The 650 O C f a t i g u e crack propagation behavior o f tu0 nickel-base superalloys, Renb 95 and Waspaloy, were studied w i t h p a r t i c u l a r emphasis placed on understanding t h e r o l e s o f creep, environment, and two key g r a i n boundary a l l o yi n g additions, boron and zirconium. Coinparison o f a i r and vacuum data showed the a i r environment t o be detrimental over a wide range o f frequencies f o r both a l l o y s . More i n depth analysis on Renb 95 showed a t lower frequencies, such as 0.02 Hz, f a i l u r e i n a i r occurred by intergranular, environinentally-assisted creep crack growth, tthilc a t higher frequencies, up t o 5.0 Hz, environmental of B and Z r i n Haspaloy was found t o be important where environmental and/or creep i n t e r a c t i o n s were present. dramatically increased crack growth and i t i s therefore p l a u s i b l e t h a t effect i v e d i l u t i o n of these elements may explain a previously observed trend i n which crack growth rates increased with decreasing g r a i n size. i n t e r a c t i o n s were s t i l l evident but creep e f f e c t s were minimized. The e f f e c t W
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