Study On Fatigue And Fracture Behavior Of Cobalt-based Superalloy

The research object of this paper is a cobalt-based alloy prepared by hot isostatic pressing process with a cobalt content exceeding 50wt%,which is mainly used for the wear-resistant layer on the surface of nuclear power thrust discs.Due to its special service conditions,it will frequently encounter the effects of cyclic load and thermal shock during operation,which will cause damage to the hard wear-resistant layer such as thermal fatigue cracks.Considering that the actual service environment water temperature is 300?,in order to maximize the practical environment,100? boiling water is used for simulated cyclic thermal shock test,and room temperature water is set as a control group to explore its failure process and mechanism.In addition,in view of the relatively few studies on fatigue of high-cobalt-based cobalt-based alloys and the increasing use of high-cobalt-based cobalt-based alloys,this article specifically supplements the three-point bending fatigue behavior of this material at room temperature.In terms of three-point bending fatigue at room temperature,we deliberately selected two typical samples,smooth samples and straight-notch samples,for three-point bending test and S-N curve measurement.The fracture morphology of different samples under different loads was observed by scanning electron microscope.The effects of notches on the fracture behavior of the specimen under both static and dynamic loading characteristics were compared,and the differences in fracture behavior under the two loading characteristics were compared.The results show that:(1)The fatigue limit of the smooth specimen was 545 MPa,which was about 25.4% of the original bending strength of 1552 MPa.The fatigue limit of the straight-notch specimen is about 100 MPa,which is about 19.1% of its corresponding bending strength of 517.6 MPa.The fatigue sensitivity of smooth specimens is 397,and that of straight-notched specimens is 31.(2)For straight-notched specimens,it was found that the fatigue cracks can initiate in the carbides aggregated regions in the near surface.Surface defects also induced the crack initiate at the specimen's surface.The crack propagation paths on the fractures of the two samples mostly manifested as transgranular fractures of carbides and intergranular fractures between cobalt matrix and carbides.At the same time,there were fewer cracks between carbides.(3)Comparing the bending and fatigue fractures,it can be found that the two are very similar,but there are also significant differences.Under the fatigue load of R = 0.1,the cobalt matrix will partially undergo martensitic transformation,resulting in a decrease in the toughness of the material.Although the overall toughness shows a certain toughness and more tear ridges,it also shows Mode of brittle fracture.Therefore,the fatigue crack propagation mode should be a mixed mode of true fatigue and static fatigue.At the same time,unlike non-notched samples,obvious crack growth areas can be found on the fractures of straight-notched samples,and fatigue fringes can be observed in the crack growth areas.In terms of cyclic thermal shock,this paper selects water at room temperature(20?)and boiling water(100?)for thermal shock tests.The influence of thermal shock process on the properties of the material is measured by means such as bending strength and fatigue life under certain load.Finally,through scanning electron microscopy,XRD and other means to analyze the damage to the material and changes in the material structure during the thermal shock process.The results show that:(1)Both 800? room temperature water and boiling water thermal shock can suppress the martensitic transformation of the cobalt matrix,so that more fcc-? phase can be retained.At the same time,it was found that the difference in crack morphology of the samples under the two processes was due to the formation of a vapor layer near the surface of the sample during the thermal shock of boiling water.The steam layer can significantly reduce the cooling rate,thereby reducing the damage to the material in a single thermal shock process,and greatly delaying the initiation and propagation of thermal cracks.Therefore,samples with thermal shock from boiling water have better surface morphology.(2)As the number of boiling water thermal shocks increases,the bending strength increases first and then stabilizes on a platform.This is due to the suppression of martensite transformation in the process of boiling water thermal shock,so that more fcc-? phase is retained and the material is strengthened.The appearance of the platform is due to the saturation of the strengthening effect and is closely related to the cooling rate.(3)Due to the strengthening effect of the boiling water thermal shock process on the material,the fatigue performance of the material increases with the number of thermal shocks,but because the fatigue performance is more sensitive to thermal shock damage,when the surface cracks of the sample are formed,the fatigue performance shows a downward trend before the bending strength.