An observation of the effect of grain structure on the appearance of kirkendall porosity

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An Observation of the Effect of Grain Structure on the Appearance of Kirkendall Porosity JOHN D. WHITTENBERGER R E C E N T r e s u l t s 1 of a r c - j e t e x p o s u r e t e s t i n g of pot e n t i a l m e t a l l i c t h e r m a l p r o t e c t i o n m a t e r i a l s for the p r o p o s e d Space Shuttle indicated s i g n i f i c a n t d i f f e r e n c e s in the a m o u n t of K i r k e n d a l l p o r o s i t y f o r m e d in two n o m i n a l l y Ni-20Cr-2ThO2* a l l o y s . In an a r c j e t t e s t , *Allcompositionsare expressedin wezghtpercent. s p e c i m e n s a r e s u b j e c t e d to a high t e m p e r a t u r e (>1200 K)-high v e l o c i t y ( a p p r o x i m a t e l y 3400 m / s ) - l o w p r e s s u r e ( a p p r o x i m a t e l y 103Pa) s t r e a m of a i r . The a r c j e t is d e s i g n e d to s i m u l a t e a p o r t i o n of a r e e n t r y t r a j e c t o r y . A f t e r 50-89h c y c l e s at n o m i n a l l y 1365 K, a D S - N i C r ( S h e r r i t t - G o r d o n M i n e s , Ltd., d e s i g n a t i o n for n o m i n a l l y N i - 2 0 C r - 2 T h O 2 alloys) s p e c i m e n had n e g l i gible K i r k e n d a l l p o r o s i t y while a T D - N i C r ( F a n s t e e l Inc. d e s i g n a t i o n for n o m i n a l l y Ni-20Cr-2ThO2 alloys) s p e c i m e n contained s i g n i f i c a n t p o r o s i t y . C e n t o l a n z i e t a l . ~ suggested that the d i f f e r e n c e in K i r k e n d a l l p o r o s i t y is due to the a b s e n c e of Cr203 p a r t i c l e s in D S - N i C r to s e r v e as pore n u c l e a t i o n s i t e s . As p a r t of a study 2 of s u b s t i t u t i o n a l diffusion in t h o r i a t e d and n o n t h o r i a t e d n i c k e l a l l o y s , s e v e r a l n o m i n a l l y Ni-20Cr-2ThO~:Ni type diffusion couples w e r e diffused at 1530 K. Upon m e t a U o g r a p h i c e x a m i n a tion one D S - N i C r : N i couple had n e g l i g i b l e K i r k e n d a l l p o r e f o r m a t i o n while the other couples contained s i g n i f i c a n t p o r o s i t y . It is b e l i e v e d that the d i f f e r e n c e in K i r k e n d a l l p o r o s i t y is r e l a t e d to the unique g r a i n

JOHN D. WHITTENBERGERis a MetallurgicalEngineer, NASALewis Research Center, Cleveland,Ohio 44135. Manuscript submitted April 12, 1972. 3038-VOLUME 3, NOVEMBER 1972

s t r u c t u r e of the p a r t i c u l a r lot of D S - N i C r used in this diffusion couple. V a r i o u s s o l i d - s o l i d diffusion s p e c i m e n s w e r e b u i l t by flanking each side of 0.3 c m thick piece of n i c k e l with n o m i n a l l y Ni-20Cr-2ThO2 a l l o y s . The m a t e r i a l s used to c o n s t r u c t the s p e c i m e n s a r e l i s t e d in T a b l e I. T h e n i c k e l p o s s e s s e d a l a r g e equiaxed g r a i n s t r u c t u r e while the thoriated a l l o y s had p a n c a k e - s h a p e d g r a i n s which w e r e p a r a l l e l to the s h e e t s u r f a c e . D S - N i C r ( g s 1)* *(gs 1) and (gs 2) designatedifferentgrainsizesand are beingusedfor zdentific

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