Nickel and cobalt eutectic alloys reinforced by refractory metal carbides
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IN an attempt to provide an interim solution to the ma-
difficulties in producing aligned composites containing terials problems of advanced gas turbines between the continuous carbide filaments or whiskers. phase out of conventional nickel and cobalt base superThe present investigation was directed toward examalloys and the possible introduction of a satisfactory ining the phase equilibria, solidification, and mechanical refractory or precious metal system, attention has been properties of nickel and cobalt pseudobinary eutectics drawn to fiber reinforced and dispersion strengthened with the monocarbides of metals from groups IVa nickel and cobalt and to some extent chromium alloys. and Va of the periodic table. While the superiority of TD nickel and cobalt alloys to conventional superalloys is clear at 2000~F and above, EXPERIMENTAL PROCEDURE this superiority is achieved only at the sacrifice of The a l l o y s studied in this i n v e s t i g a t i o n w e r e p r e strength below 1800~ with lower ductility at elevated p a r e d f r o m m a t e r i a l s whose t y p i cal c o m p o s i t i o n s a r e temperatures. l i s t e d in T a b l e If. A l l o y s along the p s e u d o b i n a r y join Previous work on the directional solidification and w e r e m e l t e d in a l u m i n a c r u c i b l e s and held a p p r o x i properties of certain eutectics, e.g. Nb-Nb2C, m a t e l y 30 rain under an a r g o n c o v e r . Heating and c o o l Ta-Ta2C,"z and NisAl(7)-Ni~Nb,s has demonstrated that ing c u r v e s w e r e obtained using P t 13 pct R h - P t t h e r alloys with improved elevated temperature mechanical properties to the existing superalloys may be developed. m o c o u p l e s!. Slabs w e r e cut f r o m the m a s t e r ingots and r e c a s t in ~ in. diam. c y l i n d r i c a l a l u m i n a c r u c i b l e s . Reinforcement of a ductile matrix was accomplished U n i 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 was a c c o m p l i s h e d by withby aligning a carbide or intermetallic phase, in the d r aw i n g the m e l t downward through e i t h e r a high f r e form of whiskers or lamellae, which possessed a greater strength and creep resistance at elevated quency induction coil or r e s i s t i v e l y heated g r a p h i t e tempe rature. furnace as described elsewhere e at velocities between 1 and I0 cm per hr. The thermal gradient in the liquid The carbides, although thermodynamically less stable was typically 60~ to I00~ per c a . than oxides, were considered most attractive as reinforcements for nickel and cobalt. The melting points Tensile testing was performed on mechanical specimens oriented such that the growth direction was parof the carbides, for example, include the highest melting points of all compositions of matter, i. e. TaC allel to the loading direction. The specimens were loaded to failure using a Tinius-Olsenfour screw test~ 4000~ Table I. They generally have high elastic ing machine with loading rates of 0.01 and 0.07 in. per moduli, low densities, and more important to the pres
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