The room temperature fatigue behavior of nickel-base superalloy crystals at ultrasonic frequency

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TIEN AND R. P. GAMBLE

P r e v i o u s i n v e s t i g a t i o n s have i n v a r i a b l y o b s e r v e d s t r a i n rate r e l a t e d deformation effects as the fatigue frequency is r a i s e d to the u l t r a s o n i c range. Through room t e m p e r a t u r e t e s t s on s t r a i n rate i n s e n s i t i v e n i c k e l - b a s e s u p e r a l l o y s i n g l e c r y s t a l s of M a r - M 2 0 0 , we have shown that a n o t h e r effect of i n c r e a s i n g the fatigue f r e q u e n c y to the u l t r a s o n i c range is in the supp r e s s i o n of the d e l e t e r i o u s influence of e n v i r o n m e n t . It was found that above a s t r e s s a m p l i tude of 30,400 psi the fatigue lives of c r y s t a l s u l t r a s o n i c a l l y fatigued in a i r i n c r e a s e with d e c r e a s i n g s t r e s s in a m a n n e r which is f u n c t i o n a l l y s i m i l a r to that of c r y s t a l s conventionally fatigued in v a c u u m . S i m i l a r l y , the f r a c t u r e s u r f a c e s of u l t r a s o n i c a l l y fatigued c r y s t a l s have a dimpled a p p e a r a n c e over m o s t of t h e i r a r e a s which is c h a r a c t e r i s t i c of locally ductile f a i l ure and i d e n t i c a l to the a p p e a r a n c e of c r y s t a l s failed at c o n v e n t i o n a l l y f r e q u e n c y in v a c u u m . T h e s e r e s u l t s , along with a kinetic a n a l y s i s of gaseous a d s o r p t i o n , indicate that the m a j o r effect of i n c r e a s i n g the fatigue frequency to the u l t r a s o n i c range is in the s u p p r e s s i o n of the i n fluence of oxygen in e n h a n c i n g the rate of c r a c k propagation. In addition, the s h o r t test t i m e s involved in r u n n i n g large n u m b e r s of c y c l e s have allowed for the d e t e r m i n a t i o n of the fatigue l i m i t in a n i c k e l - b a s e s u p e r a l l o y . This is the f i r s t indication of n o - f a i l b e h a v i o r in this type of alloy. UNDERSTANDING of the basic fatigue behavior of nickel-base superalloys has recently been advanced, by Gell and coworkers, ~-3 through studies of single crystals at fatigue frequencies between 1 and 1000 cycles per sec. However, progress in high speed machinery, such as gas turbine engines, may also require information on the fatigue of superalloys at much higher frequencies. Accordingly, one purpose of this investigation is to determine the fatigue behavior of a nickel-base superalloy in resonance at ultrasonic frequency. Another and more general purpose is to capitalize on the strain rate insensitive nature of these alloys in order to unveil manifestations of ultrasonic frequency fatigue which have heretofore not been reported. The number of metallurgical investigations on the fatigue behavior of any material at ultrasonic frequency have been limited.4-7 The major frequency dependence commonly found in all previous studies can be related to the high strain rates associated with ultrasonic fatigue frequencies. In other words, the materials tested were all

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