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Submitted on 1 Jan 1981
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THE INFLUENCE OF THERMAL CYCLING ON
THE ACOUSTIC EMISSION DURING HYDRIDE
FORMATION IN NIOBIUM
C. Cannelli, R. Cantelli
To cite this version:
JOURNAL DE PHYSIQUE
CoZZoque C5, suppZ4ment au nO1O, Tome 42, octobre 1981 page C5-947
THE INFLUENCE OF THERMAL CYCLING ON T H E ACOUSTIC EMISSION DURING
HYDRIDE FORMATION IN NIOBIUM
C . Cannelli and R. Cantelli
ConsigZio NazimaZe deZZe Ricerehe, I s t i t u t o d i Acustica "O.M. Corbino" Via Cassia 1216, 00189 Roma, I t a l y
A b s t r a c t . - T h e effect i s studied of r e p e a t e d thermal cyclings onthe a c o u s t i c acti- vity of t h e Nb-H system d u r i n g hydride p r e c i p i t a t i o n .
1 . l n t r o d u c t i o n . - Recently it h a s been shown by t h e a u t h o r s that hydride formatisn i s t h e s o u r c e of i n t e n s e a c o u s t i c emission (AE)in the Nb-H, Ta-H and V-H ~ ~ s t e m s ~ ~ ~ . ~ h e a c o u s t i c emission phenomenon was c h a r a c t e r i z e d by a n emissivity which s t a r t e d , o n c o o l i n g , a t t e m p e r a t u r e s c o f ~ r e l a t e d with the s o l v u s of the metal-hydrogen system and proceeded d u r i n g evolution of preci- pitation; or. heating no a c o u s t i c acitivity w a s p r e s e n t . O n e of the c h a r a c t e r s of the AE phenome non not sufficiently i n v e s t i g a t e d , i s the e f f e c t of a g e i n g and thermal cycling o n the behaviour of the emissivity d u r i n g p r e c i p i t a t i o n . I n o r d e r to c l a r i f y t h i s . a s p e c t , a c o u s t i c emission ex- p e r i m e n t s combined with ageing t r e a t m e n t s have been performed on the Nb-H system between 330 and 225 K
.
2 . Experimental. - F o r the p r e s e n t experiment a niobium specimen w a s c h o s e n i n which emis- s i d n had been previously stimulated by a s e v e r e h y d r i d e p r e c i p i t a t i o n . The sample w a s a c i r - c u l a r p l a t e of 99.9% p u r e p o l y c r y s t a l l i n e niobium(30 mm in d i a m e t e r and 3 mm thick).Prio13 t o the p r e s e n t hydrogen doping, the specimen w a s annealed a t 2000°C f o r I h in a vacuum of
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t h e o r d e r of 10 T o r r . T h e h y d r o g e n c h a r g e was conducted a t 550°C i n a c o n t r o l l e d 99.999% p u r e H atmosphere, and the resulting concentration of 4.0 a t % H was determined by a vacuum e x t r a c t i o n method. The a c o u s t i c emission measurements w e r e performed by Dunegan-Endevco instrumentation by v a r y i n g the t e m p e r a t u r e from 330 t o 225 K a t a r a t e of 0.6 K/min using a n iron-constantan thermocouple d i r e c t l y a t t a c h e d t o the specimen. The s i g n a l s from t h e P Z T t r a n s d u c e r w e r e p r e a m p l i f i e d . b y 4 0 d B i n the frequency band 100-300 kHz and then fed into a
l a g amplifier followed by a d i s t r i b u t i o n a n a l y z e r providing t h e t o t a l number of b u r s t s (events) Ne and t h e i r amplitude d i s t r i b u t i o n . A b u r s t s i g n a l that o c c u r r e d more than 1 0 ms a f t e r the 1% v e l o f the p r e c e d i n g b u r s t had decayed below the a d j u s t a b l e t h r e s h o l d of t h e amplitude detec- t o r , was counted a s one s e p a r a t e e v e n t . T h e e v e n t s w e r e d i s t r i b u t e d in 100 l e v e l s a c c o r d i n g t o t h e i r amplitudes e x p r e s s e d in d B above the value of 1 pV a t the t r a n s d u c e r output.
JOURNAL DE PHYSIQUE
3. R e s u l t s . - I n o r d e r t o have a more complete understandinp ot t h e emission pherromenon, i n t e r n a l f r i c t i o n , ~ - ' , a n d r e s o n a n c e frequency,f , m e a s u r e m e n t s w e r e conducted on the specimen c o n c u r r e n t l y with the AE e x p e r i m e n t s . T h i s p r o c e d u r e allowed t h e h y d r i d e p r e c i p i t a t i o n t e m p e r a t u r e Tt and t h e p l a s t i c deformation p r o c e s s e s o c c u r r i n g d u r i n g the formation of the m j
-1
st'itting hydride p r e c i p i t a t e s t o be r e v e a l e d F i g u r e 1 d i s p l a y s t h e i n t e r n a l f r i c t i o n
Q
and the t o t a l number of e v e n t s r\i, (broken l i n e s ) of the H-free specimen a f t e r the annealing a t 2000°C. After doping the specimen with 4.0 a t % H , i n t e r n a l f r i c t i o n and r e s o n a n c e f r e q u e p cy measurements w e r e simultaneously undertaken on cooling (with the same r a t e of the AE ex- ~ e r i m e r i t ) until the a b r u p t inflection of both p a r a m e t e r s o c c u r r e d ( a t T =273 K),indicating i n i t i a
t
tiorl of hydride formation. At this t e m p e r a t u r e the measurement w a s stopped and t h e specimen heated to room t e m p e r a t u r e .During subsequent cooling the A E m e a s u r e m e n t , ~ w a s conducted ( F i g . 1 from 330 t o 2 2 0 K , l s t cooling). I t i s s e e n that a c o u s t i c activity s t a r t s a t the tempe- r a t u r e 1' =269 K,which i s a l i t t l e l o w e r than the s o l v u s t e m p e r a t u r e T ,and p r o c e e d s on co-
AE t
oling with evolution of p r e c i p i t a t i o n . At 225 K the thermal cycle was inverted;during heating no a c o u s t i c activity was d e t e c t e d . T h e n , the specimen w a s a g e d . a t 150°C f o r 2 h o u r s a n d sub- sequently t h e AE cooling r u n was r e p e a t e d . T h i s p r o c e d u r e was i t e r a t e d f o r 6 t i m e s . I n F i g . 1 a r e r e p o r t e d the c u r v e s c o r r e s p o n d i n g t c the 4 t h , 5th and 7th c o o l i u g , e a c h of them was p r e c e ded by the 1 5 0 ° C ageing t r e a t m e n t . I t c a n be s e e n that the AE i n t e n s i t y d e c r e a s e s with p r o c e g
TEMPERATURE ( K 1 --- H - W E E t I. 0-' PRECOOLlHG ?--b.,
...
1_
1st CDOLmG 4 - 4 t h " ef1.r 3rd tt.15ffC'-.
5 - 5 t h " " 4 t h " 7 - 7 t h " " 6 t h-
h
.
,
-1. .
F i g . 1 : Vibration frequency f and i n t e r n a l f r i c t i o n Q i n d i c a t i n g t h e hydride formation t e m p e r a t u r e Tt of t h e Nb-4at%H system.The c u r v e s of the t o t a l n u m b e r of e v e n t s Ne d i s p l a y t h e a c o u s t i c activity of the same system with p r o c e e d i n g t h e r m a l c y c l i n g s .
- t ( Hz )
ding thermal cycling and a g e i n g , until it i s n o l o n g e r d e t e c t a b l e .
4 . D i s c u s s i o n . - The a c o u s t i c emission c u r v e s r e p o r t e d i n F i g . 1 must be d i s c u s s e d taking 1 .
i n t o account r e s u l t s previously r e p o r t e d I t was found that the t h e r m a l cycling of a same hy- d r o g e n a t e d specimen a c r o s s t h e s o l v u s t e m p e r a t u r e gradually a t t e n u a t e s the AE down t o i t s s u p p r e s s i o n in a few c y c l e s , but a t h e r m a l treatment a t 150°C p a r t i a l l y r e s t o r e s t h e emissivi- t y . This means that thermal cycling and a g e i n g affect a c o u s t i c emission in the opposite way. T h e r e f o r e t h e values of t h e Ne c u r v e s 4 , 5 and 7 of F i g . 1 a r e h i g h e r than they would be wi- thout the e f f e c t of thermal treatment a t 1 5 0 ° C , but t h i s r e c o v e r y of emissivity by a g e i n g gra-
1
C5-950 JOURNAL DE PHYSIQUE
p r e c i p i t a t i o n s c a u s e a p r o g r e s s i o n of d e t e r i o r a t i o n only when s t r e s s waves a r e emitted by the m a t e r i a l .
R e f e r e n c e s
1. G .Cannelli and R . Cantelli
,
J .Appl. P h y s.
3,
5666 (1979).2. G .Cannelli a n d R . C a n t e l l i
,
J.
Appl. P h y s.
51,
1955 (1980). 3. G .Cannelli and R. C a n t e l l i , S c r i p l a Met.3,
731 (1980). 4. G .Cannelli a n d R . C a n t e l l i , Appl. P h y s .3,
325 (1974).5. 0. B u c k , D . v . Thompson, and C.A.Wert, J . P h y s . Chem. S o l i d s