DISLOCATION RELAXATION IN THE MARTENSITIC PHASE OF THE THERMOELASTIC NiTi AND NiTiCu ALLOYS

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DISLOCATION RELAXATION IN THE

MARTENSITIC PHASE OF THE THERMOELASTIC

NiTi AND NiTiCu ALLOYS

B. Tirbonod, S. Koshimizu

To cite this version:

CoZZoque C5, suppl6ment au nOIO, Tome 42, oetobre 1981 _{page c5-1043}

DISLOCATION RELAXATION IN THE MARTENSITIC PHASE OF THE THERMOELASTIC

NiTi AND NiTiCu ALLOYS

B. ~irbonod* and S. Koshimizu

**

I n s t i t u t de G6nie Atomique, Swiss Federal I n s t i t u t e o f TeehnoZogy, Lausanne, Switzerland

Abstract.- The i n t e r n a l f r i c t i o n has been measured i n t h e m a r t e n s i t i c phase of NiTi(Cu) a l l o y s a t 1 Hz and 4 kHz. The d e t a i l e d study of t h e s t r a i n amplitude dependence of t h e i n t e r n a l f r i c t i o n and t h e whole spectrum a s a f u n c t i o n of t h e temperature i n t h e m a r t e n s i t i c phase have a1 lowed t o a t t r i b u t e t o d i s l o c a t i o n s t h e main source of i n t e r n a l f r i c t i o n .

1 . I n t r o d u c t i o n . - The a l l o y s which undergo a thermoelastic m a r t e n s i t i c transformation e x h i b i t during t h e transformation and i n t h e i r m a r t e n s i t i c phase a high value of t h e i n t e r n a l f r i c t i o n ( f i g . l ) , ( 1 , 2 ) . This phenomenon has been r e l a t e d t o t h e viscous motion of i n t e r f a c e s between t h e d i f f e r e n t v a r i a n t s of martensi t e under t h e e f f e c t of an applied s t r e s s , and more p r e c i s e l y i n t h e NiTi(Cu) a l l o y s (2), t o t h e viscous motion of twinning d i s l o c a t i o n s i n s i d e t h e martensi t e p l a t e s . However, t h e p r e c i s e nature of t h e s e mechanisms i s s t i l l not well known.

I n t e r n a l f r i c t i o n measurements a s a f u n c t i o n o f temperature o r applied s t r e s s allow t o c l a r i f y c e r t a i n mechanisnsof damping due t o d i s l o c a t i o n motion. The i n t e r n a l f r i c t i o n spectrum a s a function of temperature d i s p l a ~ s e v e r a l peaks r e l a t e d t o t h e d i s l o c a t i o n motion,

i . e . Bordoni and Hasiguti peaks (3). Pleasurements a s a f u n c t i o n of

t h e s t r a i n can be compared with t h e Granato-LUcke theory (4) o r t h e De Jonghe model

(5). These models permit t o d i s c u s s experiments conducted i n t h e m a r t e n s i t i c phase of NiTi (Cu) a l l o y s i n terms of d i s l o c a t i o n s .

2. Experimentai.- The s t u d i e d a l l o y s , provided by Brown Boveri (Baden), have been e l a b o r a t e d according t o t h e standard technique described i n ( 2 ) . The i n t e r n a l f r i c t i o n

Q-'

and t h e frequency f have been measured a s a function of t h e temperature T during l i n e a r heating ( o r c o o l i n g ) o r a s a f u n c t i o n of t h e s t r a i n amplitude E. The measure-

ments made a t 1 Hz have been done i n an i n v e r t e d t o r s i o n a l pendulum (6) on 50

mm

long and 1 mm diameter w i r e s . The measurements a t 4 kHz were done i n a resonant b a r apparatus (7) on 40

mm

x 5

mm

x 1 .5

mm

p l a t e s

.

3. Results.- A) Fleasurements a s a function of t h e s t r a i n

The measurements i n t h e range of low s t r a i n s have been c a r r i e d o u t a t 4 kHz i n a near *Present address : Abtl. Sicherhei t 32, Eidg. I n s t i

t u t Fir

Reaktorforschung, CH-5303

b!Uren? Sr,",en, 9 d i

tze;:

and

**On leave from IPEN and CNEN. Brazil

JOURNAL DE PHYSIQUE

equiatomic N i T i a l l o y which has undergone two d i f f e r e n t treatments: annealing 30 minutes a t 6000C then furnace cooled ( f i g . l a , curve I ) , annealing 2 hours a t 6 0 0 ' ~

then quenched i n c o l d water ( f i g . l a , curve 2 ) . I n each case ( f i g . 2 ) t h e value of Q-'

i n i t i a l l y low increases f o r s t r a i n s between and 5 ~ 1 0 - ~ , then remains n e a r l y constant f o r s t r a i n s up t o 3 ~ 1 0 - ~ . These r e s u l t s r e p o r t e d i n a Granato-LUcke p l o t l n ( Q - ' . c ) - ~ - ' ( f i g . 2 ) show t h a t t h e behaviour o f Q-' i n the range o f s t r a i n s

-

5 ~ 1 0 - ~ i s c o n s i s t e n t w i t h t h e Granato-Lilcke theory ( 1 in e a r p a r t o f t h e p l o t ) . T h i s means t h a t a depinning o f t h e d i s l o c a t i o n s from weak p i n n e r s occurs i n t h i s range o f s t r a i n s . A t h i g h e r s t r a i n , t h e a p p l i e d s t r e s s i s s u f f i c i e n t t o induce a d i s l o c a t i o n motion by successive depinning and

Q-'

i s n e a r l y s t r a i n independent. From these r e s u l t s , i t appears t h a t t h e d i s l o c a t i o n s a r e r e s p o n s i b l e f o r t h e i n t e r n a l f r i c t i o n i n t h e range o f s t r a i n s

-

3 ~ 1 0 - ~ .

I n the range o f h i g h s t r a i n s , t h e same k i n d o f measurements have been c a r r i e d o u t a t 1

Hz

i n t h e m a r t e n s i t i c phase o f a w t % 47 N i 46 T i 7 Cu p r e v i o u s l y annealed a t 690°C then quenched i n c o l d water ( f i g . 3 ) . Q-' i s constant i n t h e range o f s t r a i n s

-

2x10-', which i s i n good agreement w i t h t h e previous measurements, then t n - creases l i n e a r l y w i t h t h e s t r a i n from ~ X I O - ~ t o 2xlO-'+. This l a s t r e s u l t i s c o n s i s t e n t w i t h t h e De Jonghe model which p r e d i c t s a l i n e a r dependence o f Q-I w i t h t h e s t r e s s

i f t h i s s t r e s s i s l a r g e r than a c r i t i c a l one f o r t h e m a r t e n s i t e r e o r i e n t a t i o n . The r e o r i e n t a t i o n o f t h e martensi t e i s provided by t h e motion o f i n t e r f a c e d i s l o c a t i o n s between two v a r i a n t s . Thus, the i n t e r n a l f r i c t i o n i n t h e h i g h s t r a i n range should be analysed i n terms o f l o n g range motion o f d i s l o c a t i o n s .

B. D i s l o c a t i o n s and r e l a x a t i o n peaks

The low frequency i n t e r n a l f r i c t i o n spectrum measured between 15K and 260K i s r e p o r t e d i n f i g . 4 f o r two d i f f e r e n t a l l o y s . I n t h e at.% 50Ni 50Ti a l l o y which was p r e v i o u s l y annealed 1 hour a t 590°C then quenched i n c o l d water (curve I ) , Q-I increases

c o n t i n u o u s l y from 15K t o 80K and then remains constant f o r temperatures up t o 150K above which a peak appears. The wt% 49.7 Ni 45.3 T i 5 Cu a l l o y was disgased d u r i n g 8 hours a t 1 0 0 0 ~ ~ under 2 ~ 1 0 - ' ~ t o r r i n order t o e l i m i n a t e i n t e r s t i t i a l i m p u r i t i e s . I n t h i s a l l o y , the behaviour o f Q-' i s s i m i l a r t o t h e previous one between 15K and 150K, b u t no peak i s observed a t h i g h e r temperatures. For b o t h a l l o y s , no n o t i c e a b l e e f f e c t s appear on t h e frequency spectrum and Q-' i s s t r a i n independent i n t h e range o f s t r a i n s

-

2 ~ 1 0 - ~ . Q-I between 15K and 150K should be i n t e r p r e t e d by a d i s l o c -

a t i o n r e l a x a t i o n (Bordoni type peak) f o r which t h e r e l a x a t i o n time i s t h e sum o f two terms (8,3) : t h e f i r s t one T I r e l a t e d t o the temperature by an Arrehnius law i s

r e s p o n s i b l e f o r t h e i n c r e a s i n g p a r t o f

Q-'

( l o w temperature s i d e o f t h e peak) and t h e second one Tn related t o the viscous f r i c t i o n o f t h e d i s l o c a t i o n s w i t h t h e impur-

E

l

l

with

AX

=-(---)

and

6 =

om

K

Tp

T

where g2

=

orientation f a c t o r ,

A =

dislocation density,

R = dislocation length,

E

=

relaxation energy,

K = Bol tzmann constant, T =

peak temperature,

P

w

= p u l s a t i o n . Theoretical curves of

Q - I

as a function of Ax f o r several values

of

B

a r e shown in f i g . 5 . The comparison with the experimental r e s u l t s between 15K

and 150K ( f i g . 4 ) permits t o estimate

B >

1 and t o s i t u a t e the peak temperature between

50K and 80K. The density of mobile dislocations

A

can be evaluated t o 10'' cm/cm3

with the following reasonable values of the parameters: g2 = 0 . l ,

9. =

10-'cm,

B

= 1 .

This value of

A

i s too high i f ordinary s l i p dislocations are supposed t o be involved

in t h i s peak. The involved dislocations are r a t h e r imperfect dislocations with

Burgers vector smaller than the l a t t i c e parameter, t h a t i s t o say, twinning disloc-

ations (2). However the dislocations s i t u a t e d a t the i n t e r f a c e between the d i f f e r e n t

variants of martensite, a t present s t a g e , cannot be ruled out.

In the kHz range, a corresponding peak should occur a t higher temperature than i n

the Hz range. The measurements carried out in the near equiatomic NiTi alloy reported

in f i g - l a show an incertitude about the presence of a low temperature peak, except

perhaps a f t e r quenching (curve 1 ) where

Q-'

goes through a maximum between 80K and

130K. I f we a t t r i b u t e t o the peak s i t u a t e d between 50K and 80K a t 1 Hz a value of

the frequency f a c t o r of 10'' Hz and an activation energy between 0.1 and 0.2 eV, the

peak should occur i n the range of temperatures 80K-130K. This i s observed i n f i g . 1 ,

curve 1 .

4. Discussion.- I t has been shown t h a t the main source of internal f r i c t i o n in the

martensi t i c phase of the NiTi (Cu) alloys i s a r i s i n g from imperfect dislocations. The

motion of such dislocations i s s e n s i t i v e t o pinner defects and t o viscous f r i c t i o n

a r i s i n g from the l a t t i c e or some d i s t r i b u t i o n of surrounding defects. Furthermore,

the long range motion of these dislocations should provide a good explanation f o r

martensite reorientation. This assumption i s i n good agreement with r e s u l t s obtained

JOURNAL DE PHYSIQUE

REFERENCES

(1) W. De Jonghe, L . ilelaey, R . De B a t i s t and J.Van Humbeck. Metal Science,

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K.

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(7) P. Stadelmann, Thesis EPF Lausanne (1978)

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ZB,

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693 (1979)

C' . XI= TI Ni 1 6 1 Z l A 47Ni L S T I 7Cu IMz)

0.' . a ,,

20 -

I IYUEALED 6- FURNICE C m E O -5W0

2 ANUEILED m.c w~wc*ro ANNEALED 690.C OUENCHED

I t

10 - 10-

,t

/

2 : ANNEALED 600.C 2 HOURS QUENCHED

0

F i g . 2 I n t e r n a l f r i c t i o n as a f u n c t i o n of t h e s t r a i n E measured a t about 4 kHz a t

130 K i n a near equiatomic H i T i a l l o y and t h e corresponding Granato-Lucke p l o t an ( Q - ~ I Z )

-

0.' . 10' 41Ni 1678 7Cu ANNEALED 690.C QUENCHED W l o 11

Fig. 3 I n t e r n a l f r i c t i o n as a f u n c t i o n F i g . 4 I n t e r n a l f r i c t i o n and o f t h e s t r a i n E i n t h e m a r t e n s i t i c frequency spectra as a f u n c t i o n

phase o f t h e wt% 47 Ni46Ti7Cu a l l o y o f t h e temperature f o r a near o f f i g . 1 a t 1 Hz equiatomic N i T i a l l o y and a

wt%

45,3Ni49,7Ti 5Cu a1 l o y

F i g . 5 T h e o r e t i c a l p l o t o f t h e i n t e r n a l f r i c t i o n Q-l as a f u n c t i o n o f A X f o r