Formation mechanism of abnormally large grains in a polycrystalline nickel-based superalloy during heat treatment processing

Abstract

Controlling the final grain size in a uniform and controlled manner in powder metallurgy nickel-based superalloys is important since many mechanical properties are closely related to it. However, it has been widely documented that powder metallurgy superalloys are prone to suffer from growth of abnormally large grains (ALGs) during supersolvus heat treatment, which is harmful to in-service mechanical performance. The underlying mechanisms behind the formation of ALGs are not yet fully understood. In this research, ALGs were intentionally created using spherical indentation applied to a polycrystalline nickel-based superalloy at room temperature, establishing a deformation gradient underneath the indentation impression, which was quantitatively determined using finite element modelling, electron backscatter diffraction (EBSD) and synchrotron diffraction. Subsequent supersolvus heat treatment leads to the formation of ALGs in a narrow strain range, which also coincides with the contour of residual plastic strain in a range of about 2% to 10%. The formation mechanisms can be attributed to: (1) nucleation sites available for recrystallization are limited, (2) gradient distribution of stored energy across grain boundary. The proposed mechanisms were validated by the phase-field simulation. This research provides a deeper insight in understanding the formation of ALGs in polycrystalline nickel-based superalloy components during heat treatment, when subsurface plastic deformation caused by (mis)handling before super-solvus heat treatment occurs. The practical relevance of looking at small strains at room temperature this research is to understand what happens when turbine disks undergo small dents and scratches during (mis) handling before heat treatment.

As-Built and Post-treated Microstructures of an Electron Beam Melting (EBM) Produced Nickel-Based Superalloy

“…In addition, the carbide phase was also heterogeneously present and might have also contributed to the above. [48] The contour region was observed to be more prone to grain growth compared to the hatch region, as noted above for the rod specimens. Therefore, the contour region was further examined in another EBM Alloy 718 build comprising flat specimens.…”

A comparative analysis of Inconel 718 made by additive manufacturing and suction casting: Microstructure evolution in homogenization

“…The Zener pinning effect [57,58,72,75] of the fine carbide particles, NbC, at the grain boundaries is confirmed by the TEM characterization on sample AM12h, as shown in Fig. 6(k).…”

“…2(a), the NbC carbide is stable at high temperatures, even at 1200°C, and thus often co-exists with the γ matrix phase during homogenization. With the proper size and distribution range, the NbC carbide can act as the grain boundary pinning particles to impede grain growth [22,57,58], whereas the coarsened NbC particles can introduce micro-void softening effects with localized stress concentration and crack initiation to reduce grain boundaries strength [59][60][61]. The δ phase is stable up to 1034°C (Fig.…”

Effects of Deformation Processing Parameters on the Microstructure Evolution and Microhardness of GH4169 Superalloy during Annealing Treatment

“…The reasons for that can be concluded as follows. When the true strain is 0.36, the recrystallized nuclei are mostly formed by grain boundary bulging, [ 57 ] leading to a small number of nuclei and a low DRX degree. Thus, most deformation stored energy is used for δ phase dissolution, and the pinning effect on the grain growth becomes weak, as shown in Figure 8d.…”