An investigation of the shape of the titanium stress strain curves after a strain rate change
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S T R A I N r a t e cycling t e s t s a r e often employed to study the t h e r m a l l y activated flow of m e t a l s . 1'~ While the flow s t r e s s of m e t a l s , in g e n e r a l , i n c r e a s e s with an i n c r e a s e in d e f o r m a t i o n r a t e , the actual shapes of the s t r e s s - s t r a i n c u r v e s may depend on t e m p e r a t u r e and s t r a i n r a t e . The a i m of this p a p e r is to show that u s e ful i n f o r m a t i o n on the d y n a m i c p r o p e r t i e s of d i s l o c a tions m a y be obtained f r o m an a n a l y s i s of these s t r e s s strain curves. In a t e n s i l e t e s t a change in d e f o r m a t i o n speed often r e s u l t s in the flow s t r e s s v a r y i n g smoothly and c o n t i n u o u s l y to a value c h a r a c t e r i s t i c of the new r a t e . Such a b e h a v i o r is shown s c h e m a t i c a l l y in Fig. l(a) and will be r e f e r r e d to as ideal. On the other hand, as shown in Fig. l(b), it is a l s o p o s s i b l e to o b s e r v e yield d r o p s on an i n c r e a s e in d e f o r m a t i o n rate as well as n e g a t i v e yield d r o p s with a d e c r e a s e in the c r o s s head speed. 2-5 T h r e e t r u e s t r e s s - t r u e s t r a i n plots for c o m m e r c i a l p u r i t y a - T i d e f o r m e d in t e n s i o n a r e shown in Fig. 2. In t h e s e t e s t s 3 the s t r a i n r a t e was v a r i e d by e i t h e r one o r two o r d e r s of magnitude. Note that ideal b e h a v i o r is obtained at the two h i g h e s t t e m p e r a t u r e s (873 ~ and 973~ w h e r e a s t r a n s i e n t flow s t r e s s m a x i m a and m i n i m a a r e o b s e r v e d at t e m p e r a t u r e s n e a r 673~ However, s i m i l a r t r a n s i e n t flow s t r e s s peaks were a l s o o b s e r v e d by Orava, Stone, and Conrad ~ in a t e m p e r a t u r e r a n g e c e n t e r e d at about 200~ It is s i g n i f i c a n t that the t e m p e r a t u r e i n t e r v a l a r o u n d 673 ~ and 200~ a r e those in which the p h e n o m e n a a s s o c i ated with d y n a m i c s t r a i n aging 6'7 o c c u r . Yield d r o p s upon change in s t r a i n r a t e have also b e e n o b s e r v e d in single c r y s t a l s of Cu, 2's'8 A1, 2 Ag, s'a Pb, 5 and p o l y c r y s t a l l i n e Cu, s AI, 2'5 and Fe. S Bolling e t a l . 5 p o s t u l a t e d that the o b s e r v e d flow s t r e s s m a x i m a and m i n i m a m a y r e s u l t f r o m the effect of speed changes upon the t e s t i n g m a c h i n e . To t e s t t h e i r hyA. T. SANTHANAMand R. E. REED-HILLare Graduate Student and Professor, respectively, Department of Metallurgicaland MaterialsEngineering,Universityof Florida, Galnesville,Fla. V. RAMACHANDRAN, formerly Graduate Student, Department of Metallurgicaland Materials Engineering, University of Florida, Gainesville,Fla., is with the Army Mechanics and Materials Research Center, Watertown, Mass. Manuscript submitted November 21, 1969. METALLURGICALTRANSACTIONS
p o t h e s i s , s t r a i n r a t e change t e s t s
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