Effect Of P On The Low Cycle Fatigue Properties Of GH984G At Elevated Temperature

GH984G is the main material proposed for high temperature pipes,superheater and reheater at 700 degree in ultra supercritical power station.In actual service,it will be affected by high temperature,high pressure and corrosion.In addition to high temperature creep properties,fatigue properties of materials are also crucial.P element is an element that can significantly improve the high temperature creep life.It is of great significance to study the effect mechanism of P element on the high temperature low cycle fatigue life of alloys.At present,the effect of P on the fatigue properties of Ni based superalloys has not been reported.Adding 0,0.01%,0.02%and 0.04%phosphorus to GH984G alloy with phosphorus content less than 0.0005%respectively.High temperature and low cycle fatigue tests were carried out at 0.5Hz frequency and.The fatigue fracture and microstructure of the samples were analyzed by scanning electron microscopy(SEM),transmission electron microscope(TEM)and electron backscatter diffraction(EBSD).The main conclusions are as follows:(1)Overall,the fatigue performance of GH984G with 0.01%P is the best.The influence of P element on the high temperature and low cycle fatigue life of GH984G is obtained.At large strain amplitude,it first increases,then decreases;at medium and low strain amplitude,it first increases and then decreases.(2)The fatigue process of the alloys is instantaneous hardening at medium and low strain amplitude and then goes into cyclic softening-stabilization-stress rapidly decreasing fracture,while instantaneous hardening at high strain amplitude changes into cyclic hardening process.The increase of P element content increases the initial compressive stress of low strain amplitude of alloys.The ratio of cyclic softening rate and degree is also related to P content.(3)The increase of P content will make the distribution of C element more uniform at grain boundaries,reduce the large-scale carbides near grain boundaries,reduce the damage effect of large-scale carbides near grain boundaries on fatigue life,and increase the grain size of alloys after solid solution treatment;increase fine carbides near grain boundaries and the number of carbides in grains.The intragranular Vickers hardness of GH984 increases with the addition of P,but there is little difference between 0.01%P and 0.04%P.(4)As P can improve the oxidation resistance of GH984G alloy,the number of fatigue sources decreases with the increase of P content.The number of fatigue sources and the grain size before fatigue jointly restrict the fatigue life.(5)P content has influence on the distribution and number of low angle grain boundaries,the number of twins,the local misorientation of the fatigue samples.It is different near and far from the fracture surface.The change rugulation is complex.Due to the high interaction between dislocations and grain boundary twins under large strain,excessive twins will lead to the decrease of fatigue life,and excessive local orientation difference will also lead to the decrease of fatigue life.