Fracture and ductility in hot torsion of Nickel
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i m p o r t a n c e of the i n t e r a c t i o n of d e f o r m a t i o n m e c h a n i s m s and f r a c t u r e m e c h a n i s m s in the d e t e r m i n ation of hot ductility has now been r e c o g n i z e d J Both d y n a m i c r e c r y s t a l l i z a t i o n 2'~ and dynamic r e c o v e r y 4'5 have been suggested a s the operative softening m e c h a n i s m in hot working. F o r n i c k e l , the evidence sugg e s t s that d y n a m i c re c r y s t a l l i z a t i o n is operative and the k i n e t i c s have been studied. Continuous and periodic d y n a m i c r e c r y s t a l l i z a t i o n have b e e n o b s e r v e d . ~'8 While a c o n s i d e r a b l e body of work ~" lo e x i s t s on the study of d y n a m i c softening m e c h a n i s m s , l e s s i n f o r m a tion is available on hot f r a c t u r e m e c h a n i s m s and on the m e c h a n i s m s which d e t e r m i n e hot ductility.n-~a The purpose of this work i s to c o r r e l a t e hot ductility with the m i c r o s t r u c t u r e which a r i s e s from the i n t e r a c t i o n of the softening and f r a c t u r e m e c h a n i s m s . E X P E R I M E N T A L PROCEDURE
fixed t a i l s t o c k . T e n s i l e or c o m p r e s s i v e axial s t r e s s e s could be s u p e r i m p o s e d on the s h e a r s t r e s s e s developed by rotation of the headstock. T o r q u e , axial f o r c e , and angle of twist were m o n i t o r e d for each test. The device is capable of o p e r a t i n g to 2000~ at s t r a i n r a t e s from 10 -s to 102 s e c - ~ for this s p e c i m e n design. Specimens were held at test t e m p e r a t u r e for 15 rain p r i o r to t e s t i n g . A r a d i a n t q u a r t z lamp furnace was mounted on the lathe c a r r i a g e and could be quickly moved to expose the test s e c t i o n for rapid quenching by a s t r e a m of cold water d r i v e n by a CO~ c a r t r i d g e . Quenching t i m e s were u s u a l l y 2 to 5 sec. A more complete desc r i p t i o n of the t e s t i n g device and i n s t r u m e n t a t i o n is given in Refs. 11 and 14. Metallographic s p e c i m e n p r e p a r a t i o n was s t a n d a r d with a soap solution used as a l u b r i c a n t in the f i n a l polishing step. The s p e c i m e n s w e r e etched in a 50 pct n i t r i c - 5 0 pct glacial acetic acid solution applied by swabbing.
M a t e r i a l and T e s t Specimen In o r d e r to study f r a c t u r e u n d e r conditions of hot working, t o r s i o n t e s t s were c a r r i e d out on Nickel 270. T h i s m a t e r i a l i s a high p u r i t y nickel (99.97 pet Ni) with the m a j o r i m p u r i t y b e i n g c a r b o n (0.01 pet). All other i m p u r i t i e s are p r e s e n t in a m o u n t s less than 0.001 pct. P r i o r to t e s t i n g the s p e c i m e n s were annealed at 1800~ for ~ hr and oil quenched. The test s p e c i m e n s were hollow c y l i n d e r s ~ in. long,-~ in.OD and ~ in. ID. In o r d e r to confine the f r a c ture a J- in. long reduced test s e c t i o n of 0.375 in. OD and
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