The effect of hold time and waveform on the high-temperature, low-cycle fatigue properties of a Nb-A286 alloy

The effect of hold time and waveform on the high-temperature, low-cycle fatigue (HTLCF) of a Nb-modified A286 alloy was investigated at 650°C. It was found that the fatigue life with strain hold was lower than that of continuous low-cycle fatigue, and this was due to the additive creep deformation during hold time. The apparent activation energy for the creep deformation during the tensile and compressive hold times was determined to be about 260 to 270 kJ/mole from stress-relaxation curves. This value is in good agreement with the apparent activation energy required for the formation of η phase at the grain boundary in the Nb-A286 alloy. Therefore, the grain-boundary precipitates and the corresponding cavitation along the grain boundary were estimated to be the main damage of the creep-fatigue interaction of the Nb-A286 alloy and to cause the intergranular failure. Also, the driving force for grain-boundary cavitation leading to intergranular failure under a compressive strain hold is reduced compared to the case of a tensile strain hold. This fact is known to be due to the healing effect of the grain-boundary cavitation. Therefore, the strain hold in compression leads to less reduction in fatigue life than that in tension.