• PDF / 533,274 Bytes
• 5 Pages / 612 x 792 pts (letter) Page_size
• 69 Downloads / 220 Views

DOWNLOAD

REPORT

---

THE

d e f o r m a t i o n b e h a v i o r of m o l y b d e n u m s i n g l e c r y s t a l s has been e x t e n s i v e l y i n v e s t i g a t e d in r e c e n t y e a r s i n o r d e r to u n d e r s t a n d and c h a r a c t e r i z e its low t e m p e r a t u r e flow p r o p e r t i e s . l - s I n v e s t i g a t o r s have m e t with p a r t i a l s u c c e s s , but the complexity of t h e i r r e s u l t s i n d i c a t e s that m a n y of the o b s e r v e d p h e n o m e n a a r e not c l e a r l y u n d e r s t o o d . One such p h e n o m e n a conc e r n s the e a r l y stages of flow where t h e r e i s a m a r k e d change in shape of the s t r e s s - s t r a i n c u r v e of m o l y b d e n u m single c r y s t a l s with o r i e n t a t i o n and t e m p e r a t u r e . The g e n e r a l o b s e r v a t i o n s a r e that yield d r o p s o c c u r if the d e f o r m a t i o n axis is p a r a l l e l to the (110) while c r y s t a l s with (100) or c e n t r a l o r i e n t a t i o n s show s m o o t h flow c u r v e s which d i v e r g e with d e c r e a s i n g t e m p e r a t u r e . F u r t h e r m o r e , the r a t e at which the slope of the s t r e s s - s t r a i n cruve d e c r e a s e s is a function of t e m p e r a t u r e and also a f u n c t i o n of s t r a i n . T y p i c a l l y , the l a r g e s t v a l u e s for the r e s o l v e d s h e a r s t r e s s o c c u r when the d e f o r m a t i o n axis is along the (110)--(111) boundary of the s t e r e o g r a p h i c t r i a n g l e . T h e s m a l l e s t values o c c u r for o r i e n t a t i o n s n e a r the (100) c o r n e r . The o r i e n t a t i o n effect i n c r e a s e s with d e c r e a s i n g t e m p e r a t u r e and s o m e i n v e s t i g a t o r s have r e p o r t e d as much as a factor of t h r e e d i f f e r e n c e i n the m a g n i t u d e of the r e s o l v e d flow s t r e s s b e t w e e n the (110) and (100) d e f o r m a t i o n axes at 77 K. ~'~ T h e s e obs e r v a t i o n s a r e c o n t r a r y to S c h m i d ' s Law. I n v e s t i g a t o r s studying p r e y i e l d o r m i c r o s t r a i n phen o m e n a 9-~4 have long r e c o g n i z e d the fact that d i f f e r e n c e s in the shape of s t r e s s - s t r a i n c u r v e s c a u s e a p r o b l e m with the definition of the yield s t r e s s and thus the i n t e r p r e t a t i o n of the low t e m p e r a t u r e flow b e h a v i o r . The p r o b l e m a r i s e s b e c a u s e the flow p r o p e r t i e s of the bcc m e t a l s a r e s t r a i n r a t e s e n s i t i v e , and the t r u e p l a s t i c s t r a i n r a t e i s a function of the shape of the s t r e s s - s t r a i n c u r v e through the value of its slope as d e s c r i b e d by the r e l a t i o n : dp = d:T

1

E ds

'

w h e r e dp is the t r u e p l a s t i c s t r a i n r a t e , dT is the total s t r a i n r a t e , E is the a p p a r e n t modulus, and ( d a / d e T ) is

C. R. CROWE is Research Metallurgist, Naval Surface Weapons Center, White Oak Laboratory,