN BOUNDARIES). Journal de Physique Colloques, 1986, 47 (C7), pp.C7-177-C7-181.

�10.1051/jphyscol:1986732�. �jpa-00225925�

ROTATION OF GRAINS OF METAL TIPS (ANNIHILATION OF GRAIN BOUNDARIES)

M. DRECHSLER and S. RAMDANI

CRMCE-CNRS, Universite d'~ix-Marseille II, Campus de Luminy, Case 913, F-13288 Marseille Cedex 09, France

Resume - On d g c r i t d e s e x p e r i e n c e s d a n s un microscope e l e c t r o n i q u e de b a l a y a g e pour S t u d i e r i n s i t u 1 ' 6 v o l u t i o n d ' u n e p o i n t e m e t a l l i q u e (Au). L ' e x i s t e n c e d'une r o t a t i o n d e s g r a i n s d ' u n e p o i n t e q u i c o n d u i t 1 une a n n i h i l a t i o n d e s j o i n t s de g r a i n a 6 t d v i s u a l i s C e , mesurde e t i n t e r p r e t d e .

A b s t r a c t - Experiments i n a s c a n n i n g e l e c t r o n microscope a r e d e s c r i b e d which e n a b l e i n s i t u s t u d i e s o f t h e m o r p h o l o g i c a l e v o l u t i o n of m e t a l t i p s (Au). The e x i s t e n c e o f a r o t a t i o n o f t h e g r a i n s of a p o l y c r y s t a l l i n e t i p , which l e a d s t o an a n n i h i l a t i o n o f g r a i n b o u n d a r i e s , i s d i r e c t l y v i s u a l i z e d , measured and i n t e r - p r e t e d .

1. INTRODUCTION

I n 1966 t h e h y p o t h e s i s was s t a t e d : i f a g r a i n of a p o l y c r y s t a l l i n e m e t a l would b e f r e e t o r o t a t e , i t would do s o u n t i l a minimum f r e e e n e r g y p o s i t i o n would b e r e a c h e d / I / . While t h e r o t a t i o n o f a c r y s t a l on a s u p p o r t t o an e p i t a x i a l p o s i t i o n i s a known phenomenon / 2 / / 3 / / 4 / / 5 / e x p e r i m e n t a l e v i d e n c e f o r t h e r o t a t i o n of a g r a i n of a p o l y c r y s t a l have been d e s c r i b e d o n l y r e c e n t l y a s f a r a s we know 161. I n t h e c o u r s e o f r e c e n t f i e l d e l e c t r o n microscope s t u d i e s o f h e a t e d p o l y c r y s t a l l i n e t i p s (Ni,W), we founds examples of s t r i k i n g image r o t a t i o n s which we e x p l a i n e d by t h e h y p o t h e s i s of t h e e x i s t e n c e of a r o t a t i o n o f t h e t i p end g r a i n / 6 / . Now ( i n t h e c o u r s e o f a n i n s i t u s t u d y o f s u r f a c e s e l f - d i f f u s i o n on g o l d t i p s , s o f a r u n p u b l i s h e d ) we found t o o u r s u r p r i s e t h a t s u c h an h y p o t h e t i c a l r o t a t i o n i s d i r e c t l y v i s i b l e . T h i s p a p e r d e s c r i b e s a n example o f a n i n s i t u o b s e r v a t i o n o f t h i s phenomenon, a p r e l i m i n a r y a n a l y s i s and a f i r s t measurement o f a g r a i n r o t a t i o n ?peed.

2 . EXPERIMENTS

A c o n i c a l Au t i p c r y s t a l of a b o u t 1

i n l e n g t h i s p r e p a r e d by e l e c t r o l y t i c e t c h i n g from a c l e a n (99.99) Au w i r e of 0.125 d i a m e t e r . Such a t i p i s s p o t w e l d e d on a tung- s t e n w i r e h e a t i n g l o o p a s u s e d i n f i e l d e m i s s i o n microscopy. The l o o p h a s a d d i t i o - n a l l y two p o t e n t i a l l e a d s f o r a p r e c i s e t e m p e r a t u r e d e t e r m i n a t i o n by a r e s i s t e n c e measurement. l l i s d e v i c e i s mounted i n a s c a n n i n g e l e c t r o n microscope (SEM) i n such a way t h a t t h e Au t i p can be h e a t e d ( r a n g e 1000 t o 1300 K) d + l r i n g image o b s e r v a t i o n . The image and a l l image changes a r e c o n t i n u o u s l y r e g i s t e r e d by a v i d e o system. The s y s t e m i s e q u i p p e d w i t h a chronometer s o t h a t t h e t i m e of any v i s i b l e p r o c e s s i s d e t e r m i n a b l e d u r i n g o r a f t e r an i n s i t u e x p e r i m e n t .

A d i s a d v a n t a g e of t h e d e s c r i b e d e x p e r i m e n t a l method i s ( a t l e a s t i n t h e p r e s e n t v e r - s i o n ) t h a t o n l y l i t t l e i n f o r m a t i o n i s o b t a i n e d on c r y s t a l l o g r a p h i c o r i e n t a t i o n s . Only i n o u r f o r m e r i n d i r e c t g r a i n r o t a t i o n e x p e r i m e n t s a r e t h e o r i e n t a t i o n s of t h e t i p end c r y s t a l s d e t e r m i n e d from t h e f i e l d e l e c t r o n microscope image 161.

I n o u r f o r m e r e x p e r i m e n t s t h e v r a i n r o t a t i o n was o n l y a h y p o t h e s i s t o e x p l a i n f i e l d

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986732

C7-178 JOURNAL DE PHYSIQUE

e l e c t r o n microscope o b s e r v a t i o n s of an image r o t a t i o n and an image displacement 1 6 1 . I n t h e new type of exgeriments t h e e x i s t e n c e of such a g r a i n r o t a t i o n i s d i r e c t l y v i s i b l e ( f i g . 1 ) . An i n t e r p r e t a t i o n i s shown i n f i g . 2. A f t e r some a n n e a l i n g t h e g r a i n boundaries a r e sometimes v i s i b l e by t h e s u r f a c e grooves formed by s u r f a c e s e l f - d i f f u s i o n a t t h e i n t e r s e c t i o n l i n e between a g r a i n boundary and t h e s u r f a c e

Fig. 1 - T i p - c r y s t a l g r a i n r o t a t i o n as v i s i b l e i n t h e scanning e l e c t r o n microscope (photographs of t h e v i d e o monitor s c r e e n ) :

( a ) Gold t i p a f t e r e t c h i n g . Room temperature micrograph (b) Tip of ( a ) a f t e r a n n e a l i n g (1200 K , 20 seconds)

(c) Tip of Ca) a f t e r l o n g e r a n n e a l i n g time (1200 K , 30 seconds) (d) Tip of ( a ) a f t e r l o n g e r a n n e a l i n g time (1200 K , 90 seconds)

The micrographs b , c and d a r e taken a t 1200 K.

F i g . 2 - I n t e r p r e t a t i o n of f i g . 1 .

( a ) I n i t i a l c o n i c a l t i p e shape. (b) The a n n e a l i n g h a s probably e f f e c t e d g r a i n boun- dary m i g r a t i o n s l e a d i n g t o a c e r t a i n growth of t h e g r a i n s . Signs of g r a i n boundary grooving a r e v i s i b l e on t h e t i p p r o f i l e s o t h a t t h e p o s i t i o n of t h e g r a i n boundaries can b e i n d i c a t e d . The p a r a l l e l l i n e s show s c h e m a t i c a l l y t h e d i r e c t i o n of one type of l a t t i c e p l a n e s which i s d i f f e r e n t f o r each g r a i n . ( c ) Begin of t h e g r a i n r o t a t i o n . Driving f o r c e should be the r o t a t i o n induced r e d u c t i o n of t h e g r a i n boundary energy p e r cmz.

(d) The g r a i n r o t a t i o n h a s l e a d t o a g r a i n b o u n d a r y a n n i h i l a t i o n . A s i n g l e

c r y s t a l i s formed ( p a r a l l e l l a t t i c e p l a n e s ) .

where t h e g r a i n boundary i s s i t u a t e d ( f i g . 1 ) . The a n g l e of a complete g r a i n r o t a - t i o n on d i f f e r e n t t i p s v a r i e s i n t h e range between 2' and 45'. I n t h e former s t u d y a g r a i n r o t a t i o n was always a very r a r e e v e n t 161. However i n t h e new experiments g r a i n r o t a t i o n s a r e o f t e n found perhaps on 30% o r 40% of a l l t i p s . Why some g r a i n s do r o t a t e and o t h e r do n o t i s n o t s u f f i c i e n t l y c l e a r . On t i p s of g r e a t e r cone angle

( > % l o 0 ) g r a i n r o t a t i o n i s n o t found. Sometimes t h e r o t a t i o n of more than one g r a i n appears on a t i p . The appearence of 2 g r a i n r o t a t i o n s i s v i s i b l e i n f i e 1.

I n a n o t h e r example 4 g r a i n r o t a t i o n s appeared on a t i p thus forming a zig-zag shape t i p . The frequency of t h e r o t a t i o n s i s n o t a r b i t r a r y . Usually t h e g r a i n a t t h e t i p apex r o t a t e s f i r s t . A f t e r a pause r o t a t e s t h e second (neighbored) g r a i n , e t c . ..

A measurement of t h e g r a i n r o t a t i o n a n g l e a s a f u n c t i o n of time i s shown i n f i g . 3 . Such a diagram allows t h e d e t e r m i n a t i o n of t h e a n g u l a r g r a i n r o t a t i o n speed ( d 6 / d t ) a s a f u n c t i o n of t h e r o t a t i o n angle ( f i g . 4 ) . The two maxima of t h i s curve c o r r e s - pond probably t o two maxima of t h e g r a i n r o t a t i o n d r i v i n g f o r c e .

J

8 20 40 80 88

time in seconds

F i g . 3 - Measured r o t a t i o n a n g l e 6 ( s e e f i g . 2b) v e r s u s time

grain misorientation angle

rotat ion angle P

F i g . 4 - Measured r o t a t i o n speed d 6 / d t v e r s u s t h e r o t a t i o n a n g l e B

( u s i n g d a t a o.f fig'. 3)

C7-180 JOURNAL DE PHYSIQUE

4. DISCUSSION

4.1. Apparent and r e a l r o t a t i o n a n g l e

The i n i t i a l t i p a x i s l i e s i n a p l a n e which i s p a r a l l e l t o t h e microscope image plane. When a t i p end g r a i n r o t a t e s , t h e g r a i n a x i s can remain i n t h i s p l a n e . The measurable angle B ( f i g . 2) i s i n t h i s case e q u a l t o t h e r e a l r o t a t i o n angle

. ^{I n}

t h e g e n e r a l c a s e however t h e r o t a t i n g g r a i n a x i s l i e s n o t i n t h i s p l a n e . Then B i s only an a p p a r e n t r o t a t i o n a n g l e . I n t h e g e n e r a l c a s e t h e l e n g t h of t h e g r a i n a x i s (1,) must have i n t h e image a reduced l e n g t h (1,) ( f i g . 5 ) . The a n g l e ( r ) between lo and i t s image p r o j e c t i o n 1, i s

(1) cosy

1 r / l o

The a n g l e s B , y and $ form a r e c t a n g u l a r s p h e r i c a l t r i a n g l e where B and y a r e t h e s m a l l s i d e s and $ t h e hypothenuse, s o t h a t

I

(2) cos $

cosy cosB

ifl ^{cos B}

0

A s B and 1,/1, a r e measurable, t h e r e a l r o t a t i o n angle $ can b e determined approxi- mately i n t h e g e n e r a l c a s e .

Fig.5 - Scheme t o determine t h e r e a l r o t a t i o n a n g l e

The example i n f i g . 1 corresponds n e a r l y t o t h e s p e c i a l c a s e 1,

1, where t h e a p p a r e n t r o t a t i o n a n g l e i s p r a c t i c a l l y e q u a l t o t h e r e a l r o t a t i o n a n g l e .

4.2. On t h e mechanism o f g r a i n r o t a t i o n and g r a i n boundary a n n i h i l a t i o n

The d r i v i n g f o r c e f o r t h e observed g r a i n r o t a t i o n i s probably based on t h e r e d u c t i o n of t h e f r e e energy of t h e t i p c r y s t a l , t h a t i s by a continuous r e d u c t i o n of t h e g r a i n boundary energy p e r cm2 (E) down t o z e r o . It i s known t h a t E i s a f u n c t i o n of t h e m i s o r i e n t a t i o n a n g l e 0 ( s e e / 4 / / 5 / / 7 / / 8 / ) and t h e d r i v i n g f o r c e may b e p r o p o r t i o n a l t o d ~ / d @ . Consequently c o r r e l a t i o n s can b e expected between t h e measured r o t a t i o n speed ( f i g . 4) and t h e corresponding f u n c t i o n of E v e r s u s t h e m i s o r i e n t a t i o n a n g l e . The model of t h e g r a i n r o t a t i o n r e q u i r e s t h a t many atoms a r e t r a n s p o r t e d d u r i n g t h e r o t a t i o n from one s i d e of t h e t i p t o t h e o t h e r s i d e i n t h e r e g i o n of t h e g r a i n boun- dary ( a s i n t h e s i n t e r experiments of metal b a l l s /4/) o v e r d i s t a n c e s i n t h e o r d e r of one o r a few pm. Such r e l a t i v e l y long t r a n s p o r t ways may e x p l a i n why g r a i n r o t a - t i o n s a r e found only a t temperatures n e a r t h e m e l t i n g p o i n t where t h e i n t e r f a c e d i f - f u s i o n c o e f f i c i e n t must be p a r t i c u l a r g r e a t .

The g r a i n r o t a t i o n could a l s o b e e x p l a i n e d a s an "apparent" r o t a t i o n , t h a t i s a s a

p e r i o d i c a l l y r e p e a t e d s l i p ( s e e / l o / ) of a s e r i e s of p a r a l l e l l a t t i c e p l a n e s of t h e

r o t a t i n g g r a i n ( a s a l a t e r a l displacement of a pack of c a r d s ) . W e b e l i e v e t h a t such

a model do n o t apply i n our c a s e mainly f o r two reasons : ( 1 ) t h i s model r e q u i r e s a

/4/ t h i s model would r e q u i r e a deformation of t h e s p h e r e s t o e l l i p s o i d s a r e s u l t which h a s n o t been found e x p e r i m e n t a l l y .

4.3. On t h e carbon contamination

The s u r f a c e of t h e Au t i p i s probably covered w i t h a carbon l a y e r . This l a y e r h a s probably an i n f l u e n c e on t h e morphological e v o l u t i o n of t h e t i p by (1) a r e d u c t i o n of t h e s u r f a c e f r e e energy of Au, (2) a r e d u c t i o n of t h e Au s u r f a c e s e l f - d i f f u s i o n c o e f f i c i e n t ( a s i n t h e case of C on W 191) which i s confirmed by p r e l i m i n a r y measu- rements and (3) a r e d u c t i o n of t h e f r e e e v a p o r a t i o n of Au which seems t o b e c o n f i r - med by some of o u r experiments.

5. CONCLUSIONS

(1) The h y p o t h e s i s t h a t g r a i n s of m e t a l t i p s can r o t a t e a t e l e v a t e d temperature and t h a t such a r o t a t i o n l e a d s t o an a n n i h i l a t i o n of a g r a i n boundary /6/ i s d i r e c t l y confirmed b y t h e new SEM experiments.

(2) It i s shown t h a t a s p e c t a c u l a r morphological c r y s t a l change can b e produced by a g r a i n boundary d r i v i n g f o r c e .

(3) Apparent and r e a l g r a i n r o t a t i o n a n g l e s a r e measurable.

(4) The use of a video r e g i s t r a t i o n e n a b l e s a measurement of t h e g r a i n r o t a t i o n speed a s a f u n c t i o n of t h e r o t a t i o n angle.

(5) I n c o n t r a r y t o o u r former s u p p o s i t i o n s i t i s shown t h a t g r a i n r o t a t i o n s combined w i t h g r a i n boundary a n n i h i l a t i o n i s under c e r t a i n c o n d i t i o n s a v e r y f r e q u e n t pheno- menon.

(6) When a t i p h a s s e v e r a l g r a i n s t h e tip apex g r a i n r o t a t e s f i r s t . Then a f t e r a pause t h e second g r a i n may r o t a t e , e t c . I f s e v e r a l g r a i n s r o t a t e , a s i n g l e c r y s t a l of zig-zag shape can be formed.

ACKNOWLEDGEMENTS

We acknowledge f r u i t f u l d i s c u s s i o n s w i t h Prof. A. Baronnet and P r o f . J.M. Bermond.

We a l s o thank S. Nitsche and F. Q u i n t r i c f o r v e r y h e l p f u l t e c h n i c a l a s s i s t a n c e . REFERENCES

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/2/ J.J. MGtois, M. Gauch, A . Masson and R. Kern : Thin S o l i d Films 11 (1972) 205- 2 18

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