On subgrain formation during creep of 316 stainless steel
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1 +-~--3f ~
and
D 2 - 1 - - ~ -3f
11. K. Matuki,Y. Ueno,and M Yamada:J. Jap. Inst. Metals, Sendal, 1974, vol. 38, p. 219. 12. C. E. Mobley,A. H. Clauer,and B. A. Wilcox:J. Inst. Metals, 1972,vol. 100, p. 142. 13. B. Tow|eLC. P. Curler, and J. W. Edington:s Inst. Metals, 1973, vol. 10l, p. 332. 14. D. T. Gawneand G. T. Higgias:Z Mater. Sci., 1971,vol. 6, p. 403. t5. C. S. Smith: Trans. AIME, 1948,voL 175,p. 15. 16. W. A. Backofen,1. R. Turner, and D. H. Avery:Trans~ASM, 1964,voL 57, p, 980. 17. K. A. Padmanabhan and G. J. Davies:Phys. Status Solidi, 1973,vol, 18A, p. 295, 18. M. Bishopand K. E. Fletcher:InternationalMetallurgiealReviews, 1972,vol. 17, p. 95. 19. L. F. Mondolfo: Met. Rev., 1971, vol. 95, p. 95. 20, H. Ahlborn,E. Hornbogen, and N, KSster:Z Mater. Sci., 1969, voL 4, p. 944, 21. I. L, Dillamore:Recrystatlisation of Metallic Materials, F. Haessner, ed., p. 329, Riederer-Vertag,1970,
where D = average grain diameter f = v o l . f r a c t i o n of p r e c i p i t a t e d = average precipitate diameter. In the p r e s e n t w o r k , a f t e r s u p e r p l a s t i c d e f o r m a t i o n at 360~ D was m e a s u r e d as 5 ~ m , f as 0.014 and d as 0.16 ~ m ; h e n c e v a l u e s of D z and D e 3 ~ m (0.6D) and 15 /~m (2.9/)) w e r e obtained. Thus the s t r u c t u r e should be r e a s o n a b l y s t a b l e . A s i m p l e t r e a t m e n t 15 d e s c r i b i n g growth of s p h e r i c a l g r a i n s in m a t e r i a l c o n t a i n i n g a u n i f o r m d i s t r i b u t i o n of s p h e r i c a l p a r t i c l e s p r e d i c t s a m a x i m u m g r a i n s i z e given by Dma x = 2 d / 3 f = 8 ~m, which is c l o s e to the m e a s u r e d g r a i n s i z e , In p r a c t i c e n e i t h e r of the above t r e a t m e n t s would be e x p e c t e d to d e s c r i b e the p r e s e n t s i t u a t i o n a c c u r a t e l y b e c a u s e the p r e c i p i t a t e p a r t i c l e s a r e n e i t h e r s p h e r i c a l nor a r e they a l l of uniform size. No a t t e m p t h a s b e e n m a d e to o p t i m i z e s u p e r p ! a s t i c it y in the a l l o y BA708. H o w e v e r c o n t r o l of p a r t i c l e s i z e , i n i t i a l d e f o r m a t i o n , and t i m e / t e m p e r a t u r e p r o f i l e b e f o r e and d u r i n g s u p e r p l a s t i c d e f o r m a t i o n should i m p r o v e the d u c t i l i t y and f o r m i n g p r o p e r t i e s . 4. C o n c l u s i o n s . 1) It is f e a s i b l e to d e v e l o p s u p e r p l a s t i c i t y in a s t a n d a r d c o m m e r c i a l a l u m i n u m a l l o y BA 708 following a s i m p l e t h e r m o m e c h a n i c a l p r o c e s s i n g r o u t e and y i e l d ing e l o n g a t i o n s > 200 p c t o v e r a r a n g e of t e m p e r a t u r e s and s t r a i n r a t e s . 2) The fine g r a i n s i z e r e q u i r e d for s u p e r p l a s t i c i t y is p r o d u c e d by d y n a m i c r e c r y s t a l l i z a t i o n and is l o s t above 360~ 3) The a c t i v a t i o n e n e r g y s u g g e s t s g r a i n b o u n d a r y di f f
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