Effect of shape variations on the structure of directionally solidified Al-Al 3 Ni composites
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The effect on structure of some of the possible changes in shape of directionally solidified A1-AlsNi eutectic composites has been studied for two growth conditions. The shape changes investigated included both contraction and divergence in cross-section of the grown part, as well as 90 deg bends in the center-line of the composites. The experimental results showed that contractions in the solidifying cross section do not seriously affect the growth of composites, apart from some coarsening of the structure near the surface. Fiber branching took place in the case of gradual divergence in the solidifying cross section with the fibers deviating from the general growth direction by an angle determined by the shape of equitemperature contours during solidification. Sharp changes in growth direction, 90 deg bend, gave rise to nucleation of new grains of considerable misorientation and hypoeutectic a l l o y s nucleated p r i m a r y aluminum phase before the eutectic s t r u c t u r e was e s t a b l i s h e d . The r e l a t i v e l y l a r g e under cooling needed for nucleation gave r i s e to high local growth r a t e s in the 90 deg bend a r e a . As the AtsNi f i b e r s grow at right angles to e q u i t e m p e r a t u r e contours during solidification, it is concluded that control of the composite s t r u c t u r e can be achieved by controlling the growth conditions and mold design. Molten a l l o y - g r a p h i t e r e a c t i o n s r e s u l t e d in the format:ion of aluminum c a r b i d e s which w e r e m o r e extensive at higher t e m p e r a t u r e s and longer exposure t i m e s .
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of the published data on d i r e c t i o n a l l y solidified c o m p o s i t e s have been acheived in s p e c i m e n s of the rod f o r m where no change in c r o s s s e c t i o n a l a r e a of d i r e c t i o n of growth takes place. However, if these c o m p o s i t e s a r e to be s u c c e s s f u l l y used for p r a c t i c a l applications, e.g., turbine blades, m o r e c o m plicated s h a p e s than s t r a i g h t r o d s have to be grown and tested. Attempts to produce gas turbine b l a d e s by d i r e c t i o n a l s o l i d i f i c a t i o n of s u p e r a t l o y s ~-s have shown that a v a r i e t y of m i c r o s t r u c t u r e s ranging from equiaxed to columnar and p e r f e c t l y aligned g r a i n s can exist in the blade depending on the solidification conditions, alloy composition and mold design. In the a i r f o i l portion, the g r a i n s can be aligned p a r a l l e l to the leading and trailing edges or to the axis of the blade indicating that the orientation can be controlled by s e l e c t i n g the c o r r e c t growth p a r a m e t e r s . 1 In growing A1-A13Ni eutectic c o m p o s i t e s under nonlinear conditions using bat shaped 4 and hook shaped s molds it was found that the growth a x i s of the A13Ni f i b e r s is always a p p r o x i m a t e l y p a r a l l e l to the [010] d i r e c t i o n of the A13Ni l a t t i c e and is i n v a r i a b l y aligned p a r a l l e l
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