Fatigue and creep-fatigue behavior of a nickel-base superalloy at 850°C

Fully reversed total strain-controlled low-cycle fatigue tests with continuously cycling, equal hold in tension and compression, hold in tension 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 hardening 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 compression. The influence of tensile hold on fatigue life is more complicated. The opposite mean stress develops during tensile and compressive hold 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 summation 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.