Growth structures of al-4.5 wt pct cu alloys with dendrite growth directions differing from the heat flow direction
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i . INTRODUCTION W H E N an alloy is cast in a mold, copious nucleation takes place in the chill zone, and then columnar crystals are formed by competition among dendrites nucleated randomly in the chill zone.1,2 It is well known that dendrites in the cubic alloy grow in the orientation, s In directional solidification, dendrites grow in a columnar manner, and if there are two adjacent grains in which the growth directions of both dendrites differ from the heat flow direction, the grain growing with the larger deviation from the heat flow direction diminishes its size finally to disappear. Thus, when the competition of growth between neighboring grains occurs, the grain whose direction is parallel to the heat flow survives.~ On the other hand, it is often observed that when there is no grain boundary in a directionally solidified specimen, dendrites may grow freely without any competition, and in any direction. In the present work, the growth of dendrites unaffected by grain boundaries was examined by controlling the initial crystal orientation. Two different morphologies were observed and these morphologies were discussed 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 2.1. P r e p a r a t i o n of Seeds Seeds with c o n t r o l l e d o r i e n t a t i o n s of A1-4.5 wt pct Cu a l l o y s w e r e p r e p a r e d a s f o l l o w s . The m e l t was p o u r e d in an i n s u l a t i n g mold, which was heated at about 973 K (700~ and p l a c e d on a w a t e r cooled c o p p e r plate, to s o l i d i f y u n i d i r e c t i o n a l l y . The longitudinal s e c t i o n of the ingot obtained (3 cm in t h i c k n e s s , 11 cm in height and 10 cm in length) is shown in F i g . 1. It shows that the ingot is a l m o s t f i l l e d with c o l u m n a r g r a i n s growing s t r a i g h t upward. The ingot was made up into c y l i n d r i c a l s e e d s of 0.58 cm in d i a m and HIROSHI KATO is Lecturer, Department of Mechanical Engineering, Saitama University, Urawa, 338 and TAKATERU UMEDA is Associate Professor, Department of Metallurgy and Materials Science, the University of Tokyo, 113, Japan. Manuscript submitted October 18, 1977. METALLURGICALTRANSACTIONS A
about 10 cm in length with e a c h d i r e c t i o n as shown in F i g . 2. A l l the s e e d s w e r e c a r e f u l l y p r e p a r e d to have a constant d e n d r i t e a r m s p a c i n g at the i n i t i a l p o s i t i o n of the s o l i d - l i q u i d i n t e r f a c e (about 6 cm f r o m the b o t -
Fig. 1--The longitudinal section of the ingot solidified unidirectionally.
"•'Growth
Direction of Primary Arms
No.0
No.15 No.30 No,45 o.,0
Fig. 2--Schematic diagram to show the directions in which the ingot is sectioned.
1SSN 0360-2133/78/1211-1795500.75/0 9 1978 AMERICANSOCIETYFOR METALSAND THE METALLURGICAL SOCIETY OF AIME
VOLUME9A, DECEMBER 1978-1795
tom), and were n u m b e r e d so as to show the angle b e tween the p r i m a r y d e n d r i t e a r m and the s p e c i m e n axis. 2.2. Quenching O p e r a t i o n The seed
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