The Interactions Of Phosphorus And Matrix Elements On Microstructure And Mechanical Properties In Superalloys

Recent studies have found that moderate amount of phosphorus(P)can significantly improve the stress properties and creep life in some superalloys such as IN718,IN706 and GH761 alloys.Therefore,the research about benefical effects of P becomes one of hot issues.Due to the variety of alloy elements in superalloys,the effect of P and its influence mechanism are complexed,which needs to be further studied.Therefore,this paper chooses new designed alloys for experiment instead of exsiting superalloys.Firstly,the ratio of Ni and Fe is adjusted to the existing alloys in order to guarantee the y matrix.Secondly,reinforcement elements such as Al?K Ti and Nb have not been added in order to eliminate the effects of ?'and ?" While minor C has been added to improve the strength of grain boundaries.Finally,different P contents are added to find the interactions between main matrix elements and P.This experiment is helpful for understanding the real strengthening mechanism of P on improving properties and providing ways and methods for development of new alloys.In this paper,the influence of P on as-cast microstructure,as-rolled microstructure and mechanical properties of Ni-Cr alloys has been studied.The results show that the dendrite microstructure is refined and the segregation of Cr is decreased as the increase of P content.In as-rolled Ni-Cr alloys,P inhibits the precipitation of a-Cr on grain boundaries.It is because that P reduces the activity coefficient of Cr in ?-Cr phase,thus reducing the driving force of precipitation other than the "priority occupation" of P on grain boundary defects.The increment of P reduces the tensile strength but improves the impact energies of as-rolled alloys.Once a-Cr has been solutioned into matrix,P has no obvious effect on tensile properties but still enhances the impact performace.P atoms in solid solution state have a significant influence on the dislocation patterns and it is proved that P atoms are very important to alloy matrix.The effect of high P content on the nucleation and growth of Ni-Cr-15Fe alloys has been investigated and the increase of P significantly affects the as-cast dendritic structure.Obviously,the addition of P inhibites the nucleation of grains and promotes dendritic growth.A large number of phosphides are formed in the interdendritic regions and on grain boundaries,which indicates that supersaturated P atoms exist in the remaining liquid and are solidified out at the final stage.When the addition of P is more than 0.27 wt.%,M23C6 enriched in P is precipitates inner grains,which indicates that P is saturated in the matrix.EDS results show that the closer to the grain boundaries,the higher P content,which proves that P has a tendency towards grain boundaries.The change of Fe has a great effect on the solidification,segregation and mechanical properties in P-doped Ni-Cr-Fe alloys.It is indicated that the increment of Fe refines as-cast dendrite structure.The enrichment of P atoms in alloys transfer gradually from interdendritic regions to dendrite cores with the increase of Fe,showing that Fe increases the solid solubility of P atoms in the matrix.The variety of microhardness also confirms the solution strengthening effect of P in the matrix.The increase of Fe reduces the tensile properties and stress rupture life of alloys after heat treatment.The fracture modes change from transgranular mode to intergranular mode.Based on the results of previous studies,the alloys containing 15 wt.%and 30 wt.%Fe have been selected as the experimental alloys.Different contents of P are added into alloys to investigate its effects on microstructure and mechanical properties.It is found that the addition of P promotes the precipitation of M3P,which plays an important role in strengthening grain boundaries at the early stage of creep.The change of microhardness inner grains shows that the solid solution strengthening ef-fect in the matrix is the key to improve the internal strength and thus becomes the main reason for creep performance.It is confirmed that the beneficial effects of P is selective to alloy systems.In alloys with low iron content,the addition of moderate P strengthens grain boundaries through precipitation and improves solution strengthening effect inner grains,which can achieve the optimal performance.