Study of the cellular solidification structure in a continuously cast high purity copper
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IN studying mechanical
properties of materials it is necessary to have detailed knowledge of microstructural features which might influence the plastic deformation. Besides initial dislocation density and grain size, an important factor is the kind, concentration, and distribution of impurities. This paper reports a study of the microstructure of continuously cast A S A R C O * high purity copper (~-in. diana rod), *AmericanSmelting& RefiningCo., South Plainfield,NJ. Thismaterialis identifiedin orderto adequatelyspecifythe experimentalprocedureand doesnot implyendorsementby the NationalBureauof Standards. c h a r a c t e r i z e d by optical and e l e c t r o n m e t a l l o g r a p h y , as well a s a n a l y t i c a l methods. T h i s effort r e v e a l e d a c e l l u l a r solidification s t r u c t u r e in the v e r i f i e d 99.999 pct p u r i t y m a t e r i a l , see T a b l e I. O b s e r v a t i o n of such a s t r u c t u r e in high p u r i t y copper has b e e n r e p o r t e d p r e v i o u s l y 1-4 and a t t r i b u t e d i n some s t u d i e s to the p r e s e n c e of c a r b o n and oxygen i m p u r i t i e s . The effect of this c e l l u l a r s t r u c t u r e on c r e e p d e f o r m a t i o n i s significant and will be r e p o r t e d i n a l a t e r p a p e r . The phenomenon of c e l l u l a r solidification has been a t t r i b u t e d by R u t t e r and C h a l m e r s s to i m p u r i t y s e g r e gation f r o m a c o n s t i t u t i o n a l l y s u p e r c o o l e d m e l t during solidification. U n d e r a p p r o p r i a t e growth conditions, a plane s o l i d - l i q u i d i n t e r f a c e can b e c o m e u n s t a b l e and b r e a k down. The i n t e r f a c e morphology then depends m a i n l y on the r a t e of solidification, the t e m p e r a t u r e g r a d i e n t at the i n t e r f a c e , and the d i s t r i b u t i o n coeff i c i e n t of the f o r e i g n a t o m s . M o r r i s and W i n e g a r d s o b s e r v e d a t r a n s i t i o n of the i n t e r f a c e morphology with i n c r e a s i n g i n s t a b i l i t y of the s o l i d - l i q u i d i n t e r face ( i n c r e a s i n g s o l i d i f i c a t i o n rate). Starting with an a r r a n g e m e n t of individual nodes, the s t r u c t u r e develops through elongated c e l l s into, f i n a l l y , a hexagonal cell s t r u c t u r e for f a c e s close to the (111) and (100) p l a n e s . A. RUKWIEDis Physicist, EngineeringMetallurgy Section, A. W. RUFF is Chief, Lattice Defects and MicrostructuresSection, and W. A. WlLLARD is Metallurgist,EngineeringMetallurgySection, respectively, in the MetallurgyDivision,National Bureau of Standards, U. S. Department of Commerce, Washington,D. C. Manuscript submitted October 15, 1970. METALLURGICALTRANSACTIONS
Similarly, faces growing close to the (110) plane show a transition from elongated ceils into a hexagonal cell structure as the growth rate is increased. The hexagonal cell structure can be imagined to form when excess impurities are rejected by projections g
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