Strain Instability and Fracture in Sintered Compacts
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c u s s i o n , our p a p e r had a l r e a d y b e e n s e n t to the publishers. 13. R. Hultgren,R. L. Orr, P. D. Anderson,and K. K. Kelley:Selected Valuesof Thermodynamic Properties of Metals and Alloys, J. Wileyand Sons,Inc., 1963 and supplements. 14. L. S. Darken: Tran~ TMS-AIME, 1967,vol. 239, pp. 80-89. 15. E. T. Turkdogan and L. S. Darken. Trans. TMS-AIME, 1968,vol. 242, pp. 1997-2005. 16. O. J. Kleppa:J. Phys. Chem., 1955, voh 58, pp. 354-61. 17. J. F. Elhott and J. Chlpman: Trans. FaradaySoc., 1951,vol.47, pp. 138-48. 18. S. Mellgren:J. Amer. Chem. Soc., 1952, vol. 74, pp. 5037-40. 19. J. D. Esdaile:Met. Trans., 1971, vol. 2, pp. 2277-82. 20. L. S. Darken: Trans. TMS-AIME, 1967,vol.239, pp. 90-96.
Strain Instability and Fracture in Sintered Compacts H. L. GAIGHER, M. J . K O CZA K , AND A. L A W L E Y
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and L e e 1 have r e c e n t l y d e m o n s t r a t e d an a p p r o a c h to q u a n t i t a t i v e l y d e s c r i b e l o c a l i z e d s t r a i n i n s t a b i l i t i e s which p r e c e d e f r a c t u r e d u r i n g a x i a l c o m p r e s s i o n . T h e l i m i t s of p l a s t i c s t r a i n at the f r e e s u r f a c e s of c o m p r e s s i v e l y u p s e t r i g h t c i r c u l a r c y l i n d e r s w e r e shown to be a c h a r a c t e r i s t i c of c o n v e n t i o n a l e a s t and w r o u g h t m a t e r i a l s , Th e p u r p o s e of this c o m m u n i c a t i o n is to show the a p p l i c a b i l i t y of this r e f i n e d method to d e t e c t p l a s t i c s t r a i n i n s t a b i l i t i e s which p r e c e d e f r a c t u r e in s i n t e r e d p o w d e r m e t a l l u r g i c a l c o m p a c t s . A x i a l c o m p r e s s i o n of s i n t e r e d c o m p a c t s i n v o l v e s p l a s t i c flow leading to c o m p a c t d e n s i f i e a t i o n as w e l l as bulging at the f r e e s u r f a c e s . A s i m i l a r i t y in the s t r a i n c r i t e r i a f o r f r a c t u r e is shown to e x i s t f o r c o n v e n t i o n a l e a s t and w r o u g h t m a t e r i a l s as w e l l as s i n t e r e d i r o n and n i c k e l c o m p a c t s . In addition, a m o d e l f o r c r a c k i n i t i a t i o n and p r o p a g a t i o n is d i s c u s s e d f o r p o w d e r metallurgical materials. E x p e r i m e n t a l P r o c e d u r e . Two c o m m e r c i a l w e l l g r a d e d p o w d e r s MH100 i r o n and F210 n i c k e l , F i g s . 1 and 2 r e s p e c t i v e l y , w e r e c o m p a c t e d to t h e o r e t i c a l d e n s i t i e s in the r a n g e 85 to 88 pct in a f r e e floating die u t i l i z i n g zinc s t e a r a t e a s a die wall l u b r i c a n t . F o l l o w ing d e n s i f i c a t i o n , the c o m p a c t s w e r e s i n t e r e d in h y d r o gen for 3600 s (1 h) at 1038~ (1900~ Except where noted, the c o m p a c t s had a Ho/Do r a t i o of 0.5 with a height, Ho, of ~0.013 m (~0.5 in.) and a d i a m e t e r , Do, of ~0.025 m (N1.0 in.). In the c a s e of a d i f f e r e n t Ho/Do r a t i o , c o m p a c t s w e r e p r e p a r e d in an i
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