Hot-salt stress-corrosion of a titanium alloy in a dynamic air environment
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L A B O R A T O R Y i n v e s t i g a t i o n s have d e m o n s t r a t e d that t i t a n i u m alloys exhibit e m b r i t t l e m e n t a f t e r e x p o s u r e to h a l i d e s while being s t r e s s e d in the t e m p e r a t u r e r a n g e 500 ~ to 900~ (260~ to 482~ This phenomenon has b e e n t e r m e d h o t - s a l t s t r e s s - c o r r o s i o n . A field i n v e s t i g a t i o n 1 has d e m o n s t r a t e d c o n c l u s i v e l y that the q u a n t i t i e s of s a l t deposited on c o m p r e s s o r a i r f o i l s of c o m m e r c i a l and m i l i t a r y flight a i r c r a f t d u r i n g t y p i c a l o p e r a t i n g conditions a r e in e x c e s s of the m i n i m u m a m o u n t s r e p o r t e d to cause e m b r i t t l e m e n t in l a b o r a t o r y tests. Although the design s t r e s s e s and t e m p e r a t u r e s of many current aircraft gas-turbine components are within the r a n g e s where such e m b r i t t l e m e n t has b e e n o b s e r v e d in l a b o r a t o r y t e s t s , a no s e r v i c e f a i l u r e s have yet b e e n r e p o r t e d that could be c o n c l u s i v e l y a t t r i b u t e d to t h i s p h e n o m e n o n . The r a t i o n a l i z a t i o n for the lack of s e r v i c e f a i l u r e s is not c l e a r , although s o m e i n v e s t i g a t o r s have postulated that engine e n v i r o n m e n t a l c o n d i t i o n s , p a r t i c u l a r l y high a i r v e l o c i t i e s , high p r e s s u r e s , s a l t - a i r c o n d i t i o n s , oil c o n t a m i n a t i o n , a n d / o r the unique c y c l i c o p e r a t i n g c o n d i t i o n s of an engine inhibit embrittlement. A c o m m o n l a b o r a t o r y t e s t technique to e s t a b l i s h if h o t - s a l t s t r e s s - c o r r o s i o n is o c c u r r i n g involves the e x p o s u r e of s t a t i c a l l y loaded, h e a v i l y s a l t - c o a t e d s p e c i m e n s in a static a i r e n v i r o n m e n t in the t e m p e r a t u r e r a n g e 500 ~ to 900~F (260~ to 482~ for a specified t i m e p e r i o d , u s u a l l y 100 h r . The exposed s p e c i m e n s a r e then e x a m i n e d for v i s u a l e v i d e n c e of c o r r o s i o n o r c r a c k i n g , and m a y also be s u b j e c t e d to m e c h a n i c a l t e s t i n g , such a s bend or t e n s i l e t e s t s . The r e s u l t s a r e then i n t e r p r e t e d on a c r a c k - n o c r a c k or an e m b r i t t l e m e n t - n o e m b r i t t l e m e n t b a s i s . The b o u n d a r y line on a plot of s t r e s s a g a i n s t t e m p e r a t u r e , s e p a r a t i n g r e g i o n s of c r a c k i n g f r o m no c r a c k i n g o r e m b r i t t l e m e n t f r o m no e m b r i t t l e m e n t , has been t e r m e d the t h r e s h o l d c u r v e . Wide v a r i a t i o n s exist in t h r e s h o l d c u r v e s , see F i g . 1, d e t e r m i n e d by v a r i o u s i n v e s t i g a t o r s 3-~ b e c a u s e of diff e r e n c e s in t e s t conditions and the specific c r i t e r i a used for defining h o t
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