Structure and crystallography of curved Al-Al 3 Ni and Al-CuAl 2 directionally solidified eutectic alloys
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NEARLY all of the r e s e a r c h on dtrectionally solidified eutectic alloys has been directed towards straight specimens formed by translating an alloy linearly* *The terms linear and nonlinear refer to spatial curvature, not time, throughout this paper.
through a temperature gradient. However, for certain applications curved eutectic shapes could be more useful than straight rods. For example, curved composite beams for hot structures might be fabricated directly by the eutectic directional-solidification p r o c e s s . Such beams would have the composite reinforcement positioned to accept axial s t r e s s e s parallel to the beam tangent. Even in the present major projected use of structural eutectics--turbine blades--the growth process may be intentionally nonlinear to produce curved blades or unintentionally nonlinear near discontinuities. Curved shapes are also desirable for some nonmechanical applications. The present work was undertaken to investigate the morphology and crystallography of the phases in eutectic specimens which are solidified nonlinearly through a temperature gradient. Some similar experiments have been previously conducted by Jaffrey and Chadwick, 1 who solidified " h o o k - s h a p e d " ingots of A1-A13Ni horizontally, with the liquid-solid interface normal roughly in the horizontal plane. They found no s y s t e matic crystallographic relation between the A1 and A13Ni phases, although the AlsNi fibers a n d [010]AI~Ni followed the mold curvature. However, marked differences in crystallography have been found for the A1AI3Ni system between linear specimens grown h o r i zontally and vertically. ~ This result suggests that similar differences may occur in horizontally and vertically grown curved eutectics. In the present investigation the experimental apparatus was constructed so that the liquid-solid interface normal of the solidifying ingot was nearly vertical. This arrangement p r o duced samples with somewhat different c r y s t a l l o g r a phies than those reported by Jaffrey and Chadwick. 1 G. GARMONG is with Science Center, Rockwell International,
Thousand Oaks, CA 91360. Manuscript submitted August 30, 1974. METALLURGICAL TRANSACTIONS A
In addition, tile mode of curvature of both interphase and grain boundaries was studied in the A1-AI3Ni and AI-CuA12 systems. EXPERIMENTAL PROCEDURE The directional solidification apparatus for growing curved ingots was constructed in a large aluminum bell jar, Fig. 1. The bell jar was lined with a w a t e r cooled sleeve, and all vital components potentially subject to heat were also water cooled. Since the system was designed to operate with either inert atmosphere or vacuum, the drive motor was placed outside the enclosure. In order to produce the highly uniform curved motion required for the experiments a wire-and-pulley drive system was devised. No gears were used outside the motor, and heavy counterweighting helped absorb gear noise f r o m the motor. No uneven motion of the mold could be detected with dial gages calibrated in 2.5 ~ i
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