Isothermal Fatigue Behaviour of Two Nickelbase Alloys for Turbine Blades of Fighter Aircraft Engines

Turbine blades of fighter class aero-engines are subjected to repeated and unsteady throttle excursions during exploitation leading to thermo-mechanical fatigue. Failures of few turbine blades have led to premature withdrawals and major accidents. Hence, there is a need to study the high temperature fatigue behavior of these alloys. This case study deals with the isothermal fatigue behavior at 900°C for two nickel base alloys which are used for manufacturing turbine blades. The materials selected for study are precipitation strengthened wrought nickel base super alloy AP 220BD of Russian origin and western origin nickel base DS cast alloy MAR-M 002. Tensile and low cycle fatigue (LCF) tests were carried out at 900°C. LCF test was carried out at three strain ranges Δεt, i.e., ±0.3%, ±0.5% and ±0.8% using a triangular waveform of frequency 0.33 Hz with the help of MTS 800 servo hydraulic fatigue testing machine. Fractography and metallography have been done on the fracture surface to study the mode of failure and changes in morphology. Both AP220BD and MAR M 002 shows initial cyclic softening followed by cyclic hardening at low strain Δεt, i.e., ±0.3%. Stabilisation of hysteresis loop takes place at strain range Δεt ±0.5% for both the alloys. At Δεt/2 = 0.8%, AP220 BD shows cyclic hardening while MAR M 002 shows cyclic softening. MAR M 002 shows higher degree of hardening compared to that of AP220BD. The reduction in LCF life with increase in strain range is exponential for wrought alloy but for DS cast alloy it is approximately linear. LCF life has been correlated with fatigue damage parameters such as Ostergren energy(σmax Δεp) and plastic strain amplitude Δεp/2. Ostergren energy for MAR-M 002 is found to be significantly less compared to that of AP220BD. The increase in plastic strain range Δεp is significant for wrought alloys. While Ostergren energy is a good indicator of fatigue damage for wrought alloy, plastic strain amplitude Δεp/2 seems to be a better indicator for DS cast alloy. Fractography reveals inter-granular failure with initiation of cracks at at grain boundary / carbides in wrought alloy. DS cast alloy shows cleavage failure with crack initiation at pore cavities in interdendritic spacing.