STRUCTURE AND PROPERTIES OF CREEP SUBBOUNDARIES

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STRUCTURE AND PROPERTIES OF CREEP SUBBOUNDARIES

J. Martin, M. Morris, M. Carrard

To cite this version:

J. Martin, M. Morris, M. Carrard. STRUCTURE AND PROPERTIES OF CREEP SUBBOUND- ARIES. Journal de Physique Colloques, 1985, 46 (C4), pp.C4-417-C4-422. �10.1051/jphyscol:1985445�.

�jpa-00224696�

JOURNAL DE PHYSIQUE

Colloque C#, supplement au n°4, Tome 46, avril 1985 page C4-417

S T R U C T U R E A N D P R O P E R T I E S OF CREEP S U B B O U N D A R I E S

J.L. Martin, M. Morris and M. Carrard

Institut de Génie Atomique,-EPFL, CH-101S Lausanne, Switzerland

Résumé - On décrit les propriétés des sous-joints de fluage à partir d'ob- servations et expériences diverses réalisées dans un microscope électroni- que dans de Valuminium et un alliage aluminium-zinc. Leur géométrie, champs de contraintes associés, leurs propriétés dynamiques et leur rôle dans la résistance au fluage sont examinés.

Abstract - The properties of creep subboundaries are described as derived from observations and experiments in the electron microscope for aluminium and an aluminium-zinc alloy. Their geometry, associated stress fields, their dynamic properties and their contribution to the creep strength are reviewed.

Among the different types of interfaces, subboundaries are simple ones. At low misorientations at least, the constituant dislocations are visible by means of conventional electron microscopy and several of their properties can be known. The present paper reports the characteristics of subboundaries which develop during stage II of creep in Aluminium and an Al-Zn alloy. The static properties (detailed geometrical description), dynamic ones (mobility . . . ) , and their connection with the constitutive equations of creep are studied here below.

EXPERIMENTAL DETAILS

The results presented here were obtained, using 99.99 % Al single crystals in the 112 orientation, crept at 150 ° C and 400 °C /l/, 99.3 % pure Al in polycrystalline sheets of .5 mm thickness tested between 80 and 200 °C / 2 / , and Al-11 wt % Zn cylindrical polycrystals crept at 210 and 215 °C /3/. All were deformed in tension up to stage II of creep. Conventional electron microscope studies were carried out (TEM and SEM), as well as in situ creep experiments in a high voltage electron microscope (HVEM) / 4 , 5/ and dislocation pinning experiments in the load applied state / 3 / in Al-Zn. The latter techniques provide useful informations about the substructure properties under load, and are very complementary /&/.

SUBBOUNDARY STRUCTURE

The possible geometries of subboundaries (i.e. periodic networks with no long range stress fields) are analyzed in detail, in HI. The first experimental obser- vations of subboundaries were performed by decoration techniques in NaCl and KC1 /8/.

Creep subboundaries appear in stage II, when the crystal is divided into subgrains of several microns in diameter.

In the single crystals /l/, the symmetrical orientation with respect to glide, develops at 150 °C a characteristic substructure illustrated on fig. 1. A cross- grating of two types of cylinders approximately parallel to the directions of the primary Burgers vectors (i.e. [101] and [011]) appears on a (111) longitudinal section. The (112) normal cross section shows that the corresponding crystal blocks are elongated along the [111] direction. They are misoriented by 1 or

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1985445

C4-418 JOURNAL

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20 w i t h r e s p e c t t o each other, around [211] and [ I 2 1 1 r e s p e c t i v e l y . They are r o t a t e d a l t e r n a t i v e l y i n t h e p o s i t i v e and negative d i r e c t i o n s around these axes, so as t o m a i n t a i n t h e symmetrical sample o r i e n t a t i o n as i t i s s t r a i n e d . They are d i v i d e d i n t o s m a l l e r subgrains as seen i n TEM, w i t h weak r e l a t i v e m i s o r i e n t a - t i o n s (smaller than . p ) . Some o f t h e subboundaries between the c y l i n d e r s have been analysed. They appear as p e r i o d i c d i s l o c a t i o n networks. Some networks appear t o be o f pure t w i s t type, l y i n g i n t h e (111) plane w i t h 3 Burgers vectors a t 120'. The mesh s i z e s are o f t h e o r d e r o f some 100 A. A more complete i d e n t i f i c a - t i o n o f t h e o t h e r types o f subboundaries i s underway.

At 400 ^{O C ,} on t h e contrary, t h e temperature i s h i g h enough t o a l l o w g l i d e on t h e unusual [110] (001) s l i p system as evidenced by t h e s l i p l i n e s a t t h e sample sur- face. Since i t has a h i g h e r Schmid f a c t o r than t h e 111 primary systems o f i n t e r - mediate temperatures, i t i s p r i v i l e d g e d a t 400 ^{O C .} A r o t a t i o n o f t h e l a t t i c e occurs around [ l I O ] , as expected i n s i n g l e g l i d e , and t h e t e n s i l e a x i s evolves from [ I 1 2 1 towards [Ill]. The subgrains appear as f l a t b l o c k s p a r a l l e l t o (110) planes as evidenced by SEM on (1IO) sections (see t h e stereographic p r o j e c t i o n o f F i g . 1). Subboundaries e x h i b i t h i g h e r t i l t angles than p r e v i o u s l y ( 5 t o 6O).

The d i s l o c a t i o n s a r e t h e r e f o r e much c l o s e r t o each other (around 20 A and more d i f f i c u l t t o i d e n t i f y b u t seem t o belong t o t i l t boundaries w i t h

IIIO]

^as

r o t a t i o n a x i s (M. Carrard, t o be published).

I n t h e Aluminium sheets c r e p t a t i n t e r m e d i a t e temperatures, t h e f o l l o w i n g f e a t u r e s o f subboundaries were obtained /2/:

-

Pure t i l t boundaries, o r pure t w i s t ones c o n s i s t i n g o f a square g r i d o f two screw d i s l o c a t i o n f a m i l i e s are observed b u t seldom.

-

Boundaries o f mixed character, w i t h 3 Burgers vectors a t 120 ^{O C} are t h e most f r e q u e n t ones, l y i n g i n a v a r i e t y o f planes. The d i s l o c a t i o n segments appear t o l i e i n t h e i r r e s p e c t i v e s l i p planes and t h e general c o n f i g u r a t i o n of t h e n e t - works i s presented on f i g . 2 a.

-

The Frank formula which expresses t h a t a d i s l o c a t i o n a r r a y does n o t e x n i b i t any l o n g range s t r e s s f i e l d , seems t o be s a t i s f i e d f o r t h e p e r i o d i c s e c t i o n s o f t h e networks. This c o n d i t i o n t o g e t h e r w i t h t h e previous ones can l e a d t o various extreme subboundary c o n f i g u r a t i o n s as t h e subboundary plane i s r o t a t e d away from t h e Burgers v e c t o r s plane (see f i g . 2 b , c, d). The e q u i l i b r i u m o f l i n e tensions a t nodes i n t h e network i s thus n o t f u l l f i l l e d . This i n d i c a t e s t h a t i n aluminium a t l e a s t , t h e weak d i s l o c a t i o n d i s s o c i a t i o n decreases t h e c o r e energy by a l a r g e r amount than t h e subsequent increase o f l i n e energy 191. Some o f these c o n f i g u r a t i o n s had been p r e d i c t e d by B a l l and H i r s c h

/lo/.

I n t h e Al-Zn p o l y c r y s t a l , subboundaries were a l s o observed. They have been ana- lysed and most o f them c o n s i s t o f 5 d i s l o c a t i o n f a m i l i e s 131. An examples i s shown on f i g . 3. I t was a l s o observed t h a t the m i s o r i e n t a t i o n across subgrains increases w i t h s t r a i n /3/ i n agreement w i t h o t h e r measurements /12/.

Thus, according t o t h e t y p e o f sample and creep conditions, subboundaries c o n s i s t - i n g o f one t o f i v e d i s l o c a t i o n f a m i l i e s ( o u t o f 6 p o s s i b l e ones i n t h e FCC s t r u c - t u r e ) are present. This can be explained by t h e p a r t i c u l a r geometry o f t h e s i n g l e c r y s t a l , t h e various deformation c o m p a t i b i l i t y requirements i n t h e p o l y c r y s t a l s ; t h e y are two dimensional i n t h e sheet case and more severe f o r t h e c y l i n d r i c a l samples o f Al-Zn.

SUBBOUNDARY STRESS FIELDS

As mentioned above, t h e y were already checked through t h e Frank c r i t e r i o n . A d i f f e r e n t experimental approach has been developped as w e l l /11/. When i t i s p o s s i b l e t o p i n t h e d i s l o c a t i o n arrangements under load, d e f e c t s e x h i b i t bowed shapes under t h e e f f e c t s o f t h e l o c a l o r e f f e c t i v e stresses present i n t h e c r y s -

l m r

F i g . 2

-

Sketch o f subboundaries w i t h 3 bs a t 1 2 6 . a ) general case; b), c ) , d ) extreme p o s s i b l e cases /18/. XI, X2, X3 d i s l o c a t i o n segments.

t a l . The f o l l o w i n g t e c h n i q u e was used i n t h e case o f Al-Zn. A t creep temperatures o f 210 o r 250 ^{O C ,} t h e z i n c i s i n s o l i d s o l u t i o n and t h e a l l o y creeps i n t h e same way as aluminium (i.e. f o r m i n g s u b g r a i n s ) . The p i n n i n g t e c h n i q u e used, c o n s i s t s i n quenching and ageing t h e specimen under l o a d such t h a t z i n c p r e c i p i t a t e s o n t o t h e d i s l o c a t i o n s t o f r e e z e them as t h e y a r e d u r i n g t h e c r e e p t e s t / I / . The sample i s t h e n unloaded and f o i l s a r e prepared f o r TEM. From t h e d i s l o c a t i o n c u r v a t u r e s , o r d e r s of magnitude o f l o c a l e f f e c t i v e s t r e s s e s can be d e r i v e d .

They were measured a t t h e subboundary and as one moves away f r o m i t towards t h e s u b g r a i n i n t e r i o r . R e s u l t s a r e p r e s e n t e d on f i g . 4, which were o b t a i n e d from s e v e r a l s u b g r a i n s a f t e r creep a t 250 OC under an a p p l i e d s t r e s s o f 2.4 MPa and a t a s t r a i n o f 7 %. The s i g n o f t h e l o c a l s t r e s s has n o t been measured and a l l values a r e p l o t t e d as p o s i t i v e . I t appears t h a t v e r y h i g h l o c a l s t r e s s e s e x i s t a t g i v e n p o i n t s o f t h e subboundaries, s e v e r a l t i m e s l a r g e r t h a n t h e a p p l i e d s t r e s s and t h e s e decrease as a f u n c t i o n o f t h e d i s t a n c e f r o m it. I n a d d i t i o n , a g i v e n sub- boundary e x h i b i t s a l i m i t e d number of p o i n t s w i t h a h i g h l y c o n c e n t r a t e d s t r e s s , i f

JOURNAL DE PHYSIQUE

Fig. 3

-

Sketch o f a subboundary w i t h 5 d i s l o - Fig.4

-

Normalized l o c a l st;esses c a t i o n f a m i l i e s as observed i n Al-Zn. around subboundaries. e = 7% /3/.

10-

6 -

2

any Ill/. I t has a l s o been observed t h a t t h e maximum values o f l o c a l stresses i n subboudaries increase as s t r a i n increases, f o r given creep conditions, as s t r e s s increases f o r a given temperature and s t r a i n and as temperature increases f o r a given s t r e s s and s t r a i n /13/. I t t h e r e f o r e appears t h a t as d i s l o c a t i o n s are hete- rogeneously s t o r e d i n t h e c r y s t a l , a non homogeneous s t r e s s d i s t r i b u t i o n i s b u i l t up. I t i s thought t h a t these areas o f h i g h e f f e c t i v e stresses correspond t o p o i n t s a t which t h e p e r i o d i c i t y o f t h e subboundary network i s i n t e r r u p t e d as a r e s u l t of deformation: e x t r i n s i c d i s l o c a t i o n s e n t e r t h e network w h i l e others are l e a v i n g i t t h u s a l t e r i n g i t s r e g u l a r i t y .

;

T Z 2 5 0 - C

2.L MNIm2 i = 1.2 x 10-6~-'

*.

-

-I-?_J.---_~_-% ----_

..

^{--- Teff =TI}

DYNAMIC PROPERTIES OF SUBBOUNDARIES

0 4 8 Distance (&mj

These were observed d u r i n g i n s i t u creep experiments performed i n a HVEM /4/: t h e aluminium p o l y c r y s t a l l i n e sheets were submitted t o macroscopic creep t e s t s . Then, microsamples were c u t o u t o f them, w i t h t h e same t e n s i l e axis, and f u r t h e r de- formed i n t h e microscope a t t h e same temperature and s t r a i n r a t e as i n stage I 1 o f t h e preceding t e s t .

The d e s t r u c t i o n o f subboundaries (as w e l l as t h e i r f o r m a t i o n ) i s sometimes ob- served. Some examples have been r e p o r t e d p r e v i o u s l y 1141 i n which, a square g r i d o f screw d i s l o c a t i o n s (pure t w i s t boundary i n a 100 plane) was destroyed: t h e a p p l i e d s t r e s s p u l l e d both d i s l o c a t i o n f a m i l i e s a p a r t ou t h e i r g l i d e planes o f {110) type. But numerous experiments performed under s i m i l a r c o n d i t i o n s have shown t h a t t h i s phenomenon occurs r a t h e r seldom. I n a d d i t i o n t h e newly e m i t t e d d i s l o c a - t i o n s account f o r l e s s than 1 % o f t h e t o t a l s t r a i n . However, t h e m i g r a t i o n o f subboudaries i s more commonly observed. This process has been evidenced a long t i m e ago i n t h e experiments o f Washburn and Parker /15/. An extensive study was undertaken by E x e l l and Warrington, d u r i n g creep o f Aluminium between 400 and 600 ^OC, using X r a y topography and a Nomarski i n t e r f e r o m e t e r /16/. It a l s o occurs a t lower temperatures as seen d u r i n g i n s i t u s t r a i n i n g . The d i s l o c a t i o n t r a c e s v i s i b l e a t t h e f o i l surface behind t h e moving boundaries, i n d i c a t e t h a t i t s con- s t i t u e n t segments move by g l i d e . M i g r a t i o n takes p l a c e over l a r g e distances ( ? s i g n i f i c a n t f r a c t i o n o f t h e subboundary diameter) w i t h a low speed t o 10- plmn). The d i s l o c a t i o n segments having 3 Burgers vectors a t 12(P (see above), two f a m i l i e s g l i d e i n d i f f e r e n t (111) planes w h i l e t h e t h i r d one g l i d e s i n a

{loo)

t y p e plane. Indeed t h e r e are n o t t h r e e i n t e r s e c t i n g 111 planes i n t h e FCC s t r u c - ture. The geometry o f t h i s g l i d e process i s sketched on Fig. 5.

The locus o f t h e boundary nodes a r e <011> d i r e c t i o n s which as a r u l e , a r e n o t per- p e n d i c u l a r t o t h e network plane. Therefore t h e nodes move along t h i s plane d u r i n g m i g r a t i o n so t h a t t h e c o n s t i t u e n t d i s l o c a t i o n s move towards t r i p l e junctions, are c r o s s i n g through them and are c o n t i n u o u s l y t r a n s f e r r e d from one subboundary t o t h e adjacent one. T h i s complex t h r e e dimensional g l i d e mechanism, which i n v o l v e s a c o o p e r a t i v e s l i p on t h r e e d i f f e r e n t planes, i s p o s s i b l e i n t h i s case because 3 d i s l o c a t i o n f a m i l i e s o n l y are involved.

I n t h e Al-Zn p o l y c r y s t a l , m i g r a t i o n has been observed t o o Ill/. The surface t i l t s connected t o i t were observed b y s t r a i n i n g a specimen t o 15 %, p o l s h i n g i t and r e s t r a i n i n g i t another 2 %. The s u r f a c e was examined u s i n g Nomarski i n t e r f e r e n c e c o n t r a s t . An example o f such o b s e r v a t i o n s can be found i n

1171.

I n t h i s case, s i n c e subboundaries c o n s i s t o f f i v e d i s l o c a t i o n f a m i l i e s as r e p o r t e d above, and t h a t t h e y a r e n o t always s i t u a t e d i n g l i d e planes, m i g r a t i o n cannot occur b y g l i d e o n l y .

Under t h e s e v a r i o u s e x p e r i m e n t a l c o n d i t i o n s , t h e amount o f s t r a i n due t o m i g r a t i o n c o u l d be estimated. I t i s always a s m a l l f r a c t i o n o f t h e t o t a l s t r a i n i.e. 5 t o 1 0 % i n agreement w i t h o t h e r measurements 1161.

F i g . 5

-

Scheme o f subboundary m i g r a t i o n . F i g . 6

-

L o c a l creep c u r v e i n a g i v e n SP and FP: subboundary and f o i l planes. subgrain. I n s i t u experiment i n t h e X i : d i s l o c a t i o n segments. P i , y i i HVEM. A1 200 OC

/la/.

c o r r e s p o n d i n g s l i p p l a n e s and s l i p t r a - ces /4/.

SUBBOUNDARIES AND CREEP RATE

S i n c e a s m a l l amount o f s t r a i n o n l y i s due t o boundary m i g r a t i o n o r d e s t r u c t i o n , t h e m o t i o n o f i n d i v i d u a l d i s l o c a t i o n s i s r e s p o n s i b l e f o r most o f it. I t c o u l d be d i r e c t l y s t u d i e d i n aluminium p o l y c r y s t a l l i n e sheets w i t h t h e h e l p o f i n s i t u experiments, and t h i s p r o v i d e d u s e f u l i n f o r m a t i o n s about t h e p o s s i b l e i n f l u e n c e o f subboundaries on t h e r a t e c o n t r o l l i n g process d u r i n g stage I 1 o f creep

151.

The d i r e c t o b s e r v a t i o n o f moving d i s l o c a t i o n s t h r o u g h s u b g r a i n s p r o v i d e s u s e f u l i n f o r m a t i o n s on t h e i r d e n s i t i e s , on t h e e f f e c t i v e o b s t a c l e s t o t h e i r motion, as w e l l as t h e c o r r e s p o n d i n g w a i t i n g t i m e s i n v o l v e d . D i s l o c a t i o n s are observed t o g l i d e t h r o u g h t h e s u b g r a i n s a t temperatures between 8 0 and 200 OC. T h i s c o u l d be deduced from t h e t r a c e s l e f t a t t h e f o i l s u r f a c e s b y moving d e f e c t s . The f l i g h t t i m e t h r o u g h s u b g r a i n s c o u l d be measured f r o m dynamic r e c o r d i n g s and was found t o be s h o r t e r t h a n 1/50 s. The mean f r e e p a t h o f t h e d i s l o c a t i o n s i s l a r g e r t h a n t h e s u b g r a i n s i z e s i n c e moving d i s l o c a t i o n s a r e observed t o c u t t h r o u g h subbound- a r i e s . T h i s i s a much s l o w e r process and w a i t i n g t i m e s l a r g e r o r much l a r g e r than 1 s can be measured a t t h e boundaries. A l o c a l c r e e p c u r v e c o u l d be b u i l t f r o m such o b s e r v a t i o n s i n t h e f o l l o w i n g way: each d i s l o c a t i o n which shears t h e subgrain p r o v i d e s an amount o f s t r a i n E = b/L i f b i s i t s Burgers v e c t o r . By c o u n t i n g t h e number o f p a s s i n g d i s l o c a t i o n s as a f u n c t i o n o f time, t h e c u r v e o f F i g . 6 c o u l d be e s t a b l i s h e d . I t shows t h a t successive groups o f 5, 4, 1 and 5 d i s l o c a t i o n s respec- t i v e l y , have been g l i d i n g t h r o u g h t h e observed s u b g r a i n ( 2 pm i n d i a m e t e r ) d u r i n g t h e o b s e r v a t i o n p e r i o d o f 2000 s. An average d e f o r m a t i o n r a t e o f 7 . 5 * 1 0 - ~ s-' c o u l d be d e r i v e d . T h i s experiment i l l u s t r a t e s t h e j e r k y n a t u r e o f g l i d e under such creep c o n d i t i o n s 1181.

C4-422 J O U R N A L DE PHYSIQUE

Since t h e r a t e a t which t h e c r y s t a l creeps, seems t o be d i r e c t l y connected t o t h e c u t t i n g through mechanism o f mobile d i s l o c a t i o n s through subboundaries, t h i s process was s t u d i e d i n d e t a i l b o t h e x p e r i m e n t a l l y and t h e o r e t i c a l l y . I n s i t u expe- riments /5/ and pinned d i s l o c a t i o n arrangements /3/ show t h a t a cross1 i p mechanism i s i n v o l v e d f o r d i s l o c a t i o n i n s e r t i o n o r e x t r a c t i o n . The c r i t i c a l event i n e x t r a c - t i o n i s t h e breaking o f t h e f i r s t node by t h e escaping d i s l o c a t i o n . Orders o f magnitude o f t h e c r i t i c a l s t r e s s have been computed /5/ and though t h e y are s m a l l e r than t h e Orowan stress, t h e y are much l a r g e r than t h e a p p l i e d one.

The b a r r i e r s t o d i s l o c a t i o n motion f o r h i g h temperature creep i n aluminium are t h e subboundaries. They are constructed as s t r a i n i n g proceeds. I n t h e same time, t h e c r y s t a l b u i l d s up l a r g e l o c a l i n t e r n a l stresses which add t o t h e a p p l i e d s t r e s s and h e l p d i s l o c a t i o n emission a t p r i v i l e d g e d p o i n t s i n t h e boundaries.

Acknowledgements: T h i s work i s supported by "Fonds N a t i o n a l SuisseI1 and "ATP I n t e r n a t i o n a l e du CNRS'. The authors acknowledge several f r u i t f u l discussions w i t h J. Bonneville.

REFERENCES

/1/ M. Carrard and J.L. M a r t i n i n "Creep and F r a c t u r e o f Engineering M a t e r i a l s and S t r u c t u r e s " , Ed. B. W i l s h i r e and D.J. Owen, Pinebridge Press, Swansea U.K., (1984), p. 27

/2/ D. Cai l l a r d and J.L. Martin, Acta Met. 30 (1982) 437 /3/ M. M o r r i s and J.L. Martin, A c t a Met. 32 (1984) 549 /4/ D. C a i l l a r d and J.L. Martin, Acta Met. 30 (1982) 791 /5/ D. C a i l l a r d and

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Martin, Acta Met. 3 1 (1983) 813

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