Dynamic and Post-Dynamic Recrystallization of Haynes 282 below the Secondary Carbide Solvus

Abstract

Thermomechanical processes, such as forging, are important steps during manufacturing of superalloy components. The microstructural development during processing, which controls the final component properties, is complex and depends on e.g., applied strain, strain rate and temperature. In this study, we investigate the effect of process parameters on the dynamic and post-dynamic recrystallization during hot compression of Ni-base superalloy Haynes 282. Specifically, we address the effect of deformation below the grain boundary carbide solvus temperature. During deformation, discontinuous and continuous dynamic recrystallization was observed at the grain boundaries, and particle-stimulated nucleation occurred at primary carbides. Strain rate was determined to be the governing factor controlling the recrystallization fraction for strain rates up to 0.5 s−1 above which adiabatic heating became the dominating factor. Careful examination of the temperature development during deformation showed that the response of the closed-loop temperature control system to adiabatic heating can have important effects on the interpretation of the observed behavior. During a 90 s post-deformation hold, grain growth and an increasing fraction of twin boundaries significantly changed the deformation-induced microstructure and texture. The microstructure developed during post-dynamic recrystallization was mainly controlled by the temperature and only weakly coupled to the prior deformation step.

Supersolvus Recrystallization and Grain Growth Kinetics for the Fine Tuning of Grain Size in VDM Alloy 780 Forgings

“…A typical example of the resulting necklace-type structure observed after partial DRX during hot deformation is shown in Figure 3. This mechanism of recrystallization is well-reported for nickel-based superalloys, for example for Nimonic 80A from 950 C to 1180 °C at 1 s À1 , [15] Alloy 625 from 900 C to 1200 °C at 0.1 s À1 , [16] UNS N07208 nickel-based superalloy from 1060 C to 1080 °C at 0.05 to 0.5 s À1 , [17] Alloy 718 from 920 °C to 1040 °C at 0.001 to 1 s À1 . [18,19] The zoomed area of Figure 3(c) reveals that there are two types of recrystallized grains at the former grain boundaries: (i) few of them with intragranular misorientations arising from dynamic recrystallization, and (ii) others with much lower internal misorientations which are likely to have grown post-dynamically despite the quenching delay being as short as possible (1 to 3 seconds).…”

“…[18] Conversely, a longer time available for the microstructure to evolve at lower strain rates can lead to an early onset of DRX (i.e., at a lower strain value). [17] Such a decrease in dynamic recrystallization kinetics with increasing strain rate is a common observation and has been reported in many different alloys. [21,[30][31][32][33][34][35][36][37][38][39][40] The influence of strain rate on the recrystallized fraction experimentally obtained in this work is summarized in Figure 5(a).…”

“…Similar observations were found in case of another nickel-based superalloy (UNS N07208) where the post-dynamic evolution was found to be weakly coupled to the prior deformation steps and primarily dependent on temperature. [17] Such a behavior is an asset from industrial point of view as the strain level is never uniform in a forged piece, and therefore the DRX microstructure can be quite heterogeneous throughout the piece. Some time left at high temperature after forging to let PDRX proceed allows for microstructure homogenization.…”

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

“…In either case, PDRX can result in bimodal grain size distributions reported to have detrimental effects on the mechanical properties of superalloys. [54,55] A qualitative depiction of the step-by-step evolution of the resulting microstructure can be seen in Figure 13. During the deformation from Figures 13(a) and (b) the driving force for recrystallization is provided, leading to the necklace structure in Figure 13(c).…”

The Effect of Grain Boundary Carbides on Dynamic Recrystallization During Hot Compression of Ni-Based Superalloy Haynes $$282^{\mathrm{TM}}$$

“…It could therefore be suspected that the presence of carbides, which essentially cover the entire grain boundary area, could affect the nucleation stage by (i) altering the dislocation substructure in the vicinity of the boundaries and thereby the subgrain formation process; and/or (ii) by pinning the boundaries during bulging (so called Zener pinning), which has been shown to increase the length of the boundary involved in the bulging process during static recrystallization, thereby delaying nucleation. [11] Consequently, grain boundary carbides could play a decisive role in the design of hot working processes, and thereby in the need for precise temperature control. However, while the effect of particle stimulated DRX nucleation at primary carbides in coarse grained material has received some attention (see e.g., [12] ), the role of grain boundary carbides during DRX of Ni-base superalloys has not been previously reported.…”