The unusual role of surface recrystallization on creep damage behavior of GH3535 alloy weld metal

It is generally believed that surface recrystallization is detrimental to the mechanical properties of superalloys. However, in our study, surface recrystallization exerts a double-edged effect on the creep damage behavior of GH3535 alloy weld metal at 650 °C. One recrystallized layer of ∼ 20 μm depth can be formed at the surface of conventionally machined creep samples at the initial stage of creep tests. The recrystallized layer is characterized by fine grains, fine carbide particles and plentiful twin boundaries. To determine the effect of recrystallized layer, a quasi-in-situ X-ray computed tomography (X-CT) creep test was used, which provides quantizable and quasi-in-situ information of creep damage. Additional tests were carried out for the high-resolution electron microscope observations. It was found that the interface between recrystallized layer and residual strain zone acts as the crack initiation sites, which is ascribed to the residual machining strain and incompatible deformation. On the other hand, the recrystallized layer exhibits better creep damage resistance than the matrix and thus can completely withstand the crack propagation. Beneath the recrystallized layer, the crack propagates along the grain boundaries both inward and in the circumferential direction simultaneously. In our short-term tests, the creep-resisting recrystallized layer can completely isolate the cracks and air to prevent oxidation-assisted creep damage.