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MOBILITY OF DISLOCATIONS IN GRAIN BOUNDARIES
M. Grabski
To cite this version:
M. Grabski. MOBILITY OF DISLOCATIONS IN GRAIN BOUNDARIES. Journal de Physique
Colloques, 1988, 49 (C5), pp.C5-497-C5-506. �10.1051/jphyscol:1988561�. �jpa-00228058�
JOURNAL DE PHYSIQUE
Colloque C5, suppl6ment au nolO, Tome 49, octobre 1988
MOBILITY OF DISLOCATIONS IN GRAIN BOUNDARIES
M.W. GRABSKI
Institute of Materials Science and Engineering, Warsaw University of Technology, Narbutta 85, PL-02-524 Warsaw, Poland
B.&u&
-
Xous avons analyse la mobilite des dislocations dans les joints degrains (JG) en terms de la dissociation des dislocations du reseau dans les dislocations extrinsegues et leur mouvement consecutif le long du plan de joint. La cinetique de l'accomodation est confrantee ensuite avec les resultats experimentaux. La cinetique de l'accomodation est contr816e par la diffusion intergranulaire et constitue une propriete caracteristique d'un JG.
Nous resultats experimentaux recents concernant l'effet de la structure des JG et de leur composition chimique sur la cinetique de l'accomadation, ont Bte analyses du point de vue de leur influence sur les processus se produisant aux joints ainsi que de leur application pour la caracterisation du fonction de distribution des proprietes intergranulaires d'un polycrystal.
Le processus de l'accomodation est responsable de nombreux ph&no&nes d'une importance technologique, qui ont lieu aux JG et plus particulierement dans les temperatures interm+diaires. Nous avons enfin suggere des differentes possibilites d'une exploitation eventuelle des donnees concernant le phenomene de l'accomodation dans l'ingenierie des JG.
&&ra& - The mobility of grain boundary (GB) dislocations is discussed in terms of the dissociation of lattice dislocations into extrinsic GB dislocations and subsequent spreading of the dissociation products along the boundary plane. The kinetics of spreading is analysed and compared with the experimental evidence. It is shown that the motion of EGBDs is controlled by the GB diffusion and is a definite, characteristic property of an individual interface. The new experimental results related to the dependence of spreading kinetics on a GB structure and chemical composition are analysed both from the point of view of its influence on the GB processes and of its utilization for the determination of the distribution function of GB character. Activation of the spreading process is responsible for many GB dependent phenomena of engineering importance, especially in the range of intermediate temperatures. The possible ways of exploiting the data concerning spreading in the design of GBs are shown.
1. INTRODUCTION
Under the influence of imposed constrains dislocations can be created in the grain boundary (GB) which don't belong to its equilibrium structure. These are usually called extrinsic grain boundary dislocations (EGBDs) C l 1 solely to distinguish them from the intrinsic or structural GBDs (SGBDs) which accommodate the lattice misfit at interface being inherent constituent of GB structure 12i41.
Both the SGBD and EGBD posses the DSC Burgers vectors characteristic for given GB crystallography 151 but not necessarily of the same type. The distinction between SGBD and EGBD can be maintained only by the reference to the equilibrium structure of the boundary before EGBD appeared 161.
By definition the SGBD, being part of the equilibrium structure of GB cannot be responsible for any dislocation-controlled GB processes. The changes in GB structure or any processes occurring at GB as the annihilation and generation of lattice dislocations, GB sliding, GB migration or cavitation etc. must therefore be related to the processes of creation, motion, rearrangements and annihilation of EGBDs, which because of the small, non lattice Burgers vectors, are constrained to remain in the boundary when they move.
The EGBDs can be created in the pre-existing network of SGBDs by the
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988561
operation of Bardeen-Herring sources but the most efficient way is by means of dissociation of the so-called trapped lattice dislocations (TLDs) which were embedded into the GB in the course of plastic deformation or GB migration, and which posses the lattice Burgers vectors [for reviews see 2,31. This dissociation process described for the first time exactly twenty years ago 17 1 , is called spreading of TLDs, and can be directly observed in TEM as the fading of the TLD contrast when the temperature is increased.
It is believed that the ability of EGBDs to move and to relax stress concentrations on GB is responsible for many interface controlled phenomena of engineering importance, especially in the range of intermediate and high temperatures 1 for reviews see 3, gill, 121, however, direct informations concerning EGBD mobility and its dependence on GB structure are lacking. In this paper the possibility of exploiting the measurements of the spreading rate of TLDs for determination of the mobility of EGBDs is discussed from the point of view of properties of an individual GB
,
and the possible ways of application of the mobility data in the design of polycrystals are shown.2. CREATION OF EXTRINSIC GRAIN BOUNDARY DISLOCATIONS
The characteristic property of the TLD is that when the material is heated up to the temperature above 0.3 T,,,, the image of TLD as observed with TEM spreads out until it irreversibly disappears. Beginning with pioneering works of Ishida et a1. ,171 and Pumphrey and Gleiter C8,13,141, considerable experimental evidence has been gathered thus far, showing that spreading process has real physical significance 116-411. Although originally the problem aroused some controversies reviewed in 12+43, it is now becoming accepted to interpret it in terms of the dissociation of TLD into a set of extrinsic GBDs, which then move apart under the influence of the forces of mutual repulsion, as it is observed on ordered, low f boundaries 11,16,19,20,42-441. It follows from the geometry of the DSC lattice that the TLD can always dissociate into a discrete set of EGBDs with appropriate DSC vectors as the lattice Burgers vector always represents the sum of all possible DSC vectors C51, i. e. :
bL = Eni (b.,,, ), (1) As the DSC vectors are smaller than the lattice Burgers vector the reaction is energetically favourable.
The perfect DSC vectors are equal to lattice Burgers vectors only in the case of low angle Z=1 boundary; under such circumstances the dissociation can not take place and instead the TLD may be knitted into the pre-existing wall of lattice dislocations forming a boundary 1461. When Z increases, the magnitude of the DSC vectors decreases and it becomes too small to allow the detection of EGBD with conventional TEM, and then spreading manifests itself macroscopically as continuous fading of the TLDs contrast. The other problem is that when the degree of lattice matching further decreases which is the case for long periodicity boundaries, .the SGBD separation approaches the atomic spacing. As the core structure of GBDs is determined by the interatomic restraining forces, it is time independent and relatively temperature insensitive C471. It is also believed to preserve its identity for all types of GB 1481. However, it is more convenient to describe the spreading in terms of a continuous widening of the TLD core along the boundary plane 18,13,141. The uniform core-spreading can well be represented by an EGBD distribution consisting of an infinite number of EGBDs possessing infinitesiml Burgers vectors, similarly as in the Peierls model of lattice dislocation 18,16,491. It can be remarked that the discrete approach represents a microscopic point of view, which should be used in the case of analysing the structural mechanisms of the processes which involve EGBD, whereas the other, continuous approach is of macroscopic type, being useful for deriving macroscopic characteristic of the processes which f nvol17e GB behaviour C 3,10,501. The applicability of the macroscopic approach is also justified by still very incomplete knowledge concerning the structure of boundaries of low periodicity and the exact physical nature of GBD especially at high temperatures 1481.
It should be noted that the disappearance of TLDs image as observed in TEM is solely to indicate the fact that the products of dissociation spread out to
such a n e x t e n t t h a t t h e width of a r r a y h a s reached a value S,, of some hundreds a n g s t r o m [ 8 , 1 9 , 3 2 1 , Therefore t h e occurrence o r non-occurrence of c o n t r a s t should not be used a s t h e determinant of t h e s t r u c t u r a l s t a t e of t h e boundary and t h e p h y s i c a l s i g n i f i c a n c e of EGBDs does not depend on whether t h e y a r e d e t e c t a b l e i n TEM by conventional methods 1 251
.
3 . MOBILITY OF EXTRINSIC GRAIN BOUNDARY DISLOCATIONS
Only i n very s p e c i a l c a s e s when t h e boundary plape is a s l i p plane f o r GBDs, t h e motion of EGBDs may be expected t o occur by pure g l i d e a t low temperatures [521. In g e n e r a l , however, i n pure m e t a l s t h e EGBDs a r e immobile up t o 0 . 3 T,.., a l s o t h e p r o c e s s of s p r e a d i n g of TLDs is not observed a t lower temperatures, which s u g g e s t s t h a t t h e thermal s t a b i l i t y of TLDs r e s u l t s mainly from t h e k i n e t i c r e a s o n s l531. The obvious i m p l i c a t i o n is t h a t t h e Burgers v e c t o r s of t h e d i s s o c i a t i o n products a r e n o t , i n g e n e r a l , i n t h e boundary p l a n e , and t h a t t h e boundaries a r e m i c r o s c o p i c a l l y non-planar, s o t h e movement of EGBDs is non- c o n s e r v a t i v e and h a s t o be r a t e c o n t r o l l e d by d i f f u s i o n .
Although t h e t h e r m a l l y a c t i v a t e d motion and d i s s o c i a t i o n of TLDs i n t o EGBDs a s well a s t h e motion of i n d i v i d u a l EGBD was q u a l i t a t i v e l y observed d u r i n g i n - s i t u TEM experiments on low E s p e c i a l boundaries [1,15,22,42-44,553 however, t h e only d i r e c t and q u a n t i t a t i v e method of o b t a i n i n g d a t a concerning EGBDs m o b i l i t y on i n d i v i d u a l GB is from t h e measurements of t h e disappearance r a t e of t h e T L D s . Very important m e r i t of t h i s method is t h a t it can be a l s o a p p l i e d f o r t h e boundaries with l e s s o r d e r e d s t r u c t u r e s . The weakness l i e s i n t h e f a c t t h a t i t is based on a somewhat s u b j e c t i v e d e t e r m i n a t i o n s of t h e time f o r TLD c o n t r a s t disappearance, but
,
a s i t w i l l be shown l a t e r , i t is a s u r p r i s i n g l y a c c u r a t e i n d i c a t o r i n t h e measurements of t h e EGBDs motion r a t e s .
I n t h e f i r s t , ad hoc, a t t e m p t s of t h e s p r e a d i n g r a t e c a l c u l a t i o n s it was assumed t h a t t h e width S on which d i s s o c i a t i o n p r o d u c t s a r e s p r e a d is p r o p o r t i o n a l t o t h e random walk d i s t a n c e of a t o m a l o n g t h e boundary s o t h e r a t e of s p r e a d i n g was p r o p o r t i o n a l t o (D,+,,,./t)" and independent of S [ 8 , 1 6 , 1 9 , 2 0 1 , This seems u n r e a l i s t i c , because t h e mutual i n t e r a c t i o n between d i s s o c i a t i o n products is n e g l e c t e d , and was not confirmed by l a t t e r experiments. The more advanced models concerning mechanism by which EGBDs moves a p a r t a l o n g t h e i n t e r f a c e d u r i n g s p r e a d i n g of TLD, i n f a c t , can be t r e a t e d i n t h e frameworks of more g e n e r a l r e l a t i o n s concerning climb and v i s c o u s g l i d e of d i s l o c a t i o n s :
Spreading by climb i n t h e g r a d i e n t o f t h e chemical p o t e n t i a l 149,53,541, F G ~ t h i s c a s e i t is assumed t h a t EGBDs Burgers v e c t o r h a s a s t r o n g component normal t o t h e boundary p l a n e and t h e well known formula a p p l i e s :
where R is t h e atomic volume and t h e g r a d i e n t : Ap = 1 (T.,,
-
7 , ) / S l x R A s 7 S'
t h e n Ap = S ' - and f i n a l l y :where c o n s t a n t A depends on t h e d e t a i l s of t h e assumed model. This model was analysed both f o r t h e d i s c r e t e C53,541 and t h e continuous c a s e s 1491. The s o l u t i o n s o b t a i n e d d i f f e r only i n t h e numerical f a c t o r .
S p r e a d i n g by v i s c o u s g l i d e of EGBO [561. For t h e c a s e of v i s c o u s s l i p , we can w r i t e :
where M i's t h e m o b i l i t y of EGBD p r o p o r t i o n a l t o D,,,b/kT, and F is t h e d r i v i n g f o r c e r e s u l t i n g from e l a s t i c i n t e r a c t i o n between t h e p r o d u c t s of EGBD d i s s o c i a t i o n and can be p r e s e n t e d a s : F=7b, where 7 is s h e a r s t r e s s i n t h e GB plane. The d r i v i n g f o r c e F of t h e analysed process is r e s p o n s i b l e , i n t h e g e n e r a l c a s e , both f o r s l i p and f o r GB migration. I n t h e c a s e of GB whose movement is impeded, a s p r e a d i n g - l i m i t i n g
factor occurs accompanied by the existence of threshold stress T,. If T 6 T,
spreading rate! is determined by the value of coefficient H and in general case by those effects which mark the highest threshold stress and the lowest dislocation mobility on G13. In the case of boundaries with dE/dB=O the threshold stress is in minimum and for boundaries within energy cusps is in maximum, whereas the coefficient M is proportional to the GB diffusion coefficient, and behaves in the opposite way. For pure metals this leads to the formula:
dS CGb3D',,
-
=dt SkT exp (-$
where C=const. This concerns the situation when the EGBD Burgers vector is parallel to the boundary plane.
Validity of the eq. ( 5 ) has been experimentally confirmed during in-situ experiments (when the thin foils are heated up in TEM) for pure aluminium [37,49, J. Kwiecinski and al. work in progress], copper 1571, and nickel C 49,561
.
The mostextensive work was done on the austenitic steels 132,35,36,39+41,571 where the GB self-diffusion coefficients were determined and remarkable agreement with the results obtained by a radiotracer method was obtained. The eq. (61 was tested for the case of nickel C561. The experimental methods used in measurements of the TDL disappearance on thin foils are described in detail in C33,39,40 and 35,36,411 and the experimental errors committed during determination of the diffusion coefficients from spreading observations are analysed in [58,591.
The general conclusion is that from the experimental point of view, with the present method of measuring the TLD disappearance time, the models presented are practically indistinguishable E 3,39,40,50,561. It can be also concluded, that independently of the orientation of the Burgers vector towards boundary plane, the mobility of EGBD is controlled by GB diffusion, and the general relation for the time of TLD disappearance when S reaches a particular value S,. is:
t, = B (kT/De,,.) exp (Q ,,,.. /kT) (7) where B is numerical constant depending on the diffracting conditions, atomic volume, shear modulus and Burgers vector of EGBDs or GB width, depending on whether the discrete or continuous model is used.
However, is spite of the primary reports that the spreading rate is the same in thin foils as in bulk materials t131, the more recent qualitative results suggest that in the former case the stability of TLDs is higher C331. The quantitative study of the problem was undertaken in our laboratory (work in progress) with the use of the ex-post method, when heating is performed on slightly deformed bulk samples and after quenching the fraction of GBs on which TLDs are present is registred. In fact it was revealed that the higher fractions of such boundaries are obtained with the ex-post than with the in-situ method for the same times and temperatures of annealing, which yields the apparent activation energy equal rather to that of the volume than to GB diffusion.
This problem was analysed by Swiatnicki CPh. D. thesis, Warsaw University of Technology, 19861 who shown that in the case of ex-post method the considerable density of 'fLDs is created during and/or after annealing mainly due to the relaxations of dislocation pile-ups on GB, which is particularly easy in the low stacking fault energy materials as the austenitic steels under investigation. This can easily give about 25% overestimation of the fraction of GB on which TLDs are present and in consequence result in the serious overestimation of the TLDs stability. The another possibility which cannot be excuded may be related to the high efficiency of the vacancy creatiodannihilation on the juntion of GB and the foil free surface f21 which may be supported by the reports that the vacancy supersaturation destabilizes TLDs [13,141. Despite this uncertainity the results obtained until now with the use of in-situ method are giving the strong evidence for direct dependence of the spreading process on the intrinsic mobility of GBDs.
The most important experimental findings are as follows:
1. The mobility of EGBDs as represented by the spreading of TLD is an unique, characteristic property of individual interfaces.
2. Motion of EGBDs is controlled by the local diffusion, its kinetics obeys
Arrhenius plot, and the activation energy is equal to that of grain boundary selfdiffusion 1351.
3. All influences which affect the core structure of grain boundary exert very strong effect on the motion of EGBDs: the spreading rate decreases with increasing the order of grain boundary structure C 8,14,16,18,19,22,27,29,30,371.
This can be observed also on curved boundaries, as the spreading rate increases with the deviation from symmetrical orientation l unpubl. results1
.
The reports that the grain boundary migration increases the rate of spreading 1601 are most probably related to the effect of changes of the boundary plane during migration.4. On ordered boundaries the anisotropy of the spreading rate has been observed. In low Z tilt boundaries around <llO>in A1 bicrystals the spreading rate for EGBDs lying along tilt axis is much faster than for the perpendicular direction 1J.Kwiecinski and al. work in progressl which is in good agreement with the measurements of the diffusion anisotropy on the same bicrystals 1611.
5. Xobility of EGBDs is lower in alloys than in pure metals C8,14,181, however, it doesn't depend on the stacking fault energy.
6. Segregation of slowly diffusing solutes decreases the EGBDs mobility, whereas rapidly diffusing interstitials have no effect on spreading kinetics C31,361, however this may not apply to all systems [W.Swiatnicki, work in progressl.
7. Some evidence exists, but only for the case of austenitic steels that on grain boundaries which already absorbed considerable density of EGBDs, the rate of spreading is higher than that on equilibriated boundaries I19,25,27,28,621. However the more recent results relate this effect to the changes in chemical composition of the boundary C 36,41, W. Lojkowski and J. Wyrzykowski, submitted to publ. I . On the other hand, the changes in the density of the non spread TLDs do not affect the spreading kinetics C25,271.
4. RECOVERY OF GRAIN BOUNDARY STRUCTURE
The boundary loaded with EGBD exhibits an elastic field of long range and can be considered as a nonequilibrium or not stationary structure 13,13,19,62+641.
According to the Frank formula, the equilibrium Burgers vector content of the boundary is determined by the boundary crystallography:
B =
z
n,,,,b, = (V x U)e
(9)where :
V - is an arbitrary vector lying in the boundary plane u - is the versor of rotation axis
8 - is the misorientation angle, which for high angle boundary may represent a deviation from the nearest CSL orientation C41.
Each change of the Burgers vector content is equivalent to the change of misorientation of grains, if the number of structural grain boundary dislocations which are crossed by V vector is proportional to its length, i.e, if GBD are homogeneously distributed. Then the long range elastic field associated with the boundary is minimised and an equilibrium configuration is maintained.
If the EGBD is formed in the boundary, the Burgers vector content of the boundary changes, which is not necessarily followed by the instantaneous change of the misorientation, because at least at the initial stage of spreading the EGBDs are not evenly spaced. As the structure of GB is determined at any point of grain boundary by the local crystallographic parameters, then the localised changes introduced by the presence of EGBDs are equivalent to the non-stationary and non- uniform lack of correspondence between the actual values of these parameters and their average, equilibrium value, which obeys the Frank formula. This is a source of a strain field inhomogenity.
It is reasonable to assume that the reaction of lattice dislocation with the boundary will not affect significantly the core structure of structural units, since it is already of high energy. However the overall grain boundary structure may change affecting the grain boundary core dependent properties. Also the change in short and long range strain fields may occur, so the total grain boundary energy may be affected t W. Lojkowski, submitted to publ. I.
The diminishing of the surplus Burgers vector content of the boundary, or, to be more precise the convergence of the actual and equilibrium Burgers vectors at any point of the boundary, i.e. the recovery of GB structure, may occur by its annihilation, rejection or absorption 1451, all these occurrences controlled by the motion of individual EGBDs:
1. annihilation of the excess Burgers vector content by motion and mutual annihilation o:E the EGBDs with opposite signs from the same boundary or from the adjoining boundaries via triple junctions;
2. rejection of the excess Burgers vector content from the boundary by generation of the lattice dislocations at the GB.
3. absorption of EGBDs into the GB structure, which is equivalent to the transformation of GB structure due to rearrangements and reactions of EGBDs with the pre-existing network of SGBDs, similarly as in the case of low angle boundaries, until the resulting network becomes uniform, which results in the change of misorientation of abutting grains and change of the equilibrium Burgers vector contents towards the new, different equilibrium value. Alternatively a transformation of GB structure may occur by the change of type of SGBDs without affecting misc~rientation if the structural multipl'icity of GB is allowed.
The proc:ess of GB recovery is complete when:
B t Ini (bd,-#:) 4 B' (9)
where B is initial, and B' is new equilibrium Burgers vector content
This process is not expected to be instantaneous, maybe with the exception of very high (>0.8T,,,) temperatures. This comes from the fact that to recover the equilibrium, strain free GB structure, the EGBDs have to move along the boundary, and this motion has to be accommodated in the microstructure by the coupled processes of grains rotation, GB sliding and GB migration, the kinetics of these processes not necessary the same as the kinetics of EGBDs motion, the slowest one being the rate determining 145,501. A similar situation was analysed recently C651, on the assumption that the excess Burgers vector content can be accommodated in the microstructure by the Nabarro-Herring creep which was criticized is131.
In the case of bicrystal the accompanying rotations of grains and grain boundary sliding may occur more freely than in the case of polycrystal when the grain boundary is to a greater extent subjected to the constraints imposed by the microstructure. In the case of favoured boundaries, which do not posses SGBDs network, the changes in misorientation may be energetically prohibited because of the existence of the energy cusps, and,then each change of misorientation has to be related to the increase of GB energy. Similar occurrences were in fact observed by Knkawa, Watanabe and Karashima 129,661, who found during h-t creep deformation of A1 bicrystals, that the misorientation of sliding grains changed towards nearest special orientation which remained stable, and that the structure of the special low I boundary is more difficult to modify during sliding than in the case of a random
(general ) boundary.
In conclusion, when a continuous supply of lattice dislocations with differing Burgers vectors to the grain boundary takes place, the structure of grain boundary is non-stationary, undergoing extensive changes related to the dissociation of TLD and to the movement and rearrangements of the dissociation products which are accommodated by coupled processes of grain boundary sliding, grain boundary migration, generation of new lattice dislocations, changes of GB structure and triple junctions activity. These processes are very sensitive to the GB structure - at short period boundaries they occur with much smaller rate i.e. they become effective at much higher temperatures than on long period boundaries. The situation may even be mo.re complex in the presence of solute atoms and GB segregation.
5. CONCLUDING REMARKS
If it is assumed that structure dependent interface reactions are controlled by the movemen.ts of EGBDs, then polycrystal properties should be influenced by the grain boundary structure [9,10,67+701 and then the differences in the structure and properties of individual grain boundaries should be taken into account during analysing the polycrystal behaviour 1 101. The experimental data show that the
properties of grain boundaries within a population of given polycrystal are differentiated and depend on the history of the material 134-37,41,68,69,711.
Following Watanabe 1681 the function which characterizes the distribution of GB parameters will be called grain boundary character distribution (GBCD).
During the simulation of three-dimensional polycrystal stacked with grains in the form of regular tetrakaidecahedra and containing 8324 grain boundaries [A. Garbacz and al, work in progressl it was found, assuming the Brandon's criterion of specialness 1721 that for the case of random orientation of grains the coverage of L643 boundaries was about 7.3%. The most frequent orientations in descending order were: T 3,5,1/7,9,13,35/39 and the most rare: 29 and 31. Warrington and Boon [731 using the same criterion for the case of randomly generated bicrystals found for the range 3<1<25 the 9% coverage (for the same range in our work it was 5.6%).
These values are much smaller than those determined experimentally [681. The preliminary results of our simulation show, however, that the fraction of CSL boundaries can increase considerably when the randomness of the grains orientation decreases, which supports the idea that the direct relation exists between the texture of grains and the GBCD.
However, the data concerning GBCD based solely on crystallographic parameters, are of limited use in the design of materials as there is still no direct link between the crystallography and properties of GB. Moreover the full crystallographic characterization of GB population in a polycrystal can be rather difficult to obtain, especially if one takes into account the very strong dependence of GB properties not only on the misorientation of grains but also on the position of GB plane [ 12,741.
For the purpose of determining the GBCD on the basis of GB properties the diffusional data obtained from the measurements of EGBD mobility on individual GB may be exploited [35,36,37,411. As the diffusivity is a basic attribute of GB, being the property directly related to its core structure and controlling the rate of the GB reactions 1751, it can provide the appropriate basis to study the polycrystal behaviour. The GBCD for this property is of importance as the averaged parameters can mask the reality L351.
The recent works in our laboratory lead to the following conclusions:
1. The GBCD for diffusivity is continuous, which means that each boundary possesses its own distinctive structure. No sharp division of the population into high and low diffusivity GB exists (with the excepticn of first order coherent 13 twin boundaries, if they do exist). Usually about 50% of boundaries had diffusion coefficients within one order of magnitude and 75% within two orders of magnitude around the mean calculated L35,36,4l,unpubl.resultsl.
2. The GBCD for diffusivity is affected strongly by the history and chemical composition of material. This results from:
a. grain growth affected changes in the population of GB [37,76,771 resulting both from changes in misorientation and from changes in GB planes
b. texture related changes in the distribution of GB [371.
c, changes of chemical composition of grain boundaries [36, M. Tacikowski, work in progressl
.
A more general conclusion is that the commonly used classification of GB into special and non-special (or general) ones is not too well justified. The shape of the GBCD for spreading activation energies in stainless steel 136,371 demonstrates clearly that besides the tail extended towards high values, which was positively identified as related to the ordered, special boundaries of low diffusivity, the opposite tail extends towards low activation energy end. Taking this at its face value it means that apart from special boundaries, the non-ignorable fraction of
"anti-special" or "super-general" boundaries exists with very high diffusivity, whereas the most numerous or general fraction constitutes the central group. A similar conclusion can be drawn from the experiments on changes in the grain boundary energy durlng grain growth 1751, from investigations of the changes of grain size distribution [J.Mizera and al. to be published] and also from an extensive study of the grain growth dependent changes of GBCD for spreading kinetics in 5N A1 polycrystals [J.Kwiecinski et al. work in progressl where it was found that the fractions of GB with the highest activation energy disappeared
during growth, and the small anti-special fraction with the lowest activation energy dominated the process. In fact it was found many years ago that, for example 40'<111> tilt boundaries in fcc metals, which are definitely off the exact 17 relationship are of particularly high mobility C781. Identification of these anti- special GB deserves further studies.
The dranatic effect of the structure of grain boundaries on the polycrystal properties was recently shown by Watanabe 167,681, Lim and Raj C661, and also in our laboratory C37,41,79, unpubl. resultsl
.
The common conclusion of these works is that special CB i.e. GB with low diffusivity, low energy, short periodicity, ordered structures are stronger or more resistant element of microstructure than those of high diffusivity, high energy, low order structures. At low temperatures the boundaries with so defined special properties exhibit intrinsic high resistance to intergranular fracture C681. As they can sustain high stress concentrations they can be a source of nonuniformity of deformation demonstrated by Luders flow 1 81, J.
W. Wyrzykowski and a1.
unpubl. resultsl and in this way can promote the plastic flow in materials which are usually brittle at room temperatures. These properties are of structural origin, and not dependent on the mobility of EGBD which are stable at low temperatures; however, they are related to boundaries which at high temperatures exhibit very low EGBD mobility.On the other hand the high diffusivity of GB ensures the high mobility of GB dislocations at intermediate and high temperatures, and favours the occurrence of localised stress relaxation 1101. At the same time these boundaries display low fracture resistance. It means that both kinetically (high temperatures) and structurally (low temperatures) such boundaries are the weak element of the microstructure.
The practical implication is that to obtain strong polycrystal we should have GBCD shifted towards low energy, low diffusivity end, whereas for a superplastic material the beneficial effects are obtained when the shift is towards high energy, high diffusivity side.
It should also be mentioned that in many works concerning the behaviour of grain boundaries, the structural arguments were put forward to interpret the observed effects when the same boundaries displayed "special" properties at low temperatures arid "general" properties at high temperatures C811. However, assuming that at least some of these effects are related to the diffusion controlled motion of the extrinsic GBDs the differences in behaviour can be explained more simply on the kinetical basis. This was confirmed recently during studies of mechanical behaviour of polycrystalline aluminium I37, J. W. Wyrzykowski and al. unpubl. results 1 and stainless ziteels C 411.
The recent results concerning the effect of solutes on EGBDs kinetics C31,36, W. Swiqtnicki and M. Tacikowski, unpubl. results 1 , shows clearly, that by the doping of the material with appropriate solutes and by controlled segregation it is possible to change drastically the stability of TLD and mobility of EGBDs [I. In the light of the present progress in understanding the effect of solutes on GB structure and properties this seems to be a very promising way of controlling the GBCD and of GB design.
ACKNOWLEDGEMENTS
The author wishes to acknowledge the contribution of his coworkers:
W. tojkowski, W. Swiatnicki, J. W. Wyrzykowski, M. Tacikowski, K. J . Kurzydlowski, J . Kwiecihski, H. Garbacz, A. Garbacz, A. Pakiela and J. MPzera. This work was supported by the Office of the Research and Development, (Poland), under the contract CPBR 2.4.
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