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GRAIN-BOUNDARY DISLOCATIONS STRUCTURE AND MOVEMENT IN Σ=9 STUDIED BY HREM
J. Thibault-Desseaux, M. Elkajbaji
To cite this version:
J. Thibault-Desseaux, M. Elkajbaji. GRAIN-BOUNDARY DISLOCATIONS STRUCTURE AND
MOVEMENT IN Σ=9 STUDIED BY HREM. Journal de Physique Colloques, 1988, 49 (C5), pp.C5-
283-C5-288. �10.1051/jphyscol:1988533�. �jpa-00228030�
JOURNAL DE PHYSIQUE
Colloque C5, supplément au n°10, Tome 49, octobre 1988 C5-283
GRAIN-BOUNDARY DISLOCATIONS STRUCTURE AND MOVEMENT IN Z=9 STUDIED BY HREM
J. THIBAULT-DESSEAUX and M. ELKAJBAJI
Département de Recherche Fondamentale/Service de Physique, Centre d'Etudes Nucléaires, 85X, F-38041 Grenoble Cedex, France
Résumé : L'évolution, lors de la déformation, de la structure d'un joint de grains est étudiée par MEHR. La décomposition des dislocations entrantes, en dislocations du réseau DSC, est mise en évidence. La structure du coeur des dislocations DSC, ainsi que leur mouvement sont déterminés à l'échelle atomique. Le déplacement des dislocations du joint se tait par glissement et/ou par montée. La montée conservatrice est mise en évidence. La migration du joint est discutée.
Abstract : The evolution of grain-boundary structure after deformation has been studied by HREM. The decomposition of the runnlng-ln dislocations Into DSC perfect dislocations has been shown. The structure of the residual dislocations was determined and their movement established at atomic scale. Glide and climb motion are involved.
Conservative climb Is demonstrated and the GB migration examined.
Recently a number of detailed results were obtained on the interaction between g r a i n - boundary and dislocations created during the deformation of blcrystals of Si or Ge. These experiments were performed by in-sltu high voltage electron microscopy and X-Rays topography / l . 2 / . This has permitted confirmation of a number of ideas, namely, that the grain-boundary (GB) acts as a strong barrier for bulk gliding dislocations, though dislocations, with Burgers vector common to both grains slip system, can cross the GB. It was also clear that dislocations entered the GB. but their final state has not been revealed, although it was suspected that they dissociate into DSC dislocations / 3 / as shown in early work by Bollman et ai. / 4 / in stainless steel. The study in near coincidence grain-boundary has led several authors to assert that the lattice dislocations dissociate discretely into DSC dislocations within the G B / 5 . 6 , 7 / . On the other hand, the fact that the contrast of the dislocation generally vanishes when it enters a general GB. argues in favour of a continuous spreading of the core within the G B / 8 . 9 . 1 0 7 . The situation still remains unclear, (see review paper 11). However, even in the coincidence GB case, local mechanisms of entrance, dissociation and movement of the dislocations have not been yet clearly determined. High resolution electron microscopy (HREM) investigations have allowed determination of mechnisms of local interactions between dislocations and G B / 1 2 . 1 3 / . in the case of the deformation of a £=9(122) tilt bicrystal in silicon ( e = 3 8 ° . 94 around the C011] common axis). Details of the deformation and the specimen preparation conditions have been given elsewere / 1 3 . 1 4 / . The results presented here concern blcrystals strained at 850°C in compression In such a way that the dislocations gliding on the primary slip planes can be seen end-on in the microscope : they are lying along the [ O i l ] direction which is the tilt axis of the bicrystal and is parallel to the electron beam. The aim of this article is to describe the structure of the GB dislocations (GBD's) resulting from the entrance of the lattice dislocations gliding towards the GB and to make some propositions for their movement within the GB
DISLOCATION ENTRANCE AND DECOMPOSITION / 1 2 . 1 3 . 1 4 /
The deformation induced dislocations glide on the ( T T l ) | primary planes and the ( T l T ) | additional planes. They are 60° and screw dislocations. It has been clearly shown that the gliding dislocations remain dissociated into two Shockley partlals untlll they reach the GB.
"present address : Faculte des Sc!ences/D6partement de Physique AGADIR -MORROCCO
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988533
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The Burgers vector and the height of the associated step are determined,directiy from the HREM images, according to the method described in /15/. In the following. we only refer the GBD's to one of the grains (grain I). Details of all the reactions in both grains will be given ( M. ELKAJBAJI. J. THIBAULT-DESSEAUX to be published)
On the primary planes. the leading 90° Shockley partial b g o ' a / 6 1 ~ 1 i l 1 (of the incoming 600 disiocatlon) , enters the GB where it decomposes into two perfect GB dislocations. with Burgers vectors which are _eie_mentary vectors of the DSC lattice (DSCL). according to
bg0P(o ---> aI9C1221 + a / 1 8 c i i l 1
or
---
> bc + bg (fig. 1.2)bc is perpendicular to the GB plane and the 1011 I tilt axis. thus it is sessile in the GB.
wheras bg is perpendicular to tilt axis and
contained
in the GB and consequently it Is gllssiie along the GB. The 30° trailing partial b30P=a/6C121 11 enters next. Generally. the residual dislocation at the impact point is found decomposed into the two initial GBD's bc and b30 which have a rctpuisive interaction. The gllssile residue bg glides away (fig. 1). The global decomposition Isb60P(1) ---> a / 9 c i 2 2 1 + a/6ci211
+
a / 1 8 [ i i 1 ] or---
> bc + b301- + bg (fig. 1.2)( - ) index for the negative screw component of the b30 parallel to C0113 (fig. 2 ) . The matrix screw dislocations have never been detected decomposed within the GB. although both b30 DSCL residual clisiocations are in repulsive interaction. Two decompostion reactions would have been observed according to
bsc( 1) ---> b301-
+
b303- or ---> b302-+
b 3 ~ 4 -The entrance of the 60° dislocations on the additional ( i 1 i ) i glide planes leads to the foliowing residues :
b60a(l) ---> b304- + 2 bg (fig. 2)
Due to the easy giide of the two latter GBD's. oniy the isolated b304- is observed. it will be noted that , in this case (entrance on the additional planes). the initial Shockley partials do not belong lo the DSCL.
GBD INTERACTIONS
Details of all the reactions occuring are given elsewhere /13. M. ELKAJBAJI. J. THIBAULT- DESSEAUX to ble published/. We only present here two basic reactions. As the GBD bg is gllssile. it can react with other GBD's. for instance with bc, according to
bc + bg ---> bgo
We have observed other residues. which can be unamblguousiy attributed to the attractive reaction between two b30 dislocations arriving from the entrance and the decomposition of two 60° dislocations coming on both sides of the GB almost at the same impact point :
bgOl-
+
b302+---
> bc (eq. 1)As a conseqence of this, we beileved that the GBD's giide and climb within the GB. From the study of the residual GBD's detected in the GB. we also conclude. that the GB can act as a source of dislocations /13.14. M. ELKAJBAJI, J. THIBAULT-DESSEAUX to be published/ : screw dislocations can be produced during some of the reactions. so they can cross-slip onto one or tho other slip system.
GBD's STRUCTlJRE
The structure of the f=9( 122) GB has been given previously both in germanium/l6.17/ and silicon /13/. The period can be described in the structural units concept by two mirror deduced identical seven-five atom rings L and L' /18/. The structure of the GB defects will be depicted with respect to this basic conflguratlon.
We have to emphasize that the GBD cores are not extended : they are well localized and cannot be described as a continuous distribution of Burgers vectors. We have established the structure of the GBD's which are perfect elementary DSCL dislocations : bc. b30, bg, the structure of bgo will be also presented.
b, structure (fig. 3a) : this GBD is not accompanied by a GB step. it only inserts a boat- shaped six-atom ring ( T unit /18/) in a L or L' basic unit. it oniy affects half a GB period.
i. e. half a CSL period. Two bc could be introduced per period leading to a C = l l GB/13/.
Ail the bonds could be reconstructed. Futhermore. as shown already theoretically /3.19/
the structure can be easily related to the DSC lattice distortions. The overlap of the HREM image with the DSC lattice is reported fig. 4.
bg structure (fig.3b) : this GBD is accompanled by the GB step height of -a/3 perpendicular to the GB plane (initial dislocation from grain I). The sign of the Burgers vector and the one of the associated step depend in which grain the initial dislocation arrives. bg inserts a complete seven-five atom ring in the original sequence i. e. an L basic structural unit. Depending on the sign of the Burgers vector. there are two possible positions for this additional unit, between the two original L or L' units : in fact this corresponds to either an L or L' introduction. In either case the basic structure of the GB is not greatly disturbed and the bonds reconstruction is not difficult to achieve.
baO1- structure (fig. 3c) : the height of the step associated with this GBD is I . 75a/3. The b30 introduces two non complete six-atom rings. As in the case of a 300 Shockley partial in the bulk. bonds reconstruction leads to a doubling of the period along the dislocation Ilne.
The alignment of the four seven atom rings is characteristic of this defect. This does not mean that the core is extended : the defect is defined within the GB by the two additional six-atom rings which can be weir positionned.
bgo structure (fig. 3d) : this GBD is accompanled by a GB step of height -a/3. it inserts. in the original sequence, between the L and L' units. a complexe structural unit composed of a seven-five atom ring and a six atom ring : this complex unit Is the ' summ ' of the units arriving from the bc and the bg GBD's. and the step is the summ of the b, and bg associated steps
GBD's MOVEMENT WITHIN THE GB
The fact that the bc and the b30 GBD's arriving from the decomposition of an extrinsic dislocation are found at some distance from each other. and the fact that the residual GBD's can be attributed to interaction of two b30 GBD's (eq. 1). are strong arguments for the GBD's climb and giide motion
bc ciimb : the movement of bc along the GB is pure climb. The mechanism involved Is depicted on fig. 5a. During the displacement of bc by one half a CSL period along the GB plane. one cfc lattice point defect is affected, 1.8. two crystal point defects (or 2 lattice point defects per CSL period i. e. 4 crystai point defects). On the other hand bc can be considered as a perfect DSCL dislocation
.
The DSCL is denser than the bulk lattice, but iess occupied : in an a3 volume of the crystai there are 4 cfc sites. 8 atoms. whereas there are 36 DSCL sites. thus the occupancy of the DSCL is 2/9 atom per site (1/9 cfc site per dscl site). The ciimb motion in the DSCL consumes DSCL point defect which might be regarded as fractional point defect of the crystal. One DSCL point defect corresponds to a fractionai point defect : 1/9 cfc lattice point defect or 219 crystai point defect. It will be noted that as bc has no associated step. the GB does not migrate when bc moves.b glide : this GBD motion is pure giide (fig. 5b). The displacement of the core from one C%L period is obtained by a simple lateral shift of one L or L' unit which replaces the adjacent six atom ring. Bond breaking and rearrangement only occur. Depending on the direction of motion. the affected L or L' unit Is located above or below the characteristic additional L unit. Because of the associated step. the GB migrates during the GBD gliding.
It should be pointed out that GB migration occurs without the aid of point defects diffusion.
bgO1- climb and glide : the displacement of this GBD along the GB involves both climb and glide (fig. 512). The fractional point defect concept may be applied. This lead us to believe that the motion of b30 on one CSL period involved 9 perfect DSCL defects i. e. 2 crystal point defects. in this case the GB migrates when b30 moves along the GB plane and this requires diffusion of point defects.
CONSERVATIVE AND NON CONSERVATIVE CLIMB WITHIN THE GB
There is some evidence that conservative cilmb occurs In the GB In our deformation conditions.
C5-286 JOURNAL DE PHYSIQUE
Firstly. the residues left by the entrance of the primary 600 disiocation is always found decomposed into bC and b301- and the reaction between two b30 arriving from the entrance of two 600 disiocatlons on both sides of the GB generally produces a bc GBD. Secondly, the incoming screw dislocation is never found decomposed in the GB. in spite of the repulsive interaction between the two basic DSC components (which may or may not be identical to the two original Sh'ockley partials). in fact. the climb process involved in this case is not the same as that occuring in the decomposition of the residue (bc;bgO1-) or in the (b301- :b302+) reactiton. The two latter processes involve conservative climb : first bc climb ejects 4 crystal point defects which are absorbed by the b301- climb i n the opposite direction, secondly one of b30 (eq. 1) ejects 2 crystal point defects which are absorbed by the other ; the reaction is attractive and the two bgo meet by conservative ciimb (the associated steps annihilate). in both cases the climb of the GBD's is self-fed. On the other hand. screw decomposition by climb and glide is not conservative : both bgo have to absorb8 point defects to climb away from each other. Thus it might be argued that conservative climb occurs. however in our temperature conditions non conservative ciimb is much more difficult and we have to add that the GB crossing by the screw occurs easily and could mask thle eventuality of non conservative climb. Futher experimenls in different deformation conditions must be done to confirm this hypothesis : for ine'ance, asymmetric deformation forcing screw disiocatlons to accumulate within the GB.
CONCLUSION
By means of HFIEM it has been clearly shown that lattice dislocations enter the coincidence GB where they decompose into discret perfect DSC lattice disiocations. Thus their ability to emerge in the adjacent grain is considerably reduced. This explains the local hardening role of the GB. The structures of the basic DSC disiocations were established and their climb and glide motion described. it was also shown that conservative climb occurs within the GB. The fact that the GB can act as a source or a sink of point defects /20/ and that GB migration and the disiocation ciimb are related/21/ have been mentionned already.
However we emphasize that the GBD movement is not necessarily accompanied by GB migration (reciprocally pure step motion induces GB migration) and the GB migration does not always involve point defects diffusion.
REFERENCES
1. X. BAILLIN. J. PELISSIER. J. J. BACMANN. A. JACQUES. A. GEORGE, Phil.
Mag.W5( 1987) 143
2. A. JACQUE:S. A. GEORGE. X. BAiLLiN. J. J. BACMANN. Phil. Mag.h55( 1987) 165 3. W. BOLLMANN. Crystal defects and crystalline interfaces. Springer Berlin (1970) 4. W. BOLLMANN, 6. MICHAUT. G. SAINFORT, Phys. Stat. Sol. a13( 1972) 637 5. A. C. PONt). 0. A. SMITH. Phii. Mag 36( 1977) 353
6. D.J. DiNGLEY, R.C. POND. Acta. Metall. 27(1979)667
7. R. W. BALL.UFFI. A. BROKMAN. A. H. KING Acta. Metali. 30( 1982) 1453 8. P. H. PUM13HREY. H. GLEITER. Phil. Mag. 30(1974)593
9. W. LOJKOWSKi. H. 0. K. KIRCHNER. M. W. GRABSKY, Scripta. Met. 11( 1977) 1127 10. R.Z. VALIEV. V.YU GERTMAN. O.A. KAiBYSHEV. S. S. KHANNANOV. Phys. Stat. Sol.
77( 1983) 97
11. L. C. LIM. R. RAJ, J. de Phys. 46( 1986) C4-581
12. M. ELKAJBAJI. J. THIBAULT-DESSEAUX. A. BOURRET. Proc. llth Cong. Electron Microscopy Kyoto ( 1986)
13. M. ELKAJBAJi ( 1986) Thesis Grenoble-France
14. M. ELKAJBAJI, J. THIBAULT-DESSEAUX, M. MARTINEZ-HERNANDEZ. A. JACQUES, A.
GEORGE. ( 1987) Rev. Phys. Appi. In the press
15. A. H. KING. 0. A. SMITH. Acta Cryst. A35(1980)335
16. C. d'ANTEAROCHES. A. BOURRET. Phii. Mag A49(1984)783
17. A.M. PAPON, M. PETIT: J. J. BACMANN. Phii. Mag. A49(1984)573 18. A. B0URRE.T. J. J. BACMANN. Surf. Sciences 162( 1985) 495
19. R. W. BALLUFFI, (1980) Grain Boundary Structure and Kinetics p297 ASM Metals park 20. Y. KOMEN. P. PETROFF. R. W. BALLUFFI, Phil. Mag. 26(1972)239
21. R.W. BALLUFFI, J.W. CAHN. Acta. Metali. 29(1981)493
flg. 1 : Complete decomposition of a 600 d~slocatlon. arriving on the primary glide plane.
into the three basic DSC vectors. HREM JEOL 200CX
fig.2 : Schema. in the DSC lattice. of the decomposltlon of the Incoming dislocations. Sign convention :. _SF/RH. dislocation line into the paper : +.- screw components along respectively to11 1 and C O I i 1
fig. 3 : C3B structure deduced from the HREM images ( white tunnels) (a) bc. ( b ) bg. ( c ) bgol-. ( d l bgo
C5-288 JOURNAL DE PHYSIQUE
fig.4 : bc HREM image (white tunnels) related to the DSCL (122) planes distortion
fig. 5 : Motion of GBD's ( a ) bc climb. ( b ) bg glide. ( c ) bgg climb and glide The crystal point defects affected by the motion are Indicated by black dots.
