Effect of P and B compound strengthening on the evolution of phases and stress rupture properties of the IN718 superalloy during long-term aging at 650 °C

The influences of P and B compound strengthening on the precipitated phases, stress-rupture properties and deformation mechanisms of a direct-aged IN718 superalloy during long-term aging at 650 °C are investigated. The results indicate that the combined addition of P and B reduces the coarsening rate of γ″ phases and markedly inhibits the precipitation and growth of δ phases and α-Cr phases. P and B compound strengthening could improve the stress-rupture property of the alloy at 680 °C/700 MPa by increasing the strength of intragranular and grain boundary. In the grain interior, P and B compound strengthening could inhibit the intragranular diffusion and hinder the coarsening of γ″ phases, thus increasing the stability of γ″ phases that could reduce the degree of cut by dislocations. In addition, the combined addition of P and B reduces the stacking fault energy of the alloy and forms the high-density twin, improving the ability of the alloy to impede dislocation movement. At the grain boundary, P and B compound strengthening could inhibit the grain boundary diffusion, hindering the nucleation and growth of the δ phase and the α-Cr phase, and the crack initiation and propagation. After aging at 650 °C for 5000 h, the combined addition of P and B could also inhibit the precipitation of TCP-σ phases at grain boundaries, but the TCP phases have no obvious effect on the stress-rupture property of the alloys due to their lower amounts and sizes. With the increase in aging time, the permanent deformation mechanism of the two alloys changes from twinning to dislocation shear γ" phase and twinning.