Advancements in processing of Ni-based superalloys by microstructure engineering via discontinuous γ′ break-down

Abstract

Wrought γ′-strengthened Ni-based superalloys are critical materials for gas turbine disks in aircraft and power-generation applications. However, their high inherent strength makes their hot formability challenging, and this is often related to intergranular cracking. We propose that this type of crack formation may be mitigated by grain boundary serrations which has not yet been demonstrated in superalloys. Here, we introduce engineered grain boundary serrations in a Ni-based superalloy via discontinuous γ′ precipitation during slow cooling after super-solvus solutionising. We compare the formability of this microstructure to a counterpart with globular γ′ and straight grain boundaries, during sub-solvus compression at 950–1130 °C. We demonstrate that the microstructure with discontinuous γ′ shows excellent cracking resistance, while the one with globular γ′ develops severe intergranular cracks during compression ≤1100 °C. The discontinuous γ′ microstructure exhibits up to 26% lower peak flow stresses and recrystallises at temperatures ≤1100 °C, compared to the globular γ′ microstructure where recrystallisation does not start until 1130 °C. Compression of the discontinuous γ′ microstructure at 1000–1050 °C yields recrystallised fractions of >75 vol.% and results in a duplex γ-γ′ microstructure with grain diameters <4 μm. We argue that its significantly improved formability results from these engineered serrated grain boundaries, the coarsened γ′ morphology, energy absorption by the break-down of discontinuous γ′, and the resulting development of a fine-grained duplex γ-γ′ microstructure. Integrating such discontinuous γ′ precipitation into advanced manufacturing routes will facilitate or, in some cases, even enable manufacturing of parts for aircraft turbines and gas engines at lower temperatures and forces.