DEGRADATION OF SHAPE MEMORY EFFECT

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DEGRADATION OF SHAPE MEMORY EFFECT

R. Vandermeer

To cite this version:

R. Vandermeer. DEGRADATION OF SHAPE MEMORY EFFECT. Journal de Physique Colloques,

1982, 43 (C4), pp.C4-791-C4-796. �10.1051/jphyscol:19824129�. �jpa-00222112�

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DEGRADATION OF SHAPE MEMORY EFFECT

R.A. Vandermeer

The University of Tennessee, KnoxvilZe, T N 37916, U.S.A. and

Union Carbide Corporation, Nuclear Division, Oak Ridge, TN 37830, U.S.A.

(Accepted 9 August 1982)

A b s t r a c t . - An i m p o r t a n t parameter f o r d e c i d i n g whether o r n o t a SME a l l o y is s u i t a b l e f o r p r a c t i c a l a p p l i c a t i o n s i s t h e magnitude of t h e s t r a i n r e v e r s a l accompanying m a r t e n s i t e r e v e r s i o n . T h i s r e s e a r c h i s concerned w i t h e l u c i - d a t i n g m e t a l l u r g i c a l f a c t o r s t h a t c a u s e d e g r a d a t i o n of t h i s h e a t - a c t i v a t e d r e c o v e r y s t r a i n , ER. A f t e r e x p l a i n i n g what i s meant by d e g r a d a t i o n , two m a n i f e s t a t i o n s of d e g r a d a t i o n r e c e n t l y i d e n t i f i e d i n near-monotectoid ^, uranium-niobium a l l o y s w i l l b e d e s c r i b e d . The f i r s t was a s s o c i a t e d w i t h t h e o n s e t of p l a s t i c d e f o r m a t i o n o f t h e m a r t e n s i t e beyond t h e r e v e r s i b l e s t r a i n l i m i t , EL; a r e d u c t i o n of ER from 5.25% a t 8% t o t a l s t r a i n , i . e . EL, t o 2.9%

a t 12% t o t a l s t r a i n was observed. A second t y p e of d e g r a d a t i o n depended s t r o n g l y on t h e h e a t i n g r a t e d u r i n g r e v e r s i o n ; t h e ER f o r a n imposed s t r a i n of 6.95% w a s reduced from a v a l u e of 5.25% t o 1.3% when t h e h e a t i n g r a t e was d e c r e a s e d from 40 d e g / s e c t o 0.05 d e g l s e c . D e g r a d a t i o n was a t t r i b u t e d t o a change i n t h e t r a n s f o r m a t i o n p a t h and t h e i n t e r j e c t i o n of time-dependent, low t e m p e r a t u r e a g i n g r e a c t i o n s .

I n t r o d u c t i o n . - The s h a p e memory e f f e c t (SME) o b s e r v e d i n c e r t a i n a l l o y s i n v o l v e s a c y c l i c sequence of e v e n t s t h a t b e g i n s by c o o l i n g t h e a l l o y from a n e l e v a t e d tempera- t u r e , TI, t o a lower t e m p e r a t u r e , T2 such t h a t , i n t h e p r o c e s s a m a r t e n s i t i c phase t r a n s f o r m a t i o n o c c u r s . Next, s t r e s s i s a p p l i e d a t T2 c a u s i n g a r e o r i e n t a t i o n of t h e m a r t e n s i t e v a r i a n t s and a r e s u l t i n g s t r a i n . F i n a l l y , t h e deformed m a r t e n s i t e i s r e h e a t e d t o TI t r i g g e r i n g phase r e v e r s i o n and a n accompanying s t r a i n r e v e r s a l .

The n e t s t r a i n remaining i n a u n i a x i a l l y deformed a l l o y a f t e r c o m p l e t i o n of t h e SME c y c l e c a n be w r i t t e n a s

where EC i s t h e t h e r m a l c o n t r a c t i o n s t r a i n due t o c o o l i n g from T1 t o T2, E, i s t h e t o t a l s t r a i n imposed by a p p l i c a t i o n of s t r e s s t o t h e m a r t e n s i t e a t T2, and Ev i s t h e

" s p r i n g back" s t r a i n r e c o v e r e d when s t r e s s i s r e l e a s e d . I n t h e r e h e a t p o r t i o n of t h e c y c l e , t h e s t r a i n is s e p a r a t e d i n t o two components; EE i s t h e t h e r m a l expansion s t r a i n due t o h e a t i n g from T 2 t o T1 i n t h e a b s e n c e o f any s t r a i n and ER i s t h e r e v e r s a l a s s o c i a t e d o n l y w i t h t h e r e v e r s i o n of t h e s t r a i n - b i a s e d m a r t e n s i t e . The t h e r m a l s t r a i n s EC and EE a r e g e n e r a l l y e q u a l i n magnitude and o p p o s i t e of s i g n , hence Eqn. 1 r e d u c e s t o

ENet = Eu

-

^Ev

-

^ER ^{( 2 ) .}

The minus s i g n s i n f r o n t of Ev and ER i n d i c a t e t h a t t h e y a r e o p p o s i t e i n d i r e c t i o n t o E,.

I n t h e i d e a l c a s e where a n SME a l l o y e x h i b i t s o n l y r e v e r s i b l e b e h a v i o r ,

EN^^

would b e z e r o . When d u r i n g t h e d e f o r m a t i o n o r r e h e a t i n g s t a g e o f t h e c y c l e , however p l a s t i c d e f o r m a t i o n modes a r e a c t i v a t e d , E, w i l l b e g r e a t e d t h a n

I E ~ + ~d

and

EN^^

*Work s u p p o r t e d i n p a r t by t h e U. S. Dept. of Energy u n d e r C o n t r a c t W-7405-eng-26.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19824129

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JOURNAL DE PHYSIQUE

can b e d e s i g n a t e d a s Ep, t h e p l a s t i c s t r a i n . Thereupon Eqn. 2 can b e w r i t t e n

For each SME a l l o y , a r e v e r s i b l e s t r a i n l i m i t EL e x i s t s where t h e o p e r a t i v e , r e v e r s i b l e d e f o r m a t i o n modes become e x h a u s t e d and beyond which p l a s t i c d e f o r m a t i o n p r o c e s s e s predominate. The magnitude of EL i s determined by t h e n a t u r e of t h e Bain s t r a i n a s s o c i a t e d w i t h t h e m a r t e n s i t e t r a n s f o r m a t i o n and t h e o r i e n t a t i o n of p a r e n t phase c r y s t a l a x e s r e l a t i v e t o t h e a p p l i e d s t r e s s a x i s . I n a p o l y c r y s t a l l i n e a g g r e g a t e EL would b e a n a v e r a g e o f t h e d i f f e r e n t c o n t r i b u t i o n s of t h e i n d i v i d u a l g r a i n s and would depend o n t h e n a t u r e of g r a i n i n t e r a c t i o n s and t h e e x t e n t of p r e f e r r e d o r i e n t a t i o n s i n t h e m a t e r i a l .

An i d e a l l y behaving SME c y c l e can be c h a r a c t e r i z e d a s one i n which Ep = 0 f o r E,

2

EL. There c a n , however, b e o p e r a t i v e m e t a l l u r g i c a l f a c t o r s t h a t c a u s e a d e g r a d a t i o n o r d e c r e a s e of SME. D e g r a d a t i o n would r e s u l t , f o r example, when s i g n i f i c a n t Ep o c c u r s p r e m a t u r e l y , i . e . b e f o r e E, r e a c h e d EL. I n t h i s c a s e ER

+

E, E,. One s o u r c e o f d e g r a d a t i o n was e v i d e n t i n t h e work of S c h r o e d e r , e t a 1 (1) who concluded t h a t t h e p r e s e n c e of p a r e n t p h a s e g r a i n b o u n d a r i e s r e s t r i c t e d ER.

They found t h a t f o r a g i v e n s t r a i n , s i n g l e c r y s t a l s of a Cu-Zn a l l o y e x h i b i t e d a l a r g e r ER t h a n d i d p o l y c r y s t a l l i n e specimens.

Recent s t u d i e s by Baumgart, e t a 1 ( 2 ) , Vandermeer, e t a 1 ( 3 ) , and Miyazaki, e t a 1 ( 4 ) found t h a t ER i n c r e a s e d w i t h i n c r e a s i n g s t r a i n , reached a maximum v a l u e a t s t r a i n s a p p r o x i m a t e l y e q u a l t o EL and t h e n d e c r e a s e d a t h i g h e r amounts of s t r a i n . These f i n d i n g s c l e a r l y demonstrated t h a t SME p r o p e r t i e s d e t e r i o r a t e d when p l a s t i c d e f o r m a t i o n became t h e dominant d e f o r m a t i o n mode. D e g r a d a t i o n i n t h i s c a s e meant t h a t f o r E, > EL; ER

+

^Ev^<^EL. I n a d d i t i o n , Baumgart, e t a 1 ( 2 ) showed t h a t d e g r a d a t i o n was more s e v e r e when m a r t e n s i t e r e v e r s i o n was accomplished under a super-imposed e x t e r n a l s t r e s s .

I n r e c e n t y e a r s much i n t e r e s t i n d e v e l o p i n g SME a l l o y s a s working components i n p r a c t i c a l d e v i c e s h a s emerged ( 5 ) . The u s e f u l n e s s of any SME a l l o y i s d e t e r m i n e d i n l a r g e p a r t by t h e magnitudes of ER and EL. I n m u l t i p l e c y c l e a p p l i c a t i o n s , d e g r a d a t i o n of ER c o u l d have s e r i o u s i m p l i c a t i o n s r e g a r d i n g t h e u s e f u l l i f e t i m e of such a d e v i c e . There i s a need, t h e r e f o r e , t o i d e n t i f y t h e v a r i o u s s o u r c e s of SME d e g r a d a t i o n .

T h i s p a p e r r e p o r t s on t h e d e t e r i o r a t i o n of t h e SME t h a t was m a n i f e s t e d when t h e r a t e of h e a t i n g d u r i n g m a r t e n s i t e r e v e r s i o n was v a r i e d from 40°C/s down t o 0.05 deg.C/s i n a uranium

+

5.2 wt% niobium a l l o y . E x p e r i m e n t a l measurements of E,, ER, and Ep a r e p r e s e n t e d f o r a n a" m a r t e n s i t e s t r a i n e d d i f f e r e n t amounts. The ER was i n c r e a s i n g l y degraded a s t h e h e a t i n g r a t e was d e c r e a s e d . A q u a l i t a t i v e r a t i o n a l i - z a t i o n of t h i s b e h a v i o r i n terms of i n t e r a c t i o n s from low-temperature time-dependent phase t r a n s f o r m a t i o n s i s p r e s e n t e d .

E x p e r i m e n t a l Procedure.- A uranium

+

5 . 2 wt% niobium a l l o y was p r e p a r e d by i n d u c t i o n m e l t i n g t h e a p p r o p r i a t e amounts of uranium (Derby g r a d e ) and commercial uranium

+

6 wt% niobium a l l o y s c r a p . The m o l t e n a l l o y was c a s t i n t o 1 8 kg. s l a b s , homogenized a t 1375 K f o r 21.6 k s , h o t worked t o p l a t e a t 1075 K i n t h e BCC 'y p h a s e and w a t e r - quenched t o room t e m p e r a t u r e . Rod-shaped specimens f o r mechanical t e s t i n g and h e a t - a c t i v a t e d s t r a i n r e c o v e r y measurements were machined from t h e p l a t e , s o l u t i o n h e a t t r e a t e d f o r 7.2 k s a t 1075 K u n d e r vacuum and water-quenched t o form a" m a r t e n s i t e . The f u l l y m a r t e n s i t i c r o d s were deformed i n u n i a x i a l t e n s i o n t o v a r i o u s s t r a i n s between 1 and 13% a t room t e m p e r a t u r e and unloaded. The s t r a i n r a t e was 8.3 x I O - ~ S - ~ Values of E, and Ev were determined from t h e s t r e s s - s t r a i n c u r v e s . Four c y l i n d r i c a l specimens were c u t from t h e gauge s e c t i o n of each s t r a i n e d r o d . H e a t - a c t i v a t e d mar- t e n s i t i c p h a s e r e v e r s i o n was m o n i t o r e d w i t h t h e T h e t a I n d u s t r i e s D i l a t r o n i c 111 Research Model d i l a t o m e t e r ; l e n g t h and t e m p e r a t u r e were measured c o n t i n o u s l y d u r i n g i n d u c t i v e h e a t i n g under vacuum t o 1100 K a t a programmed l i n e a r h e a t i n g r a t e of e i t h e r 40, 4 , 0.8 o r 0.05 deg.C p e r s e c . The method by which ER was e x t r a c t e d from t h i s d a t a i s d e t a i l e d i n r e f e r e n c e ( 3 ) .

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y (BCC) ^-ty o ( t e t r a g o n a l ) -t a" (monoclinic) ( 4 ) . The l a s t s t e p i s m a r t e n s i t i c i n n a t u r e w i t h an Ms = 548K (275°C) and MF

-

^348K

(75°C) determined d i l a t o m e t r i c a l l y ( 7 ) .

I n t h e U

+

5.2 w t % Nb a l l o y t h e low s t r a i n t e n s i l e deformation of a" was c h a r a c t e r i z e d by s t r e s s - s t r a i n curves e x h i b i t i n g a double "knee" and by an a n e l a s t i c unloading response a t a l l s t r a i n s s t u d i e d . This behavior was q u a l i t a t i v e l y s i m i l a r t o t h a t r e p o r t e d e a r l i e r (3) f o r a U

+

6.25 w t % Nb a l l o y .

An example of t h e e f f e c t of temperature and h e a t i n g r a t e on t h e s t r a i n r e - v e r s a l p a t t e r n s a s s o c i a t e d w i t h m a r t e n s i t e r e v e r s i o n a f t e r deformation i s shown i n Fig. 1 f o r a remnant s t r a i n ( s t r a i n remaining a f t e r unloading) of 5.2%. The t o t a l

F i g . 1. I n s t a n t a n e o u s s t r a i n v e r s u s temperature f o r a U-Nb a l l o y d u r i n g t h e r e h e a t phase of t h e SME c y c l e . I n i t i a l imposed s t r a i n was 6.9%.

O O

- 1

0 200 400 600 800 1000

TEMPERATURE ("C)

imposed s t r a i n , Eo, was, i n t h i s c a s e , approximately e q u a l t o EL. The i n s t a n t a n e o u s n e t s t r a i n p l o t t e d i n t h i s f i g u r e s t i l l i n c l u d e s t h e thermal c o n t r i b u t i o n , EE. The l a r g e d e c r e a s e i n s t r a i n between 200°C and 300°C undoubtedly corresponded t o marten- s i t e r e v e r s i o n because t h i s temperature range compared f a v o r a b l y w i t h t h e AS and AF temperatures of t h e a l l o y ( 7 ) . I t i s immediately obvious t h a t t h e s t r a i n - t e m p e r a t u r e p a t t e r n became more complex and s h i f t e d t o h i g h e r s t r a i n v a l u e s a s t h e h e a t i n g r a t e was decreased. The magnitude of t h e s h i f t t o h i g h e r s t r a i n v a l u e s i n d i c a t e d a

s e v e r e d e g r a d a t i o n of SME. A s w i l l b e d i s c u s s e d l a t e r , t h e emerging c o m p l e x i t i e s i n t h e curves could b e q u a l i t a t i v e l y c o r r e l a t e d t o superimposed l e n g t h changes accompanying time-dependent phase t r a n s f o r m a t i o n s which become a c t i v a t e d d u r i n g t h e slower h e a t i n g r a t e s .

The e x t e n t of t h e h e a t i n g - r a t e - a s s o c i a t e d d e g r a d a t i o n of SME (and t h e e f f e c t of d i f f e r i n g amounts of imposed s t r a i n ) i s summarized i n Fig. 2. Here b o t h t h e remnant s t r a i n and t h e t o t a l ER determined f o r each h e a t i n g r a t e a r e p l o t t e d v e r s u s t o t a l s t r a i n . For t h e f a s t e s t h e a t i n g r a t e (40 d e g l s e c ) ER equaled t h e remnant s t r a i n up t o 7% s t r a i n , t h e r e v e r s i b l e s t r a i n l i m i t . From t h i s i t was concluded t h a t t h e a l l o y behaved more o r l e s s i d e a l l y from a n SME viewpoint a s long a s t h e h e a t i n g r a t e was high and t h e s t r a i n was l e s s t h a n EL. A s u b s t a n t i a l drop i n ER a t a l l s t r a i n s was e v i d e n t when t h e h e a t i n g r a t e was slowed, p a r t i c u l a r l y f o r t h e s l o w e s t r a t e (0.05 d e g l s e c ) . For example, a t a s t r a i n of 7%, ER dropped from 5.25%

t o 1.3% when t h e h e a t i n g r a t e was decreased from t h e f a s t e s t t o t h e s l o w e s t .

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JOURNAL DE PHYSIQUE

The d e g r a d a t i o n of ER c a u s e d by t h e o n s e t of c o p i o u s amounts o f p l a s t i c d e f o r m a t i o n when EL was exceeded was a p p a r e n t f o r each h e a t i n g r a t e , b u t was c e r t a i n l y most pronounced a t t h e f a s t e r h e a t i n g r a t e s ; e.g. t h e d e c r e a s e i n ER from a peak of 5.25% a t s t r a i n s between 7-9% down t o 2.9% a t s t r a i n s of 13% f o r a h e a t i n g r a t e of 40 d e g l s e c . The r e s u l t s a t t h e h i g h e r h e a t i n g r a t e s a r e i n q u a l i - t a t i v e agreement w i t h t h e e a r l i e r f i n d i n g r e p o r t e d f o r a U

+

6.25 wt% Nb a l l o y ( 3 ) .

10

C a ACTIVATED

r

^RECOVERY

E

2 6

2

z

Q

4

f ,

Q a Li 2

0

0 2 4 6 8 10 12 14

F i g . 2. H e a t - a c t i v a t e d r e c o v e r y s t r a i n s from f o u r d i f f e r e n t h e a t i n g r a t e s p l o t t e d a s a f u n c t i o n of imposed s t r a i n .

IMPOSED STRAIN (%I

D i s c u s s i o n . - The a" m a r t e n s i t e formed d u r i n g t h e quenching of y - s o l u t i o n - t r e a t e d , near-monotectoid U-Nb a l l o y s i s a m o n o c l i n i c d i s t o r t i o n of orthorhombic a-uranium.

A t e q u i l i b r i u m , o n l y v e r y l i m i t e d amounts of s o l u t e c a n b e d i s s o l v e d i n a-uranium.

Thus, t h e quenched U

+

5 . 2 wt% Nb a l l o y d e v i a t e s from e q u i l i b r i u m b o t h i n c r y s t a l s t r u c t u r e and s u p e r s a t u r a t i o n of s o l u t e . As s u c h , t h e a l l o y r e s p o n d s r e a d i l y t o a g i n g h e a t t r e a t m e n t s ( 8 ) . An u n d e r s t a n d i n g of t h e mechanisms of t h e v a r i o u s a g i n g r e a c t i o n s i s s t i l l r u d i m e n t a r y , b u t a TTT diagram h a s been e s t a b l i s h e d f o r a U

+

5.8 wt% Nb a l l o y u s i n g d i l a t o m e t r i c a n a l y s i s . T h i s diagram i s shown i n F i g . 3 .

F i g . 3 . Time- t e m p e r a t u r e - t r a n s f o r m a t i o n diagram f o r a n e a r monotectoid U-Nb a l l o y .

AGING TIME (SEC)

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always observed. A t t h e h i g h e s t a g i n g t e m p e r a t u r e s , a d i s c o n t i n u o u s p r e c i p i t a t i o n r e a c t i o n , H , h a s been documented ( 9 ) . The p r o d u c t of t h i s t r a n s f o r m a t i o n c o n s i s t e d of a l a m e l l a r m i c r o s t r u c t u r e of a-U and a niobium r i c h BCC y phase.

Below 658K (385'C) a n e x p a n s i o n accompanied a g i n g . I n t h i s c a t e g o r y t h e r e were a t l e a s t two o v e r l a p p i n g t r a n s f o r m a t i o n s . One of t h e s e , L1, i s t h o u g h t t o r e p r e s e n t t h e f o r m a t i o n of a time-dependent a" m a r t e n s i t e a t t e m p e r a t u r e s above t h e MS f o r athermally-formed a" (6). The o t h e r low t e m p e r a t u r e r e a c t i o n , c a l l e d L2 h a s k i n e t i c s and o t h e r c h a r a c t e r i s t i c s s u g g e s t i v e of a s o l u t e - s e g r e g a t i o n phenom- enon.

Superimposed on t h e TTT diagram a r e t h e f o u r h e a t i n g r a t e s used i n t h i s r e s e a r c h . From t h e way t h e h e a t i n g r a t e c u r v e s o v e r l a y t h e v a r i o u s t r a n s f o r m a t i o n r e g i m e s , i t was concluded t h a t t h e L-category t r a n s f o r m a t i o n s would c o n t r i b u t e i n c r e a s i n g l y a s t h e h e a t i n g r a t e was d e c r e a s e d d u r i n g t h e r e h e a t p h a s e of t h e SME c y c l e . Category H r e a c t i o n s s h o u l d a l s o b e o b s e r v e d , b u t o n l y f o r t h e s l o w e r h e a t i n g r a t e s .

For SME t o b e i d e a l , t h e phase r e v e r s i o n s t a g e s d u r i n g r e h e a t s h o u l d b e e x a c t l y t h e r e v e r s e of t h o s e t r a v e r s e d d u r i n g t h e i n i t i a l quench; i . e . t h e r e v e r s e of Eqn. 4 . The i n t e r j e c t i o n of t h e time-dependent H and L t r a n s f o r m a t i o n s , however, a l t e r e d t h e sequence of s t a t e s through which t h e SME a l l o y p a s s e d and SME was n o t i d e a l i n t h a t c a s e . F o r a v e r y s l o w h e a t i n g r a t e t h e f o l l o w i n g t r a n s f o r - m a t i o n p r o g r e s s i o n i s proposed:

a" + a " (a)

+

^{p p t}⁺^y ^(5).

The e x a c t n a t u r e of t h e i n t e r m e d i a t e s t a t e , however remains a n open q u e s t i o n . I n t e r m e d i a t e h e a t i n g r a t e s would i n v o l v e t r a n s f o r m a t i o n p a t t e r n s t h a t a r e complex c o m b i n a t i o n s o f Eqns. 4 and 5 w i t h Eqn. 5 becoming i n c r e a s i n g l y i m p o r t a n t a s t h e h e a t i n g r a t e i s slowed.

To a c c o u n t f o r t h e d e g r a d a t i o n of ER t h e i n t e r j e c t i o n of time-dependent t r a n s f o r m a t i o n s d u r i n g t h e r e h e a t i n g phase of t h e SME c y c l e e v i d e n t l y must i n v o l v e s u b s t a n t i a l amounts of p l a s t i c d e f o r m a t i o n a l o n g t h e t r a n s f o r m a t i o n p a t h . A s c h e m a t i c , g r a p h i c a l p r e s e n t a t i o n o f t h i s view i s d e p i c t e d i n F i g . 4 . I n t h i s

F i g . 4. Schematic r e p r e s e n t a t i o n of t h e s t r a i n c o n t r i - b u t i o n s d u e t o a g i n g t r a n s f o r m a t i o n s t h a t c a u s e SME d e g r a d a t i o n .

1-1

1

TEMPERATURE

(7)

C4-796 JOURNAL DE PHYSIQUE

f i g u r e t h e v a r i o u s s t r a i n c o n t r i b u t i o n s a r e p l o t t e d a s a f u n c t i o n of t e m p e r a t u r e , b o t h s e p a r a t e l y and combined. The EE r e p r e s e n t s t h e t h e r m a l e x p a n s i o n of t h e a l l o y i n t h e a b s e n c e o f s t r a i n , ER r e p r e s e n t s t h e m a r t e n s i t e r e v e r s i o n s t r a i n

F S

i d e a l l y e x p e c t e d from a s t r a i n r e o r i e n t e d m a r t e n s i t e (Eqn. 4 p a t h ) , E and EL L

r e p r e s e n t s t r a i n c o n t r i b u t i o n s due t o t h e low t e m p e r a t u r e L t r a n s f o r m a t i o n s f o r a h i g h and low h e a t i n g r a t e r e s p e c t i v e l y ( i n c l u d i n g t h e r e v e r s i o n t o y and r e - s o l u t i o n of t h e p r e c i p i t a t e a t h i g h t e m p e r a t u r e s ) , and EH r e p r e s e n t s a s i m i l a r c o n t r i b u t i o n from t h e h i g h t e m p e r a t u r e a g i n g t r a n s f o r m a t i o n . The e x p e r i m e n t a l l y observed s t r a i n b e h a v i o r would b e a p p r o p r i a t e c o m b i n a t i o n s of t h e s e . Thus, c u r v e 1 r e f e r s t o a f a s t h e a t i n g r a t e and i s t h e sum of EE, ER, and EL. F Curves 2 and 3 would c o r r e s p o n d t o t h e s l o w e r h e a t i n g r a t e s w i t h c u r v e 2 b e i n g t h e sum of EE, ER,

-

and E: and c u r v e 3 t h e same a s 2, b u t w i t h EH added. The s i m i l a r i t y of c u r v e s 1, 2 , and 3 t o t h e e x p e r i m e n t a l c u r v e s ( F i g . 1 ) s h o u l d b e a p p a r e n t . We c o n c l u d e , t h e r e f o r e , t h a t i n t e r f e r i n g , time-dependent p h a s e t r a n s f o r m a t i o n s c a n b e a s o u r c e of d e g r a d a t i o n of SME when t h e m a r t e n s i t e r e v e r s i o n t e m p e r a t u r e s a r e h i g h . Analo- gous b e h a v i o r might b e a n t i c i p a t e d i n s e v e r a l of t h e copper-base SME a l l o y s t h a t have h i g h MS t e m p e r a t u r e s .

R e f e r e n c e s .

-

1. SCHROEDER T.A., CORNELIS I . , and WAYMAN C . , Met. T r a n s .

2

(1976) 535.

2. BAUMGART F., BENSMANN G . , and HAASTERS J . , C u r r e n t Concepts of I n t e r n a l F i x a t i o n F r a c t u r e s ; E d i t o r , H. K. U h t h o f f , S p r i n g e r - V e r l a y , B e r l i n , (1980), 122.

3. VANDERMEER R. A., CARPENTER D. A . , NORTHCUTT W. G . , and OGLE J. C . , P r o c e e d i n g s I n t e r n a t i o n a l Conf. on S o l i d - S o l i d Phase T r a n s f o r m a t i o n , E d i t o r , H. U . Aaronson, ASM, M e t a l s P a r k , Ohio, (1982) i n p r e s s .

4. MIYAZAKI S., OTSUKA K . , and SUZUKI Y . , S c r i p t a Met.

15

(1981), 287.

5. WAYMAN C. M . , J . o f M e t a l s

32

( 6 ) , (1980), 129.

6. VANDERMEER R. A . , Acta Met.

28

(1980), 383.

7. VANDERMEER R. A . , OGLE J . C . , and NORTHCUTT W. G . , Met. T r a n s .

12A

(1981), 733 8 . JACKSON R. J. and MILEY D . V . T r a n s . ASM

61,

(1968), 336.

9 . DJURIC B., J. Nucl. M a t ' l s . ; 4 4 (1972), 207.

Acknowledgements.- F. L. Yearwood a s s i s t e d w i t h t h e e x p e r i m e n t s and M. B. Vandermeer typed t h e m a n u s c r i p t .