Stress-corrosion cracking of Ti-8Al-lMo-lV in aqueous environments: 1. The kinetics of subcritical crack propagation
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P r e c r a c k e d s p e c i m e n s of T i - 8 A I - I M o - I V w e r e t e s t e d in s a l t - w a t e r , and the s u b c r i t i c a l c r a c k v e l o c i t y w a s m e a s u r e d a s a function of a p p l i e d s t r e s s - i n t e n s i t y . The m i n i m u m a p p l i e d s t r e s s i n t e n s i t y which p r o d u c e s s u b e r i t i c a l c r a c k i n g , K l s c c , v a r i e s i n v e r s e l y with the a v e r a g e c h l o r i d e - i o n c o n c e n t r a t i o n of the e n v i r o n m e n t , but Klscc does not v a r y s i g n i f i c a n t l y with t e m p e r a t u r e . P l o t s of s u b e r i t i c a l c r a c k v e l o c i t y , h, a g a i n s t a p p l i e d s t r e s s i n t e n s i t y , KI, r e v e a l two t y p e s of c r a c k i n g b e h a v i o r . At low s t r e s s i n t e n s i t i e s h is a p p r o x i m a t e l y p r o p o r t i o n a l to KI (Stage I c r a c k p r o p a g a t i o n ) . At high s t r e s s i n t e n s i t i e s , h is a p p r o x i m a t e l y c o n s ta n t , independent of KI (Stage II c r a c k p r o p a g a t i o n ) . The a p p a r e n t a c t i v a tion e n e r g y for s u b c r i t i c a l c r a c k p r o p a g a t i o n is 5600 c a l - ( g - m o l e ) -1, and & cc [Cy] 1/6. It is s u g g e s t e d that the i n s t a n t a n e o u s s u b c r i t i e a l c r a c k v e l o c i t y is l i m i t e d by the r a t e of p r o d u c tion o r by the diffusion of s o m e e m b r i t t l i n g s p e c i e s . T H E p h e n o m e n o l o g y of e n v i r o n m e n t - i n d u c e d s u b c r i t i cal c r a c k p r o p a g a t i o n in t i t a n i u m a l l o y s h a s been e x t e n s i v e l y d o c u m e n t e d , and the a v a i l a b l e data have b e e n s u m m a r i z e d and d i s c u s s e d in a r e c e n t r e v i e w . 1 With a few notable e x c e p t i o n s , m u c h of the r e s e a r c h in this a r e a h as b e e n confined to d e t e r m i n i n g if a given a l l o y in a gi v en m e t a l l u r g i c a l condition is or is not s u s c e p t i b l e to s t r e s s c o r r o s i o n in v a r i o u s e n v i r o n m e n t s . * While
c r i t i c a l c r a c k v e l o c i t y is a function of al l the v a r i a b l e s that c o n t r i b u t e to the m e c h a n i s m of c r a c k e x t e n s i o n , and it is a u s e f u l q u a n t i t a t i v e m e a s u r e of the influence of e a c h v a r i a b l e on the s t r e s s - c o r r o s i o n p r o c e s s . In addition to p r o v i d i n g q u a n t i t a t i v e data for m e c h a n i s t i c m o d e l s , m e a s u r e m e n t s of s u b c r i t i c a l c r a c k v e l o c i t y i n d i c a t e how long an e n g i n e e r i n g s t r u c t u r e can s u r v i v e in an a g g r e s s i v e e n v i r o n m e n t b e f o r e f a i l u r e o c c u r s . It t u r n s out that for the c a s e of t i t a n i u m a l l o y s *The evaluationof a material's"susceptibility"to stress-corrosioncrackingis quite subjective.The usual practice is to classify a materialsusceptible to stress which
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