Grain growth in dispersion strengthened superalloys by moving zone heat treatments
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THE
high t e m p e r a t u r e strengthof d i s p e r s i o n s t r e n g t h ened m a t e r i a l s is l i m i t e d by m a c r o s c o p i c yielding through grain boundary sliding and rupture of t r a n s v e r s e grain boundary s e g m e n t s . The i n t r i n s i c high elevated t e m p e r a t u r e m a t r i x strength of d i s p e r s i o n strengthened m a t e r i a l s can be b e s t utilized through development of m a t e r i a l s with elongated g r a m s of fibrous s u b s t r u c t u r e to m i n i m i z e this grain boundary sliding.l,2 Both s h o r t and long time elevated t e m p e r a ture s t r e n g t h s have been quantitatively r e l a t e d to the d e g r e e of elongation of the s t r u c t u r e p a r a l l e l to the applied s t r e s s axis. This d e g r e e of elongation, the grain a s p e c t ratio, is given as the a v e r a g e grain bounda r y i n t e r c e p t length p a r a l l e l to the s t r e s s axis divided by the a v e r a g e grain boundary i n t e r c e p t length p e r p e n d i c u l a r to the s t r e s s axis. As a c l a s s , d i s p e r s i o n strengthened m a t e r i a l s are strongly r e s i s t a n t to r e c r y s t a l l i z a t i o n unless sufficient deformation energy from t h e r m o m e c h a n i c a l p r o c e s s i n g (TMP) is s t o r e d in the s u b s t r u c t u r e to o v e r c o m e the b a r r i e r s to grain growth. R e c r y s t a l l i z a t i o n o c c u r s suddenly and g e n e r a l l y only at v e r y high t e m p e r a t u r e s . Various techniques involving hot working and annealing, or r e c r y s t a l l i z a t i o n , have been used to achieve these s t r u c t u r e s in d i s p e r s i o n strengthened m a t e r i a l s . In general TMP produces a n i s o t r o p y in the worked m i c r o s t r u c t u r e . TD Ni is used in this form and has fine elongated g r a i n s with a strong deformation text u r e which is r e s i s t a n t to r e c r y s t a l l i z a t i o n . Other d i s p e r s i o n strengthened m a t e r i a l s such as TD N i / C r r e c r y s t a l l i z e a f t e r working and a r e used in the r e c r y s t a l l i z e d form. The s e c o n d a r y r e c r y s t a l l i z a t i o n or gram c o a r s e n i n g b e h a v i o r which f o r m s elongated g r a i n s in the d i s p e r sion strengthened s u p e r a l l o y s is not well understood. S i m i l a r types of grain growth r e s p o n s e in other alloys containing d i s p e r s i o n s such as t h o r i a t e d tungsten or SAP, have been known for many y e a r s . The grain c o a r s e n i n g b e h a v i o r of d i s p e r s i o n strengthened nickel R. L. CAIRNS,L. R. CURWICK,and J. S. BENJAMIN are Process Engineer, Research Investigator,and Research Manager, respectively, Non Ferrous Group, The International Nickel Co. Inc., SterlingForest, Suffern, N,Y. 10901. Manuscript submitted April 8, 1974. METALLURGICALTRANSACTIONS A
base alloys has been reviewed by Wilcox and C l a u e r . 3 This d i s c u s s i o n suggested that the grain c o a r s e n i
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