Unusual mechanical effects during static or cyclic creep of AI-4.6 pct Mg

PDF / 797,955 Bytes
3 Pages / 612 x 792 pts (letter) Page_size
105 Downloads / 176 Views

Unusual Mechanical Effects During Static or Cyclic Creep of AI-4.6 Pct Mg J . L. KIRK, D. K. M A T L O C K , G. R. EDWARDS, AND W. L. BRADLEY The s t a t i c and c y c l i c c r e e p b e h a v i o r of A1-4.6 pct Mg has b e e n i n v e s t i g a t e d at high s t r e s s e s in the t e m p e r a t u r e r a n g e of a p p r o x i m a t e l y 0.4 T m 1-~ U n d e r t h e s e t e s t conditions, s a m p l e s s u b j e c t e d to a c y c l i c s t r e s s ( t e n s i o n - t e n s i o n ) d e f o r m m o r e r a p i d l y than s a m p l e s s u b j e c t e d only to a s t a t i c t e n s i l e s t r e s s e q u a l in magnitude to the p e a k c y c l i c s t r e s s . 4-7 Although c y c l i c c r e e p has not r e c e i v e d m a j o r a t t e n tion to date, m a r k e d a c c e l e r a t i o n in c r e e p r a t e r e s u l t ing f r o m a c y c l i c s t r e s s has been o b s e r v e d in a wide v a r i e t y of e n g i n e e r i n g m a t e r i a l s . B e g i n n i n g with Kenn e d y ' s study of lead, 8 c y c l i c c r e e p a c c e l e r a t i o n has b e e n o b s e r v e d in s t e e l s , 9 a l u m i n u m alloys,/-7,1~ zinc, 12'13 and c o p p e r ) 4 T h e s e and o t h e r s t u d i e s a r e r e v i e w e d in a d i s s e r t a t i o n by Coutinho.4 E x p l a n a t i o n s f o r c y c l i c c r e e p a c c e l e r a t i o n a r e not yet c o m p r e h e n s i v e . Kennedy s s u g g e s t e d that e x c e s s v a c a n c i e s , c r e a t e d by c y c l i c s t r e s s i n g , enhanced the r e c o v e r y r a t e while M e l e k a ~s s u g g e s t e d that c y c l i c s t r e s s i n g i n c r e a s e d the d i s l o c a t i o n mobility. M u r a e t a116 have p r o p o s e d a m o d e l w h e r e i n m o r e u n i f o r m p l a s t i c s t r a i n r e s u l t s f r o m c y c l i c loading, a concept which has s i n c e been e x t e n d e d by Shetty e t al. ~ Knaus S and M a t l o c k and B r a d l e y 2 have c o n s i d e r e d the e f f e c t which i n t e r n a l s t r e s s t r a n s i e n t s may have on r e c o v e r y rate during cyclic creep. A n o m a l i e s r e c e n t l y o b s e r v e d by us d u r i n g s t u d i e s of c r e e p a c c e l e r a t i o n in A1-4.6 pct Mg have given a new p e r s p e c t i v e on i n t e r p r e t i n g this i n t e r e s t i n g p h e n o m e non. A c r u c i a l o b s e r v a t i o n which we have made is that L u d e r s band p r o p a g a t i o n is a m a j o r c o n t r i b u t o r to total s t r a i n if A I - 4 . 6 pct Mg is c r e p t in a t e s t r e g i m e w h e r e c r e e p a c c e l e r a t i o n o c c u r s . The o b s e r v a t i o n of discontinuous straining (Luders straining) during either constant s t r e s s or cyclic s t r e s s creep testing 1. L. KIRK, formerly Research Assistant in the Department of Metallurgical Engineering, Colorado School of Mines, is now with the MTS Systems Corporation, Minneapolis, MN 55424. D. K. MATLOCK and G. R. EDWARDSare Associate Professors in the Department of Metallurgical Engineering, Colorado School o

Data Loading...