Microdetermination of dissolved nitrogen and oxygen in a titanium alloy by secondary ion mass spectrometry
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7 CT:I.L::~J4~t/C3 dyne cm - 2
0 0
IO
20 ZlMf
30
40
Ihour s)
Fig. 1--Time dependent c r e e p c u r v e s of coated and uncoated specimens.
Table I. ComparativeSecondary Creep Rates of Anodized and Unanodized AU2GN (RR58) Alloy at 200~ and 22 H at Each Stress Level
Stress [10~dyne/cm2] Unanodized, [ lO-6h-t] %nodized, [lO-6h-1]
1.294
1.598
1.917
2.110
2.216
11.7 10.2
22.7 20.0
45.3 48.0
141.0 146.0
495.0 545.0
u n i f o r m t h r o u g h o u t t h e d i a m e t e r of t h e s p e c i m e n s . T h e h a r d n e s s w a s 1 2 . 1 7 • 10 7 d y n e / c m z f o r b o t h t h e anodized and unanodized specimens~ This is contrary to the hardness gradient which was observed in anod i z e d a n n e a l e d s p e c i m e n s a f t e r c r e e p . .8 Discussion. Several mechanisms have been proposed t o e x p l a i n t e n s i l e a n d c r e e p s t r e n g t h e n i n g d u e to a s o l i d s u r f a c e l a y e r ; a) b l o c k i n g e g r e s s of d i s l o c a t i o n s t o t h e s u r f a c e ;6 b) r e p r e s s i n g m u l t i p l i c a t i o n of d i s l o c a t i o n s f r o m s u r f a c e s o u r c e s ; 1~ c) e l a s t i c r e p u l s i o n of d i s l o cations from the surface when the coating has a higher s h e a r m o d u l u s t h a n t h a t of t h e b u l k s u b s t r a t e ; 2~ d) i m p e d i n g a p p r o a c h of i n t e r n a l l y g e n e r a t e d d i s l o c a t i o n s t o t h e s u r f a c e b y z o n e s of h i g h e r d i s l o c a t i o n d e n s i t i e s w h i c h w e r e f o r m e d a t t h e s u r f a c e b y c r a c k i n g of t h e c o a t i n g d u r i n g d e f o r m a t i o n ; z* e) a c o n t i n u o u s l y f o r m ing solid oxide layer may dull crack tips in the substrate, thus retarding crack propagation and reducing c r e e p r a t e . i~ T h e s e f i v e m e c h a n i s m s d o n o t i n c l u d e two additional mechanisms l~'n which were proposed f o r r e t a r d a t i o n of N a b a r r o - H e r r i n g type creep at very high temperatures and low stresses which are outside t h e r a n g e of o u r e x p e r i m e n t s . A p p a r e n t l y , n o n e of t h e m e c h a n i s m s a) t o d) i s a p p l i c a b l e t o o u r c a s e w h e r e the anodic coating did not improve creep rates. This b e h a v i o r m a y b e e x p l a i n e d b y t h e f a c t t h a t t h e two a l loys used in this investigation were strengthened by well distributed coherent precipitates which served a s o b s t a c l e s t o m o t i o n o f d i s l o c a t i o n s in t h e a l l o y s . Considering the difficulties dislocations experience i n m o v i n g i n t h e b u l k of a c r y s t a l l i n e s o l i d w h i c h i s at its optimal strength, it is doubtful whether a surface obstacle may serve as an additional strengthening agent. This is in contrast to the relatively unobstructed m o t i o n of d i s l o c a t i o n s i n t h e b u l k a n d t o w a r d s t h e s u r f a c e of a n n e a l e d m a t e r i a l w h e r e s o l i d s u r f a c e c o a t i n g s have been obs
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