Abstract
The hot compression curves and deformed microstructures were investigated under various hot deformation conditions in three states: hot isostatic pressing (HIP, A1), HIP+hot extrusion at 1100 ℃ (A2), and HIP+hot extrusion at 1150 ℃ (A3). The results show that A2 sample, extruded at 1100 ℃ with uniform γ+γ′ duplex microstructures, demonstrates excellent hot deformation behavior at both 1050 and 1100 ℃. The true stress-true strain curves of A2 sample maintain a hardening-softening equilibrium over a larger strain range, with post-deformation average grain size of 5 μm. The as-HIPed A1 sample and 1150 ℃ extruded A3 sample exhibit a softening region in deformation curves at 1050 ℃, and the grain microstructures reflect an incomplete recrystallized state, i.e. combination of fine recrystallized grains and initial larger grains, characterized by a necklace-like microstructure. The predominant recrystallization mechanism for these samples is strain-induced boundary migration. At 1150 ℃ with a strain rate of 0.001
Nickel-based powder metallurgy (PM) superalloys exhibit excellent comprehensive properties, high-temperature micro-structural stability and uniformity, making them appropriate for advanced aviation engine turbine disk
Thermal compression tests have been commonly used in several studies to investigate the hot deformation behavior and microstructure evolution of superallo
The FGH4113A alloy represents an innovative nickel-based PM superalloy, demonstrating exceptional operational capabilities at temperature up to 800 ℃. Depending on its processing, whether through HIP or HEX, the initial microstructures are different. Notably, with increasing the extrusion temperature, the microstructure showcases larger grain sizes and fewer coarse γ′ phases. After HIP, the dominant microstructural features are coarse γ′ phases interspersed with sporadic PPBs. These initial states, with their inherent microstructural characteristics, significantly influence the alloy's subsequent hot deformation behavior. Therefore, elucidating the relationship between deformation-induced stress-strain curves, initial microstructure, and microstructural variations is extremely important. This study aims to unravel the multifaceted influences of initial microstructures on the hot deformation behavior and microstructure evolution of FGH4113A alloy by thermal compression tests. In addition, the influence of γ′ phase on microstructure evolution and its dissolve was investigated.
The nominal composition of superalloy FGH4113A is outlined in
Co | Cr | Al | Ti | Ta | W | Mo | Nb | C, Hf, B and Zr | Ni |
---|---|---|---|---|---|---|---|---|---|
19.0 | 13.0 | 3.0 | 3.7 | 1.0 | 4.0 | 4.0 | 1.2 | Minor | Bal. |

Fig.1 Preparation process of hot compression sample: (a) alloy ingot, (b) argon atmosphere powder, (c) HIP part, and (d) hot extruded bars
A Gleeble 3180 thermo-simulation machine was utilized to conduct hot compression tests. Specimens with 8 mm in diameter and 12 mm in height were wire-cut from the extruded bars. The direction of deformation was axial, with a total deformation translating to about a 50% height reduction, equating to a true strain of 0.69. A consistent heating rate of 5 ℃/s was maintained, and a soaking duration of 3 min at the deformation temperature ensured microstructural uniformity prior to the compression test. After compression, the samples were subjected to argon cooling to retain their micro-structures. Concurrently, an automated data acquisition system captured the true stress-strain data throughout the hot defor-mation process.
The samples for initial microstructure observation were sourced from the HIP part (A1) and extruded bars (hot extruded at 1100 and 1150 ℃, marked as A2 and A3, respectively). The samples were cube with 10 mm in size.
Each compressed specimen was sectioned centrally and in alignment with the compression axis. In-depth microstructural analyses were conducted using both the NikonMM~400 optical microscope (OM) and the Sigma300 scanning electron microscope (SEM) equipped with electron back scattered diffraction (EBSD). Prior to observation, the samples were ground, polished, and chemically etched via a solution comprising HNO3, H2O, CH3COOH, and HF in a volume ratio of 3:3:3:1. Depending on the grain size, EBSD mapping was facilitated with a step size between 0.1 and 1.0 μm. Grains were characterized by a cluster more than 10 pixels with a misorientation below 10°. A grain orientation spread (GOS) threshold of 2° was set to identify the recrystallized grain
For TEM analysis, samples were firstly ground to a thickness from 50 μm to 70 μm, followed by punching to produce standard disks with 3 mm in diameter. These disks were further thinned via electrolysis using an electrolyte composed of 10% HClO4 and 90% CH3CH2OH at approximately –25 ℃. Subsequent cleaning was performed using ion-milling, with the sample tilted at an angle of 2° at 3 kV for a span of 10 min.
A1 sample was subjected to HIP at 1150 ℃, followed by furnace cooling. As shown in

Fig.2 GOS (a–c) and SEM images of samples under different initial conditions: (a, d) A1, (b, e) A2, and (c, f) A3
The true stress-true strain curves for samples under different deformation conditions are presented in

Fig.3 True stress-true strain curves of samples under different deformation conditions: (a) 1050 ℃-0.01
The typical forging temperature range for extruded samples is 1050–1100
Hot compression tests were conducted on A1 and A2 samples at a high temperature of 1150 ℃ with strain rates of 0.01 and 0.001
The SEM images and grain boundary of the samples deformed at 1050 ℃ under strain rates of 0.01 and 0.001

Fig.4 SEM images (a) and grain boundaries (b) of samples after hot deformation at 1050 ℃ with different strain rates
When comparing fractions of low-angle grain boundary (LAGB, with misorientation angles between 0°–15°, shown in black) and high-angle grain boundary (HAGB, with misorien-tation angles greater than 15°, shown in red), A2 sample exhibits a larger fraction of HAGB (83.3% at 0.01
As shown in
The RX fraction of fine-grained sample (A2) is higher than that of coarse-grained sample (A1 and A3). Furthermore, as shown in Fig.
The microstructures of A2 and A3 samples after deformation at 1100 ℃ with strain rates 0.01 and 0.001

Fig.5 SEM image, grain boundary, and KAM distribution of samples after hot deformation at 1100 ℃ with different strain rates
The microstructures of A1 and A2 samples after deformation at 1150 ℃ with strain rates of 0.01 and 0.001

Fig.6 SEM image, grain boundary, and KAM distribution of samples after hot deformation at 1150 ℃ with different strain rates
The true stress-true strain curves under different strain conditions, as well as the grain size, grain boundary misorientation distribution, GOS, and KAM distribution of the structure after hot deformation, are illustrated in

Fig.7 Grain size, grain boundary misorientation, GOS, and KAM distribution of samples after hot deformation under different conditions: (a) as-received, (b) 1050 ℃-0.01
After deformation at 1050 ℃ with strain rate of 0.01
After deformation at 1100 ℃ with strain rate of 0.01
After deformation at 1150 ℃ with strain rate of 0.01
The RX behavior of FGH4113A superalloy is sensitive to the deformation temperature. Summarily, the GOS, KAM, and grain size of FGH4113A superalloy are dramatically affected by the initial microstructure, the deformation temperature, and strain rate.
TEM images of samples under different deformation conditions are depicted in

Fig.8 TEM images and corresponding schematic diagrams of samples under different deformation conditions: (a–b, e) A1, 1050 ℃-0.01
When deformed at 1100 ℃, coexistence of recrystallized grains and deformed grains are observed in A2 sample. γ′ phases can be seen both in the grains and at grain boundaries, where they restrict grain boundary migratio
1) The A1 sample consists mainly of PPB and γ' phase. The A2 sample displays a γ+γ' duplex microstructure with uniformly distributed fine grains. The A3 sample exhibits a heterogeneous microstructure characterized by non-uniform grain sizes and the presence of residual γ' phase. All three samples, A1, A2, and A3, demonstrate complete recrystallization, with distinct average grain sizes of 6.7, 3.7, and 8.7 μm, respectively.
2) When deformed at 1050 and 1100 ℃, a fine-grained microstructure (A2) is beneficial for RX during the hot deformation process. The A2 sample exhibits complete RX, and its stress-strain curve is in a hardening-softening equilibrium state. In contrast, A1 and A3 samples display a necklace-like microstructure, corresponding to the softening region of the true stress-true strain curve. In the deformed A1 sample, dislocation tangles, sub-grains, twins, and recrystallized grains can be observed.
3) During deformation at 1150 ℃, the deformation behavior and microstructure of the A1 and A2 samples with different microstructures are similar, with average grain sizes of 13.0 and 12.4 μm, respectively.
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