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between dislocations and defects in oxides”
M. Blanchin, J. Castaing, G. Fontaine, A. Heuer, L. Hobbs, T. Mitchell
To cite this version:
JOURNAL DE PHYSIQUE
CoZZoque C3, suppZSment au n06, Tome 42, juin 1981 page C3-1
INTRODUCTORY REPORT
:
"The i n t e r a c t i o n s between d i s l o c a t i o n s and d e f e c t s i n o x i d e s "
M.G. Blanchin, J. C a s t a i n g
(*+),
G. F o n t a i n e , A.H. Heuer (*a), L.W. Hobbs (+*) and T.E. M i t c h e l l (*a)De'partement de Physique des Mate'riaux, AssociS au C.N.R.S., UniversitS Claude Bernard, 69622 V i 2 Zeurbanne Cedex, France.
f*lLaboratoire de Physique des Matdriaux, C.N.R.S., BeZZevue, 92190 Meudon, France.
(*+)Department o f Metallurgy and Materials Science, Case Western Reserve Uni- v e r s i t y , CZeveZand, Ohio, 44106, U . S.A.
1. I n t r o d u c t i o n . - During t h e l a s t t e n y e a r s , i n c r e a s i n g a t t e n t i o n h a s been g i v e n t o s t u d i e s o f p l a s t i c d e f o r m a t i o n o f r e f r a c t o r y o x i d e s . The mechanical b e h a v i o r a t e l e v a t e d t e m p e r a t u r e s i s o f p r i m a r y c o n c e r n and depends s e n s i t i v e l y o n i n t e r a c t i o n between d i s l o c a t i o n s and p o i n t de- f e c t s o r d e f e c t c l u s t e r s .
I n o r d e r t o c l a r i f y t h e s t a t e o f knowledge i n t h i s a r e a , a Workshop e n t i t l e d " I n t e r a c t i o n between P o i n t D e f e c t s and D i s l o c a t i o n s i n Oxides" was h e l d a t t h e L a b o r a t o i r e de Physique d e s M a t s r i a u x o f t h e C.N.R.S.
i n B e l l e v u e , F r a n c e , 6-9 J u n e 1978, and a r e p o r t was p u b l i s h e d i n Acta M e t a l l u r g i c a /1/ i n 1979. One o f t h e main r e s u l t s o f t h e Workshop was
t o s t i m u l a t e c o o p e r a t i v e r e s e a r c h programs on t h e s u b j e c t between s c i e n -
t i s t s from a p p r o x i m a t e l y t e n l a b o r a t o r i e s i n F r a n c e , England, Germany and t h e U.S.A. A s a consequence, new e x p e r i m e n t a l r e s u l t s were f o r t h c o - ming, e s p e c i a l l y f o r alumina, s p i n e l , r u t i l e and o l i v i n e , and a n e x t e n - s i v e r e v i e w p a p e r on h i g h t e m p e r a t u r e p l a s t i c i t y o f numerous o x i d e s was u n d e r t a k e n /2/. S i m u l t a n e o u s l y , e f f o r t s have been made t o d e v e l o p a t o - m i s t i c models t o c a l c u l a t e t h e e n e r g i e s and c o n f i g u r a t i o n s of p o i n t de-
f e c t s s i t u a t e d i n t h e c o r e r e g i o n o f d i s l o c a t i o n s i n i o n i c s o l i d s .
I t was deemed u s e f u l t o h e l d a second Workshop t o p r o v i d e a forum f o r a n open d i s c u s s i o n o f a r e a s o f knowledge and i g n o r a n c e . T h i s Work- shop, o r g a n i z e d by t h e a u t h o r s w i t h a s s i s t a l l c e o f t h e DBpartement de Physique d e s MatGriaux ( a s s o c i a t e d w i t h t h e C.N.R.S.) o f t h e U n i v e r s i t y o f Lyon, was h e l d i n Lyon, F r a n c e , 5-6 September 1980. The program o f
(+) On l e a v e a t U.S. G e o l o g i c a l Survey, Menlo P a r k , C a l i f o r n i a , 94025, U.S.A.
t h i s two day Workshop c o n s i s t e d o f c r i t i c a l s u r v e y t a l k s and of c o n t r i - b u t i o n s p r e s e n t i n g i m p o r t a n t new f i n d i n g s and i d e a s . The s u b s t a n c e o f most t a l k s and c o n t r i b u t i o n s i s r e p o r t e d i n s e p a r a t e p a p e r s a p p e a r i n g l a t e r i n t h i s i s s u e .
T h i s f i r s t p a p e r s h o u l d b e r e g a r d e d a s an i n t r o d u c t o r y r e p o r t sum- m a r i z i n g s i g n i f i c a n t r e s u l t s and c o n c l u s i o n s and emphasizing t h e main i d e a s which were d e b a t e d d u r i n g t h e Workshop.
2. P o i n t d e f e c t s . - I t was a l r e a d y c l e a r a t t h e f i r s t Workshop t h a t p o i n t d e f e c t c l u s t e r s o r complexes a p p e a r t o b e t h e r u l e i n o x i d e s , b o t h f o r t h e i n t r i n s i c d e f e c t s i n t h e r m a l e q u i l i b r i u m and f o r charge-compensating d e f e c t s produced by a l i o v a l e n t s o l u t e s . However, t h e s t a t e o f aggrega- t i o n
i s
s t i l l p o o r l y c h a r a c t e r i z e d and f u r t h e r c o r r e l a t i o n i s n e c e s s a r y between r e s u l t s o f s t u d i e s i n v e s t i g a t i n g p l a s t i c i t y o f o x i d e s and d a t a o b t a i n e d by s p e c t r o s c o p i c t e c h n i q u e s ( o p t i c a l a b s o r p t i o n , l u m i n e s c e n c e ,E . P . R . , o p t i c a l l y - d e t e c t e d magnetic r e s o n a n c e , e t c . ) . The Workshop be- gan w i t h a c r i t i q u e o f t h e r e p o r t o f t h e f i r s t Workshop by Henderson, who o u t l i n e d some e r r o r s and o m i s s i o n s which a p p e a r e d i n t h e correspon- d i n g r e p o r t / I / . F o r i n s t a n c e , V: had n o t b e e n o b s e r v e d i n Tho, nor
u
[ ~ r i ~ ~ ~ ~ 0 4 ] :
i n MgO. On t h e c o n t r a r y , i s w e l l know; i n MgO~ ~ ~ [ v ~ v G v ~ ]
h a s been o b s e r v e d i n MgO, SrO, CaO and A1203. He a l s o em-p h a s i z e d t h a t s y s t e m a t i c u s e o f t h e Krdger-Vink n o t a t i o n , s o u s e f u l i n w r i t i n g a m a s s - a c t i o n c o n d i t i o n f o r t h e c r e a t i o n o f d e f e c t s , i s unphysi- c a l and g i v e s no i n f o r m a t i o n a b o u t c h a r g e l o c a l i z a t i o n around p o i n t de- f e c t s . F o r example, t h e S c h o t t k y r e a c t i o n f o r MgO :
n u l l -+ V"
+
V;'M9
d o e s n o t mean t h a t t h e magnesium vacancy B i t e h a s t r a p p e d two n e g a t i v e c h a r g e s . I n f a c t t h e c h a r g e i s a s s o c i a t e d w i t h n e a r e s t n e i g h b o r a n i o n s . Many p a r t i c i p a n t s a l s o n o t e d t h a t v e r y l i t t l e i s known a b o u t a n i o n de- f e c t s , such a s Oi which h a s been c l a i m e d t o b e i n v o l v e d i n t h e p l a s t i c i - t y o f A1203 and COO, f o r i n s t a n c e , and a b o u t "molecular" d e f e c t s . Hen- d e r s o n a l s o reviewed t h e d e f e c t s t r u c t u r e o f s i m p l e p o i n t d e f e c t s i n o x i d e s as NgO, CaO, A1203, a s deduced by s p e c t r o s c o p i c t e c h n i q u e s .
i s s u e ) . With r e g a r d t o t h e problem o f t h e n u c l e a t i o n o f CS p l a n e s , use- f u l i n f o r m a t i o n c a n b e o b t a i n e d from o b s e r v a t i o n s by h i g h r e s o l u t i o n t r a n s m i s s i o n e l e c t r o n microscopy (TEM). The problem o f t h e s p i n e l - s t r u c - t u r e c r y s t a l s i s s t i l l r e c e i v i n g a t t e n t i o n ; n o n - s t o i c h i o m e t r y i n Mg-A1 s p i n e l s u s u a l l y i n v o l v e s a n e x c e s s o f A 1 2 0 ( e s s e n t i a l l y t h e a d d i t i o n o f
3
an a l i o v a l e n t c a t i o n o f f i x e d v a l a n c y ) and charge-compensating c a t i o n v a c a n c i e s . D i s c u s s i o n s a t t h e Workshop f o c u s s e d on s p i n e l c r y s t a l s w i t h t h e formula MgOan(Al 0 2 ) , and i t was p o i n t e d o u t t h a t t h e e x i s t e n c e o f
3
s t o i c h i o m e t r i c c r y s t a l s ( n = 1 ) i s s t i l l a m a t t e r o f d e b a t e ; q u e s t i o n s a l s o remain a b o u t t h e d e g r e e o f d i s o r d e r on t h e c a t i o n s u b l a t t i c e .
With r e g a r d t o r e s u l t s o f t h e o r e t i c a l c a l c u l a t i o n s , such computa- t i o n s a r e now r e s t r i c t e d t o p u r e l y i o n i c s t r u c t u r e s . Henderson commented t h a t c o v a l e n c y e f f e c t s have been adduced i n MgO and even i n A1203 : such e f f e c t s have been n e g l e c t e d i n t h e o r e t i c a l models. I n t h i s c o m p e t i t i o n between p u r e l y i o n i c and p u r e l y c o v a l e n t bonding, i t i s n o t c l e a r what t o t a k e a s model m a t e r i a l s , most o x i d e s b e i n g p r o b a b l y o f i n t e r m e d i a t e bond c h a r a c t e r .
3. D i s l o c a t i o n s i n o x i d e s . - We have s e l e c t e d a number o f a r e a s r e l a t i n g t o t h e g e n e r a l problem o f d i s l o c a t i o n p r o p e r t i e s i n o x i d e s and w i l l a t - tempt t o d i s t i n g u i s h between r e s u l t s on which t h e r e w a s g e n e r a l a g r e e - ment a t t h e Workshop and a s p e c t s which remain a m a t t e r f o r c o n t i n u i n g d e b a t e .
3 . 1 D i s l o c a t i o n g l i d e and d i s l o c a t i o n d i s s o c i a t i - The e x p e r i m e n t a l r e s u l t s a r e t h e f o l l o w i n g :
-f
i ) The Burgers v e c t o r b o f t h e primary s l i p system i s u s u a l l y t h e s h o r t e s t l a t t i c e v e c t o r ( e x c e p t i n T i 0 and Y203) :
&
= 1/2 <110> i n2
f . c . c . a n i o n l a t t i c e s and
&
= 1/3 < 1 1 2 0 > i n h . c . p . a n i o n l a t t i c e s .ii) The primary s l i p p l a n e ( n o t always w e l l d e f i n e d ) i s u s u a l l y t h e c l o s e s t - p a c k e d one.
bons observed by p o s t mortem TEM o b s e r v a t i o n s a r e an a r t i f a c t due t o re- arrangement d u r i n g c o o l i n g of t h e samples a f t e r deformation has ceased. R i v i e r e e t a l . /3/ have p r e s e n t e d c l e a r experimental evidence t h a t t h e d i s t a n c e between p a r t i a l d i s l o c a t i o n s and t h e p l a n e o f t h e s t a c k i n g f a u l t i n A1203 depend on t h e r a t e of c o o l i n g from t h e deformation tempe- r a t u r e . F u r t h e r q u e s t i o n s concerned why d i s l o c a t i o n s d i s s o c i a t e by climb and n o t by g l i d e , and t h e r e s p e c t i v e r o l e s of s t a c k i n g f a u l t energy, s e - g r e g a t i o n of p o i n t d e f e c t s ( v a c a n c i e s o r i m p u r i t i e s ) , and P e i e r l s s t r e s s
( f o r climb o r f o r p a r t i a l s ) .
A few q u e s t i o n s have been answered by t h e o r e t i c a l c a l c u l a t i o n s i n t h e c a s e of MgO ( c f . P u l s , t h i s i s s u e ) , which allow v i s u a l i z a t i o n of t h e c o n f i g u r a t i o n of d i s l o c a t i o n s a t t h e atomic l e v e l . The s t r e s s correspon- d i n g t o t h e P e i e r l s f o r c e h a s been c a l c u l a t e d ; t h e new v a l u e ( i n con- t r a s t w i t h p r e v i o u s l y p u b l i s h e d r e s u l t s ) i s 150 MPa.
There i s s t i l l need f o r c a l c u l a t i o n s on screw d i s l o c a t i o n s , jogs, k i n k s and o t h e r s t r u c t u r e s .
3.2
Y&e&d&~g
mechanisms.- F i r s t , it was emphasized t h a t one has t o d i s -t i n g u i s h between t h e y i e l d s t r e s s
r
( i . e . t h e s t r e s s t o overcome t h e Yb a r r i e r ) and t h e flow s t r e s s ( i . e . t h e s t r e s s t o maintain d i s l o c a t i o n s moving a t a given v e l o c i t y ) .
The r e s u l t s agreed-upon i n t h i s a r e a a r e :
i ) Only MgO-type o x i d e s a r e p l a s t i c a t low t e m p e r a t u r e s (by s l i p on (110) p l a n e s , n o t on (111) o r {001) p l a n e s )
ii) P l a s t i c behavior a t high temperature g e n e r a l l y s a t i s f i e s t h e c l a s s i - c a l e q u a t i o n :
where
7
i s t h e s t r a i n r a t e , A i s a c o n s t a n t , n a 3-5, AH i s t h e a c t i v a - t i o n e n t h a l p y f o r s e l f d i f f u s i o n , and kT has i t s u s u a l meaning, e x c e p t f o r p r i s m a t i c s l i p i n A1203,
where an unusual l o g a r i t h m i c law (Rn T~ %a T) i s observed ( s e e C a s t a i n g e t a l . , t h i s i s s u e ) . -+ iii) T i n c r e a s e s w i t h i n c r e a s i n g b , w i t h d e c r e a s i n g s p a c i n g d between Y s l i p p l a n e s , and w i t h i n c r e a s i n g c o n c e n t r a t i o n of i m p u r i t i e s . i v ) Edge d i s l o c a t i o n s a r e u s u a l l y r a t e - c o n t r o l l i n g ( e x c e p t i n U02+x a t low temperatures / 4 / ) .
The c e n t r a l q u e s t i o n i n t h i s a r e a was what determines t h e P e i e r l s
-f
3.3 Dislocations reactions and work-hardening. Correlation of T.E.M.
observations with mechanical tests show that
:i) Dislocation dipoles form by edge-trapping jog mechanism (a jog-
ged screw mechanism may be impor.cant in MgO, NiO, and A1203 at low
temperatures).
ii) Network reactions occur as expected.
iii) Cell structures are uncommon
:cells form in MgO by glide but
three-dimensional networks form by climb in MgA1204 and in A1203 defor-
med by prismatic slip
;on the other hand, dipoles and loops are obser-
ved in A1203 deformed by basal slip.
iv) Work-hardening rates are high (>~/100)
for network formation
but lower (ap/300) for loop formation and cell formation.
The critical issues in this area are a) dislocation sources
:what
are they
?are surface sources exhausted after yielding
?do networks
produce new sources
?an& b) can climb-dissociated dislocations glide
?3.4
C i _ i h ,
recovery and work-softening.- The observations are that
:i )
Different regimes of behavior can be distinguished depending on
the ratio of deformation temperature T to melting temperature Tm.
a) T
60.5 Tm
:glide mechanisms are predominant, sometimes ac-
companied by cross-slip.
b) 0.5 Tm
6
T
<0.75 Tm
:local climb plus self-climb occur.
c) T
2
0.75 Tm
:extensive climb occurs.
ii) Stress-strain curves exhibiting the shape in figure
1are often
observed.
Fig.1.- Different regions of the stress-strain curves : ( 1 ) work hardening, (2) recovery, (3) plateau (work-hardening rate = recovery rate), (4) work- softening
.
iii) Recovery at T
>0.5 T is due to climb. The mechanisms are
m
dipole/loop annealing for basal slip in A1203 and network annealing for
MgA1204 and prismatic slip in A1203.
The c r i t i c a l i s s u e s i n t h i s a r e a c o n c e r n a ) t h e r o l e and t h e impor- t a n c e o f t h e c r o s s - s l i p p r o c e s s , which h a s n o t p r e v i o u s l y r e c e i v e d much a t t e n t i o n , and b ) t h e r a t i o o f d e f o r m a t i o n due t o climb v e r s u s t h a t due t o g l i d e i n t h e o v e r a l l s t r a i n r a t e f o r d e f o r m a t i o n o f s p i n e l c r y s t a l s a t h i g h t e m p e r a t u r e . A p i c t u r e o f t h e d e f o r m a t i o n b e h a v i o r , a p p l y i n g t y - p i c a l l y f o r s t o i c h i o m e t r i c s p i n e l c r y s t a l s , was proposed i n which t h e p r e - y i e l d s t a g e i n v o l v e d g l i d e , and would b e f o l l o w e d a f t e r y i e l d i n g by a c l i m b s t a g e ( f o r example, a s t a g e where c l i m b c o n t r i b u t e s t o s t r a i n r o u g h l y as much a s g l i d e ) . D i s l o c a t i o n s would s t a r t f i r s t t o g l i d e and bow o u t u n t i l t h e y r e a c h t h o s e a t o m i c v a l l e y s where s e s s i l e d i s s o c i a t i o n i s p o s s i b l e , from which E u r t h e r motion i s p o s s i b l e o n l y by c l i m b . A l -
though some i n d i r e c t arguments were d i s c u s s e d , a q u a n t i t a t i v e l y c l e a r e s t i m a t i o n of t h e c l i m b v e r s u s g l i d e r a t i o i n a g i v e n mechanical t e s t i s s t i l l m i s s i n g . I t i s i m p e r i t i v e t o know i f climb d i s s o c i a t i o n i s r a - t e - c o n t r o l l i n g o r i r r e l e v a n t and w h e t h e r Nabarro-climb o r c l i m b - c o n t r o l - l e d g l i d e o c c u r s . 4 . I n t e r a c t i o n between d i s l o c a t i o n s and p o i n t d e f e c t s . - We d i s c u s s n e x t t h e p o s s i b l e i n t e r a c t i o n s between d i s l o c a t i o n s and d i f f e r e n t k i n d s o f p o i n t d e f e c t s . 4.1 I n t r i n s i c p o i n t d e f e c t s . - Evidence h a s b e e n p r e s e n t e d t h a t t h e s e de- f e c t s may be p l a y i n g an i m p o r t a n t r o l e i n h i g h t e m p e r a t u r e p l a s t i c i t y of MgO, A1203, o l i v i n e ((Mg, F e f Z S i 0 4 ) , and g a r n e t c r y s t a l s (Y3Fe5012,
Gd3Ga5012 / 5 / )
.
T h e o r e t i c a l a t o m i s t i c c o m p u t a t i o n s i n MgO have been r e p o r t e d by P u l s ( t h i s i s s u e ) f o r i n t e r a c t i o n between an edge d i s l o c a t i o n and a ) a s i m p l e c a t i o n vacancy, b ) an a n i o n vacancy, and c ) a (Fe' -V" -Fe' ) X
M9 M9 Mg n e u t r a l t r i m e r . The r e s u l t s g i v e a maximum b i n d i n g e n e r g y which i s i n t h e r a n g e 1-2 e V , t h e e s t i m a t e b e i n g no b e t t e r t h a n Q, 40 %. There i s
n e i t h e r q u a l i t a t i v e n o r q u a n t i t a t i v e agreement between t h e s e a t o m i s t i c c o m p u t a t i o n s and t h e u s u a l i s o t r o p i c e l a s t i c modelleven a t d i s t a n c e s o f t h e p o i n t d e f e c t from t h e d i s l o c a t i o n c e n t e r a s l a r g e a s 4b. T h i s d i s a - greement may r e f l e c t u n c e r t a i n t i e s i n t h e p a r a m e t e r s used i n t h e s i m p l e e l a s t i c (continuum) model. I n t e r n a l f r i c t i o n measurements performed by Gabbay e t a l . ( s e e t h i s i s s u e ) have l e d t o a v a l u e o f t h e b i n d i n g e n e r g y f o r t h e vacancy - d i s l o c a t i o n i n t e r a c t i o n which i s i n b e t t e r agreement w i t h t h e o r e t i c a l c a l c u l a t i o n s .
ments of the diffusion coefficient of oxygen, and there is need for mea-
surements of the diffusion coefficient of aluminum. Detailed understan-
ding of climb dissociation processes in this material is difficult.
4.2
N g q - s t o i c h i o r ~ e t r i c - p o i n t
defects.-Steady-state creep experiments
performed in non-stoichiometric oxides at temperatures for which thermal
equilibrium of the defects can be reached (say at
T >0.5 Tm, though
such a scaling was claimed by some participants to be irrelevant) have
shown that a good correlation exists between the dependence of the stea-
dy state strain rate i and that of oxygen diffusion coefficient Do on
s
oxygen partial pressure. This has been found to hold in C U ~ - ~ O ,
Ni
01
-x
/2/, and C O ~ - ~ O
(see Dominguez-Rodriguez et al. this issue). However,
the accuracy on the values of the activation energy calculated from both
sets of measurements
( Eand D
)is poor so that it is impossible to
s
0make definitive conclusions.
As to the general problem of whether the overall effect
of
non-stoi-
chiometry is hardening or softening, it was pointed out that both types
of behavior have been observed in the materials investigated so far.
Softening has been observed in C U ~ - ~ O
and C O ~ - ~ O .
On the other hand,
Ti02-x exhibits strong hardening when deformed by the easiest slip sys-
tem at T
%0.62 Tm
;this hardening is not considered at the moment to
be due to CS planes (see
Vrinat et al., this issue). The case of Nil-xO
and U02+x (see Keller et al., this issue) appears quite unclear at the
moment, since some experiments on these oxides have been performed in
non-equilibrium conditions, leading to discrepancies and to a large
scatter in the data. The roles of the impurities and of the slip aniso-
tropy have yet to be elucidated.
Deformation of nonstoichiometric Mg0.n(A1203) spinel has received
much attention in the past few years (see Donlon et al.
/8/,Duclos,
this issue, Veyssiere /2,9/). The experimental data agreed upon by the
participants at the workshop were
:i) In contrast with previous investigations, the
C
11
0
)<li0> slip
system appears to be the easiest one in nearly stoichiometric spinel
at low temperatures
/9/;on the other hand, it is not activated around
microhardness indents at room temperature.
ii) Climb dissociation modes after high temperature deformation
have been well documented by detailed TEM analysis of isolated disloca-
tions. Dislocations are observed to split into faulted ribbons off their
glide planes
;these ribbons are found either in simple crystallographic
planes, typically for n
%1, or more generally, on irrational planes for
n
>1.
had been claimed in the past few years, is now somewhat doubtful .Duclos
(this issue) showed that the n
-
1.8spinel may be as hard as the n
=1
spinel when dislocation glide is hindered and climb activated
.
As noted above, questions remain about the exact role of climb dis-
sociation in glide and climb processes, particularly if dislocations do
dissociate by climb during the deformation itself. Such processes may
be sensitive to n composition (e.g. different widths, different crystal-
lographic structures in relation to point defects, and the question whe-
ther cation vacancies introduced as charge-compensating defects allow
viscous glide of the ribbon fault when n
>1).
4.3
hpgyimies.-
The role of controlled dopants was discussed by Haasen
for alkali and silver halides, and by Hirsch for semiconductors.
In ionic crystals of NaCl structure doped with divalent impurities,
the critical resolved-shear
stress (CRSS)
T,decreases with increasing
temperature, and four temperature regions can be distinguished (Skrotzki
and Haasen, this issue). In addition, the hardness depends strongly on
the activated slip plane, (110) or (100) (both of which were activated
over a wide range of temperatures). The four regions correspond to dif-
ferent obstacles to dislocation motion, viz. the Peierl's barrier, point
defects which can be fixed or can migrate, and other dislocation debris.
The important point to be emphasized, however, is that even in the "pu-
rest" crystals, the CRSS is determined by a background of generally un-
known impurities.
In covalent semiconductors, electrically active impurities are
known to change dislocation velocities by up to an order of magnitude.
Hirsch gave a review of experimental results and some speculative ideas
about their interpretation. Dangling bonds may act as donor or acceptor
states along dislocation cores depending on the position of the Fermi
level (i.e. the concentration of impurities). Dislocation cores can give
rise to acceptor states and become negatively charged at thermal equi-
librium
;the exact localization of the charges depend on reconstruction
of bonds along the core, which is generally dissociated. The effect of
doping on dislocation mobility is considered to be due to the transition
of an electron/hole from the conduction/valence band, or from the dislo-
cations bands to the kink sites /lo/. Extension of this mechanism to
some oxides is still speculative but may explain the strong influence
of small amount of specific impurities. For example, the substantial
softening of SiOZ (quartz) by water is a matter of controversy between
the above reported mechanism and a previous suggestion by Griggs and
Blacic /11/ of hydrolised Si-0-Si bonds.
than 1500°C. These dopants reduce the flow stress of both a- axis and
c- axis sapphire, the latter by as much as 50%
.
These effects were dis-
cussed with plausible rate controlling mechanisms for the pure materials
as the background.
4.4 Irradiation defects.- Irradiation defects in spinels were reported
by Hobbs /12/. Two types of Frank dislocation loops may be created
:one
has a 1/3 <Ill> Burgers vector corresponding to a cation faults, the
O-ther a 1/6 clll> vector corresponding to both anion and cation faults.
Results of irradiation and implantation experiments in Ti02 were presen-
ted by Thevenard (see this issue).
5. Conclusion.- The Workshop demonstrated that further experimental work
is necessary for better understanding of point defect-dislocation inter-
actions.
There was general agreement that the charges on dislocations and
screening effects have to be experimentally investigated in oxides as
has been done, for instance, in alkali halides /13/. In purely ionic
crystals and in covalently-bonded semiconductors, dislocations, point
defects and surfaces are charged, giving rise to strong electrostatic
interactions between them. At thermal equilibrium, this charge is com-
pensated by a Debye-HOckel screening cloud of opposite sign with a
screening length
AD
around the defect
here
cots
is the permittivity and n the concentration of the charged
species present in the cloud. In some cases the sign of the dislocation
or surface charge changes with temperature
;at low temperatures, ex-
trinsic (impurity) effects may be important, whereas intrinsic defects
dominate at high temperatures. The isoelectric temperature is that tem-
perature at which the impurity-induced vacancy concentration is equal
to the intrinsic vacancy concentration in the absence of electrostatic
effects /14/. All these effects are well-known in alkali halides but
still not described in oxides, where the charge on dislocations has not
been measured. It was emphasized that careful measurements of disloca-
tion mobility may have to be made to really understand this effect.
interactions between dislocations and individual point defects over a
large range of concentrations (NB
:Ni has almost the same ionic radius
as Mg)
.
It should be noticed, however, that all material presently availa-
ble commercially contain transition metal ions, and it is not clear how
to eliminate from the crystals cation vacancies which compensate for the
(usual) excess positive charge of these ions.
To improve significant understanding of dislocation substructure
investigated by TEM observations, it is necessary to focus on quenching
experiments. The Workshop also demonstrated that real attention must be
paid to the role of cross-slip, which had been somewhat neglected in
previous investigations. As finally emphasized by Evans, the ultimate
success of structural ceramics can be assured not only by understanding
factors that control the strength of single crystals but also by deve-
lopment of techniques for effective prediction of failure in polycrys-
tals. Indentation tests may be as important as tensile tests from this
point of view. The Von Mises criterion that five (or perhaps four) in-
dependent active slip systems are necessary to maintain continuity of
slip displacement and stress at grains boundaries is not satisfied in
oxide polycrystals. As a consequence, grain boundary diffusion must
occur in order to relax stress concentration at pile-ups. When diffusion
occurs, appreciable deformation can take place, but only at relatively
low strain rates. The onset of such plastic behavior is generally not of
benefit for the structural performance of ceramics, as it means that
mechanical performance degrades with time.
References
/1/ Mitchell, T.E., Hobbs,
L.bJ.,Heuer, A.H., Castaing,
J.,Cadoz, J., Philibert, J., Acta Metall. 27
(1979) 1677.
/2/ Bretheau, T., Castaing, J., Rabier, J., Veyssiere,
P.,Adv. Phys. 29
(1979)
829/3/
Riviere, J.P., Cadoz, J., Philibert, J., Proc. of 7th
European Congress on Electron Microscopy, The Hague
1 (1980) 410-411.
-
/4/ Keller, K.J., Kitchell, T.E., Heuer, A.H., to be publi-
shed.
/5/
Rabier, J., Veyssiere,
P.,Garem, H., Grilh6, J., Philos.
Mag. A 2 (1979) 693.
/6/
