Acoustic emission and grain size in plastic deformation of metals

Scripta M E T A L L U R G I C A

Vol. 12, pp. 531-534, 1978 Printed in the United States

ACOUSTIC EblISSION AND (;IC¢IN SIZE

Pergamon

Press,

Inc.

IN PI,ASTlt7 Ill!FORMATION OF METALS

S. Mintzer and R. l'ascual :v Centro At6mico Bariloche Comisi6n Nacional de I/nerg~a At6mica Bariloche. Argentina and

R.bl. V o l p i l ) e p a r t a m e n t o de b l e t a l u r f , , i a C o m i s i 6 n N a c i o n a l de E n e r . ~ a A t 6 m i c a Buenos Ai r e s . A r t : e n t i n a (Received

M a r ch 28,

Introduct

1978)

i on

The o b j e c t i v e o f t i l e p r e s e n t work i s t o s t u d y t h e i n f l u e n c e o f q r a i n s 2 z e on t h e a c o u s t i c e m i s s i o n g e n e r a t e d d u r i n g p l a s t i c (leformation of polycristalline specimens. Two k i n d o f p r o c e s s e s m u s t be t a k e n i n t o a c c o u n t : g r a i n b o u n d a r y s l i d i n g and d i s l o c a t i o n m o v e m e n t , b u t , as shown in a ~revious work ( 1 ) , g r a i n boundary sliding does not contribute to acoustic emission. l ' o r t h a t r e a s o n i t c a n be e x p e c t e d t h a t , in the g r a i n s i z e range where . grain boundary sliding is most important (very small grain sizes) acoustic emiss i o n w i l l be l o w . I t i s n o t o b v i o u s w h a t t h e b e h a v i o u r s h o u l d be f o r l a r g e r g r a i n sizes. As grain boundary sliding depends strongly on temperature it is worthwhile to study the acoustic emission response as a function of grain size for different temperatures. However, and due to the experimental problems involved in this kind of experiment, it was prcferable to perform all the tests at room temperature in metals and alloys having a wide range of melting poifits (Tin). The materials selec ted where: Cd, Zn-0,4% AI, Cu and Ti, having "homologous temperatures" (lIT=q'/Tm~ of: 0,5, 0,43, 0,23 and 0,14 respectively with T= 293°K . Experimental

Procedures

A Zn-0,4% A1 alloy was air m e l t e d and chill cas~ from 99,9% Zn and AI, the latter being added in order to obtain Vine stable grain sizes at the test tern perature. The slabs obtained were first given a 60% thickness reduction at 250°and titan cold rolled to obtain finally a ] mm thick shcet. Then, they were annealed for one hour in air at temperatures between 150-300°C. Cu and Cd 99,9% and Ti specimens o a commercial ourity were prepared by cold rolling and subsequ¢nt annealing. Thickness reduction was about 80% in all cases. An adequate range of grain sizes was obtained by isochronous anneals in vacuum. Grain size determinations were made by the intercept method. The acoustic emlssion setup had been described elsewhere (I). Tensile tests were p erforr~ed at room temperature in an Instron model TTM at a strain rate of 5xl0 -~ s -1. Specimen dimensions were 65x]x7 mm in all cases. Experimental stress

Figs. ](a), ](b), and of the volume

Now at National

Results

](c) and ](d) show log-log plots of the 0,2% applied corrected accumulated acoustic emission pulses at two

Research

Council,

Physics 531

Division,

Ottawa,

Ontario,

Canada.

$32

ACOUSTIC EMISSION AND GRAIN SIZE

Vol.

12, No. 6

plastic

s t r a i n l e v e l s as a f u n c t i o n o f g r a i n s i z e . For t h e h i g h ltT p o l i c r y s t a l s [Cd and Z n - 0 . 4 % A l J t h e f l o w s t r e s s v s . g r a i n s i z e c u r v e p r e s e n t s two s t a g e s . I n t h e f i n e g r a i n s i z e t h e y e x h i b i t g r a i n b o u n d a r y w e a k e n i n g due t o tile i n c r e a s i n g i n f l u e n c e o f g r a i n b o u n d a r y s l i d i n R as g r a i n s i z e i s r e d u c e d , w h i l e i n tile c o a r s e g r a i n s i z e r e g i o n a " n o r m a l " s t r e s s .vs. g r a i n s i ze r e l a t i o n s h i p is observed, i.e., polycrystals harden with decreasing grain size, following the usual pattern of grain boundary Jlardening systems.. An o p t i c a l m i c r o g r a p h o f t h e " a s c u t " s u r f a c e o f a d e f o r m e d f i n e g r a i n e d Zn-0.4% A1 s p e c i m e n ( g r a i n s i z e = 8 , ) i s shown i n f i g . 2 where g r a i n b o u n d a r y s l i diag is clearly visible. Acoustic emission increases monotonically with grain size but the rate of increase is different in both grain size ranges, being higher in the low grain si ze region. This effect is best observed in Zn-O,4%AI where a very noticeable chart ge in the acoustic emission rata with increasing grain size is associated with t ~ maximum of the flow stress vs. grain size curve. For the low HT materials (Cu and Ti), grain boundary sliding is expected to have little influence and this is reflected in the fact ttlat the flow stress decrea ses with increasing grain size in the whole grain size range studied. The accumulated acoustic emission increases with grain size, the rate of increase being higher for the coarser grain sizes. Fig. 3 shows stress-strain and acoustic emission rate vs. strain curves for Cu and Cd polycrystals having similar grain sizes. Cu (low HTJ presents a very silarp peak in acoustic emission rate at low strains (0,4~) the rate falling rapid_ ly t'or lligher deformations. In Cd (high HT) the acoustic emission rate increases up to a strain of 6%~decreasing then much more gradually than in the case of Cu and remaining at a high level up to fracture. Discussion lye will consider first the grain size range in whicil flow stress decreases with increasing grain size. This aoplies to the coarse grain size range in high HT specimens and tile whole range stuaied for those with low HT. Grain size has the effect of limiting the mean free path of dislocations. If we deform by the same amount two polycrystals differing only in their grain size and assum~ that the mean free path is initially approximately equal to this. parameter, fewer dislocation segments should move in the specimen with the larger grain size in order to accomodate the imposed strain. This would apply only at small strains. As a consequence, a higher acoustic emission rate should be expected for ti~e specimen with tile lower grain size. This fact has previously been sug gested by Gillis (2) and is contrary to what has been found in the present work. On the other hand it has been shown (3) that the amplitude of acoustic emission pulses generated wllen dislocations move, increases with the distance tra yelled by dislocations, i.e., mean free path. As a consequence, as grain size increases, the amplitude distribution of acoustic emission pulses acctnnulated up to a given strain will be shifted toward higher amplitudes. Only a fraction of the total pulses generated will be actually detected (those with amplitudes higher tilan tile noise level of the detection system). This fraction will increase with grain size but, as was discussed above, the total amount o f pulses generated will decrease. The acoustic emission dependence on grain size will depend on the balan ce between this two processes. As acoustic emission was found to increase with grain size, it is concluded that at least for the grain size range studied in the present work the increase of amplitude with increasing mean free path is pr¢dominmlt At higher grain sizes, however, the situation may be reversed. The relationship of acoustic emission amplitude with mean free path can be seen qualitatively in Fig.3. For the Cu polycrystal the acoustic emission rate ri ses silarply at very low strains but decreases rapidly as the mean free ~ath gets smaller due to work-hardening. After a few percent strain most dislocation movement occurs over short distances and only a sma]l numb6r Of dislocation processes gene rates acoustic emission pulses of sufficient amplitude to be detected. This behaviouris to be expected in low HT materials. For the Cd specimen J due to its low |IT, dynamic recovery occurs, as reflected in a very low rate of work hardening. In that case, the mean free path of dislocations is expected to present only a mild dependence with strain even at high strain and consequently a considerable fraction of the acoustic emission

Yol, 12, No. 6

533

ACOUSTIC EMISSION AND GRAIN SIZE

p u l s e s g e n e r a t e d c a n be d e t e c t e d l e a d i n g t o a h i g h a c o u s t i c e m i s s i o n r a t e t h r o u g h out the whole t e n s i l e test. I n t h e f i n e g r a i n s i z e r e g i o n a n d f o r t h e h i g h HT m a t e r i a l s , both grain boundary sliding and dislocation movement a r e r e s p o n s i b l e for plastic deformation G r a i n b o u n d a r y s l i d i n g becomes r e l a t i v e l y l e s s i m p o r t a n t as g r a i n s i z e i n c r e a s e s a n d ' i t was a l r e a d y m e n t i o n e d t h a t i t does n o t c o n t r i b u t e ' t o acoustic emission. The h i g h e r r a t e o f i n c r e a s e o f a c o u s t i c e m i s s i o n w i t h g r a i n s i z e o b s e r v e d i n t h i s r e g i o n c a n be u n d e r s t o o d c o n s i d e r i n g t h a t , as g r a i n s i z e i n c r e a s e s , not only the amplitude of the pulses increases [ a s i n t h e c o a r s e g r a i n s i z e r a n g e ) b u t a l s o an increasing fraction of plastic d e f o r m a t i o n i s due t o d i s l o c a t i o n movement t h e r e fore increasing the acoustic emission generated for a given strain. References 1.2.3.-

R. F r y d m a n , R. P a s c u a l a n d R. V o l p i : S c r i p t a M e t a l l u r g i c a : 9(1975)1267. P.P. Gillis: A c o u s t i c E m i s s i o n , ASTM S p e c . T e c h . P u b 1 . 5 0 5 , 1 9 7 2 . p . 2 0 . K. M a l e n a n d L. B o l i n : P h y s . S t a t u s S o l i d i ( b ) 6 1 , 1 9 7 4 , 6 3 7 .

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S34

ACOUSTIC EMISSION AND GRAIN SIZE

Vol.

12, No. 6

FIG. 2 Grain boundary sliding Zn-0.4

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S t r e s s - S t r a i n curves and a c o u s t i c e m i s s i o n rate vs. s t r a i n e c u r v e s for Cu and Cd o o l y c r y s t a l s . (Grain size = 200 ~)