| Essentially,phosphorus is a trace element with strong segregation tendency and tends to be segregated at the defects such as grain boundaries.Phosphorus had been taken as a common impurity or a deleterious element in superalloys and steels for a long time.It has been well documented recently that appropriate quantities of P could significantly prolong the creep and stress rupture lives of some wrought superalloys including IN718 alloy.Boron is a typical element which can strengthening grain boundaries,and there is a strong synergistic effect between P and B on improving the stress rupture properties of IN718 alloy.The beneficial effects of P and B are attributed to their effects on grain boundaries.However,the strengthening effects of P and B on grain interiors are investigated in this work.What's more,this paper puts emphasis on the segregation behavior of P and its effect on the precipitation of ?' and ?" phases in IN718 alloy.The main conclusions are:P atoms actually are dissolved in the y matrix to a relatively high degree at the temperature when ?' and ?" phases begin to precipitate,and subsequently the precipitation of ?'and ?" phases is accelerated.The first-principle calculations indicate that P decreases the formation enthalpies of ? and ?' phases when it occupies the Ni lattice sites in the two phases,which explains the accelerating effect of phosphorus on the precipitations of ?' and ?" phases.The accelerating effect of P on ?' and ?" phases is enhanced first and then reaches a limit with the increasing P content in the alloy.The precipitation of ?" and ?' phases during air cooling is sensitive to the P content in the grain interior of IN718 alloy.The content of P in the grain interior is dominated by the segregation behavior of P.With the increase of annealing time,the content of P segregated on the grain boundary increases first and then decreases,which makes the content of P dissolved in the grain interior decreases first and then increases.As a result,the amount of ?' and ?" phases decreases first and then increases after air cooling,and the matrix micro-hardness also decreases first and then increases.In addition,with the decrease of cooling rate,the content of P segregated on the grain boundary increases,and the P dissolved in the grain interior decreases,which wakens the accelerating effect of P on the precipitation of ?' and ?" phases and decreases the matrix micro-hardness.Decreasing the cooling rate consistently,the de-segregation of P on the grain boundary occurs,and the content of P dissolved in the grain interior increases.Consequently,the precipitation of ?' and ?" phases accelerate by P and the matrix micro-hardness increases.The accelerating effect of P on the precipitation of ?" and ?' phases and the corresponding micro-hardness demonstrates the segregation behavior and distribution of P.Both the phenomena of critical time during annealing and critical cooling rate during cooling are captured,and the characterization of nonequilibrium grain-boundary segregation of P in superalloy is confirmed.What's more,a simple method for characterizing the distribution and segregation behavior of P is proposed.The accelerating effects of P on ?' and ?" phases in IN718 and IN718C are different.The grain size of IN718C is much larger,and the area of grain boundaries is smaller,which makes the capacity of grain boundaries for P atoms limited and the content of P dissolved in the grain interior higher.As a result,the accelerating effect of P on the precipitation of ?' and ?" phases in IN718C is more effective.In addition,P can increase the sizes of ?' and ?" phases in IN718C after standard heat treatment,which increases the matrix micro-hardness and tensile strength.B has no obvious effect on the precipitation of ?' and ?" phases.However,P and B can inhibite the precipitation of ? phase.What's more,the grain-boundary can be enhanced by B.So both the strength,plasticity and stress-rupture life can be improved by adding proper contents of P and B. |
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