Discontinuous γ′ nucleation due to Boron and Carbon segregation in Ni-based superalloys

Discontinuous γ′ precipitation is a solid-state phase transformation that involves nucleation at migrating grain boundaries during slow cooling, resulting in rod-like γ’. It has recently found applications in addressing hot-working challenges in Ni-based superalloys and large-scale nanorod production. However, more widespread adoption of discontinuous precipitation requires a better understanding of its nucleation and growth stages. Existing research provides inconclusive results on whether elemental segregation or secondary phases serve as nucleation sites. The nucleation mechanisms at these sites remain unclear due to two key factors. Firstly, elemental segregation reduces grain boundary energy, consequently decreasing the driving force for heterogeneous nucleation. Secondly, the growth of discontinuous γ′ is directly linked to grain boundary mobility, which is reduced by solute drag or Zener pinning. Here, we compare two Ni-based superalloys with different B and C contents to establish their role in discontinuous γ′ precipitation. We show that the high B & C variant exhibits increased susceptibility to discontinuous γ′ nucleation compared to the low B & C variant. In the high B & C variant only, discontinuous γ′ precipitation is achieved at a cooling rate of 5 °C/min. At a slower cooling rate of 1 °C/min, both variants undergo an almost complete transformation via the discontinuous reaction. Atom probe microscopy confirms that nucleation occurs at grain boundaries with increased interfacial excess of B and C, rather than at carbides or borides . CALPHAD simulations show that elevated B and C concentrations around grain boundaries locally increase the general driving force for γ′ nucleation. As growth proceeds, the morphology transitions from initial fan-like into spaghetti-like structures, alongside with emergence of misfit dislocations where differently aligned discontinuous γ′ cells intersect. We suggest that stresses cells exert on each other determine growth directions, leading to the spaghetti morphology. These insights are pivotal for controlling discontinuous precipitation and facilitating broader adoptions in advanced hot-working and nanotechnology.