Low-temperature mechanical properties of thoria dispersion-strengthened iron
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AND
J. A. LUND
Th e m e c h a n i c a l p r o p e r t i e s of i r o n , and i r o n containing a t h o r i a d i s p e r s i o n in v o l u m e f r a c t i o n s of 0.9 and 1.7 p c t , have b e e n d e t e r m i n e d o v e r the t e m p e r a t u r e r a n g e 77 to 373 K. Yield s t r e s s v a l u e s f o r Fe-ThO2 c a l c u l a t e d f r o m a m o d i f i e d O r o w a n r e l a t i o n w e r e within 8 p ct of the e x p e r i m e n t a l v a l u e s . A peak in w o r k h a r d e n i n g r a t e was o b s e r v e d in the t e m p e r a t u r e r a n g e 250 to 300 K. The peak o c c u r r e d at e a c h of the s e l e c t e d s t r a i n s , 1, 2, 4, and 8 pct, and in all t h r e e m a t e r i a l s . T h e w o r k - h a r d e n i n g r a t e s of the t h o r i a t e d a l l o y s r e m a i n e d r o u g h l y double that of i r o n at all s t r a i n s . The p a r t i c l e s w e r e c o n s i d e r e d to i n c r e a s e the r a t e of d i s l o c a t i o n m u l t i p l i c a t i o n , and have a s t r o n g but l a r g e l y i n d i r e c t e f f e c t on the w o r k - h a r d e n i n g p r o c e s s .
T H E w o r k d e s c r i b e d in this p a p e r is p a r t of a p r o g r a m i n v o l v i n g the m a n u f a c t u r e and study of the p r o p e r t i e s of F e - T h O z a l l o y s o v e r a wide r a n g e of t e m p e r a t u r e . T h e l a b o r a t o r y p r o c e d u r e s f o r p r o d u c i n g the alloys involve coprecipitation, hydrogen reduction, c o m p a c t i o n , and h o t - r o l l i n g , and have b e e n fully d e s c r i b e d e l s e w h e r e . ~ The end p r o d u c t is 0.63 m m thick b r i g h t s t r i p , and a l l the r e s u l t s g iv e n h e r e w e r e obtained f r o m t e s t s on this m a t e r i a l . T h e n a t u r e of the a l l o y s is i n d i c a t e d by the p a r a m e t e r s 1 in T a b l e I.
TENSILE
PROPERTIES
T e n s i l e t e s t s w e r e c a r r i e d out in an I n s t r o n m a c h i n e , u s i n g a s t r a i n r a t e of 4.2 • 10 -4 s e c -~ and t e m p e r a t u r e s r a n g i n g f r o m 77 to 373 K. O b s e r v e d v a r i a tions in y i e l d s t r e s s and U.T.S. with t e m p e r a t u r e a r e plotted in F i g s . 1 and 2 r e s p e c t i v e l y . The d i f f e r e n c e in y i el d s t r e s s v a l u e s b e t w e e n i r o n and F e - l . 7 p ct ThO2 was about 18 kg m m -e o v e r m o s t of the t e m p e r a t u r e r a n g e , a p a r t f r o m the r e s u l t s at 133 and 77 K, w h e r e i r o n showed r e l a t i v e l y high v a l u e s . T h e U.T.S. d i f f e r e n c e b e t w e e n i r o n and F e - l . 7 p c t ThOz was 23 kg m m -2 at 298 K and 13 kg m m -2 at 77 K. The c u r v e s for i r o n w e r e d r a w n in a s m o o t h line to 77 K, although twinning f a i l u r e s w e r e o b s e r v e d in i r o n at this t e m p e r a t u r e and a r e i n d ic a t e d by t h r e e points at the b o t t o m left of F i g . 2. Twinning was not obs e r v e d in F e - T h O 2 . The n a t u r e of the yield point changed with t e m
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