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R. D. NELSON

The effect of g r a i n size on the h a r d n e s s of alpha plutonium was m e a s u r e d at t e m p e r a t u r e s between 77 ~ and 3730K. The o b s e r v a t i o n s were: 1) f i n e - g r a i n e d m e t a l had the lowest h a r d ness at 373~ 2) f i n e - g r a i n e d m e t a l had the highest h a r d n e s s at 77~ and 3) f i n e - g r a i n e d metal had the g r e a t e s t t e m p e r a t u r e dependence of h a r d n e s s . At room t e m p e r a t u r e , f i n e grained c~-plutonium (1 ~an) had a h a r d n e s s of 235 Dph, w h e r e a s c o a r s e - g r a i n e d m e t a l (2000 gin) had a h a r d n e s s of 310 Dph. At 373~ f i n e - g r a i n e d m e t a l had a h a r d n e s s of 120 compared to 225 Dph for the c o a r s e - g r a i n e d metal. At 77~ the h a r d n e s s values were 505 and 475 Dph, the f i n e - g r a i n e d m e t a l being h a r d e r at this t e m p e r a t u r e . The h a r d n e s s was e s t i m a t e d to be independent of g r a i n size at 140~ Above 140~ g r a i n b o u n d a r i e s have a softening effect due to the c o n t r i b u t i o n of g r a i n boundary sliding to d e f o r m a t i o n . size G R is known A to affect I the creep N rate, f r a c ture c h a r a c t e r i s t i c s , yield s t r e s s , and the h a r d n e s s of m o s t metals. At high homologous t e m p e r a t u r e s the c r e e p rate is especially s e n s i t i v e to changes in g r a i n size when the g'rain size is l e s s than 10 /am. F o r i n stance, s u p e r p l a s t i c d e f o r m a t i o n has been o b s e r v e d only when the g r a i n size is l e s s than 10 /ms. Above 100 pm grain size creep rates appear to vary little with grain size. The low temperature strength and hardness of metals usually obeys the Hall-Petch equation (r (or Dph) = const + Kd -*/2, where K is a constant anddis the mean grain size. Furthermore, the notch sensitivity and fracture characteristics depend on grain size; generally, reducing the grain size raises the fracture stress and decreases the notch sensitivity of metals. The effect of grain size on the mechanical properties of alpha plutonium has not been studied in much detail. Several observations suggest that grain boundaries are quite important. Coarse grained (> 20 pro) or-plutonium is brittle and elongates less than 1 pct between room t e m p e r a t u r e (0.54 T m) and 110 ~ (0.69 Tin). l The d u c tility is i n c r e a s e d to 75 pet at 110~ in m e t a l c h i l l cast to reduce the g r a i n size to l e s s than 20 ~rn. ~ Extruded alpha plutonium, which has a grain size of 2 to 5 tun, exhibits elongations over 200 pct in tension at 110~ a The present study on the effect of grain size on hardness was appropriate since the measurements could be easily made over a wide temperature interval and all measurements for a single grain size could be made on one specimen. Furthermore, the hardness test allowed direct measurement of the effect of grain boundaries since an indentation can be made to lie entirely within a grain or to cr