ANELASTIC EFFECTS IN SINGLE CRYSTALS OF β-Cu-Zn AND β-Cu-Zn-Al ALLOYS

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ANELASTIC EFFECTS IN SINGLE CRYSTALS OF

β-Cu-Zn AND β-Cu-Zn-Al ALLOYS

A. Ghilarducci de Salva, M. Ahlers

To cite this version:

JOURNAL DE PHYSIQUE

CoZZoque C5, suppZ6ment au n O l O , Tome 42, octobre 1981

ANELASTIC EFFECTS I N SINGLE CRYSTALS OF 6-Cu-Zn AND 8-Cu-Zn-A1 ALLOYS

A. Ghilarducci de ~alva*and M. Ahlers

Centro Atomico BariZoche, Institute BaZseiro, 8400 S . C. de BariZoche, Argentina

-

Comision IITacionaZ de Energfa Atomica

Abstract.- Quenching and aging treatments strong1 a f f e c t the properties of

a

brass (1-3)and of the martensitic transformationr4-5) which occurs

i n

these alloys and which has found widespread i n t e r e s t owing t o the shape memory ef- f e c t associated with i t ( 6 ) . Aging e f f e c t s have seen observed a t room tempera- t u r e and below, where the

6

phase i s long ran e ordered. The long ranSe order has a strong influence on the atomic m o b i l i t y b - 8 ) . Since the degree of order in B brass i s s u f f i c i e n t l y high only a t low temperature where t r a c e r diffusion measurements are d i f f i c u l t t o perform, the internal f r i c t i o n technique cons- t i t u t e s a n important additional source t o study atom mobilities a t low tempera- tures.

In t h i s paper a n e l a s t i c e f f e c t s observed a f t e r quenching 8-Cu-Zn and Cu-Zn-A1 alloys are reported and the information which can deduced on the type and mobility of the defects responsible are b r i e f l y discussed. More d e t a i l s can be found i r 1 ( ~ - 1 0 ) .

1. Results.- Yost of the r e s u l t s have been obtained using ternary Cu-Zn-A1 alloys. The Cu-Zn-A1 o f f e r s several advantages over the binary Cu-Zn :

a ) They can be treated in a i r without

Z n

evaporation, which implies t h a t the quench i s f a s t e r .

b) By varying the composition, 82 or DO3 long range ordered c r y s t a l s can be studied('').

c ) The addition of A1 does not complicate the a n e l a s t i c behaviour since the l a t t e r i s the same in

Cu-Zn-A1

as well a s in

C u - Z n ,

being generally more pronounced in the ternary a1 loy.

Anelastici ty measurements have been carried out using two torsion pendul ums, one a t a frequency around 4 cycles/sec, the other around 50 cycles/sec. The internal f r i c t i o n

Q-'

and the torsion modulus have been measured f o r phase single c r y s t a l s of various orientations. From the measurements the following data were extracted :

the temperature corresponding t o the maximup of the internal f r i c t i o n peak and i t s frequency dependence ; the peak maximum, the modulus defect and t h e i r orientation dependence.

Three internal f r i c t i o n peaks have been observed a f t e r quenching be1 ow rooms temperatures: a peak a t -135", another a t -50°C and a t h i r d a t -15OC. These peaks have not been described i n the l i t e r a t u r e before. Above room temperature two peaks observing a t 100°C and 200°C have been analysed. These peaks had already been repor- ted by Clarebrough f o r CU-zn(12). By studying t h e i r orientation dependence new infor- mation on the type of defect has been obtained(lO). PI1 f i v e peaks are e s s e n t i a l l y

- -

*

On leave of absence a t I n s t i

t u t

National Polytechnique de Grenoble, France

C5-1002 JOURNAL DE PHYSIQUE

r e l a x a t i o n peaks. T h i s has been deduced by comparing t h e a c t i v a t i o n energy obtained from t h e frequency dependence o f t h e peak temperature w i t h those from t h e slopes and the w i d t h o f the peak p r o f i l e s , a l l y i e l d i n g e s s e n t i a l l y the same values.

The most i m p o r t a n t parameters which have been determined from t h e experimental r e s u l t s are l i s t e d i n t a b l e 1 f o r t h e s i n o l e c r y s t a l s . They w i l l be discussed s h o r t l y and i n c l u d e t h e a c t i v a t i o n energy AH (obtained from t h e frequency dependence o f t h e peak temperature T )

,

t h e preexponential f a c t o r T, and t h e r e l a x a t i o n moduli 6S44

P

and 6(Sll

-

S12). Also l i s t e d i n the t a b l e a r e t h e a l l o y compositions and t h e quench temperature T

Q '

-

Table 1 : A n e l a s t i c parameters o f t h e peaks Peak

-135°C

-

50°C

-

15°C

A b r i e f d e s c r i p t i o n o f each o f the peaks w i l l now be g i v e n :

a) The -135°C peak i s observed o n l y a f t e r quenching, i t decays again a f t e r aging a t room temperature. The peak h e i g h t increases w i t h p r i o r p l a s t i c deformation, i n d i c a t i n g t h a t i t i s due t o t h e i n t e r a c t i o n o f p o i n t d e f e c t s w i t h d i s l o c a t i o n s ( H a s i g u t i peak), t h a t e x i s t Dresent i n Cu-Zn-A1 and Cu-Zn.

b) The -50°C peak i s observed o n l y a f t e r a f a s t quench t o below -50°C ( i n cooled a l c o h o l ) , t h e d e f e c t which i s r e s p o n s i b l e anneals o u t r a p i d l y a l r e a d y a t O°C. I t i s observed i n B 2 ordered s i n g l e c r y s t a l s , i n Cu-Zn as w e l l as i n Cu-Zn-Al. The t e r n a r y a l l o y used has a DO3 o r d e r i n n temperature around 150°C. Quenching t h e r e f o r e from 200°C r e t a i n s 82 o r d e r ; t h i s c o u l d be v e r i f i e d by transmission e l e c t r o n micros- copy. The r e l a x a t i o n modulus 6(Sll-S12) i s zero, which i m ~ l i e s t h a t t h e defects has t r i g o n a l symmetry. The peak h e i g h t i s independent o f d i s l o c a t i o n d e n s i t y .

c ) The -15°C i s v e r y s i m i l a r t o the -50°C peak, excent t h a t i t i s found o n l y i n DO3 ordered c r y s t a l s . Since t h e d e f e c t anneals o u t so r a p i d l y i t i s l i k e l y d e f e c t s disappear d u r i n g t h e quench, e s p e c i a l l y when t h e quench temperature i s high.

d) The 100°C peak i s most pronounced a f t e r quenching from 400°C, i t s h e i g h t decreases again w i t h i n c r e a s i n g quenching t e ~ p e r a t u r e above 400°C. Since 6S44 = 0, i t i s due t o a t e t r a g o n a l d e f e c t .

2. Discussion.- It i s g e n e r a l l y accepted t h a t d i f f u s i o n i n t h e ordered

6

brass oc- c u r s through t h e movement o f vacancies. This i m p l i e s t h a t t h e r e o r i e n t a t i o n o f t h e d e f e c t s t h a t a r e r e s p o n s i b l e f o r t h e -50°C, -15OC, +lOO°C and +200°C peaks proceed by vacancy jumps, s i n c e dis1a:ations a r e n o t involved. The vacancy may be e i t h e r form p a r t o f t h e d e f e c t o r i t may be necessary o n l y i n o r d e r t h a t the d e f e c t can r e o r i e n t by t h e a p p r o p r i a t e atoln jumps. I n t h e f i r s t case .ro i s independent o f t h e vacancy concentration, s i n c e t h e jumo t i m e depends o n l y on t h e a c t i v a t i o n energy f o r vacancy movement. I n t h e second case

I-,

i s p r o n o r t i o n a l t o t h e vacancy concentration. Then, i f t h e vacancies are quenched i n , they c o n t r i b u t e t o a constant f a c t o r i n T~

independent o f the measuring temperature. I f they a r e present i n e q u i l i b r i u m t h e i r f c o n c e n t r a t i o n i s temperature dependent and can be w r i t t e n as C V = C o exp(- E,, /kT), where Co i s a preexponential f a c t o r which contains t h e f o r m a t i o n entropy o f a vacan- cy, and E~~ the formation energy o f a vacancy. This reasoning leads t o t h e f o l l o w i n g conclusions :

-

The -50°C and -15OC peaks a r e due t o d e f e c t s c o n t a i n i n g vacancies, since

T i s t o o small t o c o n t a i n the vacancy c o n c e n t r a t i o n as a p r o p o r t i o n a l i t y f a c t o r .

0

-

The +lOO°C peak i s due t o d e f e c t c o n f i g u r a t i o n s c o n s i s t i n g o f s u b s t i t u t i o - n a l atoms ; th e quenched i n vacancy c o n c e n t r a t i o n manifests i t s e l f i n t h e h i g h e r value o f .ro.

-

The +200°C peak r e s u l t s from t h e same type o f defects, except t h a t t h e temperature dependence o f t h e vacancy c o n c e n t r a t i o n leads t o a h i g h e r AH, and a lower

T Thus, w r i t i n g .r0 as the product .r0 = T ~ C ~ , where T~ i s independent o f t h e vacan- 0' cy c o n c e n t r a t i o n CV, we g e t f o r t h e 100 and 200°C peaks : 0 R 0 E: T (100°C) = T C exp

- -

RT (1 ~ ~ ( 2 0 0 ~ C ) = .rRC0 f and f o r : AH(200°C) = AH(lOO°C)

+

EV

I n s e r t i n g now t h e values from t a b l e 1 we g e t : E

: = 52 kJ/mole from t h e d i f f e r e n c e i n AH and :E = 47 kJ/mole from t h e l e s s accu- r a t e T ~ , i n good agreement w i t h each o t h e r and w i t h the estimates o f V. Paemel e t

a l ( 1 3 ) . This formation energy f o r vacancies i s q u i t e small and i m p l i e s t h a t &Cu-Zn contains a h i g h vacancy c o n c e n t r a t i o n even a t moderate temperatures. Thus vacancy concentrations o f which i n most metals e x i s t o n l y near t h e m e l t i n g p o i n t , are t o be found i n brass even around 400°C.

C5- 1004 JOURNAL DE PHYSIQUE

Cu-rich s u b l a t t i c e . The r e o r i e n t a t i o n then occurs v i a an interchange between Cu- atoms and vacancies, as shown i n f i g u r e 1

( o

vacancy,

cs

Cu atom,

I

Cu-rich and

I1 Z n - r i c h s u b l a t t i c e ) .

F i g . 1 : Relaxation mechanism f o r t h e -50°C and -15°C peaks

The peak a t 100°C and 200°C a r e b o t h due t o a t e t r a g o n a l defect. The most l i k e l y choice i s a d e f e c t which c o n s i s t s o f two Cu atoms on t h e Z n - r i c h s u b l a t t i c e i n n e a r e s t neighbour p o s i t i o n , as shown i n f i g u r e 2. The r e o r e n t i a t i o n occurs v i a the movement o f Zn atoms, which r e q u i r e s a h i g h e r AH, than f o r t h e subzero peaks.

I

I

1

I1

0 A

I

F i g . 2 : R e l a x a t i o n mechanism o f t h e 100°C

I

ancT;TOO°C peaks.(

I

Cu-rich and

I 1

Z n - r i c h s u b l a t t i c e , 0 Cu atom).

3. Conclusions

. -

1/ The formation energy f o r vacancies,

~ y f

,

which has been determined by t h e i n t e r n a l f r i c t i o n technique, i s small. The vacancy c o n c e n t r a t i o n increases r a p i d l y w i t h temperature. T h i s explains, why on quenching and aging p r e c i p i t a t e s o f an incom mensurate phase(14)or o f a y phase(15)are observed, which a r e supposed t o form i n the presence o f excess vacancies. These p r e c i p i t a t e s a r e known t o a f f e c t s t r o n g l y t h e m a r t e n s i t i c transformation, e s p e c i a l l y t h e h y ~ t e r e s i s ( ~ ) .

which a l t e r n a t e between s u b l a t t i c e I and I 1 p o s i t i o n s . The a c t i v a t i o n energy o f d i f - f u s i o n f o r t h i s process ( w i t h R2 o r d e r ) would amount t o ED = AH (-50°C)

+

E V ~

=

94 kJ/mole. Although the denree o f order would a f f e c t ED somewhat, t h i s value compa- r e s f a v o r a b l y w i t h a d i f f u s i o n energy o f 92 kJ/mole o f @ - c u - z ~ ( ~ ) . However, i f we compare ED w i t h t h e value obtained from the slope o f D versus 1/T below TB2. ( i e 150 kJ/mole), a l a r g e discrepancy i s apparent, which can be r e s o l v e d i n agreement w i t h t h e r e s u l t s o f ~ a k k e r ( ~ ) , by an order- and thus temperature dependent c o r r e l a- t i o n f a c t o r .

3/ The a n a l y s i s o f t h e 200°C peak has shown t h a t d e f e c t s w i t h a t e t r a g o n a l d i s t o r s i o n f i e l d around them e x i s t i n e q u i l i b r i u m . These d e f e c t s cause an a n i s o t r o - p i c s t r a i n f i e l d t h a t c o n t r i b u t e s t o t h e s t a b i l i t y o f t h e @ phase. Furthermore, e l a s t i c d i p o l e s i n t e r a c t w i t h t h e d i s l o c a t i o n s . The s t r o n g dependence o f t h e shear s t r e s s on composition and quenching c o n d i t i o n s may be r e l a t e d t o them.

References.

-

(1) I.S. CLARK, N. BROWN, J. Phys. Chem.

19

(1961), 291.

( 2 ) M.J. KOCZAK, H. HERMAN, A.C. DAWSK, Acta Flet. - 19 (1971), 303. (3) S. HANADA, H. YAFIAETOTO, 0. IZUFII, Acta Met.

7,

(1979), 1219.

(4) R. RAPACIOLI, ?I. AHLERS, Acta Yet.,

27,

(1979), 777.

(5)

Q.

RAPACIOLI, M. CHANDRASEKARAN, Proc. I n t . Conf. %rt. Transf.,Cambridge,

Mass.,

1979, p. 596.

(6) See : Proc. I n t . Conf. Mass Transf., Cambridge Mass., 1979.

(7) A.B. KUPER, D. LAZARUS, J.R. MANNING, C.T. TOrlIZUKA, Phys. Rev.

104,

(1956), 1536.

(8) H. BAKKER, P h i l . Mag. A

40,

(1979), 525.

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2

(1980), 1341. (10) A. GHILARDUCCI, '7. AHLERS, t o be nublished.

(11) R. RAPACIOLI, Y . RHLERS, S c r i p t a "et. - 11, (1977), 1147. (12) L.V. CLAREBROUGH, Acta Met.,

2,

(1957), 413.