Structural response to shape variations in the AI-AI 3 Ni eutectic composite
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4, 6 and 12 c m / h . At l e a s t two samples were i n d e pendently grown for each condition. The composite m a t e r i a l used was A1-A13Ni eutectic alloy prepared from 99.99 pct aluminum and 99.9 pct nickel with a composition of 6.06 wt pct Ni. 2.2. Metallographic Techniques The structures of the directionally solidified composites were studied u s i n g both n o r m a l light optical microscopy and scanning electron microscopy. The interfiber spacing (X) was calculated from the m e a s ured values of f i b e r density (p) in the c r o s s section according to the relationship:~ =
(2/3q~p)~r2.
[I]
The value of p for a given growth condition was determined by a v e r a g i n g readings from at least 10 different a r e a s w i t h about 100 counts in each a r e a . Under steady s t a t e growth conditions, the scatter in the values of p was in the r a n g e of +5 pct of the a v e r a g e . In 2. EXPERIMENTAL PROCEDURE c a s e s where the fibers were not n o r m a l t o the c r o s s 2.1. Directional Solidification section, the values of X were corrected u s i n g the f i b e r orientations measured in the longitudinal section. The The directional solidification apparatus used in the f i b e r orientations were measured in relation t o the present work is b a s e d on withdrawing a crucible conspecimen axis t o the n e a r e s t one-hall deg u n d e r a taining the molten alloy from a resistance heated furmagnification of 330 t i m e s . Equiorientation contours nace through a w a t e r cooled chill at the d e s i r e d growth were d r a w n t o pass through a r e a s of e q u a l orientation r a t e . The crucible m a t e r i a l was high p u r i t y graphite u s i n g l i n e a r interpolation. About 1000 measurements and was machined t o give the required changes in from two independently grown samples with two symshape as shown in Fig. 1. The l o c a l temperature g r a d i m e t r i c a l s i d e s in e a c h longitudinal section were inent in the straight part of the crucible away from shape volved in drawing the contours for a g i v e n growth conchanges was kept constant at about 130 K/cm and was dition ( F i g . 2). measured u s i n g a P t - P t , 10 pct Rh thermocouple, 1 mm outside tube diameter. The crucible was withThe number of f i b e r branches was counted in the drawn at a constant speed during each run at 2, 2.5, longitudinal section of deep etched specimens using the scanning electron microscope at a magnification of NAHED A. EL-MAHALLAWY is Lecturer, ProductionEngineering, 1000 t i m e s . Deep etching was c a r r i e d out electrolyAinShams University, Cairo, Egypt and MAHMOUD M. FARAG is tically in a solution of 3 ml HC1, 50 ml He0, and 50 Professor, Materials Engineering, American University, Cairo, Egypt. ml alcohol. The n u m b e r of branches at a given point Manuscript submittedJuly 17, 1978. was counted in a volume of 105 /~m long by 105 ~m ISSN 0360-2133 /79/ 0611-0707500.75 /0 @1979 AMERICAN SOCIETY FOR METALS AND VOLUME 10A, JUNE 1979-707 METALLURGICAL TRANSACTIONS A THE METALLU
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