Effects of Different Forming Preparation Processes on the Microstructure and Properties of GH4169 Nickel-Based High-Temperature Alloy
Release time:
2025-11-25
GH4169 nickel-based high-temperature alloy demonstrates excellent performance in high-temperature resistance, corrosion resistance, and fatigue resistance. Its distinctive feature lies in its ability to operate at temperatures up to 850°C and withstand short-term exposure to temperatures as high as 1100°C. The mechanical properties of this alloy are primarily influenced by two factors: precipitation strengthening from the γ" phase (Ni₃Nb) and grain boundary refinement from the δ phase (Ni₃Nb). Prior to application, heat treatment techniques are essential to effectively refine the internal microstructure, enhance hardness, strength, and overall performance, eliminate internal stresses, and prevent deformation or cracking. Corrosion testing ensures the alloy's suitability for real-world operating conditions. To maximize the application value of the aforementioned forming and preparation processes, processing parameters are rationally optimized by analyzing the effects of different aging and corrosion treatments on alloy properties.
1 Test Materials and Methods
1.1 Test Materials
The test material consists of a segment (Φ35mm × 300mm) cut from GH4169 nickel-based high-temperature alloy round bar. Its chemical composition is shown in Table 1. GH4169 nickel-based high-temperature alloy features a γ phase matrix, with primary strengthening phases being γ' (Ni₃(Al,Ti)) and γ" (Ni₃Nb), alongside minor δ phase (Ni₃Nb) and carbides. It is suitable for high-temperature, high-stress environments [1]. During heat treatment, homogenization and solution treatment are required to eliminate residual stresses and composition segregation arising from processing. Subsequent aging treatment regulates the morphology, size, and distribution of precipitates to optimize material properties. In the corrosion process, appropriate machining parameters must be selected to remove the matrix material and ensure the alloy's dimensional accuracy meets standards.
1.2 Experimental Setup
Six primary devices were used in this experiment: First, an XRD-type X-ray diffractometer with an operational range of -6° to 163°, equipped with a CuKα radiation source (λ=0.154056 nm) and a positioning speed ≥1000 rpm. Combined with JADE 6.0 software, it was used for quantitative phase analysis. Second, an FM-ARS type device, equipped with a CuKα radiation source (λ=0.154056 nm), positioning speed ≥1000 rpm, combined with JADE 6.0 software for quantitative phase analysis; second, an FM-ARS microhardness tester with measurement repeatability of ±0.8%, capable of measuring small indentations down to 15 μm, loading speed of 60 mm/sec, and load holding time of 5–99 sec; Third, a DDL-type electronic universal testing machine for high-temperature tensile testing, featuring resolution exceeding 0.001 mm and displacement speed accuracy better than ±0.5% [2]; Fourth, a SEM scanning electron microscope for observing specimen microstructure; Fifth, a TR100 roughness tester, which measures multiple symmetrically distributed points on the specimen and calculates the average value; Finally, a corrosion processing tank equipped with an agitator, thermometer, and iron stand, with the thermometer surface uniformly coated with protective adhesive.
1.3 Test Methods
1.3.1 Heat Treatment Tests
The heat treatment test procedure is as follows: First, secondary aging treatment—after homogenization and solution treatment, the alloy is first held at 718°C for a period, then furnace-cooled to 620°C, maintained at 620°C for 8 hours, and then air-cooled. Second, microstructure observation: cylindrical specimens are processed to standard dimensions, ground, polished, and etched, followed by scanning electron microscopy (SEM) examination. The etching solution consists of aqua regia [3], a mixture of concentrated hydrochloric acid and concentrated nitric acid. Third, hardness testing: specimens were examined using a hardness tester with five measurement points per specimen. During testing, the indenter was placed on the specimen surface under a 5 kg load for 10 seconds. Fourth, tensile property testing: specimens were preheated to ensure surface roughness. The initial tensile speed was 2 mm/min, and the final tensile speed was 15 mm/min. Fifth, phase analysis: Samples underwent mechanical polishing. During analysis, the scanning speed was 5 μm/min, with a step width of 0.02 μm and one overlap scan. Test data were processed using JADE 6.0 software to calibrate peak positions and calculate δ-phase content based on the integrated intensity of diffraction peaks.
1.3.2 Corrosion Testing
The corrosion testing procedure is as follows: First, place the processing fluid in the corrosion processing tank. The main components are a mixture of HNO₃ and HCl (aqua regia), with additives AN and AA. Perform activation treatment on the alloy specimens, then transfer them to the corrosion processing fluid. Control the stirring rate at 100–180 rpm and the temperature at 53 ± 2°C. Use the processing fluid volume as the variable. Second, remove the processed specimens, rinse them with cold water, immerse them in boiling water, rinse again, air-dry, and remove the protective coating. Finally, conduct inspections: first use a roughness tester to assess the specimen surface condition, then employ a universal testing machine to determine the tensile properties of the specimens.
2 Experimental Results and Discussion
2.1 Effect of Different Aging Processes on Alloy Microstructure
2.1.1 Precipitation Behavior Analysis
After aging treatment at 718°C for 1 to 32 hours, no significant grain coarsening was observed in the alloy. This is primarily because short rod-shaped δ phase (Ni₃Nb) had already precipitated at grain boundaries during the initial homogenization and solution treatment stages. Acting as a pinning phase, this effectively suppressed grain boundary migration during subsequent aging, preventing grain growth and thereby enhancing the stability of the matrix grain structure throughout the aging process. During short aging times ≤16 hours, changes in the morphology and quantity of δ phase at grain boundaries were not significant. This is because the aging temperature of 718°C is relatively low, requiring a longer nucleation period for γ" phase formation. Simultaneously, the low diffusion rates of strengthening elements like Nb and Al result in the slow precipitation and stable existence of γ“ phase within grains. At this stage, disc-shaped γ” phase uniformly distributed within grains, measuring 30–50 nm in size, with a small amount of needle-shaped δ phase scattered throughout. During extended aging times, specifically at 32 hours, the δ phase at grain boundaries exhibits increased quantity and elongated length, indicating the initiation of γ“ phase transformation into δ phase within the matrix. Intragranular γ” phase size increases to 40–110 nm, with morphology transitioning from disc-shaped to short rod-shaped, and uniformity decreasing [4]. Overall, the 16-hour aging treatment yielded the optimal results. The high-density precipitation of γ" phase strengthened the alloy, and its size fell within the strengthening peak range, effectively enhancing material strength. As shown in Figure 1, metallographic examination revealed an austenitic single-phase microstructure with a grain size rating of 5.0 and non-metallic inclusion levels of A0.5, B1.5, C0.5, and D0.5.
2.1.2 Quantitative Analysis of Precipitation
Quantitative analysis of δ-phase precipitation was conducted during aging treatments at 718°C, 770°C, and 820°C. The obtained data are shown in Figure 2. During the initial 1-hour stage, the precipitation levels at 718°C and 770°C were both around 2%, while that at 820°C was approximately 3.2%. By 32 hours, the precipitation differences among the three temperatures gradually increased, with corresponding precipitation amounts of 10.1%, 8.8%, and 5.2% from highest to lowest temperature. Overall, distinct diffraction peaks were observed for the γ matrix phase, δ phase (Ni₃Nb), and NbC carbide. However, no independent diffraction peaks were detected for the strengthening phases γ' (Ni₃(Al,Ti)) and γ" (Ni₃Nb). This is attributed to the low content of the γ' phase, whose volume fraction falls below the detection limit of XRD. Additionally, the γ" phase peak overlaps with that of the γ matrix phase, preventing their separation and identification. At identical aging times, δ phase precipitation volume exhibits a proportional relationship with temperature. Specifically, δ phase precipitation is relatively higher at 820°C and lower at 718°C. This primarily stems from the correlation between δ phase precipitation kinetics and temperature: as temperature approaches the optimum precipitation temperature, atomic diffusion rates accelerate, enhancing nucleation and growth driving forces for the δ phase, thereby increasing its volume fraction [5].
2.2 Effect of Different Aging Processes on Alloy Mechanical Properties
2.2.1 Effect on Alloy Microhardness
During aging at 718°C, the alloy hardness data are shown in Figure 3. It can be observed that hardness exhibits a characteristic pattern of initially increasing and then decreasing with aging time. The hardness peak occurs at 16 hours of aging, with a maximum value of 484.7 HV5. After 32 hours, hardness markedly decreased, reducing by approximately 30.1 HV5 compared to the 16-hour value. This hardness variation directly correlates with the coarsening process of the strengthening phase γ“ (Ni₃Nb). During the initial aging period (1–16 hours), γ” phase nucleates uniformly within the matrix and continuously grows, gradually increasing in size to 30–50 nm. At this stage, the γ“ phase remains coherent with the γ matrix, inhibiting dislocation motion and enhancing alloy strengthening, thus sustaining hardness increase. Beyond the critical aging time of 16 hours, γ” phase coarsening initiates via the Ostwald ripening mechanism, characterized by dissolution of smaller phases and growth through absorption of larger phases. When γ"-phase size exceeds 50 nm, its coherent relationship with the matrix is progressively disrupted, causing the coherent strengthening effect to vanish. Dislocation bypassing mechanisms then dominate deformation, leading to a subsequent decrease in alloy hardness.
2.2.2 Effect on Tensile Properties
Tensile property tests were conducted on specimens aged at different temperatures using the optimal aging duration. The test results are shown in Table 2. The standard requirement specifies a tensile strength ≥1275 MPa. Overall, the 718°C × 16h aging treatment yielded the best comprehensive strengthening effect, meeting the tensile strength standard while also achieving the high-temperature elongation required for forging standards. Concurrently, the 820°C × 2h aging process yielded the highest high-temperature elongation, demonstrating superior plasticity. Quantitative analysis via XRD revealed that δ phase (Ni₃Nb) content progressively increases with rising aging temperatures, a trend directly influencing the alloy's tensile properties. δ-phase precipitation consumes strengthening elements like Nb, reducing the volume fraction of γ“-phase. However, the δ-phase itself possesses weaker strengthening capability than γ”-phase. The δ-phase forms rod-like structures at grain boundaries, oriented at specific angles to the boundaries. This arrangement mediates strength differences between grain boundaries and the bulk matrix, while pinning mechanisms delay crack propagation, thereby enhancing plasticity.
2.2.3 Influence on Precipitation Kinetics
The coarsening of γ“ phase (Ni₃Nb) is driven by interfacial free energy, with its kinetics controlled by the diffusion rate of Nb in the nickel matrix. During aging treatments at 718°C, 770°C, and 820°C, the coarsening rate constant of γ” phase exhibits a linear relationship with temperature. For every 50°C increase in temperature, the rate constant increases by approximately 1.5 to 2 times. Experimental data calculations yielded an activation energy for γ“ grain growth of Q = 281.76 kJ/mol, comparable to the self-diffusion activation energy of Nb in the Ni matrix (257 kJ/mol). This confirms that γ” grain growth is primarily governed by the long-range diffusion of Nb atoms within the γ matrix, rather than short-range processes such as interfacial reactions or lattice distortion. Concurrently, the elevated coarsening activation energy indicates that the γ" phase in GH4169 alloy exhibits enhanced resistance to coarsening at elevated temperatures. Its growth rate is lower than that of conventional nickel-based alloys, demonstrating superior stability.
2.3 Effect of Different Corrosion Processes on Alloy Properties
2.3.1 Effect on Surface Roughness
With processing bath loadings set at 5, 10, 15, 20, 25, and 30 cm²/L, the surface roughness values of the specimens were 1.21, 1.23, 1.36, 1.41, 1.43, and 1.67 μm, respectively. The standard requirement is a specimen surface roughness ≤ 1.4 μm. Overall, the surface roughness of GH4169 high-temperature alloy exhibited a progressive increase with rising processing bath loading, accompanied by accelerated growth rates. To ensure both processing efficiency and quality, the processing bath loading should be controlled at ≤15 cm²/L.
2.3.2 Effect on Tensile Properties
Specimens treated with processing baths at different loadings were tested to confirm changes in tensile properties. The standard requires an elongation after fracture ≥12% and a reduction of area ≥15%. Test data are shown in Table 3. Overall, the elongation after fracture of GH4169 high-temperature alloy gradually increased with rising bath loading. When loading did not exceed 15 cm²/L, the alloy's elongation after fracture failed to meet technical standards. Meanwhile, the cross-sectional reduction rate consistently reached ≥15%.
3 Conclusions
For the performance evaluation of GH4169 nickel-based high-temperature alloy, the heat treatment process was optimized with 3 aging temperatures and 6 holding times. Comparative analysis through microscopic observation, hardness testing, high-temperature tensile testing, and phase analysis determined the optimal aging process: aging at 718°C for 16 hours, followed by furnace cooling to 620°C, followed by furnace cooling to 620°C,
an 8-hour soak, and subsequent air cooling. For the corrosion process, six different bath loadings were tested. Analysis of specimen surface roughness and tensile properties determined the optimal process parameter to be 15 cm²/L.
Reference: Vol. 45, No. 11, Metallurgy and Materials; Effects of Different Forming Preparation Processes on the Microstructure and Properties of GH4169 Nickel-Based Superalloy
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