FATIGUE AND CREEP-FATIGUE BEHAVIOR OF A NICKEL-BASE SUPERALLOY AT 850-DEGREES-C

Fully reversed total strain-controlled low-cycle fatigue tests with co ntinuously cycling, equal hold in tension and compression, hold in ten sion or in compression have been conducted at 850 degrees C in air to study fatigue and creep-fatigue behavior of nickel-base alloy GH4049. At higher strain amplitudes, the alloy exhibits cyclic strain hardenin g followed by softening while a stable cyclic stress response is shown at low strain amplitudes. A significant reduction in fatigue life is observed with hold time in compression or in both tension and compress ion. The influence of tensile hold on fatigue life is more complicated . The opposite mean stress develops during tensile and compressive hol d tests. The degree of work hardening increases during cycling due to introduction of hold time. The SEM analysis of fracture surfaces shows that crack initiation and stage I growth is transgranular, but crack growth in stage II is intergranular. In addition, a linear damage summ ation method, comprising both creep and fatigue damage, is considered to provide a good correlation of measured life with predicted life. (C ) 1998 Elsevier Science Ltd. All rights reserved.