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JO U RN A L O F M A T E RI A L S SCI E N CE 19 ( 1984) 27 9- - 290

C harac te r iz a t ion o f the m ic ros t ruc tu re o fa c o m m e r c ia l A I - C u a llo y (2 0 1 1) b yd i f f e ren t i a l scann i ng ca l o r i met ry (D SC )C. G A R C i A C O R D O V I L L A , E. L O U I SCentro de Investigaci6n g Desarro llo de Al icante, c /o E ND AS A, Ap artado 25,Al icante, S pain

In th is w ork the m icrost ructure of a commercia l A l- C u al loy (2 0 1 1 ) in several metal-lurgical states has been studied by m eans of d if feren t ial scanning ca lor im etry (DSC). Themetallurgical s tates were chosen in such a way tha t the al loy contained p redo min ant lyone of the f ou r possible phases n these al loys ( i.e. GP zone s and 0" , O' and 0 phases).The commercia l tempers T3 and T6 have also been considered. Th e inte rpretat ion of theDSC curves was aided by measuring the changes in V ickers hardness and co nd uc t iv itydu r ing a linear heat ing sim ilar to th at p rovided by the DSC apparatus; these studies al low ,for instance, a c learer d is t inct ion between the GP (Gu inier -Pre ston ) zone dissolut ionpeak and the 0 " phase disso lut ion pea k. The results are compared with those obtain edby o th er authors .

1 . Introduct ionIn the las t few years there has be en a s t rong reviva lof t he use o f c a l or i me t r i c t e chni qu es for mi c ro-s t ruc t ure cha rac t e r i z a t i on of a l umi n i um a l l oys[1-17] . The a i m of t h i s g rea t e f for t i s t o p rovet he po t en t i a l advant ages t ha t c a l or i me t ry ha s ove rt echni ques ( such a s TEM ) t rad i t i ona l l y used forsuch purpose s . Al t hough mos t o f t he work ha sb e en p e r f o r m e d o n " l a b o r a to r y a ll o y s " [ l - 6 ,9 - 1 2 ] , l a te l y a c o n s id e r ab l e n u m b e r o f s tu d i eshave bee n done on co mm erc i a l a l l oys [7 , 8 ,13-17] . Thi s i s pa r t i cu l a r l y t rue for a l l oys oft h e 7 0 0 0 s e r i e s ( A 1 - Z n - M g a n d A I - Z n - M g - C u )[ 7 , 1 3 - 1 7 ] . O n t h e o t h e r h a n d , c o m m e r c ia l a l lo y sof t he 2000 se r i e s (A1-C u) , hav i ng been i n t en-s i ve l y s t ud i ed i n t he i r b i na ry ve rs i ons [1-6] , havenot r e ce i ved so much a t t en t i on [7 , 8 ] . I t i s t hep u r p o s e o f t h e p r e s e n t p a p e r t o s t u d y , b y m e a n sof d i f fe ren t i a l s canni ng ca l or i me t ry (DSC ) andha rdness and c ond uc t i v i t y measu remen t s , t hemi c ros t ruc t ure o f t he ex t rus i on a l l oy AA-2011in di ffe rent meta l lurgica l s ta tes ; thi s a l loy hasthe spec ia l fea ture of conta ining smal l quant i t i esof low m e l t ing po i n t e l ement s ( le ad and b i sm ut h)t o i mp rove i ts machi nab i l i ty .

P rec i p i t a t i on i n A 1-C u a l loys ha s been i n t en-s i ve l y s t ud i ed [18-24] and a l t hough some prob-l ems re l a ted t o t he s t ruc t ure o f t he d i f fe ren tphases s t il l remain to be solved [21] , the sequen cemost wide ly accepted i s as fol lows. Sol id solut ion

GP(I ) zones -~ 0" phase (or C P( I I ) zones ) -+ 0 'phase -+ 0 phase . Nowadays [22] a s t he denomi -n a t i o n 0 " p h a se is p r e f e r re d o v e r G P ( I I ) z o n e s , w eshal l use G P zones fo r GP ( I ) zones and 0" phase .

T h e f o r m a t i o n o f t h e d i f f e r e n t p h as es m o d i f yt he p rope r t i e s o f t he a l l oy re l evan t fo r t he pre sen twork , t h i s i s , ha rdness and conduc t i v i t y , i n t hefo l l owi ng way . Wherea s t he 0 phase , i ncohe ren twi t h t he a ma t r i x , does no t ha rden t he a l l oy , t heo t he r t h ree phase s do , t he mo s t e f fec t i ve be i ngth e 0" phase [1 , 19] . The form a t i on o f GP zonesdec rea se s t he e l ec tr i c a l cond uc t i v i t y (0 ) o f t hea l l oy ( re fe r red t o t he so l i d so l u t i on) [19 , 24 , 25] ,whi l e t he fo rma t i on o f 0 and 0 ' phases le ads t ol a rge i nc rea se s o f o . Ve ry few da t a conce rn i nghow t he prec i p i t a t i on of t he 0" phase a f fec t s aare avai lable in the l i terature. Hirano and Iwasaki( s e e F i g . 7 o f [ 1 ] ) f o u n d f o r a n A 1 - 4 w t % C ua l l oy t ha t age i ng a t t empe ra t ure s l ower t han20 0~ leads to very small increases of a dur ing

0 0 2 2 - 2 4 6 1 / 8 4 $ 0 3 .0 0 + .1 2 9 1984 Chapman and Hall Ltd. 2 7 9

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t h e f ir s t f e w h o u r s ( p e r i o d o f t i m e d u r i n g w h i c hthe 0" pha se i s p r e f e r e n t i a l l y f o r m e d) . T h i s r e su l ti nd i c a t e s t ha t a s f a r a s c onduc t iv i t y c ha nge s a r ec on c e r ne d t he 0 " pha se i s m o r e s im i l a r t o GPz one s t ha n t o t he o the r two pha se s ; t h i s i s c on-s i s t e n t w i th t he s im i l a r i t i e s be twe e n the s t r uc tu r e so f b o t h p h a s e s f o u n d b y d i f f e r e n t a u t h o r s [ 2 1 ] .Al l these da ta wi l l be highly useful in discuss ingt h e r e s u l ts o f t h e p r e s e n t s t u d y .

T he a l l oy AA- 2011 i s supp l i e d c om m e r c i a l l ym a i n l y i n t w o t e m p e r s , i . e . T 3 a n d T 6 . A l t h o u g hs o m e c o m m e n t s w il l b e m a d e o n th e m i c r o s t r u c -tu r e o f b o th t e m p e r s , i n th i s wor k we w i ll bem a i n l y c o n c e r n e d w i t h m e t a l l u r g i c a l s t a t e so b t a i n e d t h r o u g h l a b o r a t o r y h e at t r e a t m e n t sc h o s e n i n s u c h a w a y t h a t t h e a l l o y c o n t a i n e dpr e f e r e n t i a l l y one o f t he f our poss ib l e pha se s inthe se a l l oys .2 . E xper iment a l p r ocedur esT h e m a t e r i a l u s e d i n t h i s w o r k h a s b e e n s u p p l i e db y E x t r u s i o n P r o d u c t s o f E N D A S A ( E m p r e s aN a c i o n a l d e l A l u m i n i o , S A ) i n r o d s o f 3 2 m mdia m e te r . T he a l l oy ha d be e n d i r e c t c h i l l c a s t i nb i ll e ts o f 2 0 3 m m d i a m e t e r , p r e h e a t e d a t 4 8 0 ~a n d e x t r u d e d t o t h e f in a l d i a m e t e r. It s c o m p o s i t i o ni s g ive n i n T a b le I . Sa m ple s o f two d i f f e r e n t s i z e sw e r e p r e p a r e d : ( i ) 3 2 r a m d i a m e t e r x 1 0 m mt h ic k n e s s s p e c i m e n s f o r c o n d u c t i v i t y a n d V i c k e rsh a r d n e s s m e a s u r e m e n t s a n d ( i i ) 6 m m d i a m e t e rx 1 m m t h i ck n e s s s a m p l e s f o r D S C . T h e l a t te rs a m p l e s w e r e o b t a i n e d b y p u n c h i n g 1 m m t h i c ks h e e t, r o ll e d f r o m d is cs 3 2 m m d i a m e t e r x 4 m mt h i ck n e s s ; b o t h o p e r a t i o n s w e r e p e r f o r m e d p r io rto so lu t i on h e a t t r e a tm e nt s . DSC sa m ple s c ou ldn o t b e o b t a i n e d b y m a c h i n i n g c o n d u c t i v i t ysa m ple s [ 1 ] j us t a f t e r a ge ing , a s t h i s ope r a t i oni n t r o d u c e s d e f o r m a t i o n s t h a t m i g h t a f f e c t D S Cc ur ve s [ 26] . On the o the r ha nd que nc h ing r a t ee f f e c t s a r e no t so s t r ong a s t o c a use d i f f e r e nc e sbe tw e e n the two sa m ple s use d in th i s wo r k . A llsa m ple s we r e so lu t i on he a t t r e a t e d a t 525 ~ C f o r5 h a n d w a t e r q u e n c h e d ( t o r o o m t e m p e r a t u r e )p r i o r t o a n y a g e in g h e a t - t re a t m e n t .

H e a t - t r e a t m e n t s a t t e m p e r a t u r e s l o w e r t h a n2 0 0 ~ w e r e c a r ri e d o u t in t h e r m o s t a t i c a l ly c o n -t r o l l e d o i l o r wa te r ba ths , whe r e a s f o r h igh t e m -pe r a tu r e s a n a i r f u r na c e wa s use d , the t e m pe r a tu r e

c on t r o l be ing a c c ur a t e t o -+ 1 a nd + 5 ~ C , r e spe c t -i v e l y . I f m e a s u r e m e n t s c o u l d n o t b e m a d e i m m e d i -a t e ly a f t e r h e a t t r e a t m e n t s , t h e s a m p l e s w e r e k e p tw h e n n e e d e d ( s o l u t io n h e a t t r e a t e d s a m p l e s ) , i nl i qu id n i t r oge n .

C o n d u c t i v it y w a s m e a s u re d b y m e a n s o f aS i g m a t e s t - T i n s t r u m e n t t y p e 2 . 0 6 7 a n d t h e l o a duse d t o m e a su r e t he V ic ke r s ha r dn e ss wa s 1 kg .B o t h p r o p e r t i e s w e re a l w a y s m e a s u r e d a t r o o mt e m p e r a t u r e .

T h e D S C m e a s u r e m e n t s w e re p e r f o r m e d u s in g aP e r k in E l m e r D S C - 2 C a p p a r a t u s c o n t r o l le d t h r o u g ha m in i - c om pute r . T o i nc r e a se t he se ns i t i v i t y o f t hem e a s u r e m e n t s h i g h p u r i t y a l u m i n i u m w a s u s e d a sthe r e f e r e nc e . T he r uns we r e c a r r i e d ou t a t he a t i ngr a t e s be tw e e n 2 .5 a nd 200 ~ Cm in - 1, f r om 25 to5 8 0 ~ C . A l l e x p e r i m e n t s w e r e p e r f o r m e d u n d e rd y n a m i c n i t ro g e n a t m o s p h e r e (1 l h - 1 ) . T h e i n t er -p r e t a t i o n o f t h e D S C c u r v e s w as a i d e d b y m e a s u r -i ng c o n d u c t i v i t y a n d V i c k e r s h a r d n e s s o n s p e c i m e n stha t h a d be e n l i ne a r ly he a t e d i n a sa nd fl u id i z e df ur na c e a t a he a t i ng r at e o f 5 ~ -1 up t od i f f e r e n t t e m p e r a tu r e s ( a n e r r o r o f -+ 2~ wa se s t i m a t e d ) a n d t h e n q u e n c h e d i n w a t e r a t r o o mt e m p e r a t u r e . B o t h t h e D S C m e a s u r e m e n t s a n dthe l i ne a r he a t ings t o m e a sur e o a nd VH we r ep e r f o r m e d a t l e a st t w i c e; g o o d r e p r o d u c i b i l i tyw a s f o u n d .

T he m e ta l lm gic a l s t a t e s s t ud i e d i n t h i s wor kwe r e c hose n f r om the i so the r m a l a ge ing s tud i e sde sc r ibe d i n t he ne x t s e c t i on , a nd t he r e su l t s o fo t h e r a u t h o r s [ 1 , 2 , 1 9 ].3. Resul ts and d iscussionIn this sec t ion we discuss f i r s t the i sothermala ge ing o f a l l oy AA- 2011 a t d i f f e r e n t t e m pe r a tu r e s .T he r e su l t s ob t a ine d t h r ough those s tud i e s he lpe dto c h oose t he m e ta l l u r g i c a l s ta t e s t o be s t ud i e db y c a l o r im e t r i c t e c h n i q u e s .3.1. Isothermal ageingT h e h o m o g e n i z e d a n d q u e n c h e d s a m p l e s w e r ea g ed a t t e m p e r a t u r e s in t h e r an g e 2 5 - 4 2 0 ~ C . T h ea ge ing wa s f o l l owe d by m e a sur ing c onduc t iv i t y( o ) a nd V ic ke r s ha r dne ss ( VH) . T he r e su l t s f o r t i l et e m p e r a tu r e s . c hose n f o r t he DSC s tud i e s a r eshow n in Figs . 1 and 2.

Ageing a t 25 ~ C leads to small chang es in b othT A B L E I Com pos iti on of the AA-2011 alloy used in this work (in weight per cent )Fe Cu Si Mn Ti Zn Cr Pb Bi0.31 5.41 0.12 < 0.02 < 0.02 < 0.04 < 0.04 0.35 0.44280

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3 I t

29

IEE0 2 5

Io 2 5 ~o 100 ~9 1 7 0 ~x 2 5 0 ~9 4 0 0 ~

J

- - _ . ~ A A

0 n qooo 0 H E ] O u " ' l ~ t

Figure i Change of e lectr ical conduct-ivity of alloy AA-2011 during ageing atdifferent temperatures. The alloy hadbeen homogenized at 52 5~ for 5 h andquenched in water at room temperature.Values for as-quenched (As Q) samplesare also given.

21G / I tA s Quenched 103 10a 105T IM E ( s e c )

u a n d V H [ 1 8 ] ; t h e d e c r e a s e in u c l e a r l y i n d i c a t e st h a t G P z o n e s a r e b e i n g f o r m e d [ 1 9 , 2 4 , 2 5 ] .The sm a l l i nc r e a se i n V H a t 25 ~ C doe s n o t a g r e ew i t h r e s u lt s r e p o r t e d b y V i g ie r e t a l . [ 6 ] f o r a nA 1 - 4 w t % C u a l lo y ; t h o s e a u t h o r s f o u n d t h a t V Hi n c r e a s e d n e a r l y a s m u c h a s f o r a g e in g a t 1 0 0 ~( s e e b e l o w ) . W e d o n o t u n d e r s t a n d t h i s d i s a g r e e -m e n t , b u t i t s h o u l d n o t b e a s c r i b e d t o t h e p r e s e n c eo f l ea d a n d b i s m u t h , b u t r a t h e r t o th e m i c r o -s t r u c t u r e o f t h e e x t r u d e d r o d . A t 1 0 0 ~ C w e n o t ea s t r o n g i n c r e as e o f V I I w h e r e a s a r e m a i n s n e a r l yc o n s t a n t , a l t h o u g h a v e r y s l ig h t d e c r e a se ( n o ta c c u r a t el y m e a s u r e d w i t h t h e t e c h n i q u e s u se di n t h is w o r k ) c a n b e n o t i c e d . A g a i n G P z o n e sa r e be ing f o r m e d , a nd the i r l a r ge r s i z e a nd thef a s te r k i n e t ic s , w i t h r e s p e c t t o t h o s e f o r m e d a t2 5 ~ C , c o u l d e x p l a i n t h e d i f f e r e n t b e h a v i o u r o fe [ 2 5 ] a n d V H . T h e s e r e s u l t s a r e i n a g r e e m e n tw i t h t h o s e o f o t h e r a u t h o r s , a s i t is g e n e r a l lyb e l i ev e d t h a t o n l y G P z o n e s a r e f o r m e d b e l o w1 3 0 ~ c [ ] - 8 ] .

A t 1 7 0 ~ t h e V i c k e r s h a r d n e s s i n c r ea s e ss t r o n g l y a n d f o r a g ei n g t i m e s b e y o n d 1 0 S s ec i ts l o w l y d e c r e a s e s . O n t h e o t h e r h a n d u s l i g h t l y

inc r e a se s f o r a ge ing t im e s sho r t e r t ha n 10S se ca n d m o r e r a p i d l y t h e r e a f t e r . T h e s e r e s u l t s i n d i c a t et h a t b e y o n d 1 0 5 s e c a p h a s e n o t v e r y e f f e c ti v e i nh a r d e n i n g t h e a l l o y a n d p r o m o t i n g l a r g e i n c r ea s e si n u i s b e i n g f o r m e d ; t h i s s h o u l d b e t h e 0 ' p h a s e[ 8 ] . F o r sho r t e r a ge ing t im e s a s u s l i gh t ly i nc r e a se s ,t h e 0 " p h a s e m i g h t b e p r e c ip i t a t i n g , a l t h o u g h w ec a n n o t c o n c l u d e w h e t h e r o r n o t f o r v e r y s h o r tt i m e s t h e G P z o n e s a r e b e i n g f o r m e d ( se e r e s u lt sf o r a ge ing a t 190~ in [ 8 ] ) .

A g e i n g a t 2 5 0 ~ i n d i c a t e s t h a t f o r t i m e ss h o r t e r t h a n 1 0 4 s e c t h e 0 ' p h a s e is f o r m e d a n df o r l o n g e r t i m e s t h e i n c o h e r e n t 0 p h a s e s t a r t s t op r e c ip i t a t e ( s e e F ig . 2 ) . Th e a ge ing a t 400 ~ C le a dst o t h e f o r m a t i o n o f t h e 0 p h a s e as t h e d e c r ea s e o fV H a n d i n c re a s e o f u cl e a r ly i n d i c a t e ( V H is h i g h e ri n t h e s o l u t i o n t r e a t e d s t a t e t h a n i n t h e a n n e a l e ds t a t e ) .

T o s u m m a r i z e t h e s e re s u l ts w e q u o t e t e m p e r a -t u r e s a n d t i m e s u s e d t o o b t a i n m e t a l l u r g i c a l s t a te sh a v i n g p r e d o m i n a n t l y o n e p r e c i p i t a t e p h a s e . F o rG P z o n e s b o t h 2 5 a n d 1 0 0 ~ C te m p e r a t u r e s w e r ec h o s e n , t h e a g e i n g t i m e s b e i n g 7 d a y s a n d 2 4 h ,r e s p e c t i v e l y . S a m p l e s c o n t a i n i n g t h e 0 " o r 0 ' p h a s e

281

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L

140~-o 25 ~[] 100 ~9 170 ~x 250 ~Z,00 ~

Figure 2 S a m e a s F ig . 1 f o r V i c k e r s h a r d -n e s s ( V H ) . T h e v a r i a t i o n i n V H v a l u e sf o r a s - q u e n c h e d s a m p l e s ( < 4 k g m m -2 )i s w i t h i n t h e e r r o r u s u a l l y f o u n d i nc o m m e r c i a l a l l o y s.

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6C9 A 9 . , j A i9 - - - - 9

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4 0 L ~ , E I IA s Que nched 103 104 105TIME (sec)

w e r e o b t a i n e d b y ag e in g 2 4 h a t 1 7 0 ~ o r 5 h a t2 5 0 ~ C . T h e f u l l y a n n e a l e d s t a t e d ( o n l y 0 p h a s e )w a s a c h i e v e d b y a g e i ng d u r i n g 6 h a t 4 0 0 ~ C a n ds l o w l y c o o li n g t o r o o m t e m p e r a t u r e .3 . 2 . C a l o r i m e t r i c s t u d i e s3 . 2 . 1 . F u l l y a n n e a l e d s t a t e , a g e i n g a t

4 0 0 o C ( F ig . 3 )W e n o t e d a s i n g l e e n d o t h e r m i c r e a c t i o n m t h eD S C c u r ve . Th i s r e a c t ion i s c l e a r ly r e l a t e d t o t hed i s s o l u t i o n o f t h e o n l y p h a s e p r e s e n t i n t h is s t a te( s ee a b o v e a n d [ 1 - 8 ] ) . T h i s i n t e r p r e t a t i o n iss u p p o r t e d b y t h e c o n d u c t i v i t y a n d V i c k er s h a r d -ne s s r e su l t s sh ow n in F ig . 3b . I n t h i s f i gu r e w en o t e t h a t o s ta r t s t o d e cr e a s e a n d V H t o i n c r e a ser o u g h l y a t t h e s a m e t e m p e r a t u r e t h a t t h e o n s e to f t h e D S C r e a c t i o n o c c u r s . F i n a l l y w e n o t i c e aw e a k e n d o t h e r m i c p e ak a t a r o u n d 1 2 7 ~ t h a t isr e l a t e d , as d i sc u s s e d b e l o w , t o P b - B i p a r ti c le s .3 . 2 .2 . A g e i n g a t 2 5 0 ~ C ( F i g . 4 )I n t h i s c a s e t h e D S C c u r v e s s h o w t w o e n d o t h e r m i cr e a c t i o n s o v e r l a p p i n g w i t h a n e x o t h e r m i c r e a c t i o n ;t h e s e r e a c t i o n s s h o u l d b e r e l a t e d w i t h 0 ' a n d 028 2

d i s s o l u t i o n a n d 0 p h a s e f o r m a t i o n , r e s p e c t i v e l y .A g a i n t h i s i n t e r p r e t a t i o n i s c o n f i r m e d b y t h e V Ha n d a r e s u lt s ( F ig . 4 b ) . R o u g h l y c o i n c i d i n g w i t ht h e o n s e t o f th e f i rs t e n d o t h e r m i c r e a c t i o n b o t hV H a n d a d e c r e a s e i n d i c a t in g t h a t t h e 0 ' p h a s ei s b e i n g d i s s o lv e d . B e t w e e n 3 5 0 a n d 4 0 0 ~ C b o t hV H a n d a r e m a i n n e a r l y c o n s t a n t . I n t h i s r e g i o nt h e d i s s o l u ti o n o f th e 0 ' p h a s e i s b a l a n c e d b y t h ef o r m a t i o n o f t h e 0 p h a s e . T h i s i s n o t i c e d i n t h eD S C c u r v e a s t h e r e g i o n w h e r e t h e e x o t h e r m i cr e a c ti o n p r e d o m i n a t e s . F o r h i g h e r t e m p e r a t u r e so d e c r e a s e s m o n o t o n i c a l l y d o w n t o t h e v a l u ec o r r e s p o n d i n g t o t h e s o l u t i o n t r e a t e d s t a t e ,w h e r ea s V H f ir st d e c r e as e s a n d b e y o n d 4 5 0 ~i t s t a r t s t o i nc r e a se i nd i c a t i ng t ha t t he d i s so lu t iono f t h e 0 p h a s e p r e d o m i n a t e s ( s e e t h e D S C c u r v e ).3 . 2 .3 . A g e i n g a t 1 7 0 , 1 0 0 a n d 2 5 ~ C(F ig s . 5 t o 8 )I n t h i s s u b s e c t i o n w e f a c e t h e p r o b l e m o f d is -t i n g u i s h i n g b e t w e e n G P z o n e s a n d t h e 0" p h a s e .

W e d i sc us s f ir s t t he ca se o f 170~ ( F ig . 5 ) . I nt h i s c as e , i n a d d i t i o n t o t h e r e a c t i o n s f o u n d i ns a m p l e s a g e d a t 2 5 0 ~ C w e n o t e t w o m o r e p e a k s ,

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5/12

-0.04

0.00&--bo 0 . 0 4 -%

i32!30

rIEE 28&E 26b

I I I I I

A n n e a l e dxU d

ItmL~

24 35I I J t I0 1 0 0 2 0 0 3 0 0 4 0 0 5 0 0 6 0 0

T e m p e r a t u r e ( ~Figure 3 (a) DSC curve (heating rate 5~ and(b) change of conductivity (a) and Vickers hardness (VH)during linear heating at 5~ -1, for alloy AA-2011annealed at 400 ~ C for 6 h and slowly cooled.

one endothermic reaction whose maxim um occursat 228 ~ C (see Table II for va riatio n of this maxi-mum with heating rate) and an exothermicreaction with its max imu m lying at 31 0~ (seeTable Ill). In Fig. 5b we show the variation ofVickers hardness and condu ctivity. VH starts todecrease when the reversion of the 0" phase starts(first endothermic reaction); it goes down to aweak minimum, increases slightly and finallydecreases to the solid solu tion values. As the weakminimum is above the solid solution value, it canbe concluded that not all 0" phase particles aredissolved. This suggests tha t the 0' phase can eitherbe formed through trans forma tion of the 0" phase

TABLE II DSC peak temperature (~ for the firstendothermic reaction of samples aged at 100 and 170~for different heating rates

-O.OZ,

O.OCio 0.04%

0.08

0.12

' EE

TE

I I L L I

5 h a t 2 5 0 ~ 2

- ( a )33~-3129272523

oW

0r ~Ziii

I I I i I 1 0 0E

-A 9 9 9 9 0I I, A ' a l l . A l l , ~ ~ E

70 >z

60(b)I I I i I1 0 0 2 0 0 3 0 0 4 0 0 , 5 0 0 6 0 0

Temperature (~Figure 4 Same as Fig. 3 for samples aged at 250 ~C for 5 h.

or di rectly fro m th e solid solu tion; whereas thefirst process would decrease VH, the secondincreases it, and this competition might be thereason for the weak relative maximum in VH.Nonetheless, as the max imu m occurs 20 ~ C belowthe maximum of the exothermic reaction, it mightalso be concluded that the coherency of the 0'phase with the a matrix, decreases as the tempera-ture increases. The formation of the 0' phaseduring the exothermic reaction is indicated by thefact that the latter part of the DSC curve is verysimilar to that corresponding to samples aged at250 ~ C. o remains nearly con sta nt up to 300 ~ C,when the 0' phase starts to be formed thenincreases steadily, and when the exothermicreactio n ends, it decreases indicatin g the dissol-ution of the 0' phase. In this temperature region

TA BL E I II Same as Table II for the mainexothermicpeak (0 ' phase precipitation)Peak temperature Heating rate (~ C min -1) Peak temperature Heating rate (~ C min -t)(~ C) (~ C)5 20 40 100 200 2.5 5 20 40100 170 186 196 205 222 100 323 336 372 388170 228 243 253 262 270 170 303 319 368 379

2 8 3

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r I B I i 0- 0 . 1 2 ( a ) 2 4 h a t 1 7 0 ~ xLtJ

f ~ 1 7 6

!o.o,) V ' q' , _ : o

2 4 - 5 0

2 3 0 1 ;0 2 ; 0 3 ; 0 4 ; 0 5 ; 0 6 0 0T e m p e r a t u r e ( ~

Figure 5 Sam e as Fig. 3 for samples aged at170 ~ for 24h.

t h e l a t t e r p r o c e s s c o m p e t e s w i t h t h e f o r m a t i o no f t h e 0 p h a s e a n d t h e r e f o r e o v ar ie s v e r y s l o w l y ;i t s t a r t s t o de c r e a se ve r y r a p id ly a t t he onse t o ft h e l a s t e n d o t h e r m i c r e a c t i o n . T h e f a c t t h a t b e l o w3 0 0 ~ C a r e m a i n s c o n s t a n t a n d d o e s n o t g o d o w nt o t h e a s - q u e n c h e d v a l u e m i g h t a l s o s u p p o r t t h ea s s u m p t i o n t h a t n o t a l l 0 " p h a s e p a r t i c l e s a r ed i s so l v e d ; n o n e t h e l e s s t h e n o t v e r y h ig h a c c u r a c yo f o u r a m e a s u r e m e n t s q u e s t i o n s t h i s c o n c l u s i o n .A l l t h e s e r e s u l ts i n d i c a t e t h a t t h e f i rs t e n d o t h e r m i cpe a k i s r e l a t e d t o r e ve r s ion o f t he 0 " pha sew h e r e a s t h e e x o t h e r m i c p e a k c o r r e s p o n d s t o t h ef o r m a t i o n o f t h e 0 ' p h a s e.

W e tu r n n o w t o c o n s i d e r t h e c as e o f 1 0 0 ~( F ig . 6 ) . Th e D S C c u r ve i s s im i la r t o t he onep r e v i o u s l y c o n s i d e r e d b u t t h e f i r s t e n d o t h e r m i cp e a k i s s h i f t e d 5 8 ~ d o w n w a r d s ( T a b l e I I )w h e r e a s t h e e x o t h e r m i c p e a k s h i ft s 1 7 ~ u p w a r d s( Ta b le l I I ) . The V H s t a r t s t o de c r e a se a t t he onse to f t h e f ir st e n d o t h e r m i c r e a c t i o n a n d i t a t ta i n sthe so l id so lu t ion va lue be f o r e i t i nc r e a se s a ga in .T h i s i n d i c a t e s t h a t m o s t G P z o n e s a re d i ss o l v edb e f o r e t h e 0 ' p h a s e s t a rt s t o b e f o r m e d , ( a b o v e2 8 4

3 0 0 ~ C ) . A t t h e e n d o f t h e f i rs t e n d o t h e r m i cr e a c t ion a ha s i nc r e a se d a bo ve it s so l i d so lu t io nva lue ; t h i s sugge s t s t ha t a t t he e nd o f t h i s pe a k ,s o m e o t h e r r e a c t i o n s m i g h t b e s u p e r i m p o s e d o nt o t h e d i s s o lu t io n o f G P z o n e s . A b o v e 3 0 0 ~t h e b e h a v i o u r o f V H a n d a i s r a t h e r si m i la r t o t h ep r e v ious c a se ; s l i gh t d i f f e r e nc e s m igh t be e xp la ine db y t h e d i f f e r e n t b a l a n c e b e t w e e n t h e d i s s o l u t i o no f t h e 0 ' a n d 0 p h a s e s a n d t h e f o r m a t i o n o f t h el a t t e r .

A l t h o u g h t h e V H r e s u l t s a l r e a d y d i f f e r e n t i a t eb e t w e e n t h e G P z o n e s a n d t h e 0 " p h a s e , w e h av em a d e t w o f u r t h e r s t u d i e s . F i r s t w e h a v e o b t a i n e da D S C c u r v e o f b o t h s a m p l e s ( a g e d a t 1 0 0 a n d1 7 0 ~ C ) a t a l o w h e a t i n g r a t e ( 2 . 5 ~ C m i n - 1 ) . T h eo u t s t a n d i n g d i f f e re n c e s b e t w e e n t h e m a r e r e a d i lyn o t i c e d ( F i g . 7 ) . W h e r e a s i n t h e 1 7 0 ~ c a s e ,b e l o w 3 0 0 ~ t h e r e is a s in g le e n d o t h e r m i cr e a c t i o n ( a s m a ll s h o u l d e r m i g h t b e i n d i ca t i n g t h a ts o m e g r o w t h o f t h e 0 " p h a s e t a k e s p l a c e ) , i n t h e1 0 0 ~ c a s e t h e r e a re t w o r e a c t i o n s in b e t w e e nt h e d i s s o l u t io n o f t h e G P z o n e s a n d t h e f o r m a t i o no f t h e 0 ' p h a s e . A s f i rs t p o i n t e d o u t b y Z a h r a et a l .

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-0.12

-0.08I

o - 0 . 0 4T0,0C0.04

27

2 6TEET 25Eb 24

230

I I i I I

24h at 100 ~

( a )I I I I I

I I I I I100 200 300 400 500Tempera tu re (~

oxU.l

T

lZW

105 _T

95 EE8 5 - -I>75

65600

Figure 6 Same as Fig. 3 for samples aged at100 ~ C fo r 24 h .

[3], for low heating rates, the 0" phase might beformed and dissolved during the linear heating,just after GP zones dissolution and before theexothermic peak. As a second study we haveobtained the activation energy of both dissolutionpeaks (GP zones and the 0" phases). To this endwe have used the so-called "peak method" firstintroduced by Kissinger [27] and Henderson [28].This method is based upon the assumption thatthe peak of the DSC curve coincides with themaximum reaction rate; although this assumptionis not valid for differential thermal analysis [29]it holds for DSC [30]. By assuming a Johnson-Mehl-Avrami equation for the reaction rate [28]the following equation is obtained at the peak ofthe DSC curve [27, 28].

h E ~,RIn T ~ - R r p + In E (1)

where Tp is the temperature at the peak, h theheating rate, R the gas constant and E and u thekinetic parameters in the Arrhenius equation, that

is the activation energy and frequency factor,respectively. Equation 1 holds no matter thevalue of the reaction order n. By varying theheating rate, Equation 1 can be used to obtainE and v. For obtaining the reaction order n afitting of the full DSC curve should be made [7].In the calculation of E and v we have used thepeak temperatures given it Table lI. The peaktemperatures at 2.5 ~ Cmin -1 have not beenincluded, as the DSC curve at this heating rate ismarkedly different from those at higher heatingrates. For a similar reason the peak temperaturesfor the format ion of the 0' phase at 200 ~ C and100~ -1 have not been quoted in Table ill,as at high heating rates the 0 phase is formedinstead of the 0' phase. The activat ion energiesobtained for ageing at 100 and 170~ are 1.27and 1.89 eV molecule -1, respectively. In comparingthe present values for E and u with those reportedin the literature it should be noted that E and udepend upon the equation chosen for the reactionrate [7]. Papazian [7] by using the same equation

285

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60 0Tem perature (~

-0.08

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2726

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[ I I t- ( a )

i

I I I I l(b )

)

o

i I I I I0 100 200 300 /*00 500 600Tem perature (~

oLU

g! 9 58 5 ~/EE756 5 > z55

Figure 7 DSC curves of samples aged at 100 ~ (cont inu-ous l ine ) and 170 (broken l ine ) for 24 h; hea t ing ra te of2.5~ Cm in -1Figure 8 Same as Fig. 3 for samples aged at 25~ for7 days .

f o r t h e r e a c t i o n r a t e a s h e r e o b t a i n e d 0 . 8 3 e Vm o l e c u l e -~ f o r d i s s o l u t i o n o f G P z o n e f o r m e d a tr o o m t e m p e r a t u r e ( T 3 1 t e m p e r ) ; o t h e r v a l u es l iei n t h e r a n g e 1 . 0 - 1 . 3 e V m o l e c u l e - t [ 1 8 ] . A l t h o u g hin th e m o d e l h er e w i th a s s u m e d (a J o h n s o n - M e h l -A v r a m i e q u a t i o n f o r t h e r e a c t i o n ra t e ) t h e a ct i -v a t i o n e n e r g y m i g h t i n c r e a s e a s t h e s i ze o f t h e G Pz o n e i n c r e a s e s ( l a r g e z o n e s f o r h i g h e r a g e i n g t e m -p e r a t u r e , s ee [ 7 ] ) , t h e la r g e E o b t a i n e d h e r e f o ra g ei n g a t 1 7 0 ~ ( n o t e t h a t 1 . 8 9 e V m o l e c u l e - ] isw e l l a b o v e t h e u p p e r e n d o f th e r a n ge r e p o r t e di n t h e l i t e r a t u r e ) s u g g e s t s t h a t a d i f f e r e n t p h a s ei s f o r m e d a t t h is t e m p e r a t u r e . N o n e t h e l e s s a m o r ed e t a i l e d s t u d y , o f t h e k i n e t i c s o f d i s s o l u t i o n o f G Pz o n e o r 0 " p h a s e f o r m e d a t d i f f e r e n t a g e i ng t e m -p e r a t u r e s i s r e q u i r e d b e f o r e r e a c h i n g a d e f i n i t i v ec o n c l u s i o n .

B e f o r e e n d i n g t h e d i s c u s s i o n o f a g e i n g a t t h e s et w o t e m p e r a t u r e s w e r e m a r k o n t w o f u r t h e rp o i n t s , ( i ) th e f o r m a t i o n o f t h e 0 ' p h a s e d e p e n d so n t h e h i s t o r y o f t h e s a m p l e , t h a t i s, i t i s n o tt h e s a m e f o r t h e t w o a g e in g t e m p e r a t u r e s ( s e eT a b l e 1 1) a n d , ( i i ) t h e t e m p e r a t u r e o f t h e m a x i -

m u m o f t h e 0 , p h a s e d i s s o l u t i o n p e a k i s r o u g h l yt h e s a m e f o r t h e t w o a g e i n g t e m p e r a t u r e s a n dd e p e n d s o n h e a t i n g r a t e ( T a b l e I V ) t o a l e s s e re x t e n t t h a t t h o s e c o r r e s p o n d i n g t o o t h e r r e a c t i o n s( T a b l e s I I a n d I I I ) , r e m a i n i n g c o n s t a n t f o r h e a t i n gr a t e s u p t o 4 0 ~ C m i n - t .

F i n a l l y w e d i s c u s s b r i e f l y t h e c a s e o f a g e i n g a t2 5 ~ ( F i g . 8 ) . T h e o n l y n o t i c e a b l e d i f f e r e n c e sw i t h t h e r e s u l t s f o r a g e i n g a t 1 0 0 a n d 1 7 0 ~ C o c c u rb e l o w 3 0 0 ~ C . F o r i n s t a n c e w e n o t i c e t h a t t h e G Pz o n e d i s s o l u t i o n p e a k i s n o t a s s i m p l e a s t h a t o fa g e i n g a t 1 0 0 ~ C . F i r s t i t s t a r t s a t l o w e r t e m p e r a -t u r e s a n d s e c o n d i t s h o w s a s t ro n g s h o u l d e r j u s tb e l o w 2 0 0 ~ T h e e a r l i e r s t a r t o f t h e G P z o n ed i s s o l u t i o n r e a c t i o n i n d i c a t e s t h e l o w e r s t a b i l i ty( s m a l l e r s i ze ) o f t h e G P z o n e f o r m e d a t 2 5 ~ C w i t h

T A B L E I V S a m e as T a b l e I I f o r d i s s o l u t i o n o1" t i le 0p h a s eP e ak t e m p e r a tu r e H e a t i n g r a t e ( ~ m i n - I )(~ 2.5 5 20 40 100 200100 505 505 504 512 523 528170 505 504 504 511 519 523

2 8 6

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- 0 . 0 8 ~ -

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k ,oY-6o

O.OZ, -

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heati ng rate 2.5 ~ min 1

I I100 200 i L L300 z,O0 500T E M P E R A T U R E (~

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olaJ

osZLIJ

i - - ~ ' I i I, 7 days at 25'C7 days at 25*C and punched3% ptus 7days at 25~ ~371ocand punchr / ' ~

' - " / \ \ _ s \ - i"-" \ 9" - . / " , / \ / !

r "-" " - - . _ / \ 1 Ih e a t i n g r a t e 2 0 "C m in - 1

I i . . . . . . . . . . ~ . . . . . . I . . . . I100 2 00 30 0 z.,O0 50 0TEMPERATURE ("C)

Figure 10 DSC curves forsamples s tretched 3% andn a tu ra l ly ag ed a t 2 5 ~ fo r7 d ay s (T3 h ea t t r ea tmen t )an d p u n ch ed ( co n t in u o u s l in e ) ,punched and unstretchedsamples (chain line) and sampleswi th n o p las t ic d e fo rmat io n(unpunched an d unstretch ed) .Heating rate 20 ~ C min - 1.

t u re s f o r t he T 3 t e m p e r t h a n f o r s am p l e s w i t h o u tp l a s t i c d e f o r m a t i o n ( s e e F i g . 1 0 ) . T h e s e r e s u l t si n d i c a te t h a t ( a ) G P z o n e s f o r m e d a f t e r p l a s ti cd e f o r m a t i o n a r e la r g e r a n d t h e i r s i ze d i s t r i b u t i o ni s m o r e u n i f o r m ; t h e e f f e c t o f c o l d w o r k is , a ss u g g e s t e d e l s e w h e r e [ 8 , 2 0 ] , t o f a v o u r t h e f r e e i n go f v a c a n c i e s f r o m s i n k s , a n d t h i s m e c h a n i s m m i g h ta c c e l e ra t e G P z o n e s f o r m a t i o n , a n d ( b ) c o l d w o r ka c c e l e r a t e s 0 ' f o r m a t i o n , t h is i s r e a d i ly e x p l a i n e db y t h e f a c t t h a t d i s l o c a t i o n s m i g h t a c t a s n u c l e a t i o ns i t e s f o r 0 ' p a r t i c l e s [ 2 0 ] . W e a l s o n o t i c e t h a t a st h e 0 ' p h a s e f o r m a t i o n is a c c e l e r a t e d s o i s i tsd i s s o l u t i o n , a l t h o u g h f o r m a t i o n a n d d i s s o l u t i o no f t h e 0 p h a s e a r e n o t g r e a t l y a f f e c t e d b y d e f o r -m a t i o n . F i n a l l y i t i s w o r t h r e m a r k i n g t h a t t h ep u n c h i n g o p e r a t i o n a d d s d e f o r m a t i o n t o t h e 3%s t r e tc h i n g . T h e r e f o r e t h e a c t u a l s h i f t o f t h ee x o t h e r m i c p e a k d ue t o t he 3 % d e f o r m a t i o ns h o u l d b e t h e d i f f e r e n c e b e t w e e n t h e s h i ft c o r r e -s p o n d i n g t o p u n c h e d s a m p l e s ( 4 3 ~ a n d t h a t f o rs t r e t c h e d ( p u n c h e d a l s o ) s a m p l e s ( 9 0 ~ C ) , t h i st u r n s o u t t o b e a r o u n d 4 7 ~ C .

T h e T 6 c o m m e r c i a l t e m p e r i s o b t a i n e d b ya r t i f i c i a l l y a g e i n g t h e a l l o y a t t e m p e r a t u r e s i nt he r a ng e 1 7 0 - 1 9 0 ~ C . A s i t is c o m m o n t o ha v ea d e l ay t i m e a t r o o m t e m p e r a t u r e b e t w e e n t h es o l u t i o n h e a t t r e a t m e n t a n d t h e a r t if i c ia l a g e i n g ,w e h a v e s t u d i e d t h e e f f e c t o f t h is n a t u r a l a g e in g .T o t h is e n d w e h a v e m a d e c a l o r i m e t r i c s t u d ie s o fs a m p l e s k e p t a t 2 5 ~ d u r i n g 7 d a y s b e f o r e an288

a r t i f ic i a l a g e i n g a t 1 7 0 ~ C f o r 2 4 h , a n d c o m p a r e dt h e r e s u l t s W i th t h o s e o b t a i n e d w i t h n o n a t u ra la g e i n g ( F i g . 5 ) . N o d r a m a t i c d i f f e r e n c e s a r en o t i c e d b e t w e e n t h e D S C c u r v es fo r b o t h t y p e o fs a m p l e s . W e o n l y t a o t i c e t h a t , f o r s a m p l e s w i t hn a t u r a l a g e i n g , t h e 0 " d i s s o l u t i o n p e a k i s s h i f t e d5 ~ C u p w a r d s , i n d i c a t i n g a s l i g h t ly l a r g e r s i z e o ft h e 0 " p a r t i c l e s ~ a n d t h e O' f o r m a t i o n p e a k i ss h i f t e d 5 ~ d o w n w a r d s , t h a t i s, t h e f o r m a t i o r lo f t h e 0 ' p h a s e i s a c c e l e r a t e d .3 . 4 . D i s t r i b u t i o n o f P b - B i p a r t i c l e sI n a ll s a m p l e s a n a l y s e d i n t h i s w o r k ( o v e r 1 0 0 )a w e a k e n d o t h e r m i c p e a k a t a r o u n d 1 2 5 ~ h a sb e e n f o u n d . I n v e r y f e w s a m p le s s o m e o t h e re n d o t h e rm i c p e a k s b e t w e e n 1 2 5 - 3 0 0 ~ h a vea l s o b e e n n o t i c e d . T h i s r e s u l t i n d i c a t e s t h a t m o s tP b - B i p a r ti c le s a re a r o u n d t h e e u t e c t ic c o m p o -s i t io n ( 5 6 .3 w t % P b ) , a s t h e e u t e c t i c t e m p e r a t u r ei s 1 2 5 ~ [ 3 1 ] . T h i s i s n o t s u r p r i s i n g c o n s i d e r i n gt h e w a y i n w h i c h t h o s e t w o e l e m e n t s a re u s u a l l ya d d e d t o t h e a l lo y , n a m e l y , a m i x t u r e o f P b - B i i sp o u r e d o v e r t h e l i qu i d a l l o y w h i c h i s s u b s e q u e n t l ys t ir r e d ( s e e T a b l e I f o r c o m p o s i t i o n ) .3.5. Compar i son wi t h o ther worksI n c o m p a r i n g o u r r e s u l t s w i t h t h o s e o b t a i n e d b yo t h e r a u t h o r s [ 1 - 8 ] s o m e p o i n ts a re w o r t hn o t i c i n g .

( i) T h e f e a t u r e s o f t h e D S C c u r v e s o f a l l o y

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AA-2011 a re s imi la r t o t hose o f pure A1 -C ua l l oys [1 -6] . None t he l e s s a n ou t s t a nd i ng d i f fe r -e nc e shoul d be n o t e d , na m e l y , whe rea s i n purea l l oys [3 ] t he fo rma t i on of t he 0 ' pha se a nd t hel a st t h re e re a c t i ons (d i s so l u t ion o f t he 0 ' a nd 0p h a se s a n d f o r m a t i o n o f t h e 0 p h a s e) d o n o td e p e n d o n t h e t h e r m a l h i s t o r y o f t h e a l lo y ( ar et he sa me for s a mpl e s a ge d a t 100 or 170~ C), int h e p r e s e n t a l lo y t h e y d o .( i i ) P a p a z i a n [ 8 ] h a s f o u n d f o r t h e A A - 2 2 1 9a l l oy , t ha t t he 0 ' fo rma t i on pe a k wa s sh i f t e da r o u n d 5 0 ~ d o w n w a r d s w i t h r e s p ec t t o p u r eAI-C u a l l oys [1 ] . He a sc r i be d t h i s r e su l t t o t r a c ea l l oy i ng e l e me nt s . As we ha ve no t found a nynot i c e a b l e sh i f t o f t h i s pe a k , we se e k fo r d i f fe r -e nc e s be t we e n Pa pa z i a n ' s a l l oy a nd ours . TheAA-2019 a l l oy s t ud i e d by Pa pa z i a n c on t a i ne dma nga ne se a nd z i rc on i um whe re a s ours doe s no t .As z i rc on i um shoul d be fo rmi ng i nso l ub l e Z rA13part ic les , i t i s more l ike ly tha t manganese i s thee l e me nt c a us i ng t he a c c e l e ra t i on o f 0 ' fo rma t i oni n a l l oy AA-2019 [8] . The sh i f t found by Pa pa z i a nmi ght b e a l so due t o sa mpl e p re pa ra t i on , as de for -m a t i o n i n t r o d u c e d d u r in g p u n c h in g o f D S C s am p l esc o u l d s h if t d o w n w a r d s t h e e x o t h e r m i c p e a k [ 2 6 ].( i i i ) Our re su lt s c on c e rn i ng t he mi c ros t ruc t ura le f fe c t s o f p l a s t i c de forma t i on be t we e n so l u t i onhe a t t r e a t me nt a nd na t ura l a ge ing a re ra t he rs imi la r to those found by Papaz ian [8] . Due tot he h i ghe r de forma t i on g i ve n i n t h i s work (3% )wi t h re spe c t t o Pa pa z i a n (1% ) , t he t e mpe ra t u reo f m a x i m u m r e a c t i o n r a t e f o r t h e f o r m a t i o n o fthe 0 phase i s shi f ted, wi th respec t to sampleswi t h no de forma t i on , t o a l a rge r e x t e n t i n t hepre se n t c a se (47 ~ C ) t ha n i n Pa pa z i a n 's wo rk(23 ~ C); n ot ice a lso the low er hea t ing ra te usedby Pa pa z i an (10 ~ C mi n-1) .

( i v ) The f i nal po i n t c onc e rns t he d i s so l u t i onof t he 0 pha se . Pa pa z i an [7 ] has no t e d t ha t t h ispha se re ma i ne d i n t he rmodyna mi c e qu i l i b r i umwi t h t he ma t r i x i n t he t e mpe ra t u re ra nge 300 t o50 0~ for he a t i ng ra t e s up t o 10 ~ C m i n -~. Th iswa s c onc l ude d a s t he d i s so l u t i on o f t he 0 pha seoccurred a t the same tempera ture regardless ofhe a t i ng ra t e . In Ta b l e IV we quot e t he pe a k t e m-pe ra t u re fo r d i s so l u ti on o f t he 0 pha se fo r he a t i ngra tes in the range 2.5 to 20 0~ -1. We no tet ha t t h is t e m pe ra t u re re ma i ns c ons t a n t fo r he a ti ngra tes up to 20 ~ C ra in -1. For higher hea t ing ra tes ,e ve n t he ra p i d d i f fus i on a t t he se t e mpe ra t u re s i sn o t h i gh e n o u g h t o p r e s er v e t h e r m o d y n a m i ce qui l i b r i um be t we e n t he 0 pha se a nd t he ma t r i x .

4. Concluding remarksThe fo l l owi ng c onc l us i ons e me rge f rom t he re su lt so f t h e p r e s e n t w o r k .

1. Di ffe r ent ia l scann ing ca lor ime t ry i s a rapida nd re l i ab l e too l fo r s t udy i ng t he mi c ro s t ruc t ure o fa l l oy AA-2011 . I t c a n he l p i n t he s t udy o f t hed i s tr i b u ti o n o f P b - B i p a r ti c le s .

2 . Me a sure m e nt s o f Vi c ke rs ha rdne ss a ndc ond uc t i v i t y dur i ng a l i ne a r he a t i ng s i mil a r t o t ha tp rov i de d by t he DSC a ppa ra t us a re o f g re a t va l uefor t he i n t e rpre t a t i on o f DSC c urve s . In pa r t i c u l a rt he y a id t he d i f fe re n t i a t i on be t we e n d i s so l u t i ono f G P z o n e s a n d t h a t o f t h e 0 " p h a s e .

3 . The ge ne ra l f e a tu re s o f t he DSC c urve s fo r2011 a l loy are similar to tho se of pure A1- Cua l loys [ 1 - 6] . The ou t s t a nd i ng d i f fe re nc e is t hed e p e n d e n c e o f t h e f o r m a t i o n o f t h e 0 ' p h a s e o nt he t he rma l h i s t o ry o f t he a l l oy [3 ] found i n t hepre se n t work . I t c a n be c onc l ude d t ha t , a se xpe c t e d , l e a d a nd b i smut h do no t a f fe c t p re c i p i -t a t i on a nd d i s so l u t i on p roc e sse s i n A1-C u a l l oys .

4 . C ompa r i son of t he pe a k t e mpe ra t u re s fo r t he0 ' pha se fo rma t i on found by Pa pa z i a n [8 ] fo ra l l oy 2019 wi t h t hose ob t a i ne d i n t he p re se n tw o r k f o r a l lo y 2 0 1 1 , a n d t h o s e f o u n d f o r p u r ea l l oys [ I ] , a l l ow us t o c onc l ude t ha t t he O ' phaseforma t i on i s a c c e l e ra t e d i n Pa pa z i a n ' s workt hrough t wo poss i b l e me c ha ni sms , ( i ) de forma t i on ,i n t roduc e d dur i ng punc h i ng of DSC sa mpl e s a nd( ii ) ma nga nse s c on t e n t o f a l l oy AA-2219 .5 . P l a st ic de fo rm a t i on be t we e n so l u t i on he a tt r e a t m e nt a nd na t ura l a gei ng e nha nc e s G P z oneforma t i on a nd s t rong l y a c c e l e ra t e d p re c i p i t a t i onof t he 0 ' pha se . In t he p re se n t c ase 3% de form a t i onl owe re d t he 0 ' pha se fo rm a t i on pe a k i n 47 ~ C .

6 . The s t ab l e pha se (0 ) r e ma i ns i n t he rm o-dyna mi c e qu i l i b r i um wi t h t he ma t r i x i n t het e m p e r a t u r e r a ng e 3 0 0 - 5 3 0 ~ f o r h e a ti n g r a te su p t o 2 0 ~ C m i n - 1.

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