The Mechanism of Grain Boundary Serration and Fan-Type Structure Formation in Ni-Based Superalloys

On the role of boron, carbon and zirconium on hot cracking and creep resistance of an additively manufactured polycrystalline superalloy

“…However, we believe that the asymmetric composition and precipitation profiles observed at the grain boundary may indicate that they have formed in the solid state. To support this argument, we point out that this configuration parallels, although at a finer scale, the fan-type morphologies of γ+γ' compounds found at grain boundaries of several polycrystalline nickel based superalloys cooled down from super-solvus heat-treatment at a rate below 3 K.s -1 [42,43]. In addition, it can be noticed that the composition profiles of boron, carbon and zirconium measured at grain boundaries in the as-built microstructure (Figure 7a,b) follow similar trends as those predicted by Okamoto and Ågren [44] for the evolution of carbon behind a moving phase boundary affected by solute drag in steels.…”

Evolution of γ′ Precipitation During the Early Stages of Industrial Forging of a Nickel-Based Superalloy

“…Thermodynamically, the grain boundary motion which is accompanied by discontinuous and fan-type c¢ precipitation can be driven by the free energy change of decomposition of the supersaturated c matrix ahead of the advancing boundary. [16,31] This driving force is relevant in the present case, where there is little driving force from capillarity [i.e., due to the coarse-grained starting microstructure, shown in Figure 2(a)], and little driving force from stored strain energy [shown in Figure 2(d)].…”

“…Most recently, Atrazhev et al have given a detailed model for the formation of fan-type c¢ precipitates, and shown that grain boundary mobility is the key factor favoring their formation. [16] When grain boundary mobility is high, the formation of FTS behind moving boundaries is favored. When it is lower, grain boundary c¢ precipitates form and cause grain boundary serrations to develop.…”

“…This shape has been observed to form during slow cooling, and is the result of a coarsening process in which c¢ precipitate shape evolves from a sphere, to a cube, to an octocube, and eventually to an octodendrite. [14,15] Secondly, slow cooling rates in the vicinity of the solvus temperature allow grain boundaries to move over larger distances, as their mobility is higher at higher temperatures; high grain boundary mobility has been identified as the main factor favoring the formation of discontinuous ''fan-type'' c¢ precipitates by Atrazhev et al [16] As Atrazhev et al point out, in polycrystalline samples, there will be a wide range of grain boundary mobility, so even during slow cooling a mixture of continuous (i.e., octodendritic) and discontinuous c¢ precipitation may be expected. The interactions of these two types of c¢ precipitate with recrystallization during subsequent hot forging within a single sample have not yet been investigated, although such heterogeneous microstructures are representative of the early intermediate stages of ingot to billet conversion.…”

Review of Microstructure–Mechanical Property Relationships in Cast and Wrought Ni‐Based Superalloys with Boron, Carbon, and Zirconium Microalloying Additions

“…GB segregation of C and other impurities is also believed to act as nucleation sites for fan-shaped GB-γ 0 impacting the hot deformation behavior of cast and wrought Ni-based superalloys. [118][119][120][121][122] However, the detailed nucleation mechanism has not been experimentally verified yet. First-principles calculations studying GB segregation of C in Ni or Ni-based superalloys are less frequently found when compared to B.…”