Abstract:The influencing mechanisms of P content on the creep rupture properties of GH4738 alloy and the optimum additive range of P element in this alloy were investigated by OM, SEM, TEM, and EBSD microscopic analyses. The results show that the creep rupture strength of the alloys shows a tendency of firstly increasing and then decreasing with the increase in P content. The optimal addition amount of P ranges from 0.0040wt% to 0.0091wt%, and the fracture morphology shows a mixed fracture type. With continuously increasing the P element, the endurance life is decreased by 30%–50%, the persistent plasticity is decreased by 20%–70%, and the fracture mode changes to brittle intergranular fracture. The main reasons are as follows. P, as a type of grain boundary segregation element, when its content is smaller, the segregated P element occupies the vacancy at the grain boundary to decease the free energy of grain boundary, thereby increasing the nucleation rate of carbides. Meanwhile, the uniformly dispersed M23C6-type carbides along grain boundaries can inhibit crack propagation and improve grain boundary strength, ultimately enhancing the creep resistance of the alloys. When P content is greater than the critical value, it has a tendency to solute into MC-type carbides. Consequently, with further increase in P content, the amount of MC-type carbides at grain boundaries is increased while M23C6-type carbides are decreased relatively. The blocky MC-type carbides become unevenly distributed along grain boundaries, leading to a continuous degradation in the creep resistance of the experimental alloys.