Recrystallization mechanisms and associated microstructure evolution during billet conversion of a gamma-gamma′ nickel based superalloy

Investigation and Simulation of the Effects of nm-Scale γ′ Precipitates on the Recrystallization of Ni-based Superalloys

“…Dependent on local microstructure, these c¢ precipitates can either suppress or promote recrystallization. [24,26] However, the suppression of recrystallization appears to occur more commonly due to Zener-Smith pinning, which is also expressed in increased activation energy in the presence of c. [25,27] Figure 9 does confirm the presence of distinct differences in c¢ precipitates between the Type D and Type SD profiles, indicating this as a likely explanation for the different observations. On the other hand, process conditions favoring local c¢ dissolution and re-precipitation may mitigate such pinning effects.…”

“…Isothermal holding at temperatures above the c¢ and c¢¢ solvus temperatures of Alloy 718 led to a rapid PDRX response in materials deformed under such conditions. [6] Coyne-Grell et al [24] studied interactions between recrystallization phenomena and c¢ precipitates in the superalloy AD730. This work highlights the complexity, as they found that hetero-epitaxial recrystallization of coarse c¢ precipitates can enhance recrystallization, while other forms of c¢ precipitates can suppress it.…”

“…This work highlights the complexity, as they found that hetero-epitaxial recrystallization of coarse c¢ precipitates can enhance recrystallization, while other forms of c¢ precipitates can suppress it. [24] Zhang et al [25] reported increased driving forces for DRX in the presence of c¢ precipitates, while Nishimoto et al [26] reported limited grain growth in the presence of coarse y', while recrystallization is promoted by high strains and high temperatures. In contrast, Seret et al [27] found evidence that PDRX is not fully prevented by c¢ precipitates via local dissolution and re-precipitation in proximity to the recrystallization front.…”

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

“…The size and inter-particle spacing of a precipitate distribution can affect the mechanisms and kinetics of recrystallization (i.e., the extent of recrystallization as a function of strain). [11,12] In Udimet 720, it has been shown that during two-stroke forging, applying an intermediate heat treatment, which promotes coarsening of secondary c¢, between the two forging strokes can enhance the recrystallization level achieved. [12] A similar observation has been made in AD730; in a sample with heterogeneous c¢ precipitation, regions with a coarser and sparser distribution of c¢ precipitates showed faster recrystallization kinetics (i.e., higher recrystallization levels as a function of strain) than regions with a finer and denser distribution.…”

“…[12] A similar observation has been made in AD730; in a sample with heterogeneous c¢ precipitation, regions with a coarser and sparser distribution of c¢ precipitates showed faster recrystallization kinetics (i.e., higher recrystallization levels as a function of strain) than regions with a finer and denser distribution. [11] In a billet conversion process, the c¢ precipitates which are present at the start of subsolvus forging are those that have formed during slow cooling through the solvus. In the case of industrial-scale billets undergoing controlled cooling, these slow cooling rates are on the order of 10 °C/h.…”