Dispersion strengthened superalloys by mechanical alloying

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p a r t i c l e s l e s s than 0.1 # in size upon o r within an alloy powder p a r t i c l e in such a m a n n e r as to lead to i n t e r p a r t i c l e s p a c i n g s of l e s s than 0.5 ~z in a c o n s o l i d a t e d product. It is a well known fact that the m o s t effective p r e c i p i tation h a r d e n i n g e l e m e n t s in n i c k e l - b a s e a l l o y s , a l u m i num, t i t a n i u m , and n i o b i u m , a r e e a s i l y oxidized. T h e i r oxidation would r e m o v e them as p r e c i p i t a t i o n h a r d e n e r s . T h e r e f o r e , oxygen c o n t a m i n a t i o n m u s t be kept low enough to leave s u f f i c i e n t y ' f o r m e r s in a r e d u c e d state to give adequate age h a r d e n i n g for i n t e r m e d i a t e t e m p e r a t u r e s t r e n g t h . In addition the c o m b i n a t i o n of effective oxide d i s p e r s i o n s t r e n g t h e n i n g and p r e c i p i t a t i o n h a r d e n i n g m u s t be a c c o m p l i s h e d at a r e a s o n a b l e cost. T h e r e a r e four m a j o r t e c h n i q u e s which have been used p r e v i o u s l y to c o m b i n e oxide d i s p e r s i o n s t r e n g t h ening and solid s o l u t i o n s t r e n g t h e n i n g in alloy s y s t e m s containing relatively nonreactive elements. A brief c o n s i d e r a t i o n of the n a t u r e and l i m i t a t i o n s of each of t h e s e t e c h n i q u e s will show why they a r e not suited to the t a s k of p r o d u c i n g an alloy c o n t a i n i n g ~': Simple m e c h a n i c a l m i x i n g involves the u s e of a high speed blenderS, 9 o r a ball m i l l 1~to coat the s u r f a c e of a m e t a l o r alloy powder with oxide-powders. The i n t e r p a r t i c l e spacing in the consolidated a l l o y produced f r o m these powders is l i m i t e d by the s t a r t i n g m e t a l powder p a r t i c l e size. P o w d e r s i z e s of l e s s than 5 ~t a r e r e q u i r e d to get s u f f i c i e n t l y fine i n t e r p a r t i c l e s p a c i n g s even with l a r g e m e c h a n i c a l r e d u c t i o n s d u r i n g c o n s o l i dated and s u b s e q u e n t working o p e r a t i o n s . P o w d e r s this fine c o n t a i n i n g y ' f o r m e r s such as a l u m i n u m and t i t a n i u m a r e v e r y r e a c t i v e b e c a u s e of t h e i r high specific s u r f a c e a r e a and c o m p l e t e o r n e a r l y c o m p l e t e oxidation of a l u m i n u m and t i t a n i u m can r e s u l t . Ignition s u r f a c e coating technique 11 i n v o l v e s mixing m a t r i x alloy p o w d e r s with a liquid s o l u t i o n of a salt of a r e a c t i v e m e t a l . This m i x t u r e is d r i e d and p u l v e r i z e d and the powders a r e heated in an i n e r t o r r e d u c i n g e n v i r o n m e n t c o n v e r t i n g the salt to a r e f r a c t o r y oxide. This technique also p r o d u c e s oxide coated powders which have, t h e r e f o r e , the s a m e d i s a d v a n t a g e s as powd e r s produced by the s i m p l e m e c h a n i c a l m i x i