Preprecipitation in Al-Ag alloys quenched from the liquid state
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mal diffusion, inadequate purification of the incoming gases, and inaccurate measurement of their flow rates. The partial pressure of oxygen at equilibrium with ferromonticellite and iron was determined in the temperature range from 920~ to 1100~ The activity of ferrous oxide in ferromonticellite is obtained when this value is compared to the partial pressure of oxygen at equilibrium with wfistite "FeO" and iron at the same temperature, according to Eq. [2]. aFeO = I PO2 at equilibrium with Fe and CaO. FeO. SiO2]z/2 PO2 at e q u i l i b r i u m with F e and " F e O "
1
J
ITS
[2] TURE S
rs E (-)
AIR IN
......
Fig. 2--Detaileddiagram of the oxygenprobe.
Table I. Activity of Ferrous Oxide in Ferromonticellite
Temperature, ~ 920 950 1000 1050 1100
Partial Pressureof Oxygen,atm, at Equilibriumwith: Fe and CaF~SiO4 Fe and "FeO" 1.95 X 10''7 7.39 X 10''7 6.04 X 10"16 4.09 X 10"Is 2.48 X 10"14
5.07 X 10''7 1.86 X 10"16 1.43 X 10"Is 9.38 X 10"is 5.35 X 10"14
Activityof "FeO" 0.62 0.63 0.65 0.66 0.68
c o n s t a n t e q u a l t o 23,066 c a l p e r v e q u i v a l e n t , i~ 2 (in a i r ) i s t h e p a r t i a l p r e s s u r e of o x y g e n i n a i r ( r e f e r e n c e e l e c t r o d e ) , a n d Pch (in f u r n a c e ) i s t h e p a r t i a l p r e s s u r e of o x y g e n i n t h e f u r n a c e a t m o s p h e r e . T h e stabilized-zirconia tube was frequently tested to asc e r t a i n t h e a b s e n c e of p o r o s i t y a n d l e a k s . F o r t h i s purpose, electromotive force measurements were m a d e on t h e f o l l o w i n g t h r e e c e l l s : Solid Electrolyte A) Pt, O2 (pure) (ZircomaTube) 1O2 (in air), Pt B) Pt, O2 (in furnace atmosphere Solid Electrolyte _ at equilibriumwith Ni/NlO) ] (ZirconiaTube) 102 (in air), Pt C" - (in furnaceatmosphere Solid Electrolyte 02 (in air), Pt ) Pt9o2 at equilibriumwith Fe/FeO) I (ZlrconiaTube) I T h e m e a s u r e m e n t s o b t a i n e d w i t h c e l l s (B) a n d (C) w e r e i n a g r e e m e n t w i t h t h e d a t a of C h a r e t t e a n d F l e n g a s . 4 Electromotive force measurements were made with an e l e c t r o m e t e r h a v i n g a n i n p u t i m p e d a n c e of 10 z4 o h m . At least 2 hr were allowed for each equilibration. The d e t e r m i n a t i o n of t h e o x y g e n p o t e n t i a l of t h e a t m o s p h e r e with the solid-electrolyte probe eliminated the errors t h a t a r e a s s o c i a t e d w i t h t h e c a l c u l a t i o n of t h e o x y g e n p o t e n t i a l f r o m t h e c o m p o s i t i o n of t h e i n e o m i n g m i x t u r e of g a s e s . T h e s e e r r o r s a r e p r i m a r i l y c a u s e d b y t h e r METALLURGICAL TRANSACTIONS
T h e r e s u l t s a r e s h o w n i n T a b l e I. T h e o x y g e n p o t e n t i a l s o f t h e a t m o s p h e r e s at e q u i l i b r i u m w i t h f e r r o m o n t i c e l l i t e a n d i r o n a r e s i m i l a r t o t h o s e o b t a i n e d by B u r t s e v and Filippov, 5 who i n v e s t i g a t e d the e q u i l i b r i u m b e t w e e n m i x t u r e s
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