Hydrogen Embrittlement in a Mg-AI Alloy D. G. CHAKRAPANI AND E. N. PUGH E x p e r i m e n t s a r e d e s c r i b e d w h i c h d e m o n s t r a t e that a M g - 7 . 5 wt p e t At a l l o y i s e m b r i t t l e d by both c a t h o d i c a l l y - g e n e r a t e d and g a s e o u s h y d r o g e n . I m m e r s i o n of u n s t r e s s e d s p e c i m e n s in an aqueous NaC1-KzCrO4 solution i s shown to l e a d to a b s o r p t i o n of h y d r o g e n and to a f o r m of i n t e r n a l h y d r o g e n e m b r i t t l e m e n t s i m i l a r to t h a t o b s e r v e d in BCC m e t a l s . T h u s e m b r i t t t e m e n t w a s m a n i f e s t e d b y l o s s of d u c t i l i t y in t e n s i l e t e s t s and by c l e a v a g e - l i k e fracture, s u r f a c e s ; vacuum a n n e a l i n g a f t e r i m m e r s i o n l e d to a r e d u c t i o n in h y d r o g e n c o n c e n t r a t i o n and t o a p a r t i a l r e c o v e r y of d u c t i l i t y ; and e m b r i t t l e m e n t w a s not o b s e r v e d at high s t r a i n r a t e s . E m b r i t t l e d s p e c i m e n s a l s o e x h i b i t e d d e l a y e d f a i l u r e in both c o n s t a n t - l o a d t e n s i l e t e s t s and c o n s t a n t - d e f l e c t i o n bend t e s t s . In a s e c o n d s e r i e s of e x p e r i m e n t s , slow c r a c k g r o w t h w a s o b s e r v e d when e d g e - n o t c h e d t e n s i l e s p e c i m e n s w e r e s t r e s s e d in ~ I p s i g (~ 6.9 • 103 N/m 2) d r y h y d r o g e n ; the f r a c t u r e s u r f a c e s w e r e a g a i n c l e a v a g e - l i k e in a p p e a r a n c e . The s i g n i f i c a n c e of t h e s e o b s e r v a t i o n s i s d i s c u s s e d in r e l a t i o n to the s t r e s s - c o r r o s i o n f a i l u r e of t h i s a l l o y . R E C E N T s t u d i e s have l e d to the c o n c l u s i o n that the t r a n s g r a n u l a r s t r e s s - c o r r o s i o n c r a c k i n g (SCC) of a Mg-7.5 w t p c t At a l l o y in an aqueous NaC1-K2CrO4 s o l u tion o c c u r s b y d i s c o n t i n u o u s c l e a v a g e on {3140} p l a n e s . ~'~ C r a c k i n i t i a t i o n and p r o p a g a t i o n w e r e found to be a s s o c i a t e d with c o p i o u s e v o l u t i o n of h y d r o g e n , s u g g e s t i n g t h a t c r a c k i n g m a y b e a f o r m of h y d r o g e n e m b r i t t l e m e n t (HE). T h e r e a p p e a r s to b e no e v i d e n c e in the l i t e r a t u r e that Mg a l l o y s a r e s u s c e p t i b l e to HE and h e n c e the p r e s e n t w o r k w a s u n d e r t a k e n to e x p l o r e this possibility. Specifically, experiments were carried out to d e t e r m i n e w h e t h e r e x p o s u r e of u n s t r e s s e d s p e c i m e n s to the t e s t s o l u t i o n c a n l e a d to h y d r o g e n a b s o r p tion and s u b s e q u e n t e m b r i t t l e m e n t . A f u r t h e r s e r i e s of t e s t s w a s c o n d u c t e d in which s p e c i m e n s w e r e s t r e s s e d in a g a s e o u s - h y d r o g e n a t m o s p h e r e .
EXPERIMENTAL The s t u d i e s w e r e c a r r i e d out on the h i g h - p u r i t y b i n c r y a l l o y c o n t a i n i n g 7.5 wt p e t A1 which was u s e d in the p r e v i o u s s t u d i e s . ' ' 2 T h r e e t y p e s of s p e c i m e n s w e r e m a c h i n e d f r o m sheet, n a m e l y : i) t e n s i l e s p e c i m e n s of d i m e n s i o n s 7.5 c m • 0.65 c m • 0.05 cm, with a gage s e c t i o n of 5.10 c m • 0.5 c m . ii) e d g e - n o t c h e d t e n s i l e s p e c i m e n s 8.9 e m • 2.85 c m • 0.125 c m with a 45 d e g notch e x t e n d i n g to a depth of 0.5 c m at one e d g e ; t h i s t y p e of t e s t p i e c e w a s u s e d in previous experiments. 2 iii) bend s p e c i m e n s of d i m e n s i o n s 7.5 c m • 0.25 c m • 0.25 c m , a l s o u s e d p r e v i o u s l y . T h e bend s p e c i m e n s w e r e h e a t t r e a t e d to p r o d u c e c o a r s e g r a i n s , and then s o l u t i o n t r e a t e d at 385~ f o r 4 h and w a t e r quenched. Both t y p e s of t e n s i l e s p e c i m e n s w e r e s o l u t i o n t r e a t e d and quenched f r o m 385~C, p r o ducing a g r a i n s i z e c o r r e s p o n d i n g to ASTM 8. P r i o r to t e s t i n g , the s u r f a c e s w e r e c l e a n e d by e t c h i n g with a D. G. CHAKRAPANt, formerly a Research Assistant in the Department of Metallurgy and Mining .Engineering, University of Illinois, tL, is now Corrosion-Control Analyst with the Physical Plant at that University. E. N. PUGH ~sProfessor of Metallurgical EngineerLn_gand a member of the Materials Research Laboratory at the University of Illinois, Urbana, IL 61801. Manuscript submitted June 13, 1975,
METALLURGICAL TRANSACTIONS A
s o l u t i o n c o n t a i n i n g 60 pct e t h y l e n e glycol, 20 pet g l a c i a l a c e t i c acid, 19 p c t w a t e r and 1 p c t c o n c e n t r a t e d n i t r i c acid. T y p e i) t e n s i l e s p e c i m e n s w e r e i m m e r s e d (uns t r e s s e d ) at r o o m t e m p e r a t u r e in the aqueous NaCt-K~CrO4 s o l u t i o n u s e d in the e a r l i e r s t r e s s c o r r o s i o n s t u d i e s . I ,z Only the g a u g e l e n g t h s w e r e e x p o s e d , the r e m a i n i n g s u r f a c e s b e i n g c o a t e d with L a c o m i t . C o r r o s i o n p o t e n t i a l s w e r e m e a s u r e d by m e a n s of a s a t u r a t e d c a l o m e l e l e c t r o d e i m m e r s e d in a s a t u r a t e d KCI solution, using an A g a r b r i d g e . A f t e r i m m e r s i o n , the s p e c i m e n s w e r e r i n s e d with d i s t i l l e d w a t e r and m e t h a n o l , and then d r i e d . S a m p l e s , 1.2 c m • 0.5 cm, w e r e cut f r o m the gage l e n g t h s of s p e c i m e n s i m m e r s e d f o r v a r i o u s t i m e s and a n a l y z e d f o r h y d r o g e n c o n c e n t r a t i o n by Leco T e c h n i c a l S e r v i c e s L a b o r a t o r y (St. J o s e p h , Michigan), using the i n e r t g a s f u s i o n m e thod. H y d r o g e n a n a l y s i s w a s a l s o c a r r i e d out on s p e c i m e n s i m m e r s e d f o r 24 h and then a n n e a l e d f o r 4 h at 385~ in P y r e x t u b e s w h i c h w e r e s e a l e d u n d e r a v a c u u m of ~ 10 -5 T o r r (1.33 • 10-3 Pc). V a r i o u s t e s t s w e r e c a r r i e d out on the t y p e i) t e n s i l e s p e c i m e n s a f t e r i m m e r s i o n f o r v a r i o u s t i m e s . In one s e t of e x p e r i m e n t s , s p e c i m e n s w e r e t e s t e d to f a i l u r e in a i r in an I n s t r o n m a c h i n e , using v a r i o u s s t r a i n r a t e s ; specimens were also tested after being vacuum ann e a l e d (4 h at 385~ and w a t e r quenched~ A s e c o n d s e t of s p e c i m e n s w a s t e s t e d in a i r under c o n s t a n t load. D e l a y e d f a i l u r e o c c u r r e d u n d e r t h e s e c o n d i t i o n s and the t i m e to f a i l u r e , t F, w a s d e t e r m i n e d a s a function of load. F r a c t u r e s u r f a c e s w e r e e x a m i n e d with a J E O L 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 (SEM); in s o m e c a s e s , e l e m e n t a l a n a l y s i s w a s c a r r i e d out on the f r a c t u r e s u r f a c e s u s i n g an X - r a y e n e r g y d i s p e r s i v e a n a l y s e r (Princeton Gamma-Tech). T e s t s w e r e c a r r i e d out on bend s p e c i m e n s w h i c h w e r e a l s o p r e e x p o s e d to the t e s t solution. T h e s e w e r e s t r e s s e d u n d e r c o n s t a n t d e f l e c t i o n in a i r in a t h r e e point bend j i g d e s c r i b e d p r e v i o u s l y . ~ D e l a y e d f a i l u r e a g a i n o c c u r r e d , and the a c o u s t i c e m i s s i o n d u r i n g c r a c k ing w a s m o n i t o r e d u s i n g a d i f f e r e n t i a l t r a n s d u c e r of n a t u r a l f r e q u e n c y 225 KHz (Dunegan m o d e l 140B) and a Dunegan m o d e l 702 Audio M o n i t o r and m o d e l 301 T o t a l i z e r ; the d e t a i l s of t h i s t e c h n i q u e w e r e a l s o d e s c r i b e d in the e a r l i e r p a p e r , x S t r e s s - c o r r o s i o n f a i l u r e s w e r e VOLUME 7A, FEBRUARY 1976-t 73
also produced in the bend rig using the aqueous NaCI-K~CrO4 solution, the main function being to permit elemental analysis to be carried out on the fracture surfaces using the X-ray energy dispersive analyzer. Edge-notched tensile specimens were tested in a chamber containing ~ I psig (6.90 x 103 N/m e) gaseous hydrogen; Linde Ultra-High Purity Grade Hydrogen was used with no additional purification. The specimens were stressed to specific tensile loads and the cross-head stopped, subjecting the specimen to a constant load. This type of test is commonly used to produce s l o w - c r a c k growth in s t r e s s - c o r r o s i o n t e s t s , 2'3 but to p r o d u c e c r a c k i n g in the g a s e o u s e n v i r o n m e n t it w a s n e c e s s a r y to f i r s t i n t r o d u c e a fatigue c r a c k , ~ 0.25 c m long, b y c y c l i c t e n s i l e l o a d i n g in the r a n g e 440 to 2200 N.
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Fig. 1--Relationship between hydrogen concentration and t i m e of i m m e r s i o n (unstressed) in the t e s t solution. The points r e p r e s e n t a v e r a g e s of s e v e r a l a n a l y s e s (the n u m b e r of which is given beside the point) and the b a r s give the highest and lowe s t values, r e s p e c t i v e l y .
RESULTS 200
E f f e c t s of I m m e r s i o n of U n s t r e s s e d S p e c i m e n s in the T e s t Solution E 150
C o r r o s i o n and h y d r o g e n a b s o r p t i o n . P i t s w e r e i n i t i a t e d at the e x p o s e d s u r f a c e s of the type i) t e n s i l e s p e c i m e n s a f t e r a p p r o x i m a t e l y 1 h, and the n u m b e r and depth of the p i t s i n c r e a s e d with t i m e . The c o r r o s i o n p o t e n t i a l was initially - 1220 mVH and slowly became more active as pitting proceeded, attaining a constant value of - 1280 mVH after approximately 24 h. Gas bubbles were evolved from the pits, and mass-spectrometric analysis established that it was hydrogen. Analysis of samples immersed for various times indicated that the average hydrogen concentration increased progressively with time, Fig. 1. Vacuum annealing (4 h at 385~ can be seen to partially remove the absorbed hydrogen. _Effects on t e n s i l e p r o p e ~ i e s . Monotonic t e n s i l e t e s t s w e r e c a r r i e d out in l a b o r a t o r y a i r at a s t r a i n r a t e of 0.2 • 10 -3 s - L The r e s u l t s i n d i c a t e d that both UTS and p e r c e n t a g e e l o n g a t i o n d e c r e a s e s i g n i f i c a n t l y with i n c r e a s i n g i m m e r s i o n t i m e , F i g . 2. The f r a c t u r e s u r f a c e s of i m m e r s e d s p e c i m e n s c o n s i s t e d of flat, c l e a v a g e - l i k e f a c e t s , F i g . 3 (a) and (b), with o c c a s i o n a l a r e a s s h o w ing the d i m p l e d s t r u c t u r e c h a r a c t e r i s t i c of d u c t i l e f r a c t u r e , w h e r e a s the f r a c t u r e s u r f a c e s of noni m m e r s e d s p e c i m e n s e x h i b i t e d only d i m p l e s , Fig~ 3(c). V a c u u m a n n e a l i n g (4 h at 385~ of i m m e r s e d s a m p l e s l e d to a p a r t i a l r e c o v e r y of the m e c h a n i c a l p r o p e r t i e s , F i g . 2, and c a u s e d the f r a c t u r e s u r f a c e s to b e c o m e p r e d o m i n a n t l y d i m p l e d although o c c a s i o n a l c l e a v a g e l i k e r e g i o n s p e r s i s t e d . It is e v i d e n t that the e f f e c t of i m m e r s i o n i s due p a r t l y to the r e d u c t i o n in c r o s s - s e c t i o n a l a r e a by c o r r o s i o n and p a r t l y to a r e v e r s i b l e e m b r i t t l e m e n t phenomenon. A s e r i e s of t e s t s w e r e a l s o c a r r i e d out at s t r a i n r a t e s in the r a n g e 0.2 to l l . 0 x 10-3 s -~ on s p e c i m e n s which had b e e n i m m e r s e d f o r 8 h in the t e s t s o l u t i o n ; a p a r a l l e l s e r i e s of t e s t s w e r e c o n d u c t e d on s p e c i m e n s i m m e r s e d 8 h and then v a c u u m a n n e a l e d f o r 4 h. The results demonstrated a marked strain-rate dependence, t h e t e n s i l e p r o p e r t i e s of s p e c i m e n s t e s t e d at r a t e s up to 5.5 x 10 -3 s -x b e i n g s i g n i f i c a n t l y l o w e r than t h e i r v a c u u m - a n n e a l e d c o u n t e r p a r t s , w h e r e a s t h e r e w e r e no d i f f e r e n c e s at 1 1 . 0 • 10-3 s ~, F i g . 4. F u r t h e r , the f r a c t u r e s u r f a c e s of s p e c i m e n s t e s t e d at 11o0 x 10 -3 s -~ were entirely dimpled. 174-VOLUME 7A, FEBRUARY 1976
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Fig. 2--The dependence of UTS and pct elongation on immersion time for tensile specimens tested at 0.2 x 10-3 s-I in air after immersion. The experimental points represent the averages of at least three tests, and the bars indicate the highest and lowest values.
It might be a r g u e d that the c l e a v a g e - l i k e s u r f a c e s shown in F i g . 3 r e s u l t f r o m anodic d i s s o l u t i o n d u r i n g i m m e r s i o n r a t h e r than d u r i n g t e n s i l e t e s t i n g , s i n c e d i s s o l u t i o n c a n p r o d u c e f a c e t e d s u r f a c e s of s i m i l a r g e n e r a l a p p e a r a n c e . To r e s o l v e t h i s question, e l e m e n t a l a n a l y s i s of the f r a c t u r e s u r f a c e w a s p e r f o r m e d in the SEM using an X - r a y e n e r g y - d i s p e r s i v e a n a l y s e r , and the r e s u l t s w e r e c o m p a r e d with t h o s e f o r o t h e r s u r f a c e s . It w a s found that s u r f a c e s e x p o s e d to the t e s t solution, v i z c o r r o d e d s u r f a c e s and s t r e s s - c o r r o s i o n f r a c t u r e f a c e s , d i s p l a y e d t r a c e s of both C1 and Cr, w h e r e a s the c l e a v a g e - l i k e s u r f a c e s and the d i m p l e d s u r f a c e s p r o d u c e d in n o n i m m e r s e d s p e c i m e n s showed no e v i d e n c e f o r the p r e s e n c e of t h e s e e l e m e n t s , F i g . 5. T h u s it i s c o n c l u d e d that the c l e a v a g e - l i k e s u r f a c e s a r e in f a c t p r o d u c e d d u r i n g t e n s i l e t e s t i n g , and r e s u l t f r o m brittle fracture. ~layed-failure tests. BCC metals containing hydroMETALLURGICAL TRANSACTIONS A
gen commonly undergo delayed f a i l u r e under constant load, e . g . high strength s t e e l s . 4 To d e t e r m i n e whether a s i m i l a r effect o c c u r s in the p r e s e n t case, type i) t e n s i l e s p e c i m e n s w e r e i m m e r s e d for 8 h in the t e s t s o l u tion and then s t r e s s e d in a i r under constant t e n s i l e loads. Delayed f a i l u r e o c c u r r e d , and t F was o b s e r v e d to d e c r e a s e with i n c r e a s i n g load, Fig. 6. The r e s u l t i n g fracture surfaces were predominantly cleavage-like, indistinguishable from those produced in monotonic t e n s i l e t e s t s , Fig. 3. Delayed f a i l u r e was not o b s e r v e d
in the c a s e of s p e c i m e n s which w e r e not p r e e x p o s e d to the t e s t solution, e s t a b l i s h i n g that the effect w a s due to the p r e s e n c e of i n t e r n a l hydrogen r a t h e r than to SCC in laboratory air. Delayed f a i l u r e was also produced in c o n s t a n t - d e flection bend t e s t s in a i r on s p e c i m e n s i m m e r s e d for 8 h in the test solution. The s p e c i m e n d i m e n s i o n s and s t r e s s i n g conditions were the s a m e as those u s e d in the s t r e s s - c o r r o s i o n t e s t s . D i s c r e t e acoustic e m i s sions were detected during the propagation of s t r e s s - c o r r o s i o n c r a c k s , 1 and consequently this t e c h nique was used to m o n i t o r the p r e s e n t bend t e s t s . A s m a l l background s i g n a l was detected via the audio m o n i t o r f r o m u n s t r e s s e d specimens, Fig. 7(a). A p p l i cation of the s t r e s s led i m m e d i a t e l y to the e m i s s i o n of d i s c r e t e s i g n a l s of l a r g e amplitude. These s i g n a l s w e r e m o r e frequent i m m e d i a t e l y a f t e r the application of the s t r e s s , Fig. 7 (b), becoming l e s s frequent with t i m e , e .g., F i g . 7 (c), and ceasing completely after app r o x i m a t e l y 60 min. Application of a f u r t h e r i n c r e m e n t of s t r e s s caused the p r o c e s s to be r e p e a t e d . A t y p i c a l sequence is i l l u s t r a t e d in the t o t a l i z e r d a t a of Fig. 8, in which both the count r a t e and the cumulative number of counts a r e plotted vs t i m e . In this e x p e r i m e n t , the gain was such that the background signal made little contribution to the observed count r a t e . Examination of s p e c i m e n s after s e v e r a l sequences of s t r e s s i n g r e v e a l e d the p r e s e n c e of numerous c r a c k s extending f r o m the pitted r e g i o n at the tension face. S e v e r a l s p e c i m e n s w e r e r a p i d l y f r a c t u r e d to expose the f r a c t u r e s u r f a c e s of these c r a c k s and SEM e x a m ination indicated that the s u r f a c e s w e r e c l e a v a g e - l i k e , indistinguishable f r o m those produced in t e n s i l e t e s t s on e m b r i t t l e d s p e c i m e n s . C o m p a r a t i v e t e s t s on s p e c i mens which w e r e not i m m e r s e d led neither to the d i s c r e t e acoustic e m i s s i o n nor to the o c c u r r e n c e of d e tectable cracks.
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Fig. 3--(a) and (b). Scanning e l e c t r o n m i c r o g r a p h s i l l u s t r a t ing cleavage-like f r a c t u r e s u r f a c e s in a tensile s p e c i m e n f r a c t u r e d in a i r (0.2 x 10 -3 s "l) a f t e r i m m e r s i o n (unstressed) for 8 h in the t e s t solution. (c) F r a c t u r e s u r f a c e of a nonimm e r s e d s p e c i m e n , exhibiting the dimpled s t r u c t u r e c h a r a c t e r istic of ductile rupture. METALLURGICAL TRANSACTIONS A
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Fig. 4--Effect of s t r a i n r a t e on the tensile p r o p e r t i e s of s p e c i mens tested in a i r a f t e r i m m e r s i o n (unstressed) for 8 h in the t e s t solution. The e x p e r i m e n t a l points r e p r e s e n t the a v e r a g e s of at l e a s t t h r e e t e s t s , and the b a r s indicate the highest and lowest values. VOLUME 7A, FEBRUARY ]976-175
T e s t s in gaseous hydrogen. Edge-notched tensile s p e c i m e n s were s t r e s s e d at various loads (4400 to 6850 N) beyond the elastic limit in g a s e o u s hydrogen but no slow crack growth w a s detected. Rapid s t r e s s c o r r o s i o n failure o c c u r s in this alloy under these c o n ditions of s t r e s s i n g , e'3 Specimens containing fatigue c r a c k s extending from the notches did undergo s l o w c r a c k growth in the hydrogen environments when loaded in the range 4400 to 5400 N. In air, such s p e c i m e n s exhibited elastic behavior up to loads of ~ 3550 N and fractured at ~ 9300 N; no delayed failure w a s observed
1500
i
I
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i
t
PITTEDAREAS
t c';"
1500
Mg[ Kcl
STRESS-CORROSION FRACTURE SURFACE I000
Cr'Ka
50C
s 1500
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FRACTURE SURFACE OF SPEC~MEN~
Z
TESTED IN AIR, EMBRITTLED BY IMMERSION FOR 5 H iN SOLUTION,
IO00
~
0
.... __..A~ . -g e
15OO
I~,___-A
DUCTILE FRACTURE SURFACE IOOO
500
Fig. 7 - - T y p i c a l o s c i l l o s c o p e t r a c e s f r o m a c o u s t i c - e m i s s i o n s t u d i e s o f b e n d s p e c i m e n s t e s t e d in a i r a f t e r i m m e r s i o n (uns t r e s s e d ) f o r 8 h in t h e t e s t s o l u t i o n : (a) s p e c i m e n u n s t r e s s e d , (b) i m m e d i a t e l y a f t e r t h e a p p l i c a t i o n o f t h e s t r e s s , a n d (c) 15 rain a f t e r the a p p l i c a t i o n o f s t r e s s ,
I
0
..... ..L-..-.,, ~ 0
t.O
~,~,--_J_ ~,. . . . . . }, 3,0 4.0 5,0 6.0 X-RAY ENERGY (eV x I03)
Fig. 5 - - E l e m e n t a l a n a l y s e s f a c e s p r o d u c e d by pitting. 350
~
2.0
of f r a c t u r e
/
......... 7.0
s u r f a c e s a n d of s u r to sc 6C
T
5"
_...~
4o 300 o
,< 250 o J
o
2O0
o
i iO TIME TO FAILURE (h}
o.*-.-
t00
Fig. 6 ~ R e l a t i o n s h i p b e t w e e n t i m e f o r d e l a y e d f a i l u r e in a i r a n d a p p l i e d t e n s i l e toad f o r s p e c i m e n s i m m e r s e d ( u n s t r e s s e d ) f o r 8 h in the t e s t s o l u t i o n , E a c h point r e p r e s e n t s t h e r e s u l t of a single test. 1 7 6 - V O L U M E 7A, FEBRUARY 1976
5
~0
50
35
40
60
65
TiME (rain) Fig. 8--Acoustic
emission
vs
time for bend specimen
stressed
in a i r a f t e r i m m e r s i o n ( u n s t r e s s e d ) f o r 5 h in t h e t e s t s o l u t i o m T h e a r r o w i n d i c a t e s the t i m e a t w h i c h the s t r e s s w a s a p p l i e d , METALLURGICAL TRANSACTIONS A
in air. Slow c r a c k growth in hydrogen led to a p r o g r e s s i v e d e c r e a s e in load, since the c r o s s - h e a d was fixed, and the c r a c k s eventually became a r r e s t e d . Application of a small increment of load (~250 N) caused further propagation. Generally two or three such cycles were c a r r i e d out for each specimen, p r o ducing a c r a c k length of ~0.76 cm in ~24 h. The specimens were then rapidly s t r e s s e d to produce tensile fracture of the remaining section. The resulting fracture surfaces exhibited three distinct zones, l) a striated region adjacent to the notch, corresponding to the preexisting fatigue crack, Ii) a cleavage-like region, Fig. 9, corresponding to slow c r a c k growth, and iii) the dimpled zone produced by final ductile rupture, The cleavage-like zone can be seen to be similar in appearance to the s u r f a c e s p r o duced in embrittled specimens, of., Figs. 3 and 8.
DISCUSSION Exposure of unstressed specimens to the test solution was shown to lead to pitting and copious hydrogen evolution. No attempt was made to estimate the pH within pits or s t r e s s - c o r r o s i o n cracks, but previous work by Brown s in this system indicated that the pH within the latter is ~ 11o ,At first sight, it might appear surprising that copious hydrogen evolution occurs in solutions of low hydrogen-ion concentration, but it should be noted that the corrosion potential {about - 1 2 8 0 mV~/) is sufficiently active to permit the direct reduction of water at significant rates. The rate of water reduction is probably limited by the anodic kinetics, and thus it is possible that hydrogen evolution was confined largely to pits and from within c r a c k s because of the high rates of local anodic dissolution at f i l m - f r e e regions at these sites. Analysis of corroded samples indicated that hydrogen absorption occurs, amt there is little doubt that the r e versible embrittlement in i m m e r s e d specimens r e s u l t s f r o m the presence of hydrogen. Thus the effect of ~acuum annealing, Fig. 2, can be accounted for by p~rtial outgassing of hydrogen, and the s t r a t a - r a t e dependence, Fig. 4, by the necessity for hydrogen diffusion to occur in the lattice during straining. The similarity between the fracture surfaces produced by slow c r a c k growth in i m m e r s e d specimens and in specimens tested in g a s e ous hydrogen (cf,, Figs. 3 and 9) further strengthens the view that HE can be induced by exposure to the test solution. The mechanism of HE in this system remains to be established. The acoustic-emission studies suggest that s l o w - c r a c k gro~th is discontinuous, and thus it can be argued that cracking involves repeated c y c l e s of hydrogen diffusion to the region ahead of the c r a c k tip (slow) followed by cleavage through the hydrogen rich zone (rapid). As in other c a s e s of HE, the precise role of hydrogen in inducing cleavage is not known~ The formation of brittle hydrides offers a possible explanation, and TEM studies are being undertaken to investigate this. The decohesion models 6, ~ are alternative possibilities, but it is difficult to devise experiments to evaluate them. Finally, it should be noted that hydrogen was observed to be evolved during the initiation and propagation of
METALLURGICAL TRANSACTIONS A
Fig. 9--Scanning e l e c t r o n m i c r o g r a p h s illustrating cleavage ~ like f r a c t u r e s u r f a c e s in e d g e - n o t c h e d tensile s p e c i m e n s tested in ~ t psig hydrog~en*
s t r e s s - c o r r o s i o n c r a c k s , and thus the preceding a r g u ments strongly suggest that SCC in this system is e s sentially a form of HE. Such a view is supported by the similarity between the discrete acoustic emissions occurring during SCC I and s l o w - c r a c k growth in imm e r s e d bend specimens, Fig. 7. If the mechanism of SCC and HE were essentially the same then the orientations of the fracture surfaces would be expected to be the same, It was not possible to determine orientation of the cleavage planes in the present work, but the previous study established that SCC occurs on {3140} ptanes. 1 No explanation has been given for this s o m e what irrational plane, but it may be speculated to correspond to the habit or cleavage piane of a hydride. Another factor requiring further attention is the fractography. Both SCC and the HE failures studied in this paper are c h a r a c t e r i z e d by cleavage-like fracture surfaces, but significant differences exist. In p a r t i c u lar, the facets shown in Figs. 3 and 9 are flatter than those observed in SCC of polycrystals of this alloy, 2,a and do not exhibit the "pleated" structure of the latter, Such effects are not understood, but may be related to the differences in hydrogen fugacity and kinetics of hydrogen entry in the different cases, and to the fact
VOLUME 7A~ FEBRUARY t 976-177
that anodic dissolution occurs stress-corrosion conditions.
at the crack
tip under
ACKNOWLEDGMENTS T h e a u t h o r s a r e i n d e b t e d t o H. J . E n g e l l f o r h e l p f u l discussions. This research was supported in part by the U.S. Energy Research and Development Administration, Contract AT(ll-1)-l198.
1 7 8 - V O L U M E 7A, FEBRUARY 1976
RE F E R E N C E S I. D. G. Chakrapani and E. N. }*ugh:Met. Trans. A, 1975, vol. 6A, p. 1155. 2. D. G. Chakrapani and E. N. Pugh: Corrosion, 1975, vol. 31, p. 247. 3. E. N. ]?ugh,J. A. S. Green, and P. W. Slattery: Fracture 1969, p. 387, Chapman and Hall, London, 1969. 4. A. R. Elsea and E. E. Fletcher: Hydrogen-Induced Delayed Brittle Failures of High-Strength Steels, DMIC Report 196 (1964). 5. B. F. Brown: N. R. L. Report 7168, p. 21, N. R. L., Washington, D. C., 1970. 6. A. R. Troiano: Tran~ ASM, 1960, vol. 52, p. 54. 7. R. A. Oriani: Bet. Bunsenges. Phys. CherrL, 1972, vol. 76, p. 848.
METALLURGICAL TRANSACTIONS A