EFFECT OF THERMOMECHANICAL TREATMENT ON TOUGHENING OF Al-Li-Cu-Mg-Zr ALLOY AND ITS TOUGHNESS

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EFFECT OF THERMOMECHANICAL TREATMENT ON TOUGHENING OF Al-Li-Cu-Mg-Zr ALLOY AND

ITS TOUGHNESS

M. Niinomi, K. Degawa, T. Kobayashi

To cite this version:

M. Niinomi, K. Degawa, T. Kobayashi. EFFECT OF THERMOMECHANICAL TREATMENT

ON TOUGHENING OF Al-Li-Cu-Mg-Zr ALLOY AND ITS TOUGHNESS. Journal de Physique

Colloques, 1987, 48 (C3), pp.C3-653-C3-659. �10.1051/jphyscol:1987375�. �jpa-00226606�

JOURNAL DE PHYSIQUE

C o l l o q u e C3, s u p p l b m e n t a u n 0 9 , Tome 48, s e p t e m b r e 1987

EFFECT OF THERMOMECHANICAL TREATMENT ON TOUGHENING OF Al-Li-Cu-Mg-Zr ALLOY AND ITS TOUGHNESS

M. NIINOMI, K. DEGAWA and T. ROBAYASHI

D e p a r t m e n t o f P r o d u c t i o n S y s t e m s E n g i n e e r i n g , T o y o h a s h i U n i v e r s i t y o f T e c h n o l o g y , 1-1, H i b a r i g a o k a , Tempaku-cho, T o y o h a s h i 440, J a p a n

SYNOPSIS

T h e e f f e c t o f t h e 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 f o r t o u g h e n i n g t h e Al-Li-Cu-Me-Zr

- ., ..

a l l o y was i n v e s t i g a t e d u s i n g t h e c o m p u t e r a i d e d i n s t r u m e n t e d C h a r p y i m p a c t t e s t i n g ( C A I ) s y s t e m . I n a d d i t i o n , t h e t e m p e r a t u r e d e p e n d e n c e o f t h e t o u g h n e s s o f t h i s a l l o y was a l s o i n v e s t i g a t e d .

T h e i n t e r m e d i a t e 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 r i g i n a l l y d e v e l o p e d by R o c k w e l l I n t e r n a t i o n a l Co. (RI-ITMT) was p r o v e d t o be e f f e c t i v e i n i n c r e a s i n g t h e t o u g h n e s s of t h i s a l l o y . T h e t o u g h n e s s o f t h i s a l l o y i n c r e a s e d w i t h t h e d e c r e a s e o f t h e t e m p e r a t u r e below room t e m p e r a t u r e . The t o u g h n e s s n e a r room t e m p e r a t u r e a p p e a r e d t o b e a f f e c t e d bv low m e l t i n g n o i n t m e t a l s l i k e

- .

I. INTRODUCTION

T h e t o u g h n e s s o f Al-Li-Cu-Me-Zr _{" "} a l l o v is r e l a t i v e l v low i n c o m n a r i s o n w i t h i t s s t r e n g t h . The improvement i n t o u g h n e s s i s one of t h e most i m p o r t a n t p r o b l e m f o r t h e p r a c t i c a l u s e o f t h i s a l l o y . T h e i n t e r g r a n u l a r e m b r i t t l e m e n t c a u s e d by i m p u r i t i e s (Na, K, Ca e t c ) and hydrogen , and t h e i n t e r g r a n u l a r f r a c t u r e r e l a t e d t o t h e r o u g h p l a n a r s l i p h a v e b e e n p r o p o s e d as t h e main r e a s o n s f o r t h e low t o u g h n e s s of Al-Li-Cu- Mg-Zr a l l o y [ 1 ] [ 2 ] [ 3 ] [ 4 ] . It i s c o n s i d e r e d t o be more e f f e c t i v e i n p r e v e n t i n g t h e r o u g h p l a n a r s l i p t o s h o r t e n t h e d i s t a n c e of t h e d i s l o c a t i o n p i l e up a n d d i m i n i s h t h e l o c a l s t r e s s c o n c e n t r a t i o n .

T h e 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 (TMT) seems t o b e e f f e c t i v e i n a c h i e v i n g t h i s c o n d i t i o n [ 5 ] [ 6 ] . The i n t e r m e d i a t e 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 w h i c h was d e v e l o p e d by R o c k w e l l I n t e r n a t i n a l Co. L t d . (RI-ITMT) was m o d i f i e d and a p p l i e d f o r t h e improvement o f t h e t o u g h n e s s of t h e 7075 a l l o y i n o u r l a b o r a t o r y . C o n s e q u e n t l y , t h e m o d i f i e d R I - ITMT was proved t o be e f f e c t i v e i n i m p r o v i n g t h e t o u g h n e s s of t h e 7075 a l l o y [ 7 ] .

T h e r e f o r e , t h e e f f e c t i v e n e s s of t h e m o d i f i e d RI-ITMT i n i m p r o v i n g t h e t o u g h n e s s o f t h e Al-Li-Cu-Mg-Zr a l l o y w a s examined u s i n g t h e c o m p u t e r a i d e d i n s t r u m e n t e d Charpy i m p a c t t e s t i n g (CAI) s y s t e m [ 8 ] [ 9 ] i n t h i s s t u d y . Then, t h e t e m p e r a t u r e d e p e n d e n c e of t h e t o u g h n e s s - o f t h i s a l l o y w a s - a i s o i n v e s t i g a t e d .

11. EXPERIMENTAL PROCEDURE

The h o t r o l l e d p l a t e s (6 mm t h i c k ) of two k i n d s of Al-Li-Cu-Mg-Zr a l l o y s w e r e u s e d i n t h i s s t u d y . The c h e m i c a l c o m p o s i t i o n s of t h e s e a l l o y s a r e shown i n T a b l e 1. A l l o y I w a s u s e d f o r t h e s t u d y b o t h on t h e ITMT p r o c e s s a n d t h e low t e m p e r a t u r e t o u g h n e s s . A l l o y I1 was used o n l y f o r t h e s t u d y on t h e t e m p e r a t u r e dependence of t o u g h n e s s . The h o t r o l l e d p l a t e o f t h e a l l o y I w a s p r o c e s s e d a c c o r d i n g t o t h e ITMT p r o c e s s and c o n v e n t i o n a l p r o c e s s e s , i.e.. T6 and T8 s c h e m a t i c a l l y shown i n F i g . 1 .

T h e h o t r o l l e d p l a t e o f a l l o y I1 w a s p r o c e s s e d a c c o r d i n g t o t h e c o n v e n t i o n a l p r o c e s s , T6 o n l y . The h e a t t r e a t e d m a t e r i a l s w e r e m a c h i n e d i n t o t h e C h a r p y V - n o t c h s p e c i m e n ( s i z e ( 5 5 x 1 0 x 3 ) mm) whose l o n g i t u d i n a l d i r e c t i o n i s e q u i v a l e n t t o t h e r o l l i n g d i r e c t i o n , and t e s t e d by t h e CAI s y s t e m ( c a p a c i t y : 98 J ) u n d e r t h e c o n d i t i o n o f a n i m p a c t v e l o c i t y o f 1.13 m / s a n d a n a d d e d e n e r g y c a p a c i t y o f 7 . 7 5 J . T h e s p e c i m e n s p r o c e s s e d a c c o r d i n g t o t h e ITMT and c o n v e n t i o n a l p r o c e s s e s w e r e t e s t e d a t b o t h room a n d l i q u i d n i t r c g e n t e m p e r a t u r e s . I n p a r t i c u l a r , T6 t r e a t e d s p e c i m e n s were t e s t e d a t s e l e c t e d t e m p e r a t u r e s c o v e r i n g t h e t e m p e r a t u r e r a n g e o f 7 7 t o 5 7 3 K . The f r a c t u r e d s u r f a c e w e r e c h a r a c t e r i z e d u s i n g t h e s c a n n i n g e l e c t r o n m i c r o s c o p e . M i c r o s t r u c t u r e s of a l l o y s w e r e c h a r a c t e r i z e d u s i n g t h e o p t i c a l a n d 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 h e s p e c i m e n s , ( $ 3 x 13 mm) i n s i z e , o f T6 a p p l i e d a l l o y s I and I1 w e r e f r a c t u r e d i n t h e A u g e r s p e c t r a a n a l y z e r a n d a n a l y z e d i n t h e i n - s i t u c o n d i t i o n .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1987375

JOURNAL DE PHYSIQUE

T a b l e 1 Chemical c o m p o s i t i o n s o f materials (mass %).

Li Cu M f 3 Zr Fe Si Na A1

- - - - --- -

Alloy I 2.8 1.25 0.90 0.09 0.05 0.01 0.0003bal.

Allov I1 2.54 1.12 0.70 0.12 0.0059 bal.

(1 ) Conventional procen

( a ) T6 process (Alloy F) (b) T8 process (~lloy E)

Solutionizing Solutionizing

773K-30min

Aging L63K ~ 1 0 h Aging L63K. 10h

Hot rolled Hot rolled

plate

( 2 ) ITMT process (AlloF A, B, C, D) Solutionizing

773K *30rnin n

Overaging

Hot rolled

.lop A : ,, 573 K

Recrystallization and-solutioniz~ng

~ l l o y B : ,, ^{,I 30 min}

,,

Alloy C : n 673 K 10 min

,,

Alloy D :

,,

,,

F i g . 1 S c h e m a t i c d i a g r a m o f e a c h h e a t t r e a t m e n t p r o c e s s c o n d u c t e d i n t h i s s t u d y . 111. RESULTS AND DISCUSSION

111-1. Toughness of ITMT process applied alloys 1. Results of instrumented Charpy impact test

R e s u l t s o f t h e i n s t r u m e n t e d C h a r p y i m p a c t t e s t o n a l l o y s w h e r e t h e ITMT a n d c o n v e n t i o n a l p r o c e s s e s w e r e a p p l i e d - a r e - s h o w n i n F i g . 2 . T h e s t r e n g t h o f t h e T8 a p p l i e d a l l o y E i n c r e a s e s w i t h o u t t h e d e c r e a s e o f t h e t o t a l d e f l e c t i o n Df , w h i c h i n d i c a t e s t h e d u c t i l i t y , i n c o n t r a s t w i t h t h e T6 p r o c e s s a p p l i e d a l l o y F. T h e r e f o r e , a n improvement i n t o u g h n e s s w i t h t h e i n c r e a s e o f s t r e n g t h i s p o s s i b l e w i t h t h e T8 p r o c e s s . T o t a l a b s o r b e d e n e r g i e s o f ITMT p r o c e s s a p p l i e d a l l o y s A , B a n d C a r e g r e a t e r t h a n t h a t of t h e a l l o y F. On t h e o t h e r h a n d , t h e t o t a l a b s o r b e d e n e r g y of t h e ITMT p r o c e s s a p p l i e d a l l o y D is s m a l l e r t h a n t h a t of t h e a l l o y F. Pm v a l u e s of a l l o y s

...

A a n d B , w h i c h i n d i c a t e s t r e n g t h , a r e s m a l l e r t h a n t h a t of t h e a l l o y F. P m v a l u e s o f a l l o y s A a n d D a r e g r e a t e r t h a n t h a t o f t h e a l l o y F. C o n s i d e r i n g t h e c o m b i n a t i o n between s t r e n g t h and t o u g h n e s s ; t h e ITMT p r o c e s s C i s t h e m o s t e f f e c t i v e f o r t h e i m p r o v e m e n t o f t h e t o u g h n e s s o f t h i s a l l o y as shown i n F i g . 3 . I n h i g h e r t o u g h n e s s a l l o y s , t h e t r a n s g r a n u l a r d i m p l e f r a c t u r e was p r e d o m i n a n t . I n l o w e r t o u g h n e s s a l l o y s , t h e i n t e r g r a n u l a r f r a c t u r e was predomnant. The k i n d o f t h e t o u g h n e s s w h i c h improved by t h e RI-ITMT p r o c e s s d e p e n d s on t h e o v e r a g i n g t e m p e r a t u r e d u r i n g t h e ITMT p r o c e s s . T h a t i s , t h e i n c r e a s e o f t h e E t v a l u e i n a l l o y s A a n d B w i t h t h e o v e r a g i n g t e m p e r a t u r e 573 K i s a c h i e v e d by t h e c o n s i d e r a b l e i n c r e a s e of t h e En v a l u e . T h e i n c r e a s e o f t h e Et v a l u e i n t h e a l l o y C w i t h t h e o v e r a g i n g t e m p e r a t u r e 6 7 3 K i s a c h i e v e d by t h e i n c r e a s e of b o t h Ei a n d E v a l u e s .

P

E t = E i . Ep

Delleclion D l

Specimen

1 . 4 0 b

'

0 0.6 0.8 1.0 1.2

Absorbed energy I J F i g . 3 R e l a t i o n s h i p b e t w e e n a b s o r b e d e n e r g y

and maximum l o a d (P,) a t room t e m p e r a t u r e . F i g . 2 R e s u l t o f i n s t r u m e n t e d Charpy

i m p a c t t e s t a t room t e m p e r a t u r e . 2. Microstructure

E v e r y a l l o y w a s f o u n d t o b e c o m p o s e d o f u n r e c r y s t a l l i z e d g r a i n s by TEM o b s e r v a t i o n s . The a v e r a g e s i z e o f t h e u n r e c r y s t a l l i z e d g r a i n o f t h e c o n v e n t i o n a l p r o c e s s a p p l i e d a l l o y w a s a r o u n d 5 v m . T h e a v e r a g e s i z e o f t h e u n r e c r y s t a l l i z e d g r a i n s of ITMT a p p l i e d a l l o y s was s m a l l e r t h a n t h a t o f c o n v e n t i o n a l p r o c e s s a p p l i e d a l l o y s . T h e s m a l l e s t s i z e of u n r e c r y s t a l l i z e d g r a i n was o b s e r v e d t o b e a b o u t 1 . 5 D m on a v e r a g e i n t h e a l l o y C , and t h e l a r g e s t s i z e was o b s e r v e d t o b e a b o u t 2 . 2 U m o n a v e r a g e i n t h e a l l o y D among ITMT - p r o c e s s a p p l i e d a l l o y s . T h e r e f o r e , t h e h i g h e s t t o u g h n e s s v a l u e i s n o t a t t a i n e d u n d e r t h e c o n d i t i o n w h i c h g i v e s t h e f i n e s t g r a i n s i z e . T h e s i z e o f t h e u n r e c r y s t a l l i z e d g r a i n s l i g h t l y i n c r e a s e d w i t h a n i n c r e a s e i n t i m e of t h e ( r e c r y s t a l l i z a t i o n

+

M o r p h o l o g i e s o f p r e c i p i t a t e s w e r e d i f f e r e n t among o v e r a g i n g t e m p e r a t u r e s . C u - r i c h p r e c i p i t a t e s , w h i c h w e r e a n a l y z e d by t h e EPMA, w e r e o b s e r v e d b o t h o n t h e g r a i n b o u n d a r y o f t h e worked s t r u c t u r e a n d i n g r a i n s j u s t a f t e r t h e o v e r a g i n g t r e a t m e n t a t 573 K. These p r e c i p i t a t e s were e s t i m a t e d t o be 8 (A12Cu) o r T2(A15Cu3Li) [ 1 0 ] [ 1 1 ] . On t h e o t h e r h a n d , Mg-rich p r e c i p i t a t e s w e r e o b s e r v e d w i t h t h e @ o r T2 p r e c i p i t a t e s b o t h on t h e g r a i n boundary of t h e worked s t r u c t u r e a n d i n g r a i n s . T h e s e p r e c i p i t a t e s w e r e e s t i m a t e d t o be S(Al2CuMg)[l0][12]. 8 o r T p r e c i p i t a t e s r e m a i n e d u n d i s o l v e d i n e v e r y 2 a l l o y even a f t e r t h e f i n a l s o l u t i o n a n d a g i n g t r e a t m e n t . S p r e c i p i t a t e s r e m a i n e d u n d i s o l v e d i n a l l o y C , w h e r e t h e f i n a l s o l u t i o n t r e a t m e n t t i m e was s h o r t e r ( 1 0 m i n ) , a f t e r t h e f i n a l s o l u t i o n and a g i n g t r e a t m e n t . However, t h e s e p r e c i p i t a t e s s c a r c e l y r e m a i n e d i n a l l o y D, w h e r e t h e f i n a l s o l u t i o n t r e a t m e n t t i m e was g t e a t e r ( 3 0 m i n ) , a f t e r t h e f i n a l s o l u t i o n and a g i n g t r e a t m e n t . The amount of u n d i s o l v e d p r e c i p i t a t e s i n c r e a s e d i n t h e o r d e r o f a l l o y s A , B , C and D. T h e r e f o r e , t h e r e i s a t e n d e n c y f o r t h e t o u g h n e s s t o i n c r e a s e w i t h t h e i n c r e a s e o f u n d i s o l v e d p r e c i p i t a t e s . T h e s e p r e c i p i t a t e s a r e c o n s i d e r e d t o i n h i b i t t h e f o r m a t i o n of t h e r o u g h p l a n a r s l i p , and c o n s e q u e n t l y , d i m i n i s h t h e s t r e s s c o n c e n t r a t i o n t o t h e g r a i n b o u n d a r y a s s c h e m a t i c a l l y e x p l a i n e d i n F i g . 4 . T y p i c a l m o r p h o l o g i e s and L12 d i f f r a c t i o n p a t t e r n s of t h e &'in t h e RI-ITMT and c o n v e n t i o n a l p r o c e s s a p p l i e d a l l o y s a r e s h o w n i n F i g . 5 . T h e p r e c i p i t a t i o n o f t h e 6' o n t h e A13Zr i s o b s e r v e d i n e v e r y a l l o y s i m i l a r t o t h e o t h e r r e p o r t [13]. I n a d d i t i o n , t h e p r e c i p l t a t l o n of t h e 6'on t h e r e l a t i v e l y c o a r s e C u - r i c h p r e c i p i t a t e , w h i c h was a s c e r t a i n e d by t h e EDX a n a l y s i s , was a l s o o b s e r v e d i n t h e a l l o y D i n t h i s s t u d y .

C3-656 JOURNAL DE PHYSIQUE

\ Precipitates by weraging and homogenizing ( T z , ~ , Sl

Fig.4 Schematic interpretation of transgranular and intergranular fracture : (a) Intergranular fracture is predominant. This mechanism leads to low toughness.,(b) Transgranular fracture is predominant. This mechanism leads to high toughness.

IlMT B ITMT D T6 F

Fig.5 Dark field and corresponding diffraction patterns for the A1 Li precipitates. +A indicates Cu-rich precipitate.

3

3. Low temperature toughness of RI-ITMT process applied alloys

The toughness of tvoical three allovs. that is. conventional orocess aoolied allov _" 3 A , , ^L ~- ~ r r -,

F, ITMT process applied alloys B and D which exhibited the greatest and lowest toughness values respectively, were evaluated at the liquid nitrogen temperature (77 K). Resu.lts are shown in Fig.6. T;he Et value of the conventional process applied alloy F at the liquid nitrogen temperature increases over twice as much than when at room temperature (RT). On the other hand, the ratio of the Et value of the alloys B and D at the liquid nitrogen temperature to that at RT is from 1.4 to 1.6. Therefore, the improvement ratio of the ITMT process applied alloy is lower than that of the conventional process applied alloy. The increase of the toughness of alloy D at low temperatures, which exhibits higher strength at RT, is considered to be achieved by the increase of strength because the total deflection at low temperatures scarcely increases (Fig.7;. The increase of the toughness of specimens A and B at low temperatures is achieved by the increase of both strength and deflection.

R.T. 77K

T6 F H

Deflection Df 1 m m F i g . 6 R e l a t i o n s h i p b e t w e e n maximum l o a d ( P ) ~ i ~ . 7 ~ ~ l ~ tbetween d e f l e c t i o n i ~ ~ h i ~

a n d a b s o r b e d e n e r g y (E ) a t room

t ( D f ) a n d maximum l o a d (P,) a t

t e m p e r a t u r e a n d 77 K. room t e m p e r a t u r e and 77 K.

111-2. T o u g h n e s s o f Al-Li-Cu-Mg-Zr a l l o y s

1. T e m p e r a t u r e d e p e n d e n c e o f t o u g h n e s s a n d s t r e n g t h

The t o u g h n e s s and s t r e n g t h of t h e T6 a p p l i e d a l l o y I1 i n t h e t e m p e r a t u r e r a n g e of 7 7 t o 5 7 3 K w a s i n v e s t i g a t e d i n t h i s s t u d y . The t e m p e r a t u r e dependence of t o u g h n e s s and s t r e n g t h i s shown I n F i g . 8 . I n t h e t e m p e r a t u r e r a n g e of 77 t o RT, t h e t o u g h n e s s a n d s t r e n g t h i n c r e a s e w i t h t h e d e c r e a s e of t e m p e r a t u r e . T h e r e f o r e , t h e i n c r e a s e o f t h e t o u g h n e s s i n t h i s t e m p e r a t u r e r a n g e i s c o n s i d e r e d t o b e m a i n l y c a u s e d by t h e i n c r e a s e o f s t r e n g t h . However, t h e t o t a l d e f l e c t i o n a l s o i n c r e a s e d w i t h t h e d e c r e a s e o f t e m p e r a t u r e . I n t h e 7 0 0 0 s e r i e s a l l o y s , t h e t o t a l d e f l e c t i o n d e c r e a s e s c o n s i d e r a b l y b e c a u s e t h e t e n d e n c y o f t h e i n t e r g r a n u l a r f r a c t u r e i n c r e a s e s b u t t h e s t r e n g t h i n c r e a s e s , a n d c o n s e q u e n t l y , t h e t o t a l a b s o r b e d e n e r g y d e c r e a s e s w i t h l o w e r i n g o f t e m p e r a t u r e b e l o w RT [ 1 4 ] . T h e r e f o r e , t h e i n c r e a s e of t h e t o t a l d e f l e c t i o n i n t h i s a l l o y a t l o w t e m p e r a t u r e s c a n n o t b e i g n o r e d a s a f a c t o r t h a t i m p r o v e s t o u g h n e s s . On t h e o t h e r h a n d , t h e s t r e n g t h a n d t o u g h n e s s d e c r e a s e w i t h t h e i n c r e a s e of t h e t e m p e r a t u r e i n t h e t e m p e r a t u r e r a n g e o f RT t o 4 2 3 K . T h e s t r e n g t h d e c r e a s e s f u r t h e r i n t h e t e m p e r a t u r e r a n g e a b o v e 4 2 3 K w h i l e t h e t o u g h n e s s i n c r e a s e s c o n s i d e r a b l y . T h e r e f o r e , t h e d y n a m i c s t r e n g t h d e c r e a s e s w i t h t h e i n c r e a s e o f t h e t e m p e r a t u r e i n t h e t e m p e r a t u r e r a n g e w h e r e t h e 6' h a v i n g L 1 s t r u c t u r e e x i s t s . 2

0 100 200 300 600 500 600

Testing temperature I K

F i g . 8 R e l a t i o n s h i p b e t w e e n maximum l o a d (P,) a n d a b s o r b e d e n e r g y (E ) w i t h t h e

Change o f t e m p e r a t u r e i n a l l o y 11. ^t

C3-65 8 J O U R N A L DE PHYSIQUE

T e s t temp. 77 K room t e m p e r a t u r e 573 K

Fi.g.9 T y p i c a l SEM m i c r o g r a p h s of f r a c t u r e s u r f a c e of a l l o y 11, showing t h e c h a n g e of l a m i n a t e d c r a c k w i t h t e s t i n g t e m p e r a t u r e s .

, '

1

, r ::

- .< -

. :

T e s t temp. 77 K room t e m p e r a t u r e

F i g . 1 0 O p t i c a l m i c r o g r a p h s o f l a m i n a t e d c r a c k d e p t h i n T6 s p e c i m e n o f a l l o y 11.

P r o f i l e i s p e r p e n d i c u l a r t o c r a c k p r o p a g a t i o n d i r e c t i o n .

2. Morphology o f f r a c t u r e s u r f a c e ^{h r 1 I} T y p i c a l f r a c t o g r a p h s o f t e s t e d room temp.

I I

room temp s p e c i m e n s i n t h e t e m p e r a t u r e r a n g e of 77

5

t o 5 7 3 K a r e shown i n F i g . 9 . The f r a c t u r e s u r f a c e i n t h e t e m p e r a t u r e r a n g e of l i q u i d h I-

9/ >92(Na) (NaeCu)

I I

123 K (-1504C) n i t r o g e n t e m p e r a t u r e t o 5 7 3 K e x h i b i t m

c h a r a c t e r i s t i c l a m i n a t e d c r a c k s . T h e 5

f r a c t u r e s u r f a c e a t 5 7 3 K e x h i b i t s t h e

2

d i m p l e f r a c t u r e d m o r p h o l o g y ' m i c r o s c o p i c a l l y a l t h o u g h i t e x h i b i t s t h e

-

p l a n a r s h e a r t y p e f r a c t u r e c m a c r o s c o p i c a l l y . I n t h e t e m p e r a t u r e r a n g e .P between l i q u i d n i t r o g e n and RT, t h e number of t h e l a m i n a t e d c r a c k s i n c r e a s e s w i t h t h e

%

d e c r e a s e of t e m p e r a t u r e . The d e p t h o f t h i s =;

252(K) 2 274(C)

I I

I

123K(-1500C)

t y p e o f c r a c k a l s o i n c r e a s e s w i t h t h e %!.

d e c r e a s e o f t e m p e r a t u r e a s s h o w n i n 2 u F i g . 1 0 . T h e r e f o r e , t h e t o u g h n e s s o f t h i s

a l l o y a t l o w t e m p e r a t u r e s i n c r e a s e s w i t h

&

t h e l a m i n a t e d c r a c k . T h e s e c r a c k s a r e c o n s i d e r e d t o b e p r o d u c e d b e c a u s e t h e

c r i t i c a l r e s o l v e d s h e a r s t r e s s , w h i c h 100 200 300 LOO 800 900 1000 1100 c a u s e s t h e t r a n s g r a n u l a r s l i p , i n c r e a s e s

w i t h t h e d e c r e a s e o f t e m p e r a t u r e , a n d Energy (ev)

b e c a u s e of t h i s , t h e c r a c k i n g a t t h e g r a i n F i g . 1 1 Low e n e r g y Auger s p e c t r a o f t h e b o u n d a r y i s r e l a t i v e l y p r e d o m i n a n t i n f r a c t u r e s u r f a c e o f T6 s p e c i m e n aluminum a l l o y s . From t h e m e t a l l u r g i c a l o f a l l o y 11.

274(C)

I I

992(Na)

t I

point of view, intergranular embrittlement does not take place at low temperature in aluminum alloys. It was reported by one of the authors that 7N01 and 5083 alloys are embrittled at the low temperatures when the laminated crack appears [14]. This fracture surface is the separation type and does not directly affect the crack propagation. Therefore, it seems to be difficult to explain the increase of the toughness at low temperatures by the appearance of laminated cracks.

3. Liquid metal embrittlement by low melting point metal phase

Results of the in-situ Auger electron analysis of the T6 applied alloy 11, which was fcactured under dynamic conditions at RT and 123 K, are shown in Fig.11. The Na spectrum was detected only on the fracture surface of 123K. In addition, the K spectrum was al'so detected. The Na spectrum was also detected on the grain or subgrain boundary in the analysis of the specimen surface. Therefore, it i s suggested that impurities such as K, Na and so on segregate on the grain boundary and affect the toughness near RT. It has been reported that the temperature rises locally near the crack tip in an adiabatic manner under the high strain rate and high deformation stress condition like the Charpy impact test. The rise of the adiabatic temperature at the crack tip estimated in this study is around 90 K. Considering the temperature rise, the temperature of the crack tip reaches melting point of metals recognized by Webster [15], and is nearly equal to the temperature at which the toughness begins to decrease. Consequently, it is suggested from this point that the increase of impact toughness at low temperature appears to be achieved by inhibiting the liquid metal embrittlement which is caused bv low melting point metals.

- -

rv.

(1) The RI-ITMT process is proved to be effective in improving the toughness of the Al-Li-Cu-Mg-Zr alloy.

(2)The 6' precipitates not only on the A1 Zr but also on the Cu system 3

precipitates.

(3) Since precipitates, which are produced during the overaging treatment, remain undisolved after the final solution and aging treatment, and inhibit the rough planar slip, a higher toughness value is achieved in the RI-ITMT process applied alloy.

(4) The toughness of this alloy increases with the decrease of temperature below room temperature.

(5) Low melting point metals like Na and K aDDear to affect the toughness of the A

~ l - ~ i - C U - M ~ - Z ~ alloy at about room temperature.

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ACKNOLEDGDDWI'S

The authors would like to thank Nikkei Techno-Research Co., Ltd. and Showa Aluminum Co. Ltd. for s u ~ ~ l v i n e _.A A " Al-Li-Cu-Ma-Zr allovs.

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REFWENCES

[l] A.K.Vasudevan, A.C.Miler and M.M.Kersker : Aluminum Lithium Alloys 1 1 , Met. AIME, (1983),181.

[2] D.P.Hil1, D.H.Williams and C.E.Mobley : ibid, 201.

[ 3 ] F . S . L i n , S . B . C h a k r a b o r t l y a n d E . A . S t a r k e , J r . : Met. T r a n s A, 13A(1982), 401.

[4] T.H.Sanders, Jr. and E.A.Starke, Jr. : Acta Met., 30(1982),927.

[5] M.Peters and G.Lutjering : Z.Metallkde, 67(1976),811.

161 T.H.Sanders Jr. and E.A.Starke, Jr : Met. Trans A, 76(1976),1407.

[7] M.Niinomi and T.Kobayashi : Z.Metallkde, 78(1987),39.

[8] T.Kobayashi, M.Niinomi, 1.Yamamoto and M.Kamimura : Proc. Int. Conf. on Impact Loading and Dynamic Behavior of Materials, Bremen, (1987) to be published.

[9] T.Kobayashi, 1.Yamamoto and M.Niinomi : Eng. Frac. Mech., 24(1986),773.

[lo] J.W. Bohlen and G.R. Chanani : Aluminum Lithium Alloys 11, Met. AIME, (1983),407.

[ll] R.J.Kar, J.W.Bohlen and G.R.Chanani : ibid, 255.

[12] L.F.Mondolfo : Aluminum Alloys Structure and Properties, Butterworth, London, (1976).

[13] P.L.Makin, B.Ralph : J. Mat. Sci., 19(1984),3835.

[14] T.Kobayashi and K.Takai : J. Japan Inst. Light Met., 22(1972),541.

[15] D.Webster : Metal Progress, (1984),33.