Comment on harper-dorn creep in Al alloys
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Fig. 1--Consolidated low carbon astroloy powder, Special Metals, Inc., showing cross sectional circular voids (arrows). Magnification 50 times.
Aircraft, 1977, vol. 14, pp. 613-23.
Comment on Harper-Dorn Creep in AI Alloys F A R G H A L L I A . MOHAMED The Newtonian H a r p e r - D o r n c r e e p 1-s o b s e r v e d in p u r e A1 at v e r y low s t r e s s e s and high t e m p e r a t u r e s is m a n i f e s t e d by s e v e r a l c h a r a c t e r i s t i c s that a r e inc o m p a t i b l e with those a t t r i b u t a b l e to the c l a s s i c N e w tonian N a b a r r o - H e r r i n g m e c h a n i s m . 6'7 F i r s t , the n o r m a l i z e d c r e e p r a t e s of H a r p e r - D o r n c r e e p a r e i n s e n s i t i v e to changes in g r a i n s i z e and a r e well r e p r e s e n t e d by an equation of the f o r m 3'5
1 A
1
E
Fig. 2--Block diagram of vacuum fusion apparatus. (a) Mullite crucible containing sample and gold, (b) resistance furnace, (c) mercury diffusion pump, (d) Toepler pump, (e) known volume, (f) McLeod gage, (g) mass spectrometer or gas transport vessel, (h) main vacuum system, (x) necessary valves. g a s e s a r e c o l l e c t e d for one h in a known v o l u m e by use of a m e r c u r y diffusion pump followed by a T o e p l e r pump. The quantity of the e v o l v e d g a s e s a r e d e t e r m i n e d by both the m e a s u r e m e n t of the p r e s s u r e in the known v o l u m e with a M c L e o d gage and the use of the i d e a l gas equation. The c o m p o s i t i o n of the e v o l v e d g a s e s a r e d e t e r m i n e d by the use of a c a l i b r a t e d m a s s s p e c t r o m e t e r . The technique first involves a pret r e a t m e n t of the gold and the c o n t a i n e r at 1473 K for one h followed by a p r e t r e a t m e n t of the s a m p l e at 1173 K for one h. By m o n i t o r i n g the e v o l v e d gas, this p r o c e d u r e has b e e n shown to be s u f f i c i e n t in r e m o v i n g 1342-VOLUME 9A, SEPTEMBER ~_ 78
~kT (r/G), DGb = AHD
[1]
w h e r e ~ is the s t e a d y - s t a t e c r e e p r a t e , k is B o l t z m a n n ' s constant, T is the a b s o l u t e t e m p e r a t u r e , D is the diffusivity, G is the s h e a r modulus, b is the B u r g e r s v e c t o r , AHD is a d i m e n s i o n l e s s constant having a value of 5 • 10 -1~, and r is the s h e a r s t r e s s . Second, H a r p e r - D o r n c r e e p r a t e s a r e f a s t e r than those e s t i m a t e d f r o m the t h e o r e t i c a l equation for N a b a r r o H e r r i n g c r e e p , 6,7 given by
CkT
(b) 2
DGb = ANH d
(T/G)
[2]
w h e r e ANH is a c o n s t a n t having a value of 84 and d is the g r a i n s i z e . T h i r d , while c r e e p s t r a i n of the N a b a r r o - H e r r i n g m e c h a n i s m is p r o d u c e d by s t r e s s FARGHALLI A. MOHAMEDis Assistant Professor, Departments of Materials Science and Mechanical Engineering, University of Southern California, Los Angeles, CA 90007. Manuscript submitted November 8, 1977.
ISSN 0360-2133/78
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