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ULTRASONIC STUDY ON MARTENSITIC TRANSFORMATION IN AuCd ALLOY
Zhu Jin-Song, Wang Ye-Ning, Shen Hui-Min
To cite this version:
Zhu Jin-Song, Wang Ye-Ning, Shen Hui-Min. ULTRASONIC STUDY ON MARTENSITIC TRANS- FORMATION IN AuCd ALLOY. Journal de Physique Colloques, 1983, 44 (C9), pp.C9-235-C9-240.
�10.1051/jphyscol:1983931�. �jpa-00223378�
JOURNAL DE PHYSIQUE
Colloque C9, supplCrnent au n012, Tome 44, ddcembre 1983 page C9-235
U L T R A S O N I C STUDY ON M A R T E N S I T I C T R A N S F O R M A T I O N I N A u C d A L L O Y
Zhu Jin-song, Wang Ye-ning and Shen Hui-min
I n s t i t u t e o f SoZid s t a t e Physics, Nanjing U n i v e r s i t y , Nanjing, China
A b s t r a c t - The u l t r a s o n i c a t t e n u a t i o n c u r v e s i n 4-46.5 MHz r a n g e f o r two k i n d s of AuCd a l l o y s (47.5 at$ Cd and 50 at% W ) t h r o u g h m a r t e n s i t i c t r a n s f o r m a t i o n a r e d i f f e r e n t i n ) shape. An marked peak is observed only i n 50 at% Cd a l l o y . T h i s disagreement and t h e d i f f e r e n t
behaviour of frequency dependence of a t t e n u a t i o n f o r t h e s e two a l l o y s a r e explained t a k i n g account of t h a t e i t h e r i n t e r f a c e boundary
damping o r f l u c t u a t i o n d a m p i n g p l a y s an important r o l e as a consequence of t h e a i f f e r e n t range of measured frequency. The behaviour of
damping r e l a t e d t o p r e - m a r t e n s i t i c t r a n s i t i o n was s t u d i e d i n t h e KHz a s w e l l a s MHz r a n g e .
INTRODUCTION
I n p r e v i o u s p a p e r ( 1 1 , we have r e p o r t e d t h e i n t e r n a l f r i c t i o n and e l a s t i c be- h a v i o u r i n AuCd a l l o s a t low f r e q u e n c i e s n e a r m a r t e n s i t i c t r a n s f o r m a t i o n ( M T ) . I t w a s found t h a t i n c r e a a & w i t h i n c r e a s e of i n t e r i a o e and w a s independent of t h e frequency. A s t a t i c h y s t e r e s i s l o s s mechanism a s s o c i a t e d w i t h s t r e s s - i n d u c e d motion of t h e i n t e r f a c e d i s l o c a t i o n was suggested. One of t h e a l l o y s (50 a t $ Cd) was found t o have a w e l l d e f i n e d u l t r a s o n i c a t t e n u a t i o n peak n e a r Me ( 1 ) . This i s d i f f e r e n t from Gefen and Rosen's r e s u l t s ( 2 ) t h a t no obvious peak was observed a t lOMHZ i n p o l y c r y s t a l l i n e Au-47.5 at$ Cd a l l o y but a r a p i d d e c r e a s e occured a t A, and t h e r e f o r e t h e h i g h a t t e n u a t i o n i n martensite(M) was a t t r i b u t e a t o t h e twin b ~ ~ u n d a r i e s
.
T l l t r a s o n i c s t u d y of m a r t e n s i t i c t r a n s f o r m a t i o n h a s been c a r r i e d out i n o t h e r m a r t e n s i t i c a l l o y s , such a s N i - T i ( 3 ) . In-T1 ( 4 ) , Mn-Cu ( 5 ) , AuCuZn (6), Ye-Ni ( 7 ) e t c . However, t h e e f f e c t of frequency f on t h e a t t e n u a t i o n d was i n v e s t i g a t e d only f o r I'i-Ti and In-T1 a l l o y s . Saunders proposed ( 3 ) t h a t d a s s o c i a t e d w i t h phase t r a n s i t i o n was a s c r i b e d t o t h e Akhieser-type phonon r e l a x a t i o n . Clapp ( 8 ) i n t e r p r e t e d t h e a t t e n u a t i o n due t o MT i n terms of r e s o n a n t a b s o r p t i o n of t h e l o c a l i z e d s o f t phonon mode i n t h e m a r t e n s i t e embryo r e g i o n . According t o both t h e o i e s t h e r e l a t i o n o f d p r o p o r t i o n a l t o k? would be expected i n t h e measured frequency r a n g e . But, it i s not w e l l c o i n c i d e n t w i t h t h e experimental r e s u l t s . T h e 4 peak measured by Sugimoto ( 5 ) i n Mn-Cu a l l o y and Nakanish ( 6 ) i n AuCuZn were a t t r i b u t e d t o t h e motion of i n t e r f a c e .
I t is important t o c l a r i f y , f i r s t of a l l , t h e mechanism of oC due t o MT a t h i g h frequency f o r t h e acquirement of t h e dynamical i n f o r m a t i o n of MT. I n t h e p r e s e n t paper, t h e a t t e n u a t i o n mechanism is p r o p e r l y a n a l y s e d by showing t h e e f f e c t of frequency o n d - T curves i n two AuCd a l l o y s (50 at'%, 47.5 a t $ Cd) a s w e l l a s t a k i n g i n t o account t h e r e s u l t s obtained a t KHz range. Experimental f a c t s show t h a t t h e mechanism which p l a y s major r o l e i s d i f f e r e n t i n d i f f e r e n t frequency range. We a l s o r e p o r t t h e r e l a t i o n between f and U-' o r & i n t h e t e m p e r a t u r e r e g i o n of p r e - m a r t e n s i t i c t r a n s f o r m a t i o n .
AuCd a l l o y s c o n t a i n i n g 50 at% Cd and 47.5 a t % Cd ( 5 @ a l l o y . 47.5% a l l o y respe-
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1983931
C9-236 JOURNAL DE PHYSIQUE
c t i v e l y , f o r s i m p l i t y ) were prepared by Bridgeman method ( c o a r s e c r y s t a l ) and annealed i n vacuum f o r 100 hours a t 570'~. The p u l s e echo method w a s used f o r measuring a t t e n u a t i o n d and v e l o c i t y v ( l o n g i t u d i n a l waves was w e d a l l )
.
I n KHz r a n g e , t h e p i e z o e l e c t r i c r e s o n a t o r method developed by,Marx was employed. A l l measurements were made d u r i n g h e a t i n g and c o o l i n g w i t h T a t about 0.4 co/min.EXPERIMENTAL RESULTS 1 )4
-
T curves i n two k i n d s AuCd a l l o y sT h e d a s a f u n c t i o n of T h a s been measured a t 4, 11.5, 22, 28, 36.5, 46.5, 95 MHz f o r 47.5
5
a l l o y , and a t 4 , 12, 27.5, 39.5, 46.5 MHz f o r 50 % a l l o y r e s p e c t i v e l y . T h e d-
T curve a t 11.5 MHz i n 47.5 $ a l l o y i s shown i n Pig. l ( a ) , from which i t can be s e e n t h a t a t t e n u a t i o n i n M phase is much h i g h e r t h a n t h a t i n k phase. No obvious a t t e n u a t i o n peak b u t a n a b r u p t d e c r e a s e i s observed n e a r Hs o r A s . It i s j u s t t h e same as t h e r e s u l t g i v e n by Gefen and Rosen ( 2 ) a t 1 0 MHz. However, t h eF i g . 1 . A t t e n u a t i o n as f u n c t i o n Fig. 2. A t t e n u a t i o n and v e l o c i t y of t e m p e r a t u r e f o r Au-47.5 a t $ a s f u n c t i o n of t e m p e r a t u r e f o r Od a l l o y a ) a t 11 - 5 MHz b ) . a t 95 MHz Au-50 at$ Cd a l l o y a t 17.5 MHz peakgrows up n e a r M g i f f r e q u e n c y i n c r e a s e s t o over 4 0 MHz. A markedly d e f i n e d peak is shown i n F i g . Z (b) at 95 MHz f o r 47.59 a l l o y . F i g . 2 i s t h e o(
-
T andV
-
T c u r v e s a t 17.5 MHz f o r 50 $ a l l o y . A s h a r p maximum o f @ a s w e l l a s a minimum of sound v e l o c i t y due t o MT n e a r 3 6 ' ~ ( h e a t i n g ) were observed. Comparing t h e damping p r o p e r t y of t h e two a l l o y s , i t i s noted t h a t a ) i n M phase t h e 47.5%a l l o y h a s a h i g h e r a t t e n u a t i o n t h a n t h e 50% a l l o y . b ) t h e two a t t e n u a t i o n c u r v e s a t f r e q u e n c i e s l e s s t h a n 40 MHz a r e d i f f e r e n t i n shape. The o r i g i n s of t h e d i s c r e p a n c y w i l l be d i s c u s s e d i n t h i s p a p e r .
2 ) The r e l a t i o n between d and f
The.d
-
T curves a t d i f f e r e n t f i n two a l l o y s were measured. F i g . 3 shows4 -
fand
8
( l o g a r i t h m i c decrement)-
f r e l a t i o n i n 5G% a l l o y a t some c o n s t a n t temp- e r a t u r e s n e a r Ms. It can be s e e n t h a t d i n c r e a s e s r a p i d l y w i t h f i n t h e 4-20 MHz r a n g e , but very slowly when f i s over 2 0 MHz and then i n c r e a s e s a g a i n above 40 MHz. T h e 4-
f ands -
f curves f o r 47.577 a l l o y a r e shown i n Fig. 4 . d v a r i e s very smoothly w i t h f (4-30 MHz) j u s t l i k e t h e ' c u r v e i n 20-40 MHz frequency r a n g e i n 5W7 a l l o y , a s shown i n F i g . 3, but i n c r e a s e s r a p i d l y w i t h f above 40 MHz.A t t h e same time, a d peak begins t o a p p e a r and grow upon w i t h t h e frequency
F i g . 3. A t t e n u a t i o n and decrement Fig. 4 . A t t e n u a t i o n and decrement v s f r e q u e n c y f o r Au-50 a t $ Cd v s frequency f o r Au-47.5 a t % Cd a l l o y
o
36OC s 75 . ~ O C a l l o y a t 7 6 ' ~i n c r e a s e d on t h e 4
-
T curve f o r 47.5% a l l o y .7 ) The Q-I due t o MT and a m p l i t u d e dependence i n KHz range
A s shown i n p r e v i o u s paper ( I ) , damping v a l u e s a t KHz a r e g e n e r a l l y of t h e same o r d e r of magnitude a s t h o s e a t 1 Hz. The s t a t i c h y s t e r e s i s l o s s olechanism of t h e i n t e r f a c e d i s l o c a t i o n seems t o p l a y a rnain r o l e . To v e r i f y t h i s , t h e Q-' v s T c u r v e s a t 50 KHz, 150 KHz i n two AuCd a l l o y s (47.5 at% and 52.5 at$ Cd) were measured. The v a l u e s of Q-' due t o MT a t 50 and 150 KHz i n AuCd a l l o y s a r e b a s i c a l l y i n d e n t i c a l ( ~ i g . ? . ) . I t may be b e l i e v e d tbat o s t a t i c h y s t e r e s i s l o s s mechanism of t h e i n t e r p h a s e d i s l o c a t i o n s is r e s p o n s i b l e f o r t h e damping i n KHz r a n g e . From t h i s p o i n t of view, we have d e r i v e d a n e x p r e s s i o n ( 9 ) , i n terms of t h e Mott's formula of i n t e r n a l s t r e s s of i m ~ u r i t i e s on d i e l o c a t i o n s d u r i n g t h e i r
2 b A ~ A
-
motion. a s Q-'=
rSc,
where b is Burgers v e c t o r . h i s t h e wave l e n g t h of-
Q-'and f v s T curves f o r Au-47.5 at%Cd alloy
o 50 KHz
A 150 KHz 0
rz-~
I * I0.7 2 A k x i a - 7
Pig. 6. The a m p l i t u d e dependence of i n t e r n a l f r i c t i o n f o r Au-47.5 at% Cd a l l o y . I n s e t is G-L p l o t .
JOURNAL
DE
PHYSIQUEi n t e r n a l s t r e s s f i e l d ,
A
is t h e t o t a l l e n g t h of d i s l o c a t i o n on t h e movable i n t e r f a c e ,y i s
the d i r e c t i o n f a c t o r , @ is a c o n s t a n t , C ' i s t h e s h e a r modulus C 1 = (Cll-
C I 2 ) / 2 f o r (110)TO],
E , t h e Young's modulus is a f u n c t i o n of C ' s,
which have been c a c u l a t e d f o r a few s i n g l e c r y s t a l s w i t h d i f f e r e n t o r i r l t a t i o n and f o r p o l y c r y s t a l . ( 9 ) The f o l l o w i n g f a c t s can be e x p l a i n e d from t h e 4'' expres- s i o n : a ) @ i s independent of frequency. b ) Q-' depends on t h e t o t a l l e n g t h of i n t e r p h a s e d i s l o c a t i o n . c ) Q-I is r e l a t e d t o t h e s o f t e n i n g of modulus, Q'I peak a p p e a r s when E / C ' i s a maximum.
S i n c e t h e i n t e r n a l f r i c t i o n d u r i n g MT is caused by t h e motion of i n t e r f a c e d i s l o c a t i o n , t h e d i s l o c a t i o n unpinning e f f e c t may be observ d a t lower s t r e s s amplitude. The a m p l i t u d e dependence of 9-I i n 0.7X10-7--10-9 s t r a i n a m p l i t u d e r a n g e is shown i n Fig: 6 , Q- is a m p l i t u d e dependent a t s m a l l s t r a i n amplitude
(0.7 #
-
2.2% 1 0 - 0 . A G-L p l o t ( I n~-'e,a. x ,
) which is s i m i l a r t o B e n o i t ' s work on CuZnAl (1 0 ) is i n s e r t e d i n P i g . 6 .4 ) 0-' and
o<
n e a r p r e - m a r t e n s i t i c t r a n s f o r m a t i o n t e m p e r a t u r eI t h a s been r e p o r t e d ( 1 ) t h a t t h e r e is a broad peak of low h e i g h t a t t h e h i g h t e m p e r a t u r e s i d e of MT peak i n some AuCd a l l o y s . I n t h e p r e s e n t p a p e r , t h e frequency dependence of t h e peak h e i g h t of 47.5 at$ a l l o y a t 50, 150 KHe.and 4.
1 1 , 28.5 MHz was s t u d i e d . Experimental r e s u l t s i n d i c a t e : a ) t h e peak t e m p e r a t u r e is independent of frequency. b ) The t e m p e r a t u r e i n t e r v a l between t h i s peak and MT peak is n e a r l y t h e s a m e , f o r t h r e e k i n d s of a l l d y s . c ) The peak h e i g h t
depends on f r e q u e n c y , i n c r e a s i n g a t KHz range b u t d e c r e a s i n g a t MHz range.
DISCUSSIOIJ
1 ) The a t t e n u a t i o n mechanism i n MHz range.
A s shown i n F i g . 3 . f o r 5Qd a l l o y . o( i n c r e a s e s r a p i d l y . which is followed by a slow change, and t h e n s t e e p l y i n c r e a s e s a g a i n a s f i s r a i s e d . I n MHz r a n g e t h e a m p l i t u d e is expected t o o small t o unpin t h e d i s l o c a t i o n s . T h e r e f o r e t h e s t a t i c h y s t e r e s i s l o s s is small and t h e dynamic l o s s of v i b r a t i n g d i s l o c a t i o n s t r i n g ( a t lower megahertz r a n g e ) and t h e phonon-phonon i n t e r a c t i o n ( a t h i g h e r megahertz r a n g e ) i s t h e main cause f o r t h e a t t e n u a t i o n .
I t is known from G-L v i b r a t i n g - s t r i n g model (11 ) t h a t a Q-' maximum always a p p e a r s a t &I
=dm*,
i n s p i t e of whether t h e r e l a t i o n between Q-I and f is r e s o n a n t type o r r e l a x a t i o n t y p e ( overdamped r e s o n a n c e ) . So t h a t , CXi n c r e a s e s a s W i n c r e a s e s , and, t h e n r e a c h e s a maximum a t & =dm. When d>lOmsr ot becomes f l a t and d e c r e a s e s g r a d u a l l y a t h i g h e r f
.
A s shown i n F i g .S f o r 50%a l l o y , & i n c r e a s e s w i t h f i n 4-20 MHz r a n g e , r e a c h i n g f l a t maximum and f i n a l l y r i s e s a g a i n s t e e p l y above 3 0 MHz i n s t e a d of d e c r e a s e . The r e a s o n f o r t h i s d i s - crepancy is p o s s i b l y due t o a damping mechanism i n which phonon-phonon i n t e r a c t i o n dominates above 30 MHz. I t is proposed t h a t t h e damping i n MHz frequency r a n g e
is mainly a t t r i b u t e d t o t h e d i s l o c a t i o n dynamical l o s s and phonon f l u c t u a t i o n , and t h e f a c t t h a t d i f f e r e n t mechanism dominates i n d i f f e r e n t frequency range.
The same frequency dependence of d i n 47.5% a l l o y should be observed, but it was found t h a t & v a r i e s v e r y slowly a t f i r s t and t h e n r a i s e d s t e e p l y w i t h frequency i n c r e a s e d . Maybe t h e d i s l o c a t i o n r e s o n a n t frequencyc..),,,,,(
<
4MHz) f o r 47.W;a l l o y is l o v e r t h a n t h e frequency used i n our experiment. The f i r s t s t a g e of r a p i d i n c r e a s i n g as on t h e 6
-
T curve f o r 50/0 a l l o y was n o t observed. I n a d d i t i o n , a & peak g r a d u a l l y a p p e a r s i n4
--T curve when f i n c r e a s e s . A d e f i n e d& p e a k f o r 95 MHz a t t r i b u t e d t o phonon-phonon i n t e r a c t i o n is shown i n Fig. l ( b ) . I t is n e c e s s a r y t o o b t a i n t h e frequency dependence of* i n much h i g h e r frequency r a n g e f o r e s t a b l i s h i n g t h e e x a c t mechanism of t h i s damping e f f e c t . T h i s work w i l l be done l a t e r on.
2 ) The d i f f e r e n c e of t h e u l t r a s o n i c a t t e n u a t i o n i n t h e two a l l o y s
Marked d i f f e r e n c e e x i s t s f o r t h e u l t r a s o n i c a t t e n u a t i o n c u r v e s through MT f o r t h e two a l l o y s . The 47.576 a l l o y i n M phase has a h i g h e r o( and v a r i e s v e r y slowly
w i t h T, n o d e f i n e d peak is observed. I n c o n t r a s t , i n 5W% a l l o y 4 r i s e s s t e e p l y a s T a p p r o a c h e s As and t h e n f a l l s . Thus a d e f i n e d peak can be observed.
According t o t e m p e r a t u r e dependence of t h e c r i t i c a l s t r e s s e s below and above Ms(shown i n P i g . 7 ) i n f i v e AuCd a l l o y s w i t h d i f f e r e n t Cd c o n t e n t s ( 1 2 ) . t h e l a t t i c e i n M phase f o r 47.5%
a l l o y is s o f t and c r i t i c a l s t r e s s W v a r i e s s o smoothly t h a t i t is a l m o s t a c o n s t a n t i n a l a r g e Y t e m p e r a t u r e r a n g e . Thus a s i m i l a r t e m p e r a t u r e dependence between t h e c r i t i c a l s t r e s s and s h e a r c o n s t a n t C ' s h o u l d be observed.
0
h0 1
I n t h e damping f o r m u l a of-200
-
{oO 0 400zoo
d i s l o c a t i o n r e s o n a n t (1 1 ), M e r c i e r (1 3 ) u s e d f i,
a nTemperature CC) a n i s o t r o p i c energy f a c t o r of d i s l o c a t i o n t o r e p l a c e t h e s h e a r P i g . 7. Temperature dependence of t h e m o d u l u s / U . F o r d C < < I ,
Q-LgA'
;c r i t i c a l s t r e s s e s ( 1 2 ) f o r LJK =l
,
Q-'oc,!&~ ; $A d e c r e a s e s w i t h d e c r e a s e of C * ( A o ~ C ' i n some d i r e c t i o n s ) . Theref o r e i n view of F i g 7 , o( i n M phase i n 47.55 a l l o y s h o u l d be l a r g e r t h a n t h a t i n 50b a l l o y , and t h e f o r m e r v a r i e s s l o w l y . F o r 5&a l l o y , a s T--Me,
6y
d e c r e a s e s c o n t i n o u s l y and as a r e s u l td
i n c r e a s e s g r a d u a l l y . I t can a l s o be expected from F i g . 7 t h a t t h e a t t e n u a t i o n c u r v e i n Au-46.5 a t $ Cd a l l o y may be s i m i l a r t o t h a t f o r 47.57% a l l o y i n s h a p e . F u r t h e r m o r e , f o r Au-48.5 a t $ Cd and Au-49 a t % Cd a l l o y s t h e a t t e n u a t i o n c u r v e s might be t h e same a s t h a t f o r 50h a l l o y . I t i s known t h a t t h e s t r u c t u r e of M phase of t h e 46.5% and 47-57;a l l o y s i s t h e orthorhombic, w h i l e t h a t of 48.5'7, 4% and 50% a l l o y s i s hcp.
The d i f f e r e n c e i n s h a p e between a t t e n u a t i o n c u r v e s i n MHz r a n g e ( F i g . 1 ) and i n K H z r a n g e ( P i g . 5 ) can be a t t r i b u t e d t o t h e d i f f e r e n c e of 4-I mechanisms i n two f r e q u e n c y r a n g e s . A s a l r e a d y mentioned t h e Q-' i n KHz r a n g e mainly r e s u l t s from t h e s t a t i c h y s t e r e s i s l o s s of i n t e r f a c e d i s l o c a t i o n a f t e r u n p i n n i n g , and i n MHz r a n g e i t o r i g i n a t e s mainly from t h e dynamic l o s s of d i s l o c a t i o n b e f o r e u n p i n n i n g . We can e x p e c t t h e a t t e n u a t i o n v a l u e a t MHz range t o be a b o u t 8 dB/cm i n t e r m s of t h e Q-' peak v a l u e i n K H z r a n g e f o r t h e same mechanism, s o t h e d v a l u e (1-5 d%/cm) measured i n MHz r a n g e does n o t c o n t a i n t h e s t a t i c h y s t e r s i s l o s s . I n KHz r a n g e , t h e 0'' owing t o t h e a i s l o c a t i o n dynamic damping becoming s m a l l d u e t o Q-bcld
,
t h i s l o s s , t h e r e f o r e . can be n e g l e t e d i n M phase i n K H z and H e r a n g e s . 5 ) The p r e - m a r t e n s i t i c t r a n s i t i o n peakSome p r e - m a r t e n s i t i c phenomena have been r e p o r t e d s o f a r . E l e c t r o n d i f f r a c t i o n s t u d y ( 1 4 ) i n 6 phase a l l o y i n d i c a t e s t h a t t h e r e a r e two s u c c e s s i v e p r o c e s s e s a s s o c i a t e d w i t h m a r t e n s i t i c t r a n s i t i o n . a ) 1/3 r e f l e c t i v e s p o t s a p p e a r above Ms which a r e produced by d i s t o r t e d l a t t i c e a s t h e m a r t e n s i t e embryo formed d u e t o C' s o f t e n i n g . I n t h i s p r o c e s s , n e c e s s a r y symmetry change i n MT i s accomplished.
b ) Then a l a r g e homogenous d e f o r m a t i o n forms t h e m a r t e n s i t e . Clapp ( 8 ) proposed t h a t t h e s t r a i n s n e a r l a t t i c e d e f e c t may induce l o c a l e l a s t i c s o f t e n i n g and n u c l e a t i o n i s p r e f e r e n t i a l l y l o c a t e d a t t h i s r e g i o n s .
B e n o i t ( 1 5 ) and illercier ( 1 3 ) have measured, r e s p e c t i v e l y , t h e 0-I peak a s s o c i a t e d with p r e - m a r t e n s i t i c t r a n s i t i o n i n H z r a n g e i n PaiTi a l l o y . According t o a r e c e n t s t u d y (16), t h i s peak is r e l a t e d t o t h e t r a n s i t i o n f r o m p phase t o R phase.
There i s a l w a y s a broad peak w i t h low h e i g h t f o r 47.5% a l l o y a t t h e h i g h e r t e m p e r a t u r e s i d e of MT peak i n K H z and MHz r a n g e on c o o l i n g o r h e a t i n g . I t
C9-240 JOURNAL
DE
PHYSIQUEis probably s i m i l a r t o t h a t f o r Fe-Ni a l l o y s a t MHz r a n g e obtained by IIsu e t c . ( 7 ) . What d i f f e r s from N i - T i a l l o y is that i n Hz r a n g e t h e peak corrsponding t o t h e broad peak i n KHz and MHz r a n g e has n o t y e t been observed.
By e l e c t r o n d i f f r a c t i o n s t u d y ( 1 7 ) , (1 11 ) f a c e d i f f u s e s t r e a k has been observed above Ms i n 47.Yb a l l o y . The t e m p e r a t u r e r a n g e obtained i n p r e s e n t paper i n KHz and MHz f o r t h e broad peak is f a i r l y c o n s i s t e n t w i t h t h e anomalous d e c r e a s e of
p o s i t r o n l i f e which was measured by p o s i t r o n a n n i h i l a t i o n t e c h n i q u e ( 1 8 ) above Ms and Af i n 47.5% a l l o y . So t h e broad peak probably was o r i g i n a t e d from t h e p r e - m a r t e n s i t i c t r a n s i t i o n . S i n c e t h e peak p o s i t i o n is independent of frequency, i t does n o t belong t o r e l a x a t i o n t y p e i n t e r n a l f r i c t i o n r e l a t e d t o t h e r m a l a c t i v i t y . Its h e i g h t i n c r e a s e s as f i n c r e a s e s i n KHz r a n g e and d e c r e a s e s a s f
i n c r e a s e s i n MHz range. There may be a maximum between KHz and &%Hz r a n g e , which i n d i c a t e s t h a t p r e m a r t e n s i t i c peak is perhaps t h e c h a r a c t e r i s t i c of r e s o n a n t t y p e r a t h e r t h a n s t a t i c h y s t e r s i s t y p e . It i s s t i l l hard t o t e l l t h e mechanism i n d e t a i l because of l a c k i n g o t h e r e x p e r i m e n t a l d a t a .
A CKNUW LEDGMENTS
We wish t o thank M r . Guei Jian-Min, M r . Gu l i n g and M r . Cai Tian f o r h e l p w i t h t h e experiment work.
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