A study of precipitation at elevated temperatures in a Mg-8.7 pct Y alloy
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the effect to n i t r i d e f o r m a t i o n at g r a i n b o u n d a r i e s , with r e s u l t a n t i n t e r g r a n u l a r c r a c k i n g , s i m i l a r to that obs e r v e d in the p a p e r being d i s c u s s e d . Although J o h n s o n , e t a l . , a r e c o n c e r n e d with gas c o n t e n t s at i m p u r i t y l e v e l s , and we worked with n i t r o gen at a t m o s p h e r i c p r e s s u r e , t h e r e is a m p l e evidence that only a s m a l l a m o u n t of r e a c t i v e gas is r e q u i r e d to i n c r e a s e r a t e s of c r a c k p r o p a g a t i o n s u b s t a n t i a l l y . F o r e x a m p l e , the r u p t u r e life of n i c k e l at 10 -4 t o r r of oxygen is about one half that in a v a c u u m of 10 -7 t o r r . 29 I n c r e a s i n g the oxygen p r e s s u r e p a s t 10 -4 t o r r has no f u r t h e r effect and the c u r v e , of r u p t u r e life vs p r e s s u r e , l e v e l s out to a p l a t e a u . J o h n s o n e t al. r e p o r t a n i t r o g e n p r e s s u r e of a p p r o x i m a t e l y 2 • 1 0 -6 a t m (about 10 -3 t o r r ) . It is p r o b a b l y safe to say that this p r e s s u r e is sufficient to put their data on the p l a t e a u w h e r e the e n v i r o n m e n t a l effect is a m a x i m u m . It is concluded that n i t r i d e f o r m a t i o n at g r a i n b o u n d a r i e s is the m o s t p r o b a b l e m e c h a n i s m to a c c o u n t for the e n v i r o n m e n t a l effect. 28. P. Shahmianand M. R. Achter:Proc. of the Joint Int. Conf. on Creep, 1963, Sect. 7, pp. 49-57. Inst. of Mech. Eng., 1963. 29. R. L. Stegman,P. Shahmian,and M. R. Achter: Trans. TMS-AIME, 1969, vol. 245, pp. 1759-63.
A Study of Precipitation at Elevated Temperatures in a Mg-8.7 Pct Y Alloy
Discussion of "The Effect of Environment and Grain Size on the Creep Behavior of a Ni-6 Pct W Solid Solution"" W. R. JOHNSON, C. R. B A R R E T T , AND W. D. NIX In t h e i r i n t e r e s t i n g p a p e r the a u t h o r s have r e p o r t e d , among other u s e f u l f a c t s , that the r u p t u r e life of a N i - 6 pct W alloy is s h o r t e r in c o m m e r c i a l p u r i t y a r g o n than in v a c u u m . They cite the p o s s i b i l i t y of g r a i n b o u n d a r y c o n t a m i n a t i o n by an i m p u r i t y in the gas as an e x p l a n a tion for t h e i r o b s e r v a t i o n . It is the p u r p o s e of this note to cite p r e v i o u s l y p u b l i s h e d data which s u p p o r t s this m e c h a n i s m and which can be used to identify the p r o b able i m p u r i t y in the gas used by J o h n s o n e t a l . In a study of the c r e e p of 99.8 pct Ni we found that n i t r o g e n s h o r t e n e d the r u p t u r e life, in c o m p a r i s o n to v a c u u m m o r e s h a r p l y than did oxygen. 28 We a t t r i b u t e d *W. R. JOHNSON, C. R. BARRETT, and W. D. NIX, Met. Trans., 1972, vol. 3, pp. 695-98. M. R. ACHTER is Consultant, Metallurgy Dwision, Naval Research Laboratory, Washington, D. C. 20390. Discussion submitted August 15, 1972. 3
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