Spinodal decomposition of beta brass
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phenomena of present concern may be common t o the B2 and DO3 structures. As the present work concerning the satellites will show, t h e r e are two t y p e s : sidebands a r i s i n g from a composition modulation, and diffraction patterns caused by the intersection of diffuse streaks with the E w a l d sphere. B r i e f experimental results and descriptions will be given in Section 3 t o distinguish the former from the latter, but the main p u r p o s e in this paper is to focus on the sidebands due to a composition modulation a r i s i n g from a spinodal mechanism occurring in the B2 m a t r i x phase. Spinodal decomposition in o r d e r e d phases has received less attention in that decomposition and o r d e r ing are recognized t o be conflicting trends, considering n e a r e s t neighbor effects (Brags-Williams model). However, the Fe-A1 system has been subjected t o extensive experimental study in anticipation of a spinodal decomposition of the B2 phase.25'26 In addition, Khachaturyan considered the likelihood of spinodal decomposition in o r d e r e d alloys in establishing a theory of secondary ordering. 27 In any event the important thing is t o be able to describe the c r i t i c a l temperatures of the chemical and coherent spinodals in the ordered phase, and these points will be considered in the present paper. 2. E X P E R I M E N T A L P R O C E D U R E Cu-38.9 and C u - 4 7 . 3 at. pct Z n alloys w e r e prep a r e d by melting high purity Cu and Z n in evacuated q u a r t z capsules, following w h i c h 7 m m d i a m single crystals w e r e g r o w n by the B r i d g m a n method. T h e crystal rods w e r e then cut into slabs of various orientations 1 m m thick and 4 s q u a r e m m using a low s p e e d d i a m o n d cutter followed by hand polishing to r e m o v e the m a c h i n i n g zone. Slabs with [I00], [Ii0] and [iii] n o r m a l s w e r e prepared. In most c a s e s the slabs w e r e s e a l e d in evacuated capsules and h e a t e d to 8 5 0 ° C for 5 rain as s h o w n in Fig. I. H o w e v e r , in o r d e r to inc r e a s e the quenching rate, the slabs w e r e at t i m e s heated in an atmosphere. Q u e n c h i n g w a s accomplished by ejecting the specimen from a vertical furnace into a NaOH bath at - 15°C, the capsule b e i n g broken immediately upon entering the quenchant. D u r i n g quenching, the specimens underwent ordering, resulting in
ISSN 0360-2133/79/0511-0633500.75/0 © 1979 AMERICAN SOCIETY FOR METALS AND TFIEMETALLURGICAL SOCIETYOF AIME
VOLUME IOA, MAY 1979-633
850°C x 5 min. In V a c u u m In A r
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55°and 60°C JIn Vacuum k RT i "~ In Air(02) I1[ T.E.M. I----~H3PO4+CrO3H-tJ In Air I >
I'°°/° NaOHI -15 °C
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t r a n s p a r e n c y u s i n g the j e t m e t h o d f o l l o w e d by f i n a l p o l i s h i n g u s i n g the s o l u t i o n s s h o w n i n F i g . 1 . F o r t h e m o s t p a r t , the t h i n f o i l s p e c i m e n s w e r e a g e d i n the electron microscope t o determine growth kinetics, a n d so f o r t h , but a t t i m e s t h e
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