Effect Of Strain Waveform On Fatigue Properties Of Inconel 625 Alloy

Nickel-based superalloys have excellent comprehensive properties and are widely used in the manufacture of high-temperature components of some hot end parts in aero engines and steam turbines that can withstand both long-term and stable operation in harsh working environments such as the high temperature,high speed and repeated start-up and stop.These components working at high temperatures are subjected to the alternating load and its main failure mode is low cycle fatigue.Therefore,the study concerning about the high temperature fatigue behavior has become a top priority in the study of superalloys.In this investigation,the low-cycle fatigue tests of the Inconel 625 alloy were performed at 650? under three strain waveforms including triangular waveform as well as two sawtooth waveforms with slow-tension followed by fast-compression and fast-tension followed by slow-compression.The low cycle fatigue deformation and fracture behaviors of the alloy under different strain waveforms were investigated in order to provide a necessary theoretical basis for the anti-fatigue design of Inconel 625 alloy.The results of low cycle fatigue show that the Inconel 625 alloy exhibits the cyclic stain hardening at the three different waveforms and the total strain amplitudes ranged from 0.3% to 0.7%,while the cyclic stress amplitude is the highest at the sawtooth waveforms with slow-tension followed by fast-compression.Compared with the triangular waveform,the fatigue lives of the alloys under the two sawtooth waveforms are reduced due to the presence of either tensile or compressive creep strain component.In addition,the relationships between the plastic strain amplitude,elastic strain amplitude and fatigue life are linear.Meanwhile,the relationship between the cyclic stress amplitude and plastic strain amplitude exhibits a single slope linear behavior.The dislocation configuration of the Inconel 625 alloy investigated by using the TEM reveal that the main deformation mechanism is dislocation slip in the matrix.The cyclic hardening phenomenon of the Inconel 625 alloy during the fatigue deformation is related to the interactions between dislocations,twin boundaries,grain boundaries,precipitates and dislocations.The high dislocation density occurred in the Inconel 625 alloy after fatigue deformation results in a strong interaction between dislocations.Twin boundaries and grain boundaries are effective obstacles to dislocation motion causing dislocations piled up at the grain/twin boundaries.When dislocations move in the matrix and constrain by the precipitates,a large number of dislocations accumulate at the precipitates which results in a high density of dislocations around the precipitates.The fatigue fracture surfaces characterization with the SEM reveals that the low cycle fatigue cracks for the Inconel 625 alloy under three strain waveforms are nucleated transgranularly at the free surface of specimen and also propagated in the transgranular mode.