Precipitation Hardening of Aluminum Alloys MORRIS E. FINE The a u t h o r ' s charge was to d i s c u s s r e c e n t t r e n d s in r e s e a r c h and d e v e l o p m e n t on p r e c i p i tation h a r d e n e d a l u m i n u m alloys and to indicate where r e s e a r c h is needed. This will be done for t h r e e a r e a s : fatigue, p r o p e r t i e s of g r a i n b o u n d a r i e s and i n t e r f a c e s , and s t a b i l i t y of p r e c i p i t a t e s at e l e v a t e d t e m p e r a t u r e s . P r e s e n t s t r o n g p r e c i p i t a t i o n h a r d e n e d a l u m i n u m alloys do not have high e n d u r a n c e l i m i t s . One p r o b l e m is that the s m a l l GP zones a r e cut b y the d i s l o c a t i o n s giving r i s e to highly l o c a l i z e d d e f o r m a t i o n which aids fatigue c r a c k i n i t i a t i o n . A duplex s t r u c t u r e with r e l a t i v e l y l a r g e u n i f o r m l y s p a c e d p r e c i p i t a t e s to give m o r e homogeneous d e f o r m a t i o n plus s m a l l p r e c i p i t a t e s to give high yield s t r e n g t h is a p r o m i s i n g approach. The s t r u c t u r e s of p r e c i p i t a t i o n h a r d e n e d a l u m i n u m b a s e alloys a r e e s s e n t i a l l y c o n t r o l l e d b y the s t a b i l i t i e s of the v a r i o u s p r e c i p i t a t e s and the i n t e r f a c i a l e n e r g i e s . P r e c i p i t a t e s with high i n t e r f a c i a l e n e r g i e s tend to p r e c i p i t a t e p r e f e r e n t i a l l y at g r a i n b o u n d a r i e s giving e m b r i t t l e m e n t . Low i n t e r f a c i a l e n e r g y m e a n s e a s y n u c l e a t i o n , a u n i f o r m p r e c i p i t a t e d i s t r i b u t i o n , and r e s i s t a n c e to c o a r s e n i n g at elevated t e m p e r a t u r e s . F o r elevated t e m p e r a t u r e use, the p r e c i p i t a t e m u s t be stable at elevated t e m p e r a t u r e s . P r e c i p i t a t i o n h a r d e n e d a l u m i n u m alloys do not have good elevated t e m p e r a t u r e p r o p e r t i e s b e c a u s e the h a r d e n i n g p r e c i p i t a t e s n o r m a l l y used, GP zones, a r e not stable at e l e v a t e d t e m p e r a t u r e s . Thus a low i n t e r f a c i a l e n e r g y , ductile p r e c i p i t a t e , which is stable at e l e v a t e d t e m p e r a t u r e s , is needed for a l u m i n u m . P o s s i b i l i t i e s for achieving such p r e c i p i t a t e s will be d i s c u s s e d . T H E h i s t o r y of p r e c i p i t a t i o n h a r d e n e d A l - b a s e alloys will f i r s t be v e r y b r i e f l y r e v i e w e d in o r d e r to give some p e r s p e c t i v e to r e c e n t t r e n d s and needed r e s e a r c h . Precipitation hardening in aluminumbase alloys was d i s c o v e r e d a l m o s t 70 y e a r s ago b y Alfred Wilm in G e r m a n y ' who was t r y i n g to i m i t a t e the h a r d e n i n g of s t e e l . Wilm f o r m u l a t e d D u r a l u m i n , an Al 4 pct Cu-0.5 pct Mg-0.5 pct Mn alloy. Although he d i d n ' t know it, Si was p r e s e n t as an i m p u r i t y . Wilm heated the alloy to 525~ quenched it into water. The r e s u l t i n g m e t a l was r a t h e r soft. This was 1:00 P.M. Saturday. On Monday the r e s u l t was checked and the alloy was much s t r o n g e r . The e x p e r i m e n t was continued and the c o m plete n a t u r a l aging c u r v e was obtained. Wilm published the r e s u l t in 1911 without explanation s i n c e he was not able to see any s t r u c t u r a l change in the optical m i c r o scope. The a s s o c i a t i o n of the i n c r e a s e in s t r e n g t h on aging with f o r m a t i o n of t i n y p r e c i p i t a t e s of a second phase was not made until 1919 when M e r i c a , W a l t e n b e r g , and Scott 2 published t h e i r paper which is no doubt one of the most i m p o r t a n t p a p e r s in m e t a l l u r g i cal h i s t o r y . They c o r r e c t l y a s c e r t a i n e d that the s o l u b i l i t i e s of Cu, Zn and Mg-Si in a l u m i n u m a r e much higher at 525~ than at r o o m t e m p e r a t u r e and that second phase p r e c i p i t a t e s f o r m on aging. Lack of exp l a n a t i o n did not impede the c o m m e r c i a l i z a t i o n and u t i l i z a t i o n of D u r a l u m i n . Its m o s t i m p o r t a n t e a r l y use was for zeppelin f r a m e s and c o m p o n e n t s ; 97 of t h e m were b u i l t d u r i n g World War I. M e r i c a , W a l t e n b e r g and Scott's p a p e r , however, c l e a r l y showed the way to d e v e l o p m e n t of other p r e c i p i t a t i o n h a r d e n i n g MORRIS E. FINE is WalterP. Murphy Professor of Materials Science, Northwestern University, Evanston, Ill. 60201. This paper is based on an invited presentation made at a symposium on "Advances in the Physical Metallurgyof Aluminum Alloys" held at the Spring Meeting of TMS-IMDin Philadelphia, Pennsylvania,on May 29 to June 1, 1973. The symposium was co-sponsoredby the Physical Metallurgy Committee and the Non-Ferrous Metals Committee of TMS-IMD. METALLURGICALTRANSACTIONSA
alloys and s i n c e then h u n d r e d s of p r e c i p i t a t i o n h a r d ening alloys have b e e n d i s c o v e r e d and developed for technological utilization. Since the s t r u c t u r a l changes o c c u r r i n g on n a t u r a l aging of A l - C u - M g - S i alloys a r e s u b - o p t i c a l m i c r o scopic in s i z e , a c o n t r o v e r s y e x i s t e d as to the exact n a t u r e of the p r e c i p i t a t e s . M e r i c a c o n s i d e r e d t h e m to be p r e p r e c i p i t a t e s or " k n o t s ''s while J e f f r i e s and A r c h e r s thought they w e r e s u b m i c r o s c o p i c p a r t i c l e s of the e q u i l i b r i u m p h a s e s , CuA12, 0, and Mg2Si. Jeff r i e s and A r c h e r p r o p o s e d that such p r e c i p i t a t e s " k e y e d " the slip p l a n e s . This c o n t r o v e r s y c o n c e r n i n g the n a t u r e of the p r e cipitates was, of c o u r s e , r e s o l v e d through u t i l i z a t i o n of m o r e advanced r e s e a r c h t e c h n i q u e s , m a i n l y diffuse X - r a y diffraction, 4 e l e c t r o n d i f f r a c t i o n , and t r a n s m i s sion e l e c t r o n m i c r o s c o p y . 5 F o r example, in the b i n a r y A1-Cu s y s t e m , it is now known that a s e q u e n c e of four d i f f e r e n t p r e c i p i t a t e s o c c u r : G u i n i e r - P r e s t i o n zones (GPI), 0" ( s o m e t i m e s r e f e r r e d to as GPII), 0', and 0. The f i r s t t h r e e p r e c i p i t a t e s a r e m e t a s t a b l e . GP zones a r e the p r e c i p i t a t e s which f o r m f i r s t on aging at r o o m t e m p e r a t u r e or m o d e r a t e l y elevated t e m p e r a t u r e s ; the o t h e r s r e q u i r e a r t i f i c i a l aging, i.e., aging at elevated t e m p e r a t u r e s . All four p r e c i p i t a t e s yield p r e c i p i t a t i o n h a r d e n i n g . The GP zones which f o r m at r o o m t e m p e r a t u r e are knownto be thin {001} platelets of Cu-enriched regions, one or two at. diam thick, and perhaps 15 at. diam in radius, however, their structure and composition are not really known. The Cu atoms are 13 pct smaller than the Al atoms givingrise to strain energy. The GP zones form as thin platelets on the {I00} planes of the A1 to minimize strain energy. The thickness of the GP zones in Al-Cu, i.e., 1 or 2 at. diam, deduced by Guinier4from the X-ray diffraction streaks was confirmed by Nicholson and Nutting6 and Phillips7 who obtained transmission electron micrographs showing GP zones in a sample aged for 16 h at 130~ and in a slowly cooled sample, respectively. VOLUME 6A, APRIL 1975 -625
[oo,]
STRUCTURE OF GP ZONES AND RELATION TO THE HARDENING Gerold 8 has given a qualitative model of the GP zones in A1-Cu. He a s s u m e d the zones to be a single disc of copper a t o m s and c a l c u l a t e d the n o r m a l d i s p l a c e m e n t of the adjacent a l u m i n u m p l a n e s f r o m the r e l a t i v e i n t e n t e n s i t i e s of the X - r a y diffraction s t r e a k s a r i s i n g f r o m the zones. G e r o l d ' s model a s s u m e s t h e r e is no l a t e r a l d i s t o r t i o n in the plane of the zone n o r m a l to the t h i c k n e s s d i r e c t i o n . Some d i s t o r t i o n would be expected in this d i r e c t i o n too, and due to the m i s m a t c h in a t o m i c size, a s e m i c o h e r e n t i n t e r f a c e between the zone and m a t r i x might be expected. F o r example, an e x t r a row of Cu a t o m s e v e r y 7 or 8 rows in the plane of the zone would take up the l a t e r a l m i s m a t c h . If the s t r u c t u r e does not r e l a x l a t e r a l l y , the i n t e r n a l s t r e s s would be m o r e than 1 x 106 psi (7 x 103 MN/m 2) which is u n a c ceptable. Cohen and Mourikas 9 attempted to d e t e r m i n e the r e a l s t r u c t u r e of the GP zones in A l - C u f r o m a b solute i n t e n s i t y m e a s u r e m e n t s mapped over r e c i p r o c a l space but the d i s p l a c e m e n t was too l a r g e to allow s e p a r a t i o n of the c o m p o n e n t s of the X - r a y i n t e n s i t y . F u r t h e r r e s e a r c h is thus r e q u i r e d to d e t e r m i n e the s t r u c t u r e of GPI. We do not yet know the t r u e s t r u c t u r e of this i m p o r t a n t p r e c i p i t a t e . In o r d e r to r e a c h a detailed u n d e r s t a n d i n g of the p r e c i p i t a t i o n h a r d e n i n g , it is n e c e s s a r y to know the s t r u c t u r e of the G P zones b e c a u s e cutting of the GP zones by d i s l o c a t i o n s gives the s t r e n g t h i n g . We need to know what happens to the zones d u r i n g cutting. Before this can be a s c e r t a i n e d , it is n e c e s s a r y to know the s t r u c t u r e of the zones t h e m selves. In A1 b a s e - A g alloys where the m i s m a t c h in atom size is only about ~1 pct, s t r u c t u r e of the GP zones has been determined. Transmission electron micrographs and e a r l y X - r a y diffraction r e s u l t s had b e e n i n t e r p r e t e d to m e a n s p h e r i c a l GP zones. R e c e n t l y Gragg and Cohen 1~ m e a s u r e d the diffuse X - r a y s c a t t e r i n g f r o m a single c r y s t a l of Al-5 at. pct Ag aged 14 h at l l 0 ~ on an absolute s c a l e and s e p a r a t e d the data into its c o m p o n e n t s (static d i s p l a c e m e n t s and c l u s t e r i n g ) . The W a r r e n s h o r t - r a n g e o r d e r p a r a m e t e r s were c a l culated and then the GP zone s t r u c t u r e was d e t e r m i n e d using c o m p u t e r s i m u l a t i o n . A t y p i c a l GP zone is shown in Fig. 1. On aging below 170~ the GP zones in Al-5 at. pct Ag a r e o c t a h e d r a shaped and contain ~ 6 8 at. pct Ag. T h e r e is no i n t e r n a l o r d e r . Also the s t r u c t u r e is d i s t o r t e d in the r e g i o n of the zone. The GP zones f o r m e d above 170~ B a u e r and Gerold, 11 a r e s p h e r i c a l in shape, contain about o n e - h a l f the Ag of the lower t e m p e r a t u r e zones, and t h e r e is no m e a s u r a b l e d i s t o r t i o n . One is now f i n a l l y able to t r e a t p r e c i p i t a t i o n h a r d e n i n g in A1-Ag in a detailed m a n n e r . This has not b e e n done. Gragg and C o h e n ' s r e s u l t does show that d e s t r u c t i o n of o r d e r in the A1-Ag zones d u r ing cutting of the zones b y d i s l o c a t i o n s cannot be a s o u r c e of s t r e n g t h e n i n g . The zones a r e not o r d e r e d . A d i f f e r e n c e in s t a c k i n g fault e n e r g y b e t w e e n the zones and p r e c i p i t a t e s m a y play a role.12 A l t e r n a t e l y , the s t r e n g t h e n i n g m a y a r i s e f r o m the i n c r e a s e in s u r f a c e a r e a when the p r e c i p i t a t e s a r e cut b y d i s l o c a t i o n s . This type of absolute X - r a y i n t e n s i t y a n a l y s i s should be extended to the G P zones in A l - Z n where the m i s m a t c h in atom size is 3 pct and the zones a r e thought to be s p h e r i c a l . One might a n t i c i p a t e o c t a h e d r a shaped 626-VOLUME 6A, APRIL 1975
_
Ag
Q-
[olol
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Fig. 1--Exploded view of typical Guinier-Preston zone in AI5 at, pct Ag formed on aging at ll0~ From J. E. Gragg and J. B. Cohen. l~ GP zones with A I - Z n as well as with AI-Ag alloys. F u r t h e r r e s e a r c h is needed to d e t e r m i n e the d e t a i l s of the GP zone s t r u c t u r e s in the i m p o r t a n t t e r n a r y a l loys and c o m m e r c i a l alloys in o r d e r to b e t t e r u n d e r stand the o r i g i n of the p r e c i p i t a t i o n h a r d e n i n g f r o m GP zones in c o m m e r c i a l alloys. FATIGUE Alloy d e v e l o p m e n t for m e c h a n i c a l s t r e n g t h has l a r g e l y been b a s e d on the p a r a m e t e r s d e t e r m i n e d in the t e n s i l e test, such as yield s t r e s s , t e n s i l e s t r e n g t h and pct elongation. Yet, f r e q u e n t l y , the loading c o n d i tions a r e cyclic in n a t u r e and a l a r g e f r a c t i o n of the f a i l u r e s a r e in fatigue. The r e l a t i o n s h i p b e t w e e n the t e n s i l e p r o p e r t i e s and the fatigue p r o p e r t i e s a r e obs c u r e at p r e s e n t and while p r e c i p i t a t i o n h a r d e n i n g has r e s u l t e d in some v e r y s t r o n g A1 b a s e alloys, the b e n e fit in i m p r o v i n g the e n d u r a n c e l i m i t is only s m a l l . An i n d i c a t i o n of this is given in Table I. ~3 F o r example, the fatigue s t r e s s / t e n s i l e s t r e n g t h r a t i o in 7075-T6 is only 0.28 while in 5086-0, which is solid solution h a r d ened only (0.45 pct Mn and 4.0 pct Mg), this r a t i o is METALLURGICALTRANSACTIONSA
Table I. Tensile and Fatigue Properties of Alunimum Base Alloys TS
SolidSolutionandWorkHardened
YS
in psi
FS
FS/TS
1100O 1100H18 5083 O 5083 H321 5086 O 5086 H34 PrecipitationHardened
5,000 22,000 21,000 33,000 17,000 37,000
13,000 24,000 42,000 46,000 38,000 47,000
5,000 9,000 22,000 22,000 21,000 23,000
0.38 0.38 0.52 0.48 0.55 0.49
2024 O T3 T4 7075 O T6
l 1,000 50,000 47,000 15,000 73,000
27,000 13,000 70,000 20,000 68,000 20,000 33,000 17,000 83,000 23,000
0.48 0.29 0.29 0.51 0.28
YS= 0.2 pct offset strength;FS is basedon 500 X 106cyclesin rotatingbeam machine. FromJ. A. Neck,Jr. ~3 1MN/m2= 1A5 • 10-4 psi.
0.55. Alloy 5086 m a y be w o r k - h a r d e n e d f r o m 38,000 to 47,000 p s i (260-320 M N / m 2) t e n s i l e s t r e n g t h with an a c c o m p a n y i n g i n c r e a s e of fatigue s t r e n g t h f r o m 21,000 to 23,000 p s i (145 to 160 MN/mZ). 7075-T6, which has a t e n s i l e s t r e n g t h of 83,000 psi (570 MN/m2), has a fatigue l i m i t only of 23,000 p s i (160 MN/mZ). Thus f r o m the fatigue point of view, alloy 5086, which is l e s s highly alloyed than 7075 and r e q u i r e s no heat t r e a t m e n t , is just as good as heat t r e a t e d 7075. It should be pointed out that c o m p e t i n g m a t e r i a l s such as s t e e l s have F S / T S r a t i o s of 0.5 for t e n s i l e s t r e n g t h s in e x c e s s of 100,000 psi (690 MN/mZ). The d e s i r a b i l i t y of i m p r o v e d fatigue s t r e n g t h in a l u m i n u m b a s e alloys has led to a high level of c u r r e n t r e s e a r c h activity. A n u m b e r of p a p e r s in this s y m p o s i u m a r e devoted to this s u b j e c t . One p r o m i s i n g approach is through cont r o l l e d t h e r m o m e c h a n i c a l t r e a t m e n t . O s t e r m a n n 14 obt a i n e d a 25 to 50 pct i n c r e a s e in the fatigue l i m i t of smooth s p e c i m e n s of 7075 by cold working after aging and then r e a g i n g ; however, no i m p r o v e m e n t was obs e r v e d in notched s p e c i m e n s or in r e d u c i n g c r a c k p r o p a g a t i o n r a t e s (see Montulli, R e i m a n n and P o s s n e r , this s y m p o s i u m ) . A few y e a r s ago fatigue r e s e a r c h c o n s i s t e d of m e a s u r i n g the S-N c u r v e . The m o d e r n r e s e a r c h p r o c e d u r e is to divide the fatigue p r o c e s s into its component p a r t s , a) p l a s t i c d e f o r m a t i o n p r e c e d i n g c r a c k initiation, b) c r a c k i n i t i a t i o n and c) crack p r o p a g a t i o n , and to study t h e s e s e p a r a t e l y . F u r t h e r , a s t r a i n c o n t r o l l e d low cycle fatigue p r o c e d u r e has b e e n worked out which app r o x i m a t e s the t i m e c o n s u m i n g d i r e c t d e t e r m i n a t i o n of the S-N c u r v e . ~s The p r o c e d u r e i n c l u d e s d e t e r m i nation of the cyclic s t r e s s - s t r a i n c u r v e and s e v e r a l cycles to f a i l u r e t e s t s . In the p a s t p h y s i c a l m e t a l l u r g i s t s , b y r e l a t i n g the i n t e r n a l s t r u c t u r e of m a t e r i a l s to the m e c h a n i c a l p r o p e r t i e s d e t e r m i n e d in the t e n s i l e test, were able to m a x i m i z e t h e s e p r o p e r t i e s . It is now i m p o r t a n t to r e l a t e the i n t e r n a l s t t u c t u r e of m a t e r i a l s to fatigue p r o p e r t i e s such as the t i m e for c r a c k i n i t i a t i o n under v a r i ous s t a n d a r d conditions and the c r a c k propagation r a t e d a / d N v s the s t r e s s i n t e n s i t y amplitude AK. It is also i m p o r t a n t to r e l a t e other s t r e n g t h p a r a m e t e r s such as Kc, the u l t i m a t e t e n s i l e s t r e n g t h , elongation, and the METALLURGICALTRANSACTIONSA
work h a r d e n i n g r a t e to the p a r a m e t e r s d e t e r m i n e d in the fatigue e v a l u a t i o n of m a t e r i a l s . The s t r e s s i n t e n s i t y f a c t o r itself, Kc, is a c t i v e l y b e i n g r e l a t e d to the s t r u c t u r e of a l u m i n u m b a s e a l l o y s . K e r r n a n r e c e n t l y showed that o v e r a g e d s t r u c t u r e s in 7075 give lower K c v a l u e s than u n d e r a g e d s t r u c t u r e s . 16 This was m a i n l y a t t r i b u t e d to an e m b r i t t l i n g p r e c i p i tate at the g r a i n b o u n d a r i e s . The s i g n i f i c a n c e of the d y n a m i c s t r e s s - s t r a i n c u r v e is not c l e a r l y u n d e r s t o o d . It is d e t e r m i n e d b y c y c l i n g a s a m p l e in s t r a i n c o n t r o l at a given s t r a i n amplitude until a stable h y s t e r e s i s loop is o b s e r v e d . This gives a point on the d y n a m i c s t r e s s - s t r a i n c u r v e . The s t r a i n a m p l i t u d e is changed and the p r o c e s s r e p e a t e d a n u m b e r of t i m e s to g e n e r a t e the d y n a m i c s t r e s s - s t r a i n c u r v e . Since f r a c t u r e would soon e n s u e if the cycling at any of t h e s e higher s t r a i n l e v e l s w e r e continued, the concept of a stable s p e c i m e n state c h a r a c t e r i z e d by a s t a b l e h y s t e r e s i s loop is open to question. 1~ T r a n s m i s s i o n e l e c t r o n m i c r o s c o p e s t u d i e s h e r e would be v e r y useful. In r e l a t i n g m e t a l l u r g i c a l s t r u c t u r e to fatigue, l a r g e effects of m e t a l l u r g i c a l s t r u c t u r e a r e expected on c r a c k i n i t i a t i o n and on c r a c k p r o p a g a t i o n at low v a l ues of A K w h e r e the c r a c k moves slowly. At high AK, the c r a c k p r o p a g a t i o n r a t e is c l o s e l y r e l a t e d to K c. It is c u s t o m a r y to r e p o r t c r a c k p r o p a g a t i o n r a t e data in plots of log d L / d N vs log AK. In the i n t e r m e d i a t e r a n g e of AK the P a r i s e m p i r i c a l equation, 18
dL dN - c a K .
[1]
is u s u a l l y c o n s i d e r e d to hold. In Eq. [1] C and n a r e e m p i r i c a l c o n s t a n t s and the s t r e s s i n t e n s i t y amplitude = 2cra4--ffY(L , W), w h e r e % is the s t r e s s amplitude, L is the c r a c k length and Y(L, W) (W is the s p e c i m e n width~ is a c o r r e c t i o n factor which depends on the s p e c i m e n g e o m e t r y . In the b e h a v i o r u s u a l l y o b s e r v e d , after a t h r e s h o l d value of AK, d L / d N i n c r e a s e s r a p i d l y with ~rf until the r e g i m e of the P a r i s r e l a t i o n , and f i n a l l y at v e r y high AK, d L / d N i n c r e a s e s m o r e r a p i d l y than given by the P a r i s r e l a t i o n . The actual log d L / d N v s log AK c u r v e and C and n v a r y with s p e c i m e n t h i c k n e s s and s p e c i m e n g e o m e t r y as well as a t m o s p h e r e , so one m u s t be c a r e f u l to keep t h e s e constant in t r y ing to evaluate the effects of s t r u c t u r e . The g e n e r a l log d L / d N vs log ~ b e h a v i o r j u s t d e s c r i b e d d o e s n ' t always hold in a l u m i n u m b a s e alloys, as shown in Fig. 2, for 2024-T6 in d r y argon. T h e r e is a r e g i o n of AK where d L / d N is a l m o s t independent of AK. S i m i l a r b e h a v i o r has b e e n r e p o r t e d by E1Soudani and Pelloux. 19 Study of d L / d N vs AK for diff e r e n t kinds of s p e c i m e n s is b e i n g a c t i v e l y p u r s u e d in a n u m b e r of l a b o r a t o r i e s and a c l e a r c h a r a c t e r i z a t i o n of the b e h a v i o r vs s t r u c t u r e should e m e r g e over the next few y e a r s . As p r e v i o u s l y m e n t i o n e d , when the p r e c i p i t a t e s or G P zones a r e s m a l l , they a r e cut by the glide d i s l o c a tions d u r i n g p l a s t i c d e f o r m a t i o n . While this is the condition for m a x i m u m yield s t r e s s , once the p a r t i c l e s a r e cut, d i s l o c a t i o n s continue to p a s s through the p a r t i c l e s on the active slip plane and the local w o r k - h a r d ening is s m a l l . The d e f o r m a t i o n tends to be l o c a l i z e d on a few active slip p l a n e s . This would s e e m to be v e r y d e l e t e r i o u s to both fatigue and s t r e s s c o r r o s i o n VOLUME 6A, APRIL 1975 627
in a l u m i n u m b a s e alloys a r e f a v o r a b l e r e g i o n s for c r a c k n u c l e a t i o n . The c r a c k s in 2024-T6 or 7075-T6 a r e u s u a l l y not b r a n c h e d and the c r a c k p r o g r e s s e s in a m o r e continuous m a n n e r . The a v e r a g e c r a c k p r o p a gation r a t e s in d r y a r g o n were n e a r l y the s a m e in all t h r e e . D i s p e r s i o n of l a r g e p a r t i c l e s which a r e ductile and c o h e r e n t with the m a t r i x would p r o b a b l y be m o r e f a v o r a b l e for e n h a n c i n g fatigue r e s i s t a n c e than the 8 p a r t i c l e s in the 6.3 pct Cu alloy.
-2.400-
2024 - T6 -3.200
?
I N I T I A L MAX. STRESS = 1 7 K S I / + DRY ARGON
J r )-
ELEVATED TEMPERATURE MECHANICAL P R O P E R T I E S
"-'-4.000' z _.1 "o o 1
-4.800
-5.600
1.200
t
1.400 1.600 LOG A K ( K G / M M 312)
1.800
F i g . 2 - - L o g dL/dN v s l o g z2df i n 2 0 2 4 - T 6 i n d r y a r g o n . S p e c i m e n 12 cm long, 2 cm wide and 0.3 cm thick and side notched,
before heat treatment using a V cross section milling cutter was loaded in pull-pull at a nominal maximum stress of 17 ksi and minimum stress of 1 ksi (computed on basis of original cross section). c r a c k i n g . On the other hand, if the p r e c i p i t a t e p a r t i cles a r e l a r g e and spaced far apart, the p r o c e s s b y which the d i s l o c a t i o n s b y p a s s the p a r t i c l e s leads to r a p i d w o r k - h a r d e n i n g and the p l a s t i c d e f o r m a t i o n tends to be g e n e r a l l y d i s t r i b u t e d throughout the s p e c i m e n . However, the yield s t r e s s is low. One i n t e r e s t ing p o s s i b i l i t y is to have a d i s p e r s i o n of two kinds of second phase p a r t i c l e s , s m a l l c l o s e l y spaced p a r t i c l e s to give high yield s t r e s s plus l a r g e p a r t i c l e s to d i s t r i b ute the p l a s t i c d e f o r m a t i o n throughout the m a t e r i a l . We a r e t e s t i n g the concept that two kinds of second phase p a r t i c l e s m a y be b e t t e r than one for fatigue r e s i s t a n c e . Our i n i t i a l study is with a b i n a r y A1-6.3 pct Cu alloy.* The m a x i m u m s o l u b i l i t y of Cu in Al is ex*Supplied by Kaiser Aluminum Center for Technology, Pleasanton, Calif.
ceeded in this alloy so that e x c e s s O p a r t i c l e s a r e a l ways p r e s e n t . After quenching f r o m 535~ the u n d i s solved O p a r t i c l e s a r e 5 to 10 g m in diam. After aging at 135~ a t e n s i l e s t r e n g t h of 60,000 psi (415 M N / m 2) was obtained. To date we have studied c r a c k p r o p a g a tion in notched s p e c i m e n s in this alloy. The c r a c k is highly b r a n c h e d and at low v a l u e s of &/
Another a r e a which may be f r u i t f u l for f u r t h e r r e s e a r c h is i m p r o v e d m e c h a n i c a l p r o p e r t i e s at elevated t e m p e r a t u r e s . Nickel alloys have b e e n developed which a r e useful to 0.75T m where T m is the absolute m e l t i n g t e m p e r a t u r e . On a s i m i l a r b a s i s , one might p r e d i c t that a l u m i n u m alloys could be developed which would be useful to 400~ The Ni b a s e s u p e r a l l o y s which a r e used at high t e m p e r a t u r e s depend upon the p r e s e n c e of y' p r e c i p i t a t e s which a r e c o h e r e n t and cop l a n a r with the m a t r i x and a r e m o d e r a t e l y ductile. They a r e also t h e r m o d y n a m i c a l l y stable at high t e m p e r a t u r e s . Needed for a good ductile a l u m i n u m b a s e alloy which would be useful to 400~ a r e stable c o h e r e n t - c o p l a n a r ductile p r e c i p i t a t e s or s e c o n d - p h a s e p a r ticles. T h e r e a r e a n u m b e r of r e a s o n s why p r e c i p i t a t e s for s t r e n g t h e n i n g at elevated t e m p e r a t u r e s m u s t be t h e r m o d y n a m i c a l l y stable and c o h e r e n t and c o p l a n a r with the m a t r i x . Obviously, for e l e v a t e d t e m p e r a t u r e p r e cipitation s t r e n g t h e n i n g the p r e c i p i t a t e s m u s t be t h e r m o d y n a m i c a l l y stable at elevated t e m p e r a t u r e s ; o t h e r wise they would tend to d i s s o l v e . One of the p r o b l e m s with p r e s e n t p r e c i p i t a t i o n h a r d e n e d a l u m i n u m alloys is that the p r e c i p i t a t e s used a r e all m e t a s t a b l e and d i s a p p e a r at e l e v a t e d t e m p e r a t u r e s . Thus the s t a n d a r d p r e c i p i t a t i o n h a r d e n e d alloys such as 2024-T81 and 7075-T61 have 100 h r u p t u r e s t r e n g t h s of only 6000 psi (40 M N / m 2) at 325~ c o m p a r e d to 66,000 and 76,000 psi (455 MN/m ~ and 525 M N / m 2) r e s p e c t i v e l y at 100~ is When the p r e c i p i t a t e is c o h e r e n t and c o p l a n a r with the m a t r i x , d i s l o c a t i o n s in the m a t r i x can r e a d i l y t r a v e r s e through the p r e c i p i t a t e p a r t i c l e s and if the p r e c i p i t a t e and m a t r i x have d i f f e r e n t s t r u c t u r e s then the d i s l o c a t i o n s which t r a v e r s e the p r e c i p i t a t e c r e a t e s t a c k i n g faults in the p r e c i p i t a t e . Since a l a r g e amount of e n e r g y is r e q u i r e d to c r e a t e the s t a c k i n g fault and the t r a v e r s i n g d i s l o c a t i o n c r e a t e s m o r e s t a c k i n g fault with each unit of slip, this is a v e r y effective s t r e n g t h e n i n g m e c h a n i s m for elevated t e m p e r a t u r e s . Also, the c o h e r e n t - c o p l a n a r c o r r e s p o n d e n c e m e a n s that the p r e c i p i t a t e - m a t r i x i n t e r f a c e is a low e n e r g y i n t e r f a c e and t h e r e is little t e n d e n c y for c o a r s e n i n g or c o a l e s c e n c e of the p r e c i p i t a t e . The d r i v i n g force for c o a l e s c e n c e is r e d u c t i o n of s u r f a c e e n e r g y , and if the i n t e r f a c e e n e r g y is s m a l l , t h e r e is little d r i v i n g f o r c e for the coalescence. What a r e the p o s s i b i l i t i e s for o b t a i n i n g t h e r m o d y n a m i c a l l y stable c o h e r e n t - c o p l a n a r s e c o n d - p h a s e in a l u m i n u m ? The i n t e r m e d i a t e s i z e d (500 to 5000A diam) p r e c i p i t a t e s which f o r m in 7075 c o n t a i n i n g Cr a r e at l e a s t p a r t i a l l y c o h e r e n t with the m a t r i x , e~ T r a n s mission electron microscopy indicates a strain contrast effect c h a r a c t e r i s t i c of c o h e r e n t or p a r t i a l l y c o h e r e n t METALLURGICAL TRANSACTIONS A
p a r t i c l e s where the l a t t i c e s p a c i n g in the two phases a r e s l i g h t l y different. The c o m p o s i t i o n of the p a r t i cles a r e thought to be Cr2Mg3A118. zl As shown in Table II, this t e r n a r y compound has a cubic s t r u c t u r e and t h e r e is a 10 pct m i s m a t c h in l a t t i c e p a r a m e t e r with four t i m e s the l a t t i c e p a r a m e t e r of A1. ZrAI3 is also p a r t i a l l y c o h e r e n t with the A1 m a t r i x . 2z As shown in Table H, the l a t t i c e p a r a m e t e r in the a o d i r e c t i o n is n e a r l y the s a m e as that in A1. The v o l u m e f r a c t i o n s of p a r t i c l e s of both Cr2Mg3A1,8 and ZrA13 a r e l i m i t e d to b e i n g s m a l l b e c a u s e the s o l u b i l i t i e s of Cr and Zr in A1 a r e s m a l l , the m a x i m u m b e i n g 0.72 and 0.28 wt pet in solid b i n a r y A1-Cr and A1-Zr alloys, r e s p e c t i v e l y . T h e s e might p o s s i b l y be i n c r e a s e d through p r o p e r choices of t h i r d solute e l e m e n t s and c o n c e n t r a t i o n s . In b i n a r y A1-Mg a l l o y s , MgzA13 is s t a b l e to 451~ the m a x i m u m s o l u b i l i t y of Mg in A1 is 17 wt pet and this d e c r e a s e s to 7 pet at 300~ Thus a l a r g e v o l u m e f r a c t i o n of p r e c i p i t a t e m a y be f o r m e d . Mg2Ala is cubic and, as shown in T a b l e II, t h e r e is a v e r y good m a t c h b e t w e e n 7ao in the compound and a o in A1. By alloying, it m a y be p o s s i b l e to a d j u s t the l a t t i c e p a r a m e t e r s as with the Ni b a s e s u p e r a l l o y s so that the p r e c i p i t a t e s b e c o m e c o h e r e n t with the a l u m i n u m m a t r i x . Some additional compounds which m a y be of i n t e r e s t f r o m this point of view a r e also given in T a b l e H. P a r t i c u l a r l y n o t e w o r t h y a r e TiA13 where a o d i f f e r s only 4 pet f r o m that of A1 (this compound is t e t r a g o n a l so full c o h e r ency could not be achieved) and Mn3SiAI~2 where t h e r e is only a 4 pet d i f f e r e n c e with 3aA1. The stable second phase in the AI-Cu s y s t e m (0, CuA12) has b e e n used to i m p r o v e e l e v a t e d t e m p e r a t u r e p r o p e r t i e s . Alloy 2219, which is e s s e n t i a l l y a b i n a r y A1-Cu alloy c o n t a i n i n g 6.3 wt pct Cu, is useful to higher t e m p e r a t u r e s than alloys with lower Cu content. z3 As p r e v i o u s l y m e n t i o n e d , e x c e s s 0 is always p r e s e n t in the solid alloys of this c o m p o s i t i o n . While this is not a p r e c i p i t a t i o n effect, it does i l l u s t r a t e the e f f e c t i v e n e s s of stable p h a s e s in i m p r o v i n g high t e m p e r a t u r e p r o p e r t i e s . Another e x a m p l e is the addition of 2 wt pet Ni to a 4 wt pet C u - l . 5 wt pet Mg for i m p r o v e m e n t of high t e m p e r a t u r e s t r e n g t h in alloy 2218.~3 The s o l u b i l i t y of Ni in solid A1 is v e r y s m a l l , 0.05 wt pet at 640~ and the high t e m p e r a t u r e s t r e n g t h e n i n g
Table II. Some Second Phase Compounds Formed in Aluminum a
c
Compound
Structure (in A) (in ~)
AI Mg~AI3 Mg:Si TiA13 CuAI~ NiAI3 ZrAI3 Cr2MgaAlls (Cr,Mn)AI]: Cr4Si4AI13 CuMg4A16 CuaNiAI6 Cu3ZnAla MgaZn~A12 MnaSiAI12
Cubic Cubic Cubic C.P. Tet Tet Ortho Tet Cubic Cubic Cubic Cubic Cubic Cubic Cubic Cubic
acomp/aAi Stableas Solidto ~
4.05 28.16 6.35 3.85 8.60 6.07 4.88 6.61 4.81 4.01 17.32 14.55 7.51 10.92 14.31 14.6 2.91 14.19 12.65
From L. A. Willey.21 1A= 10-1om. METALLURGICALTRANSACTIONSA
6.9 1.6 0.95 1.5 1.6 0.99 3.6 1.85 2.7 3.5 3.6 0.72 3.5 3.1
660 452 1102 1340 591 854 1580 >460 >550 >550 "475 ~820 >360 ~530 >460
d e r i v e s f r o m A13Ni which is s t a b l e to 854~ MgzSi, which m e l t s at 1102~ might also be useful in this regard. The whole field of s t r e n g t h e n i n g of a l u m i n u m b y s e c ond p h a s e s , which a r e t h e r m o d y n a m i c a l l y stable at e l e vated t e m p e r a t u r e s , is one which is worthy of c o n s i d e r a b l e study, not only for elevated t e m p e r a t u r e s t r e n g t h but also for p o s s i b l y i m p r o v e d fatigue p r o p e r t i e s as previously mentioned. INTERFACIAL ENERGY AND P R E C I P I T A T E MORPHOLOGY The final r e s e a r c h a r e a to be d i s c u s s e d c o n c e r n s the i n t e r f a c i a l e n e r g y between the p r e c i p i t a t e and m a t r i x . The r o l e of i n t e r f a c i a l e n e r g y in c o a r s e n i n g of p r e c i p i tate s t r u c t u r e s at e l e v a t e d t e m p e r a t u r e s has a l r e a d y b e e n mentioned. Low i n t e r f a c i a l e n e r g y is a p r e r e q u i site for low c o a r s e n i n g r a t e s . N u c l e a t i o n mode is an additional c o n s i d e r a t i o n . P r e c i p i t a t e s with high i n t e r f a c i a l ene{'gies have high a c t i v a t i o n e n e r g i e s for n u c l e ation and, t h e r e f o r e , tend to p r e c i p i t a t e at g r a i n b o u n d a r i e s where the n u c l e a t i o n is catalyzed, that is, h e t e r o geneous n u c l e a t i o n . Such p r e c i p i t a t i o n t e n d s to make the s t r u c t u r e b r i t t l e and f o r m a t i o n of e m b r i t t l i n g p r e c i p i t a t e s at g r a i n b o u n d a r i e s is a v e r y i m p o r t a n t p r o b l e m in such a l u m i n u m alloys as 2024 and 7075 when they a r e quenched slowly or aged at e l e v a t e d t e m p e r a t u r e s . The effect of such p r e c i p i t a t e s in r e d u c i n g the c r i t i c a l s t r e s s i n t e n s i t y factor has a l r e a d y b e e n m e n tioned. On the other hand, low i n t e r r a c i a l e n e r g y m e a n s low a c t i v a t i o n e n e r g y , e a s y n u c l e a t i o n , homogeneous n u c l e ation, and a u n i f o r m p r e c i p i t a t e d i s t r i b u t i o n . The G P zones which f o r m in a l u m i n u m b a s e alloys, of c o u r s e , have low i n t e r r a c i a l e n e r g i e s ; they n u c l e a t e h o m o g e n e ously, and a r e d i s t r i b u t e d u n i f o r m l y in the m a t r i x . T h e r e is no g r a i n b o u n d a r y e m b r i t t l e m e n t . The i n t e r f a c i a l e n e r g i e s of the v a r i o u s p r e c i p i t a t e s and s e c o n d - p h a s e p a r t i c l e s which f o r m in A l - b a s e a l loys have b e e n studied r e l a t i v e l y l i t t l e and i n t e r f a c i a l e n e r g y has not always b e e n c o n s i d e r e d in d e s i g n of A1b a s e alloys and t h e i r heat t r e a t m e n t . More r e s e a r c h on m o d i f i c a t i o n of i n t e r r a c i a l e n e r g i e s by alloying would appear to be v e r y fruitful. An e x a m p l e of the s o r t of effect which can be achieved is the r e c e n t work of Boyd and Nicholson 24 on Cd-doped A1-Cu. Addition of 0.1 pct Cd to an A1-4 pct Cu alloy r e d u c e d the s u r face e n e r g y of O' f r o m 1.5 to 0.25 J / m 2. This r e d u c e d the c o a r s e n i n g r a t e b y a f a c t o r of 5 and also r e s u l t e d in o c c u r r e n c e of 0' p r e c i p i t a t i o n at lower t e m p e r a t u r e s than in undoped A1-4 pct Cu. The addition of Ag to A I - Z n - M g m a y lower the i n t e r f a c i a l e n e r g y of the ~?' p r e c i p i t a t e and allow it to preci]~itate m o r e r e a d ily. m According to P e t e r s ~ 7' n u c l e a t e s at e l e v a t e d t e m p e r a t u r e s only on p r e v i o u s l y f o r m e d G P zones in undoped alloys, but when Ag is p r e s e n t 77' can n u c l e a t e independently. Staley et al~ have also studied the effect of Ag on p r e c i p i t a t i o n in s i m i l a r alloys. S o m e t i m e s GP zones avoid g r a i n b o u n d a r i e s and the r e g i o n n e a r g r a i n b o u n d a r i e s due to a r e d u c t i o n in the e n h a n c e d diffusion r e s u l t i n g f r o m the q u e n c h e d - i n v a c a n c i e s . (Depletion of solute due to f o r m a t i o n of a g r a i n b o u n d a r y p r e c i p i t a t e , such as d u r i n g a slow quench, can also give a p r e c i p i t a t e f r e e zone n e a r g r a i n b o u n d a r i e s f ~) The e q u i l i b r i u m diffusion r a t e s VOLUME 6A, APRIL 1975-629
a r e t o o s l o w f o r f o r m a t i o n of G P z o n e s a t low a g i n g t e m p e r a t u r e s , b u t in q u e n c h e d a l l o y s t h e d i f f u s i v i t y is amplified by quenched-in vacancies allowing GP z o n e s to f o r m at r e d u c e d a g i n g t e m p e r a t u r e s . G r a i n b o u n d a r i e s a r e s i n k s for v a c a n c i e s and thus under slow quenching and e l e v a t e d t e m p e r a t u r e aging conditions a rather large region near the grain boundary sometimes b e c o m e s d e v o i d of v a c a n c i e s . T h i s r e g i o n i s d e n u d e d of p r e c i p i t a t e s b e c a u s e a m p l i f i e d d i f f u s i o n c a n n o t t a k e place here. Since the denuded region is a weak region, it i s t h o u g h t t o b e d e l e t e r i o u s t o m e c h a n i c a l p r o p e r t i e s . It s h o u l d b e p o i n t e d out t h a t t h e r e w i l l a l w a y s b e a denuded region near grain boundaries. The denuded z o n e w i l l b e v e r y s m a l l if t h e q u e n c h i s r a p i d a n d t h e a g i n g t e m p e r a t u r e i s low b e c a u s e t h e v a c a n c y d e p l e t e d r e g i o n n e a r g r a i n b o u n d a r i e s is v e r y s m a l l , S i n c e p l a s tic deformation atso generates diffusion-aiding point d e f e c t s , o n e of t h e p o s s i b l e b e n e f i t s of t h e r m o m e c h a n ical t r e a t m e n t is p r e v e n t i n g the p r e c i p i t a t e - f r e e zone n e a r g r a i n b o u n d a r i e s and t h e r e b y a c h i e v i n g a m o r e u n i f o r m p r e c i p i t a t e d i s t r i b u t i o n . A n o t h e r a p p r o a c h is t o w e a k l y t i e up v a c a n c i e s into s o l u t e - v a c a n c y c o m plexes which later act as vacancy sources. G r o v e a n d J u d d ~ r e c e n t l y f o u n d t h a t a d d i t i o n of 0.04 p c t Ti t o A1-5.32 p c t Z n - l . 6 6 p c t M g r e t a r d e d t h e k i n e t i c s of p r e c i p i t a t i o n a n d p r e v e n t e d s e g r e g a t i o n of solute to grain boundaries during slow quenching. They s u g g e s t e d t h a t Ti a t o m s t i e up v a c a n c i e s s o t h a t f e w e r v a c a n c i e s a r e a v a i l a b l e t o a i d d i f f u s i o n of M g a n d Zn a t o m s . M g a n d Zn a t o m s t h e n d i f f u s e m o r e s l o w l y s o that f e w e r r e a c h grain b o u n d a r i e s and the GP z o n e s form more slowly. T h e e f f e c t s of m i n o r a l l o y a d d i t i o n s t h u s a p p e a r t o b e w o r t h y of c o n s i d e r a b l e f u r t h e r s t u d y , not o n l y f o r g r a i n r e f i n e m e n t but to change p r e c i p i t a t e m o r p h o l o g y t h r o u g h c h a n g i n g s u r f a c e e n e r g y so t h e r e is l e s s t e n d e n c y f o r g r a i n b o u n d a r y e m b r i t t l e m e n t . It i s a l s o p o s s i b l e t o c o n t r o l t h e e n h a n c e m e n t of d i f f u s i o n b y e x c e s s vacancies by adding a solute element which forms comp l e x e s w i t h v a c a n c i e s . In s o m e c a s e s t h e s e m a y l a t e r become vacancy sources. In s u m m a r y , in s p i t e of t h e l a r g e a m o u n t of r e s e a r c h p u b l i s h e d on p r e c i p i t a t i o n a n d p r e c i p i t a t i o n h a r d e n i n g
630-VOLUME 6A, APRIL 1975
of A l - b a s e a l l o y s , m u c h g o o d r e s e a r c h r e m a i n s to b e d o n e . T h i s b r i e f r e v i e w h a s i n d i c a t e d a n u m b e r of r e search areas which appear promising.
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METALLURGICAL TRANSACTIONS A