AN 18R TO HEXAGONAL TRANSFORMATION IN CuZnAl

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AN 18R TO HEXAGONAL TRANSFORMATION IN CuZnAl

M. Sade, R. Rapacioli, F. Lovey, M. Ahlers

To cite this version:

M. Sade, R. Rapacioli, F. Lovey, M. Ahlers. AN 18R TO HEXAGONAL TRANSFOR- MATION IN CuZnAl. Journal de Physique Colloques, 1982, 43 (C4), pp.C4-647-C4-652.

�10.1051/jphyscol:19824104�. �jpa-00221959�

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CoZZoque C4, suppZdment au n o 12, Tome 43, de'cembre 1982

AN 18R

TO

HEXAGONAL TRANSFORMATION IN CuZnAl

M. Sade, R. Rapacioli, F.C. Lovey and M. Ahlers

Centro A t h i c o BariZoche, ~omisio'n NacionaZ de Energz'a

Atomics,

8400-S.C. de Bariloche, Argentina

(Revised text accepted 23 September 1982)

Abstract: In the present paper studies of a transformation from the orthorhombic 18R to a hexagonal 2H martensite in Cu-Zn-A1 are reported.

This structural change was obtained by applying tensile stresses to 18R single crystals which in turn had been stress induced from 5 phase single crystals of various tensile orientations all deviating more than 28O from a <100>5 axis. Two variants of a 2I1 structure were observed: a) the basal plane parallel to that of the 18R martensite, and also to the habit plane; and b) the basal plane in the 2H perpendicular to that of the 18R and inclined to the habit plane. The hexagonal structures were analized by TEE1 and optical microscopy.

I. Introduction.- The mechanical properties of shape memory materials in the martensitic state have received considerable attention recently, since they do not only deform plastically by slip, but are also found to transform to a variety of other structures, whose selection depends, among other factors, on the orientation of the stress system with respect to the crystal axes. In this way it has been possible to transform 18R martensite to an fct or a hexagonal structure (1-7). (Due to the long range order, the hexagonal martensite in fact is orthorhombically distorted, nevertheless it will be referre\d to as hexagonal in the following).

When an 18R single crystal of Cu-Zn or Cu-Zn-Al, which has been obtained by tensile stressing from the 5 phase, is stressed in tension, an fct martensite is obtained for orientations near in the original 5 phase (1-3). The shear occurs on a [100]~(001)~ system of the orthorhombic lattice (corresponding to a (111)fcc<112>fcc system in the fcc lattice). It is expected and has been found, in fact, that after an appropriate cutting and stres-sing of the 18R crystal, also a hexagonal 2H structure can be induced by a [100]~(001)~ shear (4). It should not be possible, however, to induce a 2H crystal by tensioning a 5 single crystal, whatever its orientation, since the formatisn of the hexagonal phase is associated with a shortening of the sample for a [100]~(001)~ system.

This expectation seems to be at variance with the results obtained by Tas et al.

( 5 ) , who stressed in tension multivariant Cu-Zn-A1 martensitic crystals and found that they had transformed into a nearly single crystalline hexagonal variant. It is not clear whether this is due to the interaction of the stresses with the boundaries, or whether another than the [100]0(001)0 system is activated.

In this paper it is shown that indeed by simple tensile experiments a hexagonal phase is obtained from a monovariant 18R crystal. Its crystallo- graphic 'aspects are analized by optical and transmission electron microscopy

(TEM).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19824104

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C4-648 JOURNAL DE PHYSIQUE

11. E x p e r i m e n t a l procedure.- The 13 phase s i n g l e c r y s t a l s were grown i n t h e u s u a l way by t h e Bridgman t e c h n i q u e , a s d e t a i l e d e l s e w h e r e (3). The composition was Cu-12at% Zn-18at%Al, w i t h a n Ms t e m p e r a t u r e a t 50°C. The o r i e n t a t i o n s of

t h e B phase were determined by t h e x-ray Laue t e c h n i q u e a t 80°C.

The monovariant s i n g l e c r y s t a l s were o b t a i n e d i n t h e f o l l o w i n g s t e p s : 1. A 15 min a n n e a l i n g a t 800°C. 2. Slow a i r c o o l i n g t o 100°C. 3. S t r e s s i n d u c i n g t h e 18R s i n g l e c r y s t a l by a p p l y i n g a t e n s i l e s t r e s s a t 100°C.

4. Cooling under s t r e s s t o below Ms. Thus, when t h e sample was removed from t h e t e s t i n g machine t h e r e g i o n between t h e g r i p s had become a s t a b l e monovariant m a r t e n s i t i c s i n g l e c r y s t a l a t room t e m p e r a t u r e .

The s i n g l e c r y s t a l s which were f i n a l l y used f o r t h e deformation s t u d i e s were of two t y p e s . The f i r s t type had been s p a r k machined i n t h e c e n t e r p a r t t o 3 mm d i a m e t e r and 20 mm l e n g t h b e f o r e t h e f i n a l t r e a t m e n t was a p p l i e d , t h u s t h e s h o u l d e r s of t h e samples t h a t f i t i n t o t h e g r i p s remained p o l y v a r i a n t . The second t y p e was s p a r k machined a f t e r t h e t r e a t m e n t , r e s u l t i n g i n a m a r t e n s i t i c s i n g l e c r y s t a l i n c l u d i n g t h e s h o u l d e r s . A f i n a l e l e c t r o p o l i s h followed i n a l l c a s e s .

The t e n s i l e t e s t s were c a r r i e d out i n a n I n s t r o n t e s t i n g machine model 1123 a t room t e m p e r a t u r e and a t a d e f o r m a t i o n r a t e of 0.025 min-l. A f t e r u n l o a d i n g , t h e samples were s t u d i e d by X-rays, o p t i c a l and e l e c t r o n microscopy.

111. Experimental r e s u l t s . - F i g u r e 1 shows t h e o r i e n t a t i o n s of t h e samples used i n t h e e x p e r i m e n t s ( t h e r e s u l t s f o r o t h e r o r i e n t a t i o n s not i n c l u d e d i n t h e f i g u r e w i l l be r e p o r t e d e l s e w h e r e ) . A l l samples e x c e p t 9 and 10 were of t h e f i r s t t y p e ( w i t h p o l y v a r i a n t s h o u l d e r s ) .

F i g u r e 1: O r i e n t a t i o n s of t h e t e n s i l e a x e s a s r e f e r r e d t o t h e bcc l a t t i c e . Sample 1, 11,12 ( * ) ; 2 ( 0 ) ; 3 , 5 ( t ) ; 4 ( 0 ) ; 6(n); 7(%); 8 ( t ) ; 9(x); l o @ ) .

Type 1 samples: When g r i p s were used which s u p p o r t e d t h e s h o u l d e r s of t h e samples from below, f r a c t u r e o c c u r r e d n e a r t h e heads a t low s t r e s s e s . T h e r e f o r e t h e sample ends were clamped r i g i d l y between two s p l i t g r i p s , a l t h o u g h it was c l e a r t h a t t h i s could l e a d t o c o n s i d e r a b l e bending s t r e s s e s . S i n c e s l i d i n g i n t h e g r i p s sometimes o c c u r r e d , t h e s t r e s s s t r a i n c u r v e s had i r r e g u l a r i t i e s . However no w e l l marked r e p r o d u c i b l e s t a g e of low hardening could be observed which might have been a s s o c i a t e d w i t h t h e f o r m a t i o n of t h e s u r f a c e markings.

The samples r u p t u r e d a t a s t r e s s of about 540 FfiV/m2.

By o p t i c a l microscopy of t h e f r a c t u r e d samples bands were v i s i b l e . Those s e e n i n sample 3 ( F i g . 1 ) were analyzed i n d e t a i l . They a r e shown i n Fig. 2.

By m u l t i s u r f a c e a n a l y s i s , i t i s shown t h a t t h e bands a r e p a r a l l e l t o t h e b a s a l p l a n e of t h e 18R m a r t e n s ~ t e (and i n c l i n e d t o t h e f r a c t u r e p l a n e ) . For t h e p r e s e n t o r i e n t a t i o n t h e a n g l e between t h e sample a x i s and t h e b a s a l p l a n e i s s m a l l . T h e r e f o r e a p r e c i s e d e t e r m i n a t i o n of t h e s h e a r d i r e c t i o n was d i f f i c u l t . I n F i g . 3 a r e shown i n a s t e r e o g r a p h i c p r o j e c t i o n t h e t e n s i l e a x i s (A), t h e

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- t h e s u r f a c e . The range w i t h i n which t h e shear d i r e c t i o n i s l o c a t e d c e n t e r s

around 60° away from

[ .

Figure 2: Bands observed i n sample Figure 3: Stereographic p r o j e c t i o n 3 a f t e r f r a c t u r e ( b a r corresponds showing t h e observed h a b i t planes

t o 8 0 ~ ) . (HP), shear d i r e c t i o n s ( d ) , and t h e

corresponding t e n s i l e axes ( A ) .

By TEM i t was observed t h a t t h e bands were hexagonal. It was confirmed t h a t t h e h a b i t plane was p a r a l l e l t o t h e b a s a l plane, and t h a t t h e basal plane of t h e 18R and 2H s t r u c t u r e s were p a r a l l e l t o each o t h e r w i t h i n 3 O (no change i n d i s t a n c e between b a s a l planes and between atoms i n t h e b a s a l plane were observed f o r t h e two s t r u c t u r e s ) . The r a t i o s f o r t h e 2H L a t t i c e parameters a i n

LOO]^,

b i n [ 0 1 0 ] ~ , and c i n [001] were determined t o be

Figure 4: S e l e c t e d a r e a d i f f r a c t i o n diagram. L e f t : 18R (Zone a x i s [ O I O ] ~ ~ ~ . Right: hexagonal p a t t e r n (zone a x i s [010] 2H ( i .e.

[ ll30]

)

.

The o t h e r samples ( 1 , 2 , 4 t o 9,11 and 12) have shown 18R twins and a second v a r i a n t of a 2H s t r u c t u r e ( v a r i a n t 2 from now on) t h a t w i l l be described i n t h e next paragraphs. I n t h i s work we s h a l l only be concerned with t h e 2H s t r u c t u r e s .

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Type 2 samples: These samples c o u l d be deformed i n g r i p s t o u c h i n g t h e s h o u l d e r s from below. A s t r e s s - s t r a i n c u r v e i s shown i n F i g . 5. A h i g h h a r d e n i n g o c c u r s up t o t h e f r a c t u r e s t r e s s , b e f o r e f r a c t u r i n g some l o a d d r o p s a r e n o t i c e d , t h e y a r e a s s o c i a t e d however w i t h bands p a r a l l e l t o t h e f r a c t u r e p l a n e , and t h u s a r e n o t r e l a t e d t o t h e o t h e r bands which a r e v i s i b l e .

F i g u r e 5: S t r e s s - s t r a i n curve F i g u r e 6: Micrographs showing one of t h e bands f o r sample 10 (sample t y p e 2 ) . observed a f t e r f r a c t u r e i n samples of t y p e 2.

( L i n e s c r o s s i n g t h e band a r e p a r a l l e l t o t h e f r a c t u r e p l a n e ) .

The bands t h a t were s e e n by o p t i c a l microscopy ( F i g . 6 ) were a n a l y z e d by m u l t i s u r f a c e a n a l y s i s . The r e s u l t s a r e p l o t t e d i n t h e s t e r e o g r a p h i c p r o j e c t i o n of F i g . 3. The h a b i t p l a n e i s i n c l i n e d by 28" w i t h r e s p e c t t o t h e b a s a l p l a n e , and t h e s h e a r d i r e c t i o n i s normal t o t h e [ 0 1 0 ] ~ d i r e c t i o n . These bands a r e t h u s d i f f e r ? n t from t h e s e d e s c r i b e d i n d e t a i l f o r t y p e 1 samples. I n t h e f o l l o w i n g t h e y w i l l be denoted by " v a r i a n t 2".

By TEM i t h a s been v e r i f i e d t h a t t h e s e bands have a 2H, o r t h o r h o m b i c a l l y d i s t o r t e d h e x a g o n a l , untwinned s t r u c t u r e ( t h e same a s t h e v a r i a n t I ) , and t h a t t h e b a s a l p l a n e i s normal t o t h a t of t h e 18R. I n F i g . 7 a r e shown some s e l e c t e d a r e a d i f f r a c t i o n diagrams t o prove t h i s : F i g . 7a r e p r e s e n t s a d i f f r a c t i o n p a t t e r n of t h e 2H phase w i t h t h e zone a x i s p a r a l l e l t o

The ( 0 2 % ~ and (040)2H s p o t s a r e marked ( t h e y would c o r r e s p o n d t o (0002) and (2110), r e s p e c t i v e l y i n t h e u s u a l hexagonal n o t a t i o n ) . An a d j a c e n t 18R c r y s t a l shows s i m u l t a n e o u s l y , w i t h o u t a d d i t i o n a l r o t a t i o n , a d i f f r a c t i o n p a t t e r n w i t h a zone a x i s p a r a l l e l t o [ 0 0 1 ] ~ . The o r i e n t a t i o n of t h e and [ 0 1 0 ] ~ a x e s of t h e 18R w i t h r e s p e c t t o t h e 2H a r e a l s o i n d i c a t e d i n Fig. 7 and a r e marked by a. and b o , r e s p e c t i v e l y . Thus from 7a f o l l o w s t h e f o l l o w i n g o r i e n t a t i o n r e l a t i o n s h i p between 18R and 2H:

[ l o o ]

18Ru[0011

2 ~ ; [ ~ ~ ~ 1 1 8 ~ ~ ' [ ~ ~ ~ 1 2 ~ ; [ ~ ~ ~ 1 1 8 ~ " [

"01

2H The p a t t e r n s 7b and 7c a r e shown i n o r d e r t o prove t h a t t h e s t r u c t u r e is 2H.

They a r e o b t a i n e d a f t e r r o t a t i n g t h e 18R c r y s t a l around t h e and [ 0 1 0 ] ~ a x e s , r e s p e c t i v e l y , by t h e r e q u i r e d amount, and i n d e e d show t h e e x p e c t e d 2H d i f f r a c t i o n d i a g r a m s , a s s e e n i n F i g s . 7b and 7c. (More d e t a i l s on t h e 2H s t r u c t u r e i n Cu-Zn-Al w i l l be p u b l i s h e d e l s e w e r e ) .

IV. D i s c u s s i o n : The p r e s e n t r e s u l t s have shown- t h a t t h e hexagonal phase c a n be s t r e s s induced n o t o n l y by a s h e a r on t h e [ 1 0 0 ] ~ ( 0 0 1 ) , , s y s t e m , but a l s o by t e n s i o n i n g B phase s i n g l e c r y s t a l s , l e a d i n g t o t h e two d i f f e r e n t 2H v a r i a n t s : ( v a r i a n t 1 ) (001)188 p a r a l l e l (001)2H w i t h i n 3 ' . H a b i t p l a n e p a r a l l e l (001)

s h e a r direction -60" from [ 1 0 0 ] ~ ~ ~

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Figure

7:

Selected are diffraction diagrams of the 2H variant 2: a) Zone axis

[

(in ortho_rhombic notation, corresponding to [OLIO] in hex indices). Arrows denoted by a. and bo parallel to [100]~ and [ O L O ] ~ of an adjacent L8R crystal. b) Same sample after rotation around 1 ~ 0 1 ~ R; zone axis [ 2 1 0 ] ~ ~ (i.e.

1

^l2l0fheX

^.

c) After rotation around O ~ O ] 8R, zone axis [ i 0 1 ] ~ ~ (i.e.

/O1ll!hex')

It is to be noted that variant 1 is formed for the same orientations as 2, the only difference being that variant 1 appeared less frequently. It is likely that bending stresses exist in these samples, and that therefore the shear stress component from the applied stress is not the only acting stress.

Nevertheless, the fact that the shear direction is not parallel to [100]~

implies that a different transformation mechanism is responsible, and not [i00]~(001)~. An interpretation of this result has to take into account that the Burgers vector of the shear on the (001)18R plane should be the smallest possible (excluding the [100]~?~) and should not change long range order. The DOg order-disorder transformatlon in this alloy occurs at 683 K, and thus cannot be changed by the shear stresses available. The simplest choice is an average shear on each sixth plane with Burgers vector a/3 [ 2 3 0 ] ~ ~ ~ . This is 30 degrees away from [ 0 1 0 ] ~ ~ ~ and is consistent with the experimentally determined shear direction. This transformation to the hexagonal phase leads to an elongation of the sample as required in the tensile test. If instead of a shear on each sixth plane, each third plane is sheared, an 18R twinned crystal is obtained, associated with an elongation. Since, as seen in Fig.3, the Schmid orientation factor is small, and since only a small fraction of the sample transform to the hexagonal phase, it is reasonable to expect no marked change in the stress-strain curve when the transformation starts, and therefore critical strssses are difficult to determine (even when bending stresses are absent).

Variant 2 is more difficult to explain. A similar habit plane had also been observed by Otsuka and Shimizu (7) in CU-Al-Ni, They interpreted their results by an additional ( 1 0 ~ ) ~ ~ twin shear (i.e. a [ 1 0 1 2 ] ~ ~ ~ twinning). Since,

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however, in the present results no twinning has been found, their interpretation, if applied to the Cu-Zn-A1 system, is not very convincing, and at the most can only serve as a formal description of the transformation. Therefore the interpretation of variant 2 must be sought elsewhere. Research on this problem is in progress.

The present results can rationalize the observations by Tas et al. ( 5 ) , that for 13 phase crystal orientations near the (111)~ to (011)~ line of the unit triangle, a hexagonal phase can be induced in originally polyvariant 18R martensites, an observation which has been difficult to reconcile with a [fOO]O(OO1)O transformation shear. The present results have made it clear, that the selection of the variants depends not only on the orientation of the crystals, that therefore bending stresses in the present case, or interface boundaries in that of Tas et al. (5) may play an important role. This is corroborated by the difference in orientation range, for which hexagonal structures are found after deformation in both studies.

Acknowledgements: Helpful advice by Dr. M. Chandrasekaran is gratefully acknowledged.

References

1. ARNEODO, W. and AHLERS, M., Scripta Met.

1

(1973) 1287.

2. SCHROEDER, T.A. and WAYMAN, C.M., Acta Met.

3

(1978) 1745.

3 . BARCELO, G., AHLERS, 11. and RAPACIOLI, R., Z.Metallkde

70

(1979) 732.

4. BARCELO, G. and AHLERS, M., Scripta Met.

16

(1982) 1

5. TAS, H., DELAEY, L. and DERUYTTERE, A., Z. Metallkde

66

(1973) 855.

6. OTSUKA, H., SAKAMOTO, H. and SHIMIZU, H., Scripta Met. (1976) 983.

7. OTSUKA, H. and SHIMIZU, H., Proc. ICOMAT 79, Cambridge, p.607.