EFFECT OF GRAIN-BOUNDARY STRUCTURE ON STRESS CORROSION CRACKING IN αCu-Al ALLOY BICRYSTALS

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EFFECT OF GRAIN-BOUNDARY STRUCTURE ON STRESS CORROSION CRACKING IN αCu-Al

ALLOY BICRYSTALS

T. Mimaki, M. Yamashita, S. Hashimoto, S. Miura

To cite this version:

T. Mimaki, M. Yamashita, S. Hashimoto, S. Miura. EFFECT OF GRAIN-BOUNDARY STRUC-

TURE ON STRESS CORROSION CRACKING IN

Physique Colloques, 1988, 49 (C5), pp.C5-693-C5-698. �10.1051/jphyscol:1988591�. �jpa-00228088�

EFFECT OF GRAIN-BOUNDARY STRUCTURE ON STRESS CORROSION CRACKING IN aCu-A1 ALLOY BICRYSTALS

T. MIMAKI'

'

Department of Mechanical Engineering, Faculty of Engineering, Doshisha University , Kyoto 602, Japan

' Department of Engineering Science, Faculty of ~nkineering , Kyoto University, Kyoto 606, Japan

Intergranular stress-corrosion-cracking (SCC) of Cu-9at.XAl alloy was studied in <llO>-symmetrical tilt bicrystals having low-angles 3.7;

8 . 6 3 CSL/C=3(111). C=9(221), C=11(311) boundaries with various grain

boundary energies. T h e C = 3 , 9 and 11 bicrystals were fractured inter- granularly. T h e intergranular f r a c t u r e s did not occur until the applied s t r e s s e s higher than the yield stresses. The grain boundary energy reflected t h e susceptibility t o intergranular S C C in a lower s t r e s s level. Especially, the C = 3 boundary showed a higher resistance t o the initiation o f intergranular SCC. T h e small-angle tilt bicrystals had an extremely lov s u s c e ~ t i b i l i t ~ ~ t o intergranular S C C even under a h i g h level of s t r e s s ratios.

1. INTRODUCTION

Various properties of metallic materials a r e affected by the s t r u c t u r e of grain boundary. In many c a s e s , s t r e s s corrosion cracking (SCC) initiates and propagates intergranularly. Logan [11 reported that more than 7 0 percent of t h e c r a c k s examined in a b r a s s , originated a t the grain boundaries, which were considered t o have higher interfacial energies. Ohtani and Kon [2] found that the fracture time of Al-l%Cu alloy bicrystals decreased with increasing the tilt and twist angles. F u r t h e r m o r e , Minaki e t a1.[3] pointed o u t that S C C susceptibility depended on the grain boundary s t r u c t u r e o f a C u - A 1 alloy bicrystals. Kovacs and Low [4] reported that s l i p behavior between t h e neighboring grains affected the intergranular SCC.

Murakami and Ikai [51 showed in a b r a s s bicrystals t h a t c r a c k s were

O N O W

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988591

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generated at the boundary where deformation was restricted. We have recently reported f61 that the susceptibility to intergranular S C C o f a C u - A 1 alloy bicrystals increased by the s t r e s s concentration a t t h e

boundary due to the plastic deformation.

Grain boundary s t r u c t u r e has been studied L7.81 intensively and is classified precisely s u c h a s the C o i n c i d e n c e S i t e L a t t i c e (CSL) model. Therefore, it is expected to employ well-defined bicrystals in order to obtain more precise information about the mechanisms of intergranular SCC. The objective of the present research is to e x a m i n e the effect of grain boundary s t r u c t u r e on S C C in bicrystals' having

<llO>-symmetrical tilt boundaries with various C-values.

2,EXPERIMENTAL PROCEDURES

The material used in t h i s experiment was an as-rolled s h e e t o f Cu-9at.%Al alloy p o l ~ c r y s t a l s . The bicrystals were grown f r o m the seed c r y s t a l s using the Bridgman method in a vacuum. T h e grain boundaries of bicrystal s p e c i m e n s were normal to the tensile axis. T h e c r y s t a l - lographic c h a r a c t e r s and yield s t r e s s e s of s p e c i m e n s a r e shown in Table 1. A l l g r a i n boundaries of bicrystals were <llO>-symmetrical tilt CSL boundaries and the deviation a n g l e s f r o m the CSL relationship were smaller than the respective allowable v a l u e s , A e c , which c a n be accommodated by grain boundary dislocations, calculated by the Brandon's equation [91, a s A 0 c - 1 5 (C)-1'2 degrees. The s p e c i m e n s were cyclically annealed in the r a n g e of 1023-1223K f o r 7 2 0 k s

(7.2ks/cycle).

Table :I Crystallographic characters and yield stresses of bicrystals tested.

T h e bicrystals were corroded in a (Br2 : 2 m l + H C 1 : 6 0 m l + C H ~ C O O H : 2 O m l + H20;lOOml) s o l u t i o n f o r 3 k s t o evaluate relative values of the interfacial e n e r g i e s from the measurement of groove depth using optical-cut method.

Tensile t e s t s were carried out with a s t r a i n rate of 4 . 2 ~ 1 0 - ~ s - ' at 303'2 K , in order t o observe the s l i p behavior in the early s t a g e of deformation. T h e yield s t r e s s (UY) was defined a s tensile s t r e s s at which the primary s l i p system(s) was activated in the component crystal.

S C C tests were carried o u t in a ( N H ~ O H : 2 0 0 m l + N a O H : 1 0 0 g + H ~ O ~ 8 O O ~ l ) s o l u t i o n a t 303'2 K under constant loads, which vere chosen in the range of ( 0 . 9 - 1 . 6 ) ~ ~ .

3. RESULTS AND DISCUSSION

3.1 Measurement o f relative grain boundary energy

The relative grain boundary e n e r g i e s were evaluated from the depth o f intergranular corrosion. a s s h o w n in Fig.1. T h e r e s u l t is similar t o the measurement of grain boundary energy in aluminium by Hasson e t al. [lo].

D , and rotation a n g l e (C-value). D o f C=1(8=3.71) bicrystal was estimated to be less than 0.1 um. Dotted line is t h e experimental c u r v e of r e l a t i v e energies. 7 / 7 0 . of t i l t boundaries about [011] in aluminium by Hasson e t al. [lo].

Table 2 Fracture time of specimen.

Warks (-1 =an that the specimens were not cracked a t 800 hours.

3.2 Effects o f grain boundary energy and plastic deformation T h e fracture t i m e of s p e c i m e n is listed in T a b l e 2.

The C = 3 , 9 and 1 1 boundaries were attacked and intergranu- lar fractures occurred under applied s t r e s s e s higher than the yield s t r e s s e s except a C = 3 bicrystal loaded 1 . 1 ~ ~ . it

is concluded t h a t t h e plastic deformation is necessary f o r intergranular SCC and the initiation of c r a c k s a t the grain boundary having a lower energy r e q u i r e s a larger amount of plastic deformation.

T h e elongation V.S. time curve measured. which is schemati- cally shown in Fig.2, could be divided into two r e g i o n s , a linear part and a s t e e p s l o p e part. It was proved from the measurement of elongation due t o d u c t i l e deformation a s will be discussed in detail later, that the second region corre- s p o n d s to the t i m e needed for ductile f r a c t u r e , td

.

f o r e , the first one is considered to be the time needed f o r initiation and propagation o f the c r a c k ,

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tci+~). T h e t(i+~) is plotted against the normal s t r e s s t o grain boundary plane in Fig.2. The t i m e f o r initiation and propagation of SCC was decreased with increasing the normal stress. It is suggested that the is controlled by t h e normal stress. More experiments are necessary in higher s t r e s s level to obtain further information about the effect of the normal stress. In the F i g u r e , the t(i+~, of C=ll boundary tends to be longer than that o f C=9 boundary in a lower stress level. It s e e m s likely that grain boundary energy reflects the susceptibility t o intergranular SCC in a lower s t r e s s level.

On the contrary, all of the C = 1 (8=3.7f) bicrystals were not cracked even under the highest stress. An 8.58'bicrystal which tested under the s t r e s s ratio of 1.6 was fractured in an intragranular manner

Normal stress t o g r o i n b o u n d a r y plone ( M P o ) C = I (B=3.7Ie) 1.1 (8.8.58.)

0 2 0 3 0 4 0 5 0

Yield stress ( M P a )

Fig. 2 Relation between the time needed for initiation and propagation o f SCC, t(i+~) , and the normal s t r e s s to grain boundary plane. A s c h e m a t i c elongation V.S. time c u r v e is a l s o shown. Arrows mean that the s p e c i m e n s were not fractured at 8 0 0 hours.

low susceptibility to intergranular S C C e v e n under a high level of s t r e s s r a t i o s which leads a large amount of plastic deformation. It is suggested that intragranular cracking can occur a t s t r e s s c o n c e n t r a - tion s i t e s , e.g. s l i p s t e p s or dislocation pile-ups against obstacles such a s the Lomer-Cottrell barriers. a s reported p r e v i o u s l ~ [11.121.

3.3 Morphology of intergranular crack

A typical example of intergranular fracture in the bicrystals having high-angle grain boundaries is s h o w n in Fig.3. The fracture surface is characterized by a f l a t plane and a remarkable deformation region located about the center of the surface. T h e s e correspond to brittle and d u c t i l e fracture s u r f a c e s , respectively. Intergranular S C C caused by the former and the latter resulted only f r o m subsequent mechanical fracture. The surface markings which are parallel to the intersections o f the activated s l i p planes o n t o the grain boundary, a s s h o w n in Fig.S(c), were thought to be s e l e c t i v e dissolution and crack formation a t s t r e s s concentration s i t e s s u c h a s the p i l e d - U P group of dislocations o n s l i p planes.

Fig. 3 Intergranular S C C observed in a C = l l bicrvstal applied 29.1 MPa f o r 3 9 ks. The fracture s u r f a c e is s h o w n in (b). T h e region A is ductile fracture surface. The region B of brittle fracture s u r f a c e is shown a t higher magnification in (c). T h e s u r f a c e markings parallel to the intersections of the activated s l i p planes o n t o the grain boundary are clearly shown.

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C O N C L U S I O N S

S t r e s s c o r r o s i o n c r a c k i n g t e s t s w e r e c a r r i e d o u t o n Cu-9at.%Al a l l o y . b i c r y s t a l s in a ( N H ~ O H ; 2 0 0 m l + N a O H ; 1 0 0 ~ + H z 0 ; 8 0 0 m l ) s o l u t i o n a t 3 0 3 2 2 K. T h e r e s u l t s a r e s u m m a r i z e d a s f o l l o w s ;

1) T h e : C = 3 , 9 a n d 11 b i c r y s t a l s w e r e f r a c t u r e d i n t e r g r a n u l a r l y . T h e i n t e r g r a n u l a r f r a c t u r e s d i d n o t o c c u r u n t i l t h e a p p l i e d s t r e s s e s h i g h e r t h a n t h e y i e l d s t r e s s e s . T h e g r a i n b o u n d a r y e n e r g y r e f l e c t e d t h e s u s c e p t i b i l i t y t o i n t e r g r a n u l a r S C C in a lower s t r e s s level.

E s p e c i a l l y , t h e ,Y=3 b o u n d a r y s h o w e d a h i g h e r r e s i s t a n c e t o t h e i n i t i a t i o n o f i n t e r g r a n u l a r S C C .

2 T h e s m a l l - a n g l e t i l t b i c r y s t a l s h a d a n e x t r e m e l y l o w s u s c e p t i - b i l i t y t o i n t e r g r a n u l a r S C C e v e n u n d e r a h i g h level o f s t r e s s r a t i o s .

A C K N O W L E D G E M E N T S

T h e a u t h o r s w i s h t o t h a n k t h e S u n i t o m o L i g h t M e t a l I n d u s t r i e s , Ltd., f o r s u p p l y i n g m a t e r i a l s . T h e f i n a n c i a l s u p p o r t g i v e n b y t h e L i g h t M e t a l E d u c a t i o n a l F o u n d a t i o n , Inc., i s a c k n o w l e d g e d . T h a n k s a r e a l s o d u e t o Mr. H. M o r i g a m i f o r h i s h e l p in t h e c o u r s e o f t h e e x p e r i - m e n t a l work.

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