Fiber/matrix interface porosity formation in tungsten fiber/copper composites on thermal cycling
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i n d i c a t e that the m e c h a n i s m s invoked by Sikka e t al, 3 involving d i f f e r e n t i a l c o n c e n t r a t i o n of a l l o y i n g e l e m e n t s and the r e q u i r e m e n t of a c a s t s t r u c t u r e a r e not r e l e v a n t . It would a p p e a r that the e a r l i e r r e s u l t s of A n d r a d e ~ and of Lubahn and F e l g a r ~ could a l s o be c o n s i d e r e d in t e r m s of d y n a m i c s t r a i n aging e f f e c t s , the l a t t e r , in p a r t i c u l a r , being s u p p o r t i v e of t h i s h y p o t h e s i s on the b a s i s of the o c c u r r e n c e of j u m p s in p a r t i a l l y a g e d m a t e r i a l . The r e a s o n why s u c h f e a t u r e s have not b e e n o b s e r v e d m o r e f r e q u e n t l y in the p a s t m a y r e l a t e to the i n f r e q u e n c y with which c r e e p d e f o r m a t i o n is r e c o r d e d c o n t i n u o u s l y r a t h e r than at d i s c r e t e t i m e i n t e r v a l s . While no d e t a i l e d c o n c l u s i o n c a n be d r a w n , it is s u g g e s t e d that the p h e n o m e n o n m a y be m o r e w i d e s p r e a d than is c o m m o n l y b e l i e v e d , and f u r t h e r s t u d i e s a r e continuing on o t h e r a l l o y s in an a t t e m p t to identify the m e c h a n i s m s in m o r e d e t a i l .
Fig. 3-Stress-temperature plot with the tests and limit conditions. The high and low temperature regions where jumps occur are shadowed. high t e m p e r a t u r e j u m p s at low s t r e s s e s c o n d i t i o n s a s shown in F i g . 3. It is not p o s s i b l e at the p r e s e n t t i m e to g i v e an a d e q u a t e and d e t a i l e d e x p l a n a t i o n of the o b s e r v e d j u m p s . H o w e v e r , it is b e l i e v e d b y the a u t h o r s that they a r e r e l a t e d to d y n a m i c s t r a i n aging. C e r t a i n l y , t h e y
1. E. N. Da C. Andrade: Proc. ofRoy. Soc., 1910, vol. 84-A, pp. 1-12. 2. J. D. Lubahn and R. P. Felgar: Plasticityand Creep of Metals, pp. 409-10, John Wiley and Sons, 1961. 3. V. K. Sikka, S. A. David, E. H. Lee, C. W. Hauch, and B. L. Booker: Private communication, Oak Ridge National Laboratory, Oak Ridge, TN 37830, 1978. 4. T. L. Silveira and S. N. Monteiro: Metalurgia-ABM, a Journal of the Brazilian Association for Metals~ 1979,vol. 35, pp. 533-36.
Fiber/Matrix Interface Porosity Formation in Tungsten Fiber/Copper Composites on Thermal Cycling
of i n t e r n a l s t r e s s e s w h i c h a r e g e n e r a t e d upon t e m p e r a t u r e c y c l i n g due to d i f f e r e n t i a l t h e r m a l e x p a n s i o n of the c o n s t i t u e n t p h a s e s . H o w e v e r , no s a t i s f a c t o r y a n s w e r was given to
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