Static strain-aging in commercial purity alpha titanium
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THE mechanical properties of titanium are strongly influenced by the presence of interstitials in solid solution. Although several investigators have demonstrated that interstitials produce strain-aging effects in this metal, an investigation of the kinetics and energetics of static strain-aging in titanium has been lacking. The r e s u l t s of such a study, made on a c o m mercial purity titanium, are reported here. EXPERIMENTAL PROCEDURES The t i t a n i u m used in this i n v e s t i g a t i o n was p u r c h a s e d f r o m Reactive Metals, Inc., as r o l l e d plate. Its p r i n c i pal i m p u r i t i e s in atomic p e r c e n t w e r e F e - 0 . 1 4 , O-0.41, C-0.080, N-0.034, and H-0.26/0.44. Square b a r s app r o x i m a t e l y 1.0 by 1.0 cm in c r o s s - s e c t i o n w e r e cut f r o m the plate p a r a l l e l to its r o l l i n g d i r e c t i o n and w e r e swaged d i r e c t l y to 0.55 cm d i a m c y l i n d r i c a l r o d s . Threaded-end tensile specimens were machined from the r o d s . These were a n n e a l e d under a d y n a m i c v a c uum of about 1.3 x 10 -3 Pa at 1073 K for 30 m i n u t e s to p r o d u c e an equiaxed g r a i n s t r u c t u r e with a m e a n l i n e a r i n t e r c e p t of 17 ~M. The gage s e c t i o n s of the t e n s i l e s p e c i m e n s had a n o m i n a l d i a m e t e r of 0.36 cm and a length of 2.54 cm. Static s t r a i n - a g i n g e x p e r i m e n t s w e r e conducted in situ at four t e m p e r a t u r e s , 555, 575, 600 and 630 K. All t e s t s w e r e conducted under a high purity, oxygeng e t t e r e d a r g o n a t m o s p h e r e in o r d e r to m i n i m i z e s p e c i m e n c o n t a m i n a t i o n . A n o m i n a l s t r a i n - r a t e of 3.3 x 10 -4 s -I was used. The t e n s i l e s p e c i m e n s w e r e p r e s t r a i n e d to 0.02 s t r a i n b e f o r e aging. When this p r e s t r a i n was r e a c h e d , the s p e c i m e n s were unloaded to a s t r e s s of about 10 MPa in o r d e r to m a i n t a i n t h e i r a l i g n m e n t in the t e n s i l e jig. They w e r e then aged in situ for l e n g t h s of t i m e r a n g i n g f r o m l 50 to about 2.55 x 105 s (68 h). The r e c o r d e d aging t i m e was m e a s u r e d from the i n s t a n t that the unloading of the s p e c i m e n was s t a r t e d to that t i m e when, on r e l o a d i n g , the load b e c a m e equal to the value of the preload. J. R. DONOSO is Professor, UniversidadSanta Maria, Casilla t IO-V, Valparaiso, Chile and R. E. REED-HILL is Professor, Department of Materials Science and Engineering,University of Florida, Gainesvilte, FL 3261 l. Manuscript submitted September 29, 1976. METALLURGICALTRANSACTIONSA
The m a j o r i t y of the s p e c i m e n s were d e f o r m e d to f r a c t u r e d u r i n g the r e l o a d i n g p o r t i o n of the t e s t s . The aging t e m p e r a t u r e was m a i n t a i n e d to within ~=1 K for p e r i o d s of l e s s than 5000 s, and to within • K for longer times. The p a r a m e t e r s employed in e v a l u
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