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SCARLIN

Creep and fatigue c r a c k growth r a t e s and t h r e s h o l d s t r e s s intensity amplitudes have been m e a s u r e d for a d i r e c t i o n a l l y solidified c a r b i d e - e u t e c t i c alloy, C73. Fatigue testing t e m p e r a t u r e s have been ambient, 750 and 950~ for c r a c k i n g p e r p e n d i c u l a r and p a r a l l e l to the solidification d i r e c t i o n . In the f o r m e r c r a c k i n g d i r e c t i o n c o m p a r a t i v e propagation r a t e s may be understood in t e r m s of the p r o p e r t i e s of the m a t r i x , which shows a phase t r a n s f o r m a t i o n f r o m hexagonal to cubic above ~900~ A situation where c r a c k growth r a t e s d e c r e a s e with i n c r e a s i n g a p p a r e n t s t r e s s intensity amplitudes (AK) has been found to exist for propagation p a r a l l e l to the s o l i d i f i c a tion d i r e c t i o n at low AK values and high t e m p e r a t u r e s only. This phenomenon can be r e lated to the o c c u r r e n c e of c r a c k branching and multiple c r a c k i n g of the carbide f i b e r s . Considerations of p l a s t i c zone s i z e s and c r i t i c a l defect s i z e s for c r a c k propagation a r e consistent with the conditions n e c e s s a r y for such c r a c k d e c e l e r a t i o n to occur. T r a n s f o r m a t i o n of the M7C3 f i b e r s , p r e s e n t in the a s - c a s t condition, to M2sCs at cell boundaries of the solidification s t r u c t u r e o c c u r s at a t e m p e r a t u r e of 950~ Although M23Cs c a r b i d e s a r e e a s i l y c r a c k e d and t h e r e f o r e probably reduce propagation r a t e s by causing s e c o n d a r y cracking, their p r e s e n c e is known to be d e t r i m e n t a l to creep p r o p e r ties. High cycle fatigue t h r e s h o l d s t r e s s intensity amplitudes for C73 in e i t h e r loading d i r e c tion at room t e m p e r a t u r e , 750 and 950~ a r e ~20 pct lower than for the cast n i c k e l - b a s e alloy, IN 738LC, i.e. c r i t i c a l defect s i z e s a r e ~10 pct s m a l l e r in C73. Despite the known s e n s i t i v i t y of c r a c k i n g r a t e s and t h r e s h o l d values to f a c t o r s such as minor fluctuations in loading amplitude it is believed that these d i f f e r e n c e s a r e significant.

ALLOYS

used for gas turbine blades and vanes need to p o s s e s s a good combination of c r e e p strength and c o r r o s i o n r e s i s t a n c e . F o r i n c r e a s e d efficiency of operation higher operating t e m p e r a t u r e s a r e r e q u i r e d so that m a t e r i a l s capable of withstanding higher s e r vice t e m p e r a t u r e s a r e sought. It a p p e a r s that little f u r t h e r i m p r o v e m e n t can be achieved by development of the c u r r e n t wrought and cast s u p e r a l l o y s , however, c e r t a i n d i r e c t i o n a l l y solidified (DS) eutectic cobalt and n i c k e l - b a s e alloys have c o n s i d e r a b l y higher c r e e p strengths. T h r e e distinct c l a s s e s of suc