HIGH RESOLUTION STUDY OF DIFFERENT STRUCTURAL UNITS IN A Mo TILT BOUNDARY

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HIGH RESOLUTION STUDY OF DIFFERENT STRUCTURAL UNITS IN A Mo TILT BOUNDARY

J. Penisson, T. Nowicki

To cite this version:

J. Penisson, T. Nowicki. HIGH RESOLUTION STUDY OF DIFFERENT STRUCTURAL UNITS IN A Mo TILT BOUNDARY. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-305-C1-310.

�10.1051/jphyscol:1990148�. �jpa-00230307�

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HIGH RESOLUTION STUDY OF DIFFERENT STRUCTURAL UNITS IN A M O TILT BOUNDARY

J.M. PENISSON and T. NOWICKI*

Departement d e Recherche Fondamentale, Service de Physique, CENG, 85X, F-38041 Grenoble Cedex, France

Ecole Nationale Superieure des Mines, 158 Cours Fauriel, F-42023 Saint Etienne Cedex, France

Resume ^-Le coeur des d i s l o c a t i o n s primaires d'un j o i n t de f l e x i o n dans du molybdene a e t e e t u d i e par microscopie e l e c t r o n i q u e i haute r e s o l u t i o n . L e s micrographies ont 6 t e p r i s e s dans des c o n d i t i o n s a s s u r a n t une p r o j e c t i o n e x a c t e de l a s t r u c t u r e . La mesure, r e a l i s e e s u r 55 d i s l o c a t i o n s , des d i s t a n c e s interatomiques dans l e coeur montre que l a grande m a j o r i t e d ' e n t r e e l l e s s u i t l e modele c l a s s i q u e du prisme i base t r i a n g u l a i r e . Un p e t i t nombre montre u n c o n t r a s t e d i f f e r e n t dans lequel une colonne atomique supplementaire e s t p r e s e n t e . Les images experimentales de c e s 2 types de coeur s o n t comparees a des images simulees e t l e s f a c t e u r s gouvernant l a v i s i b i l i t e de c e t t e colonne supplementaire s o n t analyses.

Abstract - The d i s l o c a t i o n core s t r u c t u r e has been s t u d i e d by HREM. Using s u i t a b l e c o n d i t i o n s , an exact p r o j e c t i o n of t h e s t r u c t u r e can be obtained. 55 d i s l o c a t i o n images were s t u d i e d and i f most of them a r e i n agreement with t h e capped t r i a n g u l a r prism model a small number e x h i b i t s a d i f f e r e n t c o n t r a s t which i s i n t e r p r e t e d as t h e presence of an e x t r a atomic column i n s i d e t h e core. Experimental images a r e compared t o simulated ones and t h e f a c t o r s governing t h e v i s i b i l i t y of t h i s e x t r a atomic column a r e discussed.

1 - INTRODUCTION

The physical p r o p e r t i e s of g r a i n boundaries a r e s t r o n g l y dependant on t h e i r atomic s t r u c t u r e and t h e determination of t h i s s t r u c t u r e i s an important problem. In metals and metal1 i c a l l o y s , t h e main atomic spacings a r e i n t h e 2 A range and i t i s only s i n c e t h e a p p a r i t i o n of t h e new generation of ' v e r y high r e s o l u t i o n microscopes t h a t s t r u c t u r e determination i s p o s s i b l e /1,2/. I t has been shown /3/ t h a t i f well defined experimental conditions a r e used

,

t h e high r e s o l u t i o n images r e p r e s e n t a t r u e p r o j e c t i o n of t h e s t r u c t u r e . A d i r e c t comparison between c a l c u l a t e d models and experimental images i s then p o s s i b l e . Atomistic c a l c u l a t i o n s c l e a r l y reveal t h a t several metastable c o n f i g u r a t i o n s of t h e s t r u c t u r a l u n i t s a r e p o s s i b l e /4/ and t h a t t h e d i s l o c a t i o n c o r e s can be a p r e f e r e n t i a l s i t e f o r segregation /5/. In t h i s paper, a [l001 molybdenum t i l t boundary has been observed using 400 kV high r e s o l u t i o n e l e c t r o n microscopy. Atomistic models have been computed using a s t a t i c r e l a x a t i o n method and several d i f f e r e n t i n t e r a t o m i c p o t e n t i a l s were used f o r pure molybdenum a s well a s f o r t h e i n t e r a c t i o n between MO and t h e segregated i m p u r i t i e s ( mainly oxygen ) . The c o n t r a s t of t h e d i s l o c a t i o n c o r e s i s analysed i n d e t a i l and compared t o simulated images. This comparison r e v e a l s t h a t a g r e a t number of t h e d i s l o c a t i o n c o r e s i s i n agreement with t h e model of an empty capped t r i a n g u l a r prism /6/.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990148

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Cl-306 COLLOQUE DE PHYSIQUE

A small number o f them however has a d i f f e r e n t c o n t r a s t which can be i n t e r p r e t e d as t h e presence i n s i d e t h e core o f an e x t r a atomic column. T h i s column can be t o t a l l y o r p a r t i a l l y f i l l e d .

11 E l e c t r o n m i c r o s c o ~ v and irnaqe s i m u l a t i o n s

Specimens were observed i n a JEOL 4000-EX microscope working a t 400 kV. A l l t h e d e t a i l s concerning t h e b i c r y s t a l e l a b o r a t i o n and specimen p r e p a r a t i o n have already been pub1 ished /l/. The observed boundary i s a pure t i l t w i t h a 14 " angle &d i t s geometry can be found

i n r e f /l/.

II 1 Imasinq c o n d i t i o n s

A l l t h e images were taken a t Scherzer defocus ( az = - 450 A a t 400 kV ) so t h a t t h e atomic p o s i t i o n s are b l a c k i n t h e 50 t o 150 A t h i c k n e s s range which i s t h e usual thickness f o r our specimens /7/. Atomic p o s i t i o n s were manually p l o t t e d on t r a n s p r a r e n c i e s . The accuracy o f t h i s p l o t i s estimated t o .25 A and represents t h e l i m i t a t i o n o f t h e method.

A1 l the i n t e r a t o m i c d i s t a n c e s accross t h e boundary were measured. The exact d i s l o c a t i o n core p o s i t i o n can be determined using t h e f a c t t h a t t h e b a s i s o f t h e p r i s m i s t h e smallest i n t e r a t o m i c d i s t a n c e across the boundary as i t w i l l be seen l a t e r .

E I m a q e simul a t i o n s

A m u l t i s l i c e program i n which t h e absorption can be i n c l u d e d was used t o simulate t h e h i g h r e s o l u t i o n images and t o c a l c u l a t e t h e amplitudes and phases o f t h e d i f f r a c t e d beams which c o n t r i b u t e t o t h e images. A l l t h e images were c a l c u l a t e d i n a 128 X 128 s i z e which contains one p e r i o d o f t h e boundary. I n these c o n d i t i o n s

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t h e sampling i s 0.22 A and i t should be remarked t h a t t h i s value i s s m a l l e r than t h e experimental e r r o r i n the p o s i t i o n i n g o f t h e atomic columns. Two d i f f e r e n t thicknesses were choosen : 50 and 100 A.

The s l i c e t h i c k n e s s i s 3.14 A corresponding t o t h e p e r i o d i c i t y along t h e [l001 a x i s . A l l t h e parameters concerning t h e microscope s e t t i n g a r e those determined by Bourret and Penisson /8/.

UI A t o m i s t i c c a l c u l a t i o n s

A l l these c a l c u l a t i o n s have been done using a s t a t i c r e l a x a t i o n method d e r i v e d from t h e one described by Hasson e t a l . /g/. 3 d i f f e r e n t coincidence boundaries which are i n t h e v i c i n i t y o f t h e experimental t i l t angle were c a l c u l a t e d :

llI I I n t e r a t o m i c ~ o t e n t i a l s .

Two d i f f e r e n t i n t e r a t o m i c p o t e n t i a l s were used t o d e s c r i b e t h e MO-MO i n t e r a c t i o n . The f i r s t one i s t h e Mie p o t e n t i a l g i v e n by the expression :

A and B are constants which have been adjusted t o t h e vacancy energy f o r m a t i o n and t o t h e b u l k modulus. T h i s p o t e n t i a l i s t r u n c a t e d between t h e second and t h i r d neighbors.

The second p o t e n t i a l has been described by F i n n i s ans S i n c l a i r /10/. I n t h i s p o t e n t i a l , t h e r e p u l s i v e i n t e r a c t i o n i s described by a c e n t r a l p a i r p o t e n t i a l w h i l e t h e cohesive p a r t i s an N-body term.

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E 2 Grain boundarv structures

Pure boundaries : In the ³different coincidence boundaries and with the 2 potentials, the primary dislocation core has the same structure. It is constituted by a capped triangular prism identical to the description given by Vitek et al. /6/. The grain boundary plane is a symmetry plane and there is no rigid body translation between both crystals. A careful inspection of the dislocation core structures show that the differences between the different coincidences are too small to be experimentally detected. The basis of the prism is equal to 0.82a

.

This value is smaller than the first nearest neighbors distance which is 0.87a in the MO bcc structure. This fact has already pointed out by Wolf /11/. If all the interatomic distances across the boundary plane are measured, it can be seen that the basis of the prism is the smallest : fig 1. This remark is very useful1 to determine the exact position of the dislocation core.

a b

Fig 1 : Calculated models : a Z = 41 b Z = 2 5

The interatomic distances across the boundary are listed in a units. In both cases, the basis of the prism is the smallest distance.

Seqreqated boundaries : the segregation sites have been determined using the same calculation procedure as for the pure boundary. Two kinds of sites were investigated :

interstitial and substitutional ones. The most favorable interstitial site is the center of the prism and when substitution is envisaged

,

the site labelled 6 on fig 1 is favored.

E Results and inter~retation

Fig 2 represents a high resolution image of the grain boundary. This picture was taken at Scherzer defocus so that the atomic positions are black. The atomic calculations showed that the basis of the prism representing the dislocation core is the smallest interatomic distance across the boundary. All these distances were measured on the pictures and 55 primary dislocation cores have been analysed. In order to determine if interstitial segregation can be detected, images have been simulated from both models with the same set of parameters as the experimental ones. Several defocus values were used and it results that only Scherzer defocus give a true projection of the structure. If, for example, the the second defocus band giving white atomic positions is used, some distortions appear

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along t h e boundary and t h e i n t e r p r e t a t i o n becomes hazardous.

F i g 2 : 400 kV e l e c t r o n micrograph o f t h e 14 " t i l t boundary. Scherzer defocus. t h e atomic p o s i t i o n s are b l a c k .

The empty p r i s m i s e a s i l y recognised because o f t h e presence o f a l a r g e w h i t e t r i a n g u l a r d o t i n s i d e t h e prism. T h i s f e a t u r e i s very c h a r a c t e r i s t i c and appears o n l y a t Scherzer defocus and i f t h e specimen t h i c k n e s s i s l e s s than 150 A. The s u p e r p o s i t i o n t o t h e simulated image o f t h e s e t o f atomic p o s i t i o n s shows t h a t t h e image i s a t r u e p r o j e c t i o n o f the s t r u c t u r e . I n p a r t i c u l a r , t h e b a s i s o f t h e prism i s reproduced w i t h t h e r i g h t value ( f i g 3 a )

When i n t e r s t i t i a l segregation i s present i n t h e c e n t e r o f t h e p r i s m ( f i g 3b), t h e c o n t r a s t i s d i f f e r e n t : t h e w h i t e t r i a n g u l a r spot i s replaced by a b l a c k spot l o c a t e d on the segregated atom. This s t r o n g d i f f e r e n c e i n d i c a t e s t h a t these two d i f f e r e n t s t r u c t u r e s should be e x p e r i m e n t a l l y d i f f e r e n c i a t e d .

When t h e experimental images are compared t o t h e simulated ones, t h e y can be arranged i n 3 d i f f e r e n t classes. The g r e a t m a j o r i t y o f t h e cores are i n agreement w i t h t h e empty prism.

I n t h i s case, t h e w h i t e t r i a n g u l a r spot i s present as i n f i g 4 a. The mean basis o f the prism i s 0.846a. T h i s value i s l a r g e r than t h e value deduced from t h e models ( 0.82a ) b u t smaller than t h e f i r s t nearest neighbor d i s t a n c e (0.866a ) . 70 % o f t h e cores belong t o t h i s c l a s s .

I n 15 % of t h e images t h e c o n t r a s t can be a t t r i b u t e d t o t h e presence o f an e x t r a atomic column i n s i d e t h e core. F i g 4b represents an example o f t h i s c l a s s o f images. The former white t r i a n g u l a r spot i s no more present and i n t h e c e n t e r o f t h e p r i s m a b l a c k spot i s now present. The c o n t r a s t a t t h e c e n t e r can be s l i g h t l y d i f f e r e n t from one d i s l o c a t i o n t o another one. This f a c t suggests t h a t e i t h e r t h e occupancy o f t h e segregation s i t e i s n o t constant along the column o r t h e segregated atom i s c h e m i c a l l y d i f f e r e n t . This p o i n t has been i n v e s t i g a t e d u s i n g t h e s i m u l a t i o n . I t appears t h a t , f o r a 100 A t h i c k specimen and using oxygen, carbon and molybdenum as segregant species, no d i f f e r e n c e has been found between oxygen and carbon because these elements are very close t o each o t h e r i n the p e r i o d i c t a b l e . A small d i f f e r e n c e i s present between 0 and MO b u t i t seems very d i f f i c u l t t o r e l y t h i s d i f f e r e n c e t o t h e experimental s i t u a t i o n . T h e occupancy o f t h e c e n t e r o f t h e prism has been t e s t e d o n l y using oxygen: t h e presence o f segregation i s d e t e c t a b l e i f a t

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b : an oxygen atomic column i s present i n t h e c e n t e r o f t h e p r i s m

The c o n d i t i o n s used i n these images are : V = 400 kV; 6z = - 450 A; specimen t h i c k n e s s t =

100 A; CS = 1 mm; defocus spread Az = 70 A; beam divergence 0 = .0007 mrad

a

F i g 4 Experimental images o f both types o f d i s l o c a t i o n cores b

a : empty capped t r i a n g u l a r prism

b : an e x t r a atomic column i s present i n t h e c e n t e r o f t h e p r i s m

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least half the column is filled. The measurement of the basis of the prism did not reveal the small relaxation introduced by the segregated atoms. In fact, the calculations of the different structures showed that this relaxation is in any case less than 0.15 A so that it is smaller than the experimental precision in the determination of the atomic positions.

The last 15 % of the dislocation core images have a different contrast. In general, the image is not symmetric with respect to the grain boundary plane. and the core appear distorted. Among the possible causes of this distortion, a substitutional segregation on the 6 sites and a possible inclined dislocation line are the most likely causes.

P Conclusion

When the conditions which give a true projection of the structure are used, it can be shown that several types of dislocation images are present in the boundary. The bicrystal being very pure, the majority of the primary dislocations is in agreement with the capped triangular prism with a basis slightly smaller than the first nearest neighbor distance.

For this type of images there is a good correspondance between experimental and calculated images. But if other conditions are used, some imaging artefacts are present which can lead to an erroneous interpretation of the images. Among the dislocation images which are different from the empty prism, a small number can be interpreted as containing an extra atomic column in the center. The chemical nature of this column could not be determined so that it is not possible to say if a small of segregation is present or i f this empty core is the image of a different structural unit.

REFERENCES

/l/ PENISSON J.M., NOWICKI T.N. and BISCONDI M. Phil. Mag. A 58,947,1988.

/2/ MILLS M. and STADELMAN P.to be published.

/3/ BOURRET A., ROUVIERE J.L. and PENISSON J-M. Acta Cryst. A44,838,1988.

/4/ WANG G.J., SUTTON A. P. and VITEK V. Acta Met. 32,1093,1984.

/5/ NOWICKI T.N., PENISSON J-M. and BISCONDI M. Journal de Physique C5,403,1988.

/6/ VITEK V., SMITH D.A. and POND R.C. Phil. Mag. A5,649,1981.

/7/ PENISSON J-M. Journal de Physique C5,87,1988.

/8/ BOURRET A. and PENISSON J. -M. JEOL News 25E, 1,1987.

/9/ HASSON G., BOOS J.-Y., HERBEUVAL I., BISCONDI M. and GOUX C. Surf. Sci. 31,115,1972.

/10/ FINNIS M. W. and SINCLAIR J. E. Phil. Mag. A50,45,1984.

/11/ WOLF D. to be published.