Directionality of fatigue properties in a low carbon alloy steel
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Fig. 4---4 pct Au alloy heated at 900~ times.
Magnification 810
Magnification 820
b i l i t y l i m i t r e p r e s e n t s the r a n g e of t e m p e r a t u r e s c o n s i d e r e d . Fig. 1 shows that the slope of the c u r v e r e p r e s e n t i n g the b o u n d a r y b e t w e e n the y and the + Au r e g i o n s is f a i r l y s t e e p below 1000~ but d e c r e a s e s at higher t e m p e r a t u r e s . T h i s m a k e s a c c u r a t e location of the s o l i d u s p o r t i o n of the b o u n d a r y of the s i n g l e - p h a s e y r e g i o n m o r e difficult s i n c e a s m a l l e r r o r in l a t t i c e p a r a m e t e r would r e s u l t in a l a r g e r e r r o r in gold s o l u b i l i t y . A m i n i m u m of two d i f f r a c t i o n p a t t e r n s was m a d e for each a l l o y - t e m p e r a t u r e c o m b i n a t i o n , and both points a r e plotted in F i g . 1 for those s a m p l e p a i r s for which s i g n i f i c a n t d i f f e r e n c e s w e r e obtained. The d i f f r a c t i o n p a t t e r n s showed no e v i d e n c e of e i t h e r a bcc ~ phase or an o r d e r - d i s o r d e r r e a c t i o n , both of which a p p e a r on the F e - C o b i n a r y phase d i a g r a m . s H o w e v e r , the l o w e s t t e m p e r a t u r e s t r u c t u r e f r o m which d i f f r a c t i o n p a t t e r n s w e r e made was 600~ and it is p o s s i b l e that the ot phase would d e v e l o p only at l o w e r t e m p e r a t u r e s . The l o w e r l i m i t of the ~ + Au + L r e g i o n or the s o l i d u s for this s e r i e s of a l l o y s , F i g . 2, was e s t a b l i s h e d by d i f f e r e n t i a l t h e r m a l a n a l y s i s . S p e c i m e n s w e r e h e a t e d under an a r g o n a t m o s p h e r e at a r a t e of 1472-VOLUME 1, MAY 1970
Fig. 5--6 pet Au alloy heated at 900~ times.
Magnification 810
20~ p e r m i n . A phase change was o b s e r v e d at 1015 ~ :L 6~ for the 10, 14, and 18 pct Au a l l o y s . An e x t r a p o l a t i o n of the s o l i d u s i n d i c a t e s that the l i m i t of the ), + Au + L r e g i o n o c c u r s at a p p r o x i m a t e l y 5 pct Au and d e l i n e a t e s a r e g i o n of r e t r o g r a d e s o l u b i l i t y at h i g h e r t e m p e r a t u r e s . M e t a l l o g r a p h i c o b s e r v a t i o n s s u p p o r t e d the gold s o l v u s d e t e r m i n e d by the X - r a y l a t t i c e p a r a m e t e r m e a s u r e m e n t s . F i g s . 3, 4, and 5 w e r e p h o t o m i c r o g r a p h s of the 2, 4, and 6 pct Au a l l o y s which w e r e h e a t e d at 900~ The d i f f e r e n c e s in the q u a n t i t i e s of p r e c i p i t a t e s a r e r e a d i l y apparent, and the n e a r a b s e n c e of gold p a r t i c l e s in the 2 pct Au s a m p l e c o r r e l a t e s well with F i g . 2 which shows that the s o l u b i l i t y of gold at 900~ is just s l i g h t l y l e s s than 2 pct. The a u t h o r s e x p r e s s t h e i r a p p r e c i a t i o n to R. P. M e n i c h e l l i of the W e s t e r n E l e c
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