Role of diffusion-induced grain boundary migration in the oxidation response of a Ni-30 Cr alloy

Surface deformation of Ni and Fe alloys often contributes to improved oxidation resistance at low and intermediate temperatures. While this beneficial effect has been attributed to fast diffusion mediated by high densities of dislocations and grain boundaries, their contributions ought to be limited due to the very rapid recovery and recrystallization at these temperatures. To elucidate the mechanisms responsible for the beneficial effect of deformation, a Ni-30 at.% Cr alloy in electropolished and ground conditions was oxidized in air at 600 °C for times up to 50 h. The electropolished surface produced a duplex scale with nodules consisting of external NiO and internal Cr ₂O ₃ oxides. Limited bulk Cr diffusion sustained Cr ₂O ₃ thickening, leading to a continuous thin Cr-depletion zone (<50 nm at 50 h) beneath the scale away from nodules. Alternatively, the ground surface generated a single oxide Cr ₂O ₃ layer. Since recovery and recrystallization took place within less than 10 min, the contribution of dislocations remained limited. Instead, the significantly faster Cr transport to the surface yielding exclusive selective Cr oxidation was enabled by diffusion-induced grain boundary migration (DIGM). Rather than diffusion along stationary grain boundaries as previously reported, DIGM and consequently grain growth of the sub-surface recrystallized region resulted in the originally deformed region to become uniformly Cr depleted over several micrometers. DIGM also plays a dominant role in the electropolished sample with the formation of a near Cr ₂O ₃ single layer and limited external NiO above grain boundaries.