Multiscale Characterization of Phase Evolution in the Fusion Zone of an Electron Beam-Welded Dissimilar Alloy 718/718Plus Joint During Long-Term Thermal Exposure at 760 °C

The fusion zone (FZ) of a dissimilar Alloy 718/Alloy 718Plus joint is chemically intermediate between the two base materials. It retains the segregated microstructure inherited from weld-pool solidification, so that its thermal stability—and not that of either base metal—limits the life of the joint. An autogenous electron beam-welded 718/718Plus butt joint was therefore exposed at 760 °C for up to 1000 hours in the as-welded condition, as a model upper-bound exposure representing accumulated service overtemperature excursions, and the fusion zone was characterized across four length scales. Quantitative SEM- and TEM-EDX established the local compositions of the dendrite cores and of the interdendritic regions (2.6 and 16.5 wt pct Nb), which served as separate inputs to CALPHAD equilibrium, Scheil and TC-PRISMA precipitation-kinetics calculations; SEM-BSE imaging and microhardness resolved the transformation kinetics over the complete exposure series; FIB-SEM tomography at 8 nm isotropic voxel size reconstructed the reaction products in three dimensions; and SAED combined with HRSTEM-HAADF imaging and image simulation identified the phases and their interfaces at atomic resolution. At 760 °C, the Laves phase lies outside its stability field, yet remains far below the temperature at which it would dissolve; it is consequently replaced in situ by a δ + η + σ assemblage, with the released niobium consumed locally rather than returned to the matrix. δ and η grow on {111}γ habit planes across semicoherent interfaces, the γ/η pair obeying the Blackburn orientation relationship, and σ nucleates on existing η platelets—crystallography that accounts for the slow thickening of the platelets (20–55 nm after 250 hours, 60 to 160 nm after 1000 hours) and for their persistence. Tomography reveals that these products form a partially continuous three-dimensional network confined to the former interdendritic regions, a feature not accessible from two-dimensional sections. Microhardness rises from ~ 300 to 490 to 500 HV0.5. Then it falls to ~ 420 HV0.5, tracking γ′ precipitation ( V V ≈ 7.3 pct, D eq = 77 ± 34 nm) and its coarsening combined with the irreversible transfer of niobium into non-strengthening phases. The fusion zone does not homogenize: it diverges from both base materials, and microstructural stability data measured on wrought Alloy 718 cannot be transferred to the weld.