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Low Cycle Fatigue And Fatigue Failure Analysis Of Inconel 625 Alloy At High Temperature

Posted on:2017-03-31Degree:MasterType:Thesis
Country:ChinaCandidate:Y Y WangFull Text:PDF
GTID:2271330482975645Subject:Materials engineering
Abstract/Summary:PDF Full Text Request
Because of their high tensile strength and good creep resistance as well as excellent fatigue and hot corrosion resistances at elevated temperatures, nickel base superalloys are widely used in aviation, power and petrochemical industries for making the key components operating under the uncharitable conditions such as high temperature and high speed in aviation, power and petrochemical industries. It is thought that high temperature low cycle fatigue is one of the main failure forms for those high temperature components. In this dissertation, the low cycle fatigue behaviors of Inconel 625 superalloy and its welding joint in the temperature range from 25?C to 760?C were investigated in order to provide a reliable theoretical basis for the fatigue-resistant design of Inconel 625 alloy and components.The results of low-cycle fatigue tests show that at 25?C, Inconel 625 alloy exhibits the cyclic strain hardening in the early stage of fatigue deformation and the cyclic strain softening in the later stage of fatigue deformation. At 650?C, the alloy exhibits the cyclic strain hardening throughout the entire process of low-cycle fatigue deformation. At 760?C, the alloy exhibits the cyclic strain hardening in the early stage of fatigue deformation, while the alloy exhibits either cyclic strain softening or cyclic strain hardening in the later stage of fatigue deformation. For the Inconel 625 superalloy welding joint, the continuous cyclic strain softening is observed at 25?C, while the cyclic strain hardening occurs at 760?C. It is noted that for both Inconel 625 alloy and welding joint, the cyclic stress-strain relation at 25?C shows a double slope linear behavior, while the cyclic stress-strain relation at high temperatures shows a single slope linear behavior. The relationship between both plastic and elastic strain amplitudes as well as reversals to failure for both Inconel 625 alloy and welding joint shows a linear behavior. At all total strain amplitudes used in this investigation, the fatigue lives of Inconel 625 alloy at 25?C are the longest, the fatigue lives of Inconel 625 alloy at 760?C are the shortest, and the fatigue lives of Inconel 625 alloy at 650?C are between those at 25?C and 760?C.The dislocation substructures in Inconel 625 superalloy after the low cycle fatigue failure under strain controlled condition at 25?C, 650?C and 760?C were analyzed using transmission electron microscope(TEM). The main deformation mechanism for Inconel 625 superalloy is dislocation slipping in the matrix. The cellular substructures can form during the fatigue deformation. The dislocation density inside the cellular substructure is low, while the dislocations on the cell walls tangle up and the dislocation density is high. With the slip of dislocations, the dislocations on the cell walls gradually evolve into the dislocation networks. In addition, the carbides can hinder the movement of grain boundaries and dislocations, and the dislocation pileups can form.The morphology of fracture surfaces of Inconel 625 superalloy and welding joint after the low cycle fatigue failure were analyzed using scanning electron microscope(SEM). For the Inconel 625 alloy, the low-cycle fatigue cracks initiate and propagate in a transgranular mode at all temperatures used in this investigation. For the Inconel 625 alloy welding joint, the low-cycle fatigue cracks initiate transgranularly at the free surface of fatigue specimens and propagate in a transgranular mode at 25?C. However, the low-cycle fatigue cracks initiate transgranularly at the free surface of fatigue specimens, and propagate in a mixed transgranular and intergranular mode at 760?C.
Keywords/Search Tags:Inconel 625, Low-cycle fatigue, Fatigue life, Dislocation, Cyclic stress response
PDF Full Text Request
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