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Tensile behavior of a mechanically-alloyed Al-4.0 Mg powder alloy
Scrlpta
METALLURGICA
Vol. 16, pp. 799-802 1982 Printed in the U.S.A.
B E H A V I O R OF A H E C H A N I C A L L Y - A L L O Y E D
TFNSILF
YOUNG-Wr3tl *
**
KIM* AND
I_. R.
Pergamon
AI-".0Hg
Press
Ltd
POWDER A L L r ) Y
BIDWFLL**
H e t c u t - M a t e r i a l s R e s e a r c h G r o u p , P . O . B o x 33511, W r i q h t - P a t t e r s o n A i r Force Rase, n i l , q5L133. and M a t e r m l s L a b o r a t o r v , A i r Force W r i q h t A e r o n a u t i c a l W r i q h t - P a t t e r s o n A i r Force Rase, OH, q5q3q.
Labs.
( R e c e i v e d March 17, 1982) ( R e v i s e d A p r i l 28, i 9 8 2 ) INTRNrbI]CTION Ilse of t h e mechanical a l l o v i n q [ ~ l A I p r o c e s s has r e c e n t l v a c h i e v e d r e m a r k a b l e s t r e n o t h i n c r e a s e s in A I - H q allm, s ( 1 - 6 ) . A l l o v IN-9051 ( A I - L I . 0 M q - 0 . 6 0 - 0 . S C / developed bv Inco I . T D . f o r a i r c r a f t s t r u c t u r a l a p p l i c a t i o n s is one such e x a m p l e . T h t s n o n - h e a t t r e a t a b l e allm/ m p r e p a r e d h v m e c h a n i c a l l y a t t r i t i n q element p o w d e r s (AI and ~ q ) in ,~mbtent a i r in the p r e s e n c e of a h y d r o c a r b o n lubricant. It e x h i b i t s a t y p i c a l HA s t r u c t u r e c o n s i s t i n q olf i n e l y - d i s p e r s e d o x i d e s a n d c a r b i d e s and a c o m p l e x , e x t r e m e l y f i n e , d i s l o c a t i o n s u b s t r u c t u r e . P r e v i o u s e x p e r i m e n t s {7) s h o w e d t h a t t h e ma}or s t r e n q t h e n i n a r e l a t i v e to i n q o t AI-Hc] allo\,5, stems from t h e MA s t r u c t u r e . D i s p e r s o i d s c o n t r i h u t e to a lesser e x t e n t . T h e ~IA s t r u c t u r e ts v e r y s t a b l e . It s u r v i v e s s e v e r e t h e r m o m e c h a n i c a l p r o c e s s i n q vacuum deqasslng hot c n m p a c t m n , e x t r u s i o n , a n d f o r g m g at t e m p e r a t u r e s r a n q m q from 380 C to q50 C. The tensile s t r e n o t h a f t e r t h i s p r o c e s s i n n r a n q e s from 520 to 590HPa. T h i s is to be c o m p a r e d wrth a r a n q e of 2q0 tn tl00MPa f o r 5000 series inr]ot a l u m i n u m a l l o y s w i t h a s i m i l a r Dq c o n t e n t . While t h e s e u n u s u a l l y htqh s t r e n r ] t h s h a v e been n o t e d p r e v i o u s l v ( 1 - 5 , 8 , 9 / ; t h i s is t h e f i r s t r e p o r t on t h e e q u a l l y u n u s u a l t e n s i l e b e h a v i o r nf t h e allox, as a f u n c t i o n of s t r a i n - r a t e and temperature. A d d i t i o n a l d e t a i l s will be n t J b l i s h e d e l s e w h e r e (11~). •
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EX PEP IP4FI'IT,A I_ H a t e r i a l w i t h a nominal c o m p o s i t i o n of A I - 4 . 0 H q - I , 5 ~ h - 0 . R C waq ~,upplled b v N o v a m e t , I n c o , I T D , as a h o t - c o m p a c t e d h i l l e t . It s h o u l d be n n t e d t h a t t h e o x v q e n o ~ n t e n t of t h e b i l l e t was h i q h e r t h a n t h a t t y p i c a l l y f o u n d in IH-9051. After machininq, the as-received b i l l e t was e x t r u d e d 20:1 at q00'GC to a 1.7cm diame+er r o d . T e n s i l e specimens p r e p a r e d from t h e e x t r u s i o n w e r e t e s t e d at d i f f e r e n t s t r a i n r a t e s in l a b o r a t o r y a i r at t e m p e r a t u r e s from PT te ~;50°C. A l l o y m i c r o s t r u c t u r e s , b o t h b e f o r e and aCter testlno,, w e r e o b s e r v e d h v o p t i c a l and transmiqslon electron microsconv. PFSIILTq
AND D I F C I r c . S I n ~
Room T e m p e r ; ~ t u r e P r o p e r t i e s A t v p m a l room t e m p e r a t u r e s t r e s s - ¢ t r a i n curve of t h e a s - e x t r u d e d It~-9051 is c o m p a r e d w i t h t h a t f o r a n n e a l e d m a t e r i a l in Fiq. 1. As can r e a d i l y he qeen, t h e b e h a v i o r of the as-extruded m a t e r i a l is q u i t e u n c o n v e n t i o n a l . A f t e r n n l v a small a m o u n t of p l a s t i c s t r a t a ( R e , inn I ) , d u r i n g w h i c h w o r k harctenincl t a k e s p l a c e , t h e a l l o v r e a c h e s its u l t i m a t e t e n s t l e strenqth (A). With a d d i t i o n a l s t r a i n [ R e q i o n I I ) , t h e s t r e s s d r o p s r a p i d l y to a l o w e r v i e l d nornt [ I 3 ) . F u r t h e r s t r a i n h a r d e n s t h e m a t e r i a l to C. T h e m a q n i t u d e of t h e , q e l d p o i n t d r o p helow a qmooth c u r v e AC v a r i e s from specimen to specimen at a q i v e n s t r a i n r a t e . On t h e averaqe, however, it a p p e a r s to d e c r e a s e w i t h d e c r e a s i n q s t r a t a r a t e . P e o i o n I II is characterized hv sliqht strain-softeninq ( - 0 . 1 < n <0). IJnstable p l a s t i c flaw b e o i n s at the i n f l e c t i o n p o i n t D a n d t h e specimen n e c k s ( R e q i o n 1V/ to u l t i m a t e f r a c t u r e . T h e u n u s u a l d e f o r m a t i o n b e h a v i o r of a s - e x t r u d e d Ihl-9051 mav be a s c m b e d to t h e m i c r o -
799 0036-9748/82/0~70799-04S03. 00/0
800
TENSILE
BEHAVIOR
OF AI-Mg ALLOY
Vol.
16, No.
7
s t r u c t u r e d e v e l o n e d d u r i n g t h e m e c h a n i c a l allovinel (MA) p r o c e s s , T h i s p r o c e s s r e s u l t s in a h i q h l y d e f o r m e d m a t r i x t h a t is c h a r a c t e r i z e d b y a c o m p l e x d i s l o c a t i o n n e t w o r k t h a t f o r m s I,~oorlv d e f i n e d , fine (0.3~um) c e l l s , Fig. 2a. As n o t e d e l s e w h e r e ( 7 ) , t h e d i s l o c a t i o n s f o r m i n q t h e s e cell b o u n d a r i e s , as well as t h o s e p r e s e n t w i t h i n t h e c e l l s , form a h i q h l y s t a b l e a r r a y , p r e s u m a b l y stabilized by the f i n e l y - d i s p e r s e d oxides and c a r b i d e s that are p r e s e n t . Durinq a t e n s i l e t e s t , t h e h i g h l y d e f o r m e d m a t r i x cluicklv r e a c h e s its uIHmate t e n s i l e s t r e n g t h with v e r v little a d d i t i o n a l p l a s t i c flow. T h e s u b s e q u e n t r a p i d d r o p in s t r e s s is b e l i e v e d to r e s u l t from t h e c o l l e c t i v e m o v e m e n t of a l a r g e n u m b e r of t h e more mobile, p r e v i o u s l y p i n n e d ~A dislocations. T h e s e d i s l o c a t i o n s may face loss r e s i s t a n c e to motion o n c e t h e y hecome unpinned. As d e f o r m a t i o n p r o c e e d s , a d d i t i o n a l d i s l o c a t i o n s b r e a k f r e e a n d t h e material s t r a i n - s o f t e n s until t h e o n s e t of n e c k i n c j . It has been well d o c u m e n t e d t h a t Al-~4g s o l i d - s o l u t i o n a l l o y e x h i b i t s t r a i n - a n i n c l and d i s c o n t i n u o u s d e f o r m a t i o n b e h a v i o r w h e n t e s t e d at low s t r a i n - r a t e s , e v e n below ror~m temperature (11-14). A s e p a r a t e e x p e r i m e n t was c o n d u c t e d in o r d e r to d e t e r m i n e i f such s o l u t e - d i s l o c a t i o n i n t e r a c t i o n s p l a y a role in t h e u n u s u a l d e f o r m a t i o n b e h a v i o r of IH-9051. E x t r u d e d material was h e a t - t r e a t e d at 530°C f o r 18h to decrease its h a r d n e s s from P'B of 82 to 68. T h i s r e s u l t s in p a r t i a l r e c o v e r y of t h e a l l o y (7) and increases the cell size f r o ~ ~ O.3um to 0.9urn. As can be seen in F i q . 2b, cell b o u n d a r i e s become b e t t e r d e f i n e d and the d i s l o c a t i o n d_~n~ity i f r e d u c e d . T h e t e n s i l e c u r v e f o r t h i s m a t e r i a l , o b t a i n e d at a s t r a i n r a t e of 1.7 x 10 s is shown in F i g . 1. T h e a l l o y now shows m a r k e d l y r e d u c e d s t r e n q t h and a complete absence o f a t v p i c a l d e f o r m a t i o n b e h a v i o r . This clearly indicates that the interaction b e t w e e n mobile d i s l o c a t i o n and t h e ~4A s t r u c t u r e (dislocations) is t h e d o m i n a n t f a c t o r responsible for the deformation behavior of the as-extruded material. Elevated Temperature Properties E l e v a t e d t e ~ p e ~ - a t u r e t e n s i l e t e s t s w o r e c o n d u c t e d at s t r a i n r a t e s of 5 x 10 -7 a n d 5 x In -4 a n d 5 x 1 0 - ~ , - , e x c e p t at 237°C w h e r e a d d i t i o n a l s t r a i n r a t e s w e r e u s e d . Fiq. 3 s h o w s t h e v a r i a t i o n of s t r e n g t h with t e s t t e m p e r a t u r e a n d s t r a i n r a t e . It can b e s e e n t h a t t h e d e p e n d e n c e o f t h e s t r e n q t h on t e m p e r a t u r e a n d t h e d e p e n d e n c e o f t h e s t r e n g t h on s t r a i n r a t e a r e b o t h most p r o n o u n c e d at i n t e r m e d i a t e t e m p e r a t u r e s . The d i f f e r e n c e between the yield and t h e u l t i m a t e s t r e n g t h , w h i c h is s i g n i f i c a n t at lower t e m p e r a t u r e s , is v i r t u a l l y nil a b o v e 3O0°C. T h e s e c h a r a c t e r i s t i c s s u q q e s t t h a t at e l e v a t e d t e m p e r a t u r e , t h e ~4A s t r u c t u r e no I o n q e r p l a y s a role a s a b a r r i e r to mobile d i s l o c a t i o n s a n d t h e d i s p e r s o i d s may not e f f e c t i v e l y pin t h e m down. R e p r e s e n t a t i v e s t r e s s - s t r a i n c u r v e s for t e n s i l e t e s t s c o n d u c t e d at f i v e d i f f e r e n t s t r a i n r a t e s a t 232°C a r e s h o w n in F i g . 4. T h e s t r e s s - s t r a i n b e h a v i o r a t e l e v a t e d t e m p e r a t u r e is s o m e w h a t similar to t h a t s e e n at room t e m p e r a t u r e in t h a t t h e u l t i m a t e t e n s i l e s t r e n q t h is r e a c h e d a f t e r minimal p l a s t i c flow a n d t h e material s u b s e c l u e n t l v s t r a i n - s o f t e n s ( - 0 , 1 < n < 0 ) . One n o t a h l e d i f f e r e n c e , h o w e v e r , is t h e a b s e n c e o f t h e r a p i d p o s t - U T S d r o p in s t r e s s a n d t h e lower y i e l d p o i n t s e e n at room t e m p e r a t u r e . T h e c o i n c i d e n c e b e t w e e n t h e i n f l e c t i o n p o i n t (D) a n d t h e o n s e t o f n e c k i n g was e s t a b l i s h e d b y c a r e f u l m i c r o s c o p i c o b s e r v a t i o n s d u r i n g many tests. P o s t - t e s t TEM o b s e r v a t i o n s of t h e m i c r o s t r u c t u r e n e a r t h e f r a c t u r e s u r f a c e s h o w e d t h a t t h e i n i t i a l , a s - e x t r u d e d m i c r o s t r u c t u r e was v i r t u a l l y u n c h a n g e d d u r i n g t h e t e s t s . T h e u n i f o r m e l o n q a t i o n seen in Fig. 4 ( d e l i n e a t e d b y t h e d o t t e d line) is r o u g h l y p r o p o r t i o n a l to t h e t o t a l e l o n q a t i o n B o t h a r e a f u n c t i o n of s t r a i n r a t e . I r~ter~estinqly, t h e maximum e l o n g a t i o n at 232 C o c c u r s at a r e l a t i v e l y h i g h s t r a i n ~-at~ (5 x 1 0 - J s - - ) , w h i l e t h e lowest e l o n g a t i o n o c c u r s at t h e slowest s t r a i n r a t e used. (5 x 10- s- ). Fig. 5 shows the total e l o n g a t i o n as a f u n c t i o n of t e m p e r a t u r e and s t r a i n r a t e . As can be seen, t h e s t r a i n r a t e f o r maximum e l o n g a t i o n increases w i t h t e m p e r a t u r e . 2 ~'he l a r q e s t total e l o n g a t i o n , o b s e r v e d o c c u r r e d at 288°C at a s t r a i n r a t e of 5 x 1 0 - s and seems to be a b o u t t h e maximum o b t a i n a b l e w i t h a s - e x t r u d e d IN-9051. F u r t h e r increases in t e m p e r a t u r e and s t r a i n r a t e do n o t increase the elongation siqnificantlv (10). •
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O
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CONCLIISIONS The mechanically-alloyed (MA) AI-4.0Mg alloy, c o n d i t i o n , is m a r k e d l y s t r e n g t h e n e d b v t h e MA s t r u c t u r e . r e s u l t s in u n u s u a l t e n s i l e b e h a v i o r as f o l l o w s :
IN-9051, in the as-extruded The presence of this structure
Iol.
lo,
No.
'I'ENSILE BI!tb~V]OR ()} A1-51g ALLOY
8[)1
1. The alloy e x h i b i t s a lower y i e l d point (Fiq. 11 at room t e m p e r a t u r e which reqult from th~ c o l l e c t i v e motion of previnLmly p,nned IHA d~slocations. ?. At all t p m p e r a t u r e s ,nvPstiqated the alloy rr, P.ches its a{h,r minimal plastic strain. Subsequently, the mater,al ~--0.1
~s t h o u g h t to
ultimate tensile s t r e n q t h strain-softens uniformly
]. At elovatpr! tpmr)erature, the MA s t r u c t u r e affects deformation such that the s t r a i n rat('~ f~r m,qximum elonqat,on ~ncreases with increasmq temr)erature. Uniform elonq~tinP ~ncreases w,th increasinc~ temperature. I Iniform elonqation and total ~k~nqation are rouqhlv proportional. The maximum uniform and total elonqation r,b~ervpH are 4n 0 nil(! 1357, r e s p e c t i v e l y . RFFERENCES 1. n. 3. 4. q. 6. 7. 8. 9. 10. 11. 17. 13. 14.
J. S. r~enlamln, IJ.S Patent 3,591,362 (1968). J. S. Beniamin and J. J. Romford, .~et. T r a n s . 8A, 1301 [1977). P,. F. S , n q e r . w. C. N l i v e r and W. D. N i x , ~let. T r a n s . l l A , 1895 [1980). D. I_. F r i c h , A i r Force J'.~aterials L a b o r a t o r y , AFWAL-TR-79-4210, 1980. J. S. Penjamm ariel K. D. S c h e l l e r y , Met. T r a n s . (in p r e q s ) . P. S. G,Iman and W. D. N i x , Het. T r a n s . 12A, 813 1"1981}. y-w Kim and L. R. B i d w e l l , S c r i p t a Met. 15, 483 [1981}. Y-W. Kim and L. P.. R i d w e l l , AI~IE Annual I~,leetIna, Feb. 24-28, 1980, Las Veqas, Nevada. Y-W. Kim and L. R. B i d w e l l , T M S - A I H E Fall Meetinq, Oct 5. 1980, P i t t s b u r q h , PA. Y-W. Kim and I.. R. ,n;idwell, /'let. T r a n s . , to be s u b m i t t e d . S. P. ,~larewan and P. Ramaswanl, Phil. Haq. 21, 1025 (1970). P,. J. r~;rinklev and P. 3. W o r t h i n q t n n , Acta Met. 17, 1357 r1969/. A. T. Thomas, Acta Het. 14, 1363 (1966/. H. Fui)ta and T. T a h a t a , 25, 793 [19~7~.
FIG. 2 TEM microstructures of: a) as-extruded and b) post-extrusion heat-treated (530oC/18h) IN-9051.
802
TENSILE
BEIIAVIOR OF Al-blg ALLOY
Vol.
600|-
550[-"
[AS EXTRUDED1
A
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STRAIN
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FIG. 1
I
I
I
I00 200 300 4oo TEST TEMPERATURE(°C)
o
Room temperature tensile stress-strain curves of as-extruded and post-extrusion heat-treated IN-9051.
FIG. 3 T e n s i l e s t r e n g t h o f a s - e x t r u d e d IN-9051 as a f u n c t i o n o f t e s t temperature and strain rate.
500
,
~
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K~ 2 E S TEMP T 232%
20C I00 F
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STRAIN
RATE(S5 x 10-2 5x I0"3 5x 10"_ 4 8x I0~ 3x I0-o
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80
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STRAIN (%) 0 FIG. 4 Tensile stress-strain curve of IN-9051 at 232°C as a function of strain rate.
0
IO0
200
1 300
400
TEST TEMPERATURE (QC)
FIG. 5 Total elongation of as-extruded IN-9051 as a function of test temperature and strain rate.
METALLURGICA
Vol. 16, pp. 799-802 1982 Printed in the U.S.A.
B E H A V I O R OF A H E C H A N I C A L L Y - A L L O Y E D
TFNSILF
YOUNG-Wr3tl *
**
KIM* AND
I_. R.
Pergamon
AI-".0Hg
Press
Ltd
POWDER A L L r ) Y
BIDWFLL**
H e t c u t - M a t e r i a l s R e s e a r c h G r o u p , P . O . B o x 33511, W r i q h t - P a t t e r s o n A i r Force Rase, n i l , q5L133. and M a t e r m l s L a b o r a t o r v , A i r Force W r i q h t A e r o n a u t i c a l W r i q h t - P a t t e r s o n A i r Force Rase, OH, q5q3q.
Labs.
( R e c e i v e d March 17, 1982) ( R e v i s e d A p r i l 28, i 9 8 2 ) INTRNrbI]CTION Ilse of t h e mechanical a l l o v i n q [ ~ l A I p r o c e s s has r e c e n t l v a c h i e v e d r e m a r k a b l e s t r e n o t h i n c r e a s e s in A I - H q allm, s ( 1 - 6 ) . A l l o v IN-9051 ( A I - L I . 0 M q - 0 . 6 0 - 0 . S C / developed bv Inco I . T D . f o r a i r c r a f t s t r u c t u r a l a p p l i c a t i o n s is one such e x a m p l e . T h t s n o n - h e a t t r e a t a b l e allm/ m p r e p a r e d h v m e c h a n i c a l l y a t t r i t i n q element p o w d e r s (AI and ~ q ) in ,~mbtent a i r in the p r e s e n c e of a h y d r o c a r b o n lubricant. It e x h i b i t s a t y p i c a l HA s t r u c t u r e c o n s i s t i n q olf i n e l y - d i s p e r s e d o x i d e s a n d c a r b i d e s and a c o m p l e x , e x t r e m e l y f i n e , d i s l o c a t i o n s u b s t r u c t u r e . P r e v i o u s e x p e r i m e n t s {7) s h o w e d t h a t t h e ma}or s t r e n q t h e n i n a r e l a t i v e to i n q o t AI-Hc] allo\,5, stems from t h e MA s t r u c t u r e . D i s p e r s o i d s c o n t r i h u t e to a lesser e x t e n t . T h e ~IA s t r u c t u r e ts v e r y s t a b l e . It s u r v i v e s s e v e r e t h e r m o m e c h a n i c a l p r o c e s s i n q vacuum deqasslng hot c n m p a c t m n , e x t r u s i o n , a n d f o r g m g at t e m p e r a t u r e s r a n q m q from 380 C to q50 C. The tensile s t r e n o t h a f t e r t h i s p r o c e s s i n n r a n q e s from 520 to 590HPa. T h i s is to be c o m p a r e d wrth a r a n q e of 2q0 tn tl00MPa f o r 5000 series inr]ot a l u m i n u m a l l o y s w i t h a s i m i l a r Dq c o n t e n t . While t h e s e u n u s u a l l y htqh s t r e n r ] t h s h a v e been n o t e d p r e v i o u s l v ( 1 - 5 , 8 , 9 / ; t h i s is t h e f i r s t r e p o r t on t h e e q u a l l y u n u s u a l t e n s i l e b e h a v i o r nf t h e allox, as a f u n c t i o n of s t r a i n - r a t e and temperature. A d d i t i o n a l d e t a i l s will be n t J b l i s h e d e l s e w h e r e (11~). •
.
.
"o
o
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.
EX PEP IP4FI'IT,A I_ H a t e r i a l w i t h a nominal c o m p o s i t i o n of A I - 4 . 0 H q - I , 5 ~ h - 0 . R C waq ~,upplled b v N o v a m e t , I n c o , I T D , as a h o t - c o m p a c t e d h i l l e t . It s h o u l d be n n t e d t h a t t h e o x v q e n o ~ n t e n t of t h e b i l l e t was h i q h e r t h a n t h a t t y p i c a l l y f o u n d in IH-9051. After machininq, the as-received b i l l e t was e x t r u d e d 20:1 at q00'GC to a 1.7cm diame+er r o d . T e n s i l e specimens p r e p a r e d from t h e e x t r u s i o n w e r e t e s t e d at d i f f e r e n t s t r a i n r a t e s in l a b o r a t o r y a i r at t e m p e r a t u r e s from PT te ~;50°C. A l l o y m i c r o s t r u c t u r e s , b o t h b e f o r e and aCter testlno,, w e r e o b s e r v e d h v o p t i c a l and transmiqslon electron microsconv. PFSIILTq
AND D I F C I r c . S I n ~
Room T e m p e r ; ~ t u r e P r o p e r t i e s A t v p m a l room t e m p e r a t u r e s t r e s s - ¢ t r a i n curve of t h e a s - e x t r u d e d It~-9051 is c o m p a r e d w i t h t h a t f o r a n n e a l e d m a t e r i a l in Fiq. 1. As can r e a d i l y he qeen, t h e b e h a v i o r of the as-extruded m a t e r i a l is q u i t e u n c o n v e n t i o n a l . A f t e r n n l v a small a m o u n t of p l a s t i c s t r a t a ( R e , inn I ) , d u r i n g w h i c h w o r k harctenincl t a k e s p l a c e , t h e a l l o v r e a c h e s its u l t i m a t e t e n s t l e strenqth (A). With a d d i t i o n a l s t r a i n [ R e q i o n I I ) , t h e s t r e s s d r o p s r a p i d l y to a l o w e r v i e l d nornt [ I 3 ) . F u r t h e r s t r a i n h a r d e n s t h e m a t e r i a l to C. T h e m a q n i t u d e of t h e , q e l d p o i n t d r o p helow a qmooth c u r v e AC v a r i e s from specimen to specimen at a q i v e n s t r a i n r a t e . On t h e averaqe, however, it a p p e a r s to d e c r e a s e w i t h d e c r e a s i n q s t r a t a r a t e . P e o i o n I II is characterized hv sliqht strain-softeninq ( - 0 . 1 < n <0). IJnstable p l a s t i c flaw b e o i n s at the i n f l e c t i o n p o i n t D a n d t h e specimen n e c k s ( R e q i o n 1V/ to u l t i m a t e f r a c t u r e . T h e u n u s u a l d e f o r m a t i o n b e h a v i o r of a s - e x t r u d e d Ihl-9051 mav be a s c m b e d to t h e m i c r o -
799 0036-9748/82/0~70799-04S03. 00/0
800
TENSILE
BEHAVIOR
OF AI-Mg ALLOY
Vol.
16, No.
7
s t r u c t u r e d e v e l o n e d d u r i n g t h e m e c h a n i c a l allovinel (MA) p r o c e s s , T h i s p r o c e s s r e s u l t s in a h i q h l y d e f o r m e d m a t r i x t h a t is c h a r a c t e r i z e d b y a c o m p l e x d i s l o c a t i o n n e t w o r k t h a t f o r m s I,~oorlv d e f i n e d , fine (0.3~um) c e l l s , Fig. 2a. As n o t e d e l s e w h e r e ( 7 ) , t h e d i s l o c a t i o n s f o r m i n q t h e s e cell b o u n d a r i e s , as well as t h o s e p r e s e n t w i t h i n t h e c e l l s , form a h i q h l y s t a b l e a r r a y , p r e s u m a b l y stabilized by the f i n e l y - d i s p e r s e d oxides and c a r b i d e s that are p r e s e n t . Durinq a t e n s i l e t e s t , t h e h i g h l y d e f o r m e d m a t r i x cluicklv r e a c h e s its uIHmate t e n s i l e s t r e n g t h with v e r v little a d d i t i o n a l p l a s t i c flow. T h e s u b s e q u e n t r a p i d d r o p in s t r e s s is b e l i e v e d to r e s u l t from t h e c o l l e c t i v e m o v e m e n t of a l a r g e n u m b e r of t h e more mobile, p r e v i o u s l y p i n n e d ~A dislocations. T h e s e d i s l o c a t i o n s may face loss r e s i s t a n c e to motion o n c e t h e y hecome unpinned. As d e f o r m a t i o n p r o c e e d s , a d d i t i o n a l d i s l o c a t i o n s b r e a k f r e e a n d t h e material s t r a i n - s o f t e n s until t h e o n s e t of n e c k i n c j . It has been well d o c u m e n t e d t h a t Al-~4g s o l i d - s o l u t i o n a l l o y e x h i b i t s t r a i n - a n i n c l and d i s c o n t i n u o u s d e f o r m a t i o n b e h a v i o r w h e n t e s t e d at low s t r a i n - r a t e s , e v e n below ror~m temperature (11-14). A s e p a r a t e e x p e r i m e n t was c o n d u c t e d in o r d e r to d e t e r m i n e i f such s o l u t e - d i s l o c a t i o n i n t e r a c t i o n s p l a y a role in t h e u n u s u a l d e f o r m a t i o n b e h a v i o r of IH-9051. E x t r u d e d material was h e a t - t r e a t e d at 530°C f o r 18h to decrease its h a r d n e s s from P'B of 82 to 68. T h i s r e s u l t s in p a r t i a l r e c o v e r y of t h e a l l o y (7) and increases the cell size f r o ~ ~ O.3um to 0.9urn. As can be seen in F i q . 2b, cell b o u n d a r i e s become b e t t e r d e f i n e d and the d i s l o c a t i o n d_~n~ity i f r e d u c e d . T h e t e n s i l e c u r v e f o r t h i s m a t e r i a l , o b t a i n e d at a s t r a i n r a t e of 1.7 x 10 s is shown in F i g . 1. T h e a l l o y now shows m a r k e d l y r e d u c e d s t r e n q t h and a complete absence o f a t v p i c a l d e f o r m a t i o n b e h a v i o r . This clearly indicates that the interaction b e t w e e n mobile d i s l o c a t i o n and t h e ~4A s t r u c t u r e (dislocations) is t h e d o m i n a n t f a c t o r responsible for the deformation behavior of the as-extruded material. Elevated Temperature Properties E l e v a t e d t e ~ p e ~ - a t u r e t e n s i l e t e s t s w o r e c o n d u c t e d at s t r a i n r a t e s of 5 x 10 -7 a n d 5 x In -4 a n d 5 x 1 0 - ~ , - , e x c e p t at 237°C w h e r e a d d i t i o n a l s t r a i n r a t e s w e r e u s e d . Fiq. 3 s h o w s t h e v a r i a t i o n of s t r e n g t h with t e s t t e m p e r a t u r e a n d s t r a i n r a t e . It can b e s e e n t h a t t h e d e p e n d e n c e o f t h e s t r e n q t h on t e m p e r a t u r e a n d t h e d e p e n d e n c e o f t h e s t r e n g t h on s t r a i n r a t e a r e b o t h most p r o n o u n c e d at i n t e r m e d i a t e t e m p e r a t u r e s . The d i f f e r e n c e between the yield and t h e u l t i m a t e s t r e n g t h , w h i c h is s i g n i f i c a n t at lower t e m p e r a t u r e s , is v i r t u a l l y nil a b o v e 3O0°C. T h e s e c h a r a c t e r i s t i c s s u q q e s t t h a t at e l e v a t e d t e m p e r a t u r e , t h e ~4A s t r u c t u r e no I o n q e r p l a y s a role a s a b a r r i e r to mobile d i s l o c a t i o n s a n d t h e d i s p e r s o i d s may not e f f e c t i v e l y pin t h e m down. R e p r e s e n t a t i v e s t r e s s - s t r a i n c u r v e s for t e n s i l e t e s t s c o n d u c t e d at f i v e d i f f e r e n t s t r a i n r a t e s a t 232°C a r e s h o w n in F i g . 4. T h e s t r e s s - s t r a i n b e h a v i o r a t e l e v a t e d t e m p e r a t u r e is s o m e w h a t similar to t h a t s e e n at room t e m p e r a t u r e in t h a t t h e u l t i m a t e t e n s i l e s t r e n q t h is r e a c h e d a f t e r minimal p l a s t i c flow a n d t h e material s u b s e c l u e n t l v s t r a i n - s o f t e n s ( - 0 , 1 < n < 0 ) . One n o t a h l e d i f f e r e n c e , h o w e v e r , is t h e a b s e n c e o f t h e r a p i d p o s t - U T S d r o p in s t r e s s a n d t h e lower y i e l d p o i n t s e e n at room t e m p e r a t u r e . T h e c o i n c i d e n c e b e t w e e n t h e i n f l e c t i o n p o i n t (D) a n d t h e o n s e t o f n e c k i n g was e s t a b l i s h e d b y c a r e f u l m i c r o s c o p i c o b s e r v a t i o n s d u r i n g many tests. P o s t - t e s t TEM o b s e r v a t i o n s of t h e m i c r o s t r u c t u r e n e a r t h e f r a c t u r e s u r f a c e s h o w e d t h a t t h e i n i t i a l , a s - e x t r u d e d m i c r o s t r u c t u r e was v i r t u a l l y u n c h a n g e d d u r i n g t h e t e s t s . T h e u n i f o r m e l o n q a t i o n seen in Fig. 4 ( d e l i n e a t e d b y t h e d o t t e d line) is r o u g h l y p r o p o r t i o n a l to t h e t o t a l e l o n q a t i o n B o t h a r e a f u n c t i o n of s t r a i n r a t e . I r~ter~estinqly, t h e maximum e l o n g a t i o n at 232 C o c c u r s at a r e l a t i v e l y h i g h s t r a i n ~-at~ (5 x 1 0 - J s - - ) , w h i l e t h e lowest e l o n g a t i o n o c c u r s at t h e slowest s t r a i n r a t e used. (5 x 10- s- ). Fig. 5 shows the total e l o n g a t i o n as a f u n c t i o n of t e m p e r a t u r e and s t r a i n r a t e . As can be seen, t h e s t r a i n r a t e f o r maximum e l o n g a t i o n increases w i t h t e m p e r a t u r e . 2 ~'he l a r q e s t total e l o n g a t i o n , o b s e r v e d o c c u r r e d at 288°C at a s t r a i n r a t e of 5 x 1 0 - s and seems to be a b o u t t h e maximum o b t a i n a b l e w i t h a s - e x t r u d e d IN-9051. F u r t h e r increases in t e m p e r a t u r e and s t r a i n r a t e do n o t increase the elongation siqnificantlv (10). •
•
O
"
CONCLIISIONS The mechanically-alloyed (MA) AI-4.0Mg alloy, c o n d i t i o n , is m a r k e d l y s t r e n g t h e n e d b v t h e MA s t r u c t u r e . r e s u l t s in u n u s u a l t e n s i l e b e h a v i o r as f o l l o w s :
IN-9051, in the as-extruded The presence of this structure
Iol.
lo,
No.
'I'ENSILE BI!tb~V]OR ()} A1-51g ALLOY
8[)1
1. The alloy e x h i b i t s a lower y i e l d point (Fiq. 11 at room t e m p e r a t u r e which reqult from th~ c o l l e c t i v e motion of previnLmly p,nned IHA d~slocations. ?. At all t p m p e r a t u r e s ,nvPstiqated the alloy rr, P.ches its a{h,r minimal plastic strain. Subsequently, the mater,al ~--0.1
~s t h o u g h t to
ultimate tensile s t r e n q t h strain-softens uniformly
]. At elovatpr! tpmr)erature, the MA s t r u c t u r e affects deformation such that the s t r a i n rat('~ f~r m,qximum elonqat,on ~ncreases with increasmq temr)erature. Uniform elonq~tinP ~ncreases w,th increasinc~ temperature. I Iniform elonqation and total ~k~nqation are rouqhlv proportional. The maximum uniform and total elonqation r,b~ervpH are 4n 0 nil(! 1357, r e s p e c t i v e l y . RFFERENCES 1. n. 3. 4. q. 6. 7. 8. 9. 10. 11. 17. 13. 14.
J. S. r~enlamln, IJ.S Patent 3,591,362 (1968). J. S. Beniamin and J. J. Romford, .~et. T r a n s . 8A, 1301 [1977). P,. F. S , n q e r . w. C. N l i v e r and W. D. N i x , ~let. T r a n s . l l A , 1895 [1980). D. I_. F r i c h , A i r Force J'.~aterials L a b o r a t o r y , AFWAL-TR-79-4210, 1980. J. S. Penjamm ariel K. D. S c h e l l e r y , Met. T r a n s . (in p r e q s ) . P. S. G,Iman and W. D. N i x , Het. T r a n s . 12A, 813 1"1981}. y-w Kim and L. R. B i d w e l l , S c r i p t a Met. 15, 483 [1981}. Y-W. Kim and L. P.. R i d w e l l , AI~IE Annual I~,leetIna, Feb. 24-28, 1980, Las Veqas, Nevada. Y-W. Kim and L. R. B i d w e l l , T M S - A I H E Fall Meetinq, Oct 5. 1980, P i t t s b u r q h , PA. Y-W. Kim and I.. R. ,n;idwell, /'let. T r a n s . , to be s u b m i t t e d . S. P. ,~larewan and P. Ramaswanl, Phil. Haq. 21, 1025 (1970). P,. J. r~;rinklev and P. 3. W o r t h i n q t n n , Acta Met. 17, 1357 r1969/. A. T. Thomas, Acta Het. 14, 1363 (1966/. H. Fui)ta and T. T a h a t a , 25, 793 [19~7~.
FIG. 2 TEM microstructures of: a) as-extruded and b) post-extrusion heat-treated (530oC/18h) IN-9051.
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TENSILE
BEIIAVIOR OF Al-blg ALLOY
Vol.
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Room temperature tensile stress-strain curves of as-extruded and post-extrusion heat-treated IN-9051.
FIG. 3 T e n s i l e s t r e n g t h o f a s - e x t r u d e d IN-9051 as a f u n c t i o n o f t e s t temperature and strain rate.
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STRAIN (%) 0 FIG. 4 Tensile stress-strain curve of IN-9051 at 232°C as a function of strain rate.
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TEST TEMPERATURE (QC)
FIG. 5 Total elongation of as-extruded IN-9051 as a function of test temperature and strain rate.