Effect of heat treatment process parameters on the microstructure and properties of GH4720Li superalloy

The solution temperature and aging temperature of GH4720Li superalloy are the critical factors in determining its microstructure stability and mechanical properties. However, the matching of solution temperature, primary aging temperature, and second aging temperature greatly influences the final microstructure and mechanical properties, and the mechanism was rarely studied. In this paper, the effects of solution temperature, aging temperature, and other process parameters on the grain size, precipitated phase, and mechanical properties of GH4720Li superalloy were systematically studied by orthogonal experiment, and a set of optimal heat treatment process parameters were found. The results show that the sample was solution treated in the temperature range of 1060 °C ∼ 1120 °C. When the solution temperature was 1160 °C ∼ 1100 °C, although the primary γ ′ phase gradually dissolved, the secondary γ ′ phase gradually increased, and the primary γ ′ phase was pinned at the grain boundary to hinder the grain growth, and the hardness of the alloy gradually increases. When the solution temperature exceeded 1100 °C, the primary γ ′ phase dissolved in large quantities, the grains grew up rapidly, and the hardness of the alloy decreased. The sample was subjected to two-stage aging treatment in the temperature range of 650 °C ∼ 770 °C and 760 °C ∼ 880 °C. As the aging temperature increased, the primary γ ′ phase of the sample gradually grew, and the ability to pin the grain boundary weakened. The volume fraction of the secondary γ ′ phase of the sample gradually decreased and coarsened, resulting in grain growth and a gradual decrease in the hardness of the sample. When the samples were treated by 1100 °C/OC × 4 h + 650°C /AC × 8 h + 760 °C/AC × 8 h, the grain size of the sample was the smallest, the average grain size was 4.5 μm, the distribution of γ ′ phase was the most uniform, and the mechanical properties are the best, reaching 47 HRC.


Introduction
GH4720Li superalloy is a precipitation-strengthening superalloy that usually needs heat treatment after forging. The primary purpose is to promote the precipitation and growth of the γ ′ phase, increase the volume fraction of the γ ′ phase, and then improve the strength of the alloy [1][2][3]. Different heat treatment processes directly affect the grain size of the alloy and the morphology, size, and volume fraction of the γ ′ phase. The change in microstructure has a crucial influence on the alloy's microstructure stability and mechanical properties [4,5].
Therefore, the heat treatment process of GH4720Li superalloy has been studied extensively. It has been found that the distribution of secondary and tertiary γ strengthening phases can be adjusted by preheating and heat treatment of the sample, which can avoid the growth of abnormal grains and realize the manufacture of the alloy [6]. It was also believed that the γ ′ phase of the alloy is precipitated and grown up during the heat treatment Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
process. The relationship between the size of the γ ′ phase and the mechanical properties were analyzed, and a new aging system for the U720LI heat treatment was determined to obtain the best mechanical properties [7,8]. Studies have shown that aging time significantly influences the microstructure stability and mechanical properties of GH4720Li alloy. During the long-term aging process at 720°C, the primary γ ′ phase almost did not change, the secondary γ ′ phase coarsens, and the hardness gradually decreased. During the long-term aging process at 800°C, the primary γ ' and the secondary γ ′ phases were obviously coarsened [9,10]. Other studies have shown that the cooling method of the alloy during solution treatment had a crucial influence on the microstructure and properties. It was determined through experiments on different cooling methods of the alloy during solution treatment that the microstructure and properties obtained by oil cooling are ideal [11,12]. In the above literature, although the composition, microstructure stability, growth mechanism, distribution of γ ′ phase, and its influence on mechanical properties of the alloy were studied from the aspects of solution treatment, aging time, and cooling method, there was a lack of research on the influence of temperature on the evolution of microstructure and mechanical properties at different heat treatment stages, and there was a lack of systematicness.
This study aims to study the effect of temperature on the microstructure evolution and mechanical properties of GH4720Li alloy at different heat treatment stages by designing a variety of solid solution and aging treatment processes and providing a theoretical reference for optimizing the heat treatment process of the alloy.

Experimental material and methods
The ingots were made by vacuum induction melting (VIM) and vacuum self-consumption remelting (VAR). The chemical composition of the alloy is shown in table 1. Through the gradient hot working process shown in figure 1. The billet was heated to 1160°C and then quickly transferred to a 1-ton free forging hammer for forging. The initial forging temperature of the billet was 1140°C, the final forging temperature was 1080°C, and the total deformation was 79%. To ensure the consistency of the original state of the organization, the use of an electric spark wire cutting machine cutting cake diameter 1/4 position 10 mm × 10 mm sample as the experimental object, the heat treatment process.
The heat treatment process is divided into solution treatment and aging treatment. The solution treatment process scheme is as shown in table 2: the samples of four combinations of gold were first subjected to solution treatment experiments at different temperatures of 1060°C ∼ 1120°C for 4 h, and then subjected to the same process at 650°C × 8h/AC + 760°C × 8h/AC two-stage aging treatment. The microstructure and mechanical properties of the alloy samples after heat treatment were analyzed, and the optimal solution treatment  temperature was obtained. The first aging treatment process is shown in table 3: Four groups of samples were selected. Firstly, the best solution temperature obtained in table 2 was treated, and then the first aging treatment at different temperatures was carried out at 650°C ∼ 770°C. Finally, the four groups of samples were subjected to secondary aging treatment at 760°C × 8 h/AC under the same process. The evolution of microstructure and mechanical properties of the alloy samples after heat treatment was analyzed, and the best first aging treatment temperature was obtained. The secondary aging treatment process scheme is shown in table 4: four samples were aged twice at 760°C ∼ 880°C using the optimum solution temperature and first aging treatment. The microstructure and mechanical properties of the alloy samples after heat treatment were analyzed to obtain the optimum secondary aging temperature. The electrolyte used for electrolytic polishing of the sample is 20 ml H 2 SO 4 + 80 ml CH 3 OH. The polishing process parameters are DC voltage 25V, current 1-1.5A, and time 5-12s. The mixed solution to etch the metallographic sample is 20 ml HCl + 30 ml C 2 H 5 OH + 3g CuCl 2 . The corrosion process of the metallographic sample was to use the absorbent cotton soaked in the etching solution to wipe and polish the smooth sample surface for about 1 min. The mixed solution was 170 ml H 3 PO 4 + 10 ml H 2 SO 4 + 15g CrO 3 . The DC voltage of electrolytic corrosion was 5v, the current was 1-2.5A, and the time was 5-12s. Preparation of TEM sheet samples was first mechanically polished to 50-60 μm and then ion-thinned. The test plane of the sample and the corresponding plane were ground to be parallel and smooth, and the mechanical properties were measured on an HR-150A Rockwell hardness tester after electrolytic polishing. Figure 2 shows the microstructure of the original forgings. As can be seen from the figure, the grain size of the forging is small and uniform, with an average grain size of about 4μm. There are a large number of primary γ ′ phase and secondary γ ′ phase in the forgings, with the primary γ ′ phase mainly distributed on the grain boundaries and the secondary γ ′ phase mainly distributed in the grain. A large amount of the primary γ ′ phase can effectively inhibit the bending of the original grain boundary and the grain boundary migration of the recrystallized grains, so the grain size is minimal [13,14]. The Rockwell hardness of the sample is 45HRC because of fine grain and precipitation strengthening. Since the secondary phase in the forging is not ultimately precipitated at this time, the effect of the secondary phase on the strength of the forgings is relatively small, so the forgings need to be further strengthened by heat treatment. Figure 3 shows the microstructure of samples after solid solution treatment at different temperatures. When the solid solution treatment temperature is less than 1080°C, the grain size of the sample is the finest and most Table 2. Solution temperature treatment process.

Scheme
Solution treatment One-time aging treatment Secondary age treatment Table 3. Aging treatment process.

Scheme Solution treatment
One-time aging treatment Secondary age treatment Optimum solution temperature The optimum primary aging temperature uniform with about 4 μm average grain size, and lots of primary γ ′ phase are precipitated around the grain boundary. With the solid solution treatment temperature rising, the grains grow slowly, and the primary γ ′ phase is dissolved gradually. When the solid solution temperature exceeds 1120°C, the grains grow fastly to about 8.3 μm, and a large amount of the primary γ ′ phase is dissolved. Due to the primary γ ′ phase dissolving  and disappearing with the solid solution treatment temperature increase, the 'pinning' effect of the primary γ' phase on grain boundaries is gradually reduced, which makes grain growth. Figure 4 shows samples' γ ′ phases morphology after solid solution treatment at different temperatures. The volume fraction of the total γ ′ phase is around 40%. With the solid solution treatment temperature rising from 1060°C to 1120°C, the volume fraction of the primary γ ′ phase reduces from 33.5% to 21.5%. The higher the solid solution treatment temperature is, the faster the γ ′ phases of the samples dissolve. The secondary γ ′ phase is dissolved first [15][16][17][18], and the primary γ ′ phase is dissolved later. When the solid solution temperature reaches 1120°C, only the primary γ ′ phase with a larger size remains in the samples, and the secondary γ ′ phase almost completely disappears. Figure 5 shows the relationship between the volume fraction of the primary and secondary γ′ phases and the solid solution temperature. With the increase of solution temperature, the volume fraction of the primary γ ′ phase gradually decreases, and that of the secondary γ ′ phase gradually increases. This result is also proved by other researchers [19][20][21]. Figure 6 shows the relationship between the Rockwell hardness of samples and solution temperatures. It can be seen from figure 6 that when the solution temperature increased from 1060°C to 1080°C, the Rockwell hardness of the sample also increased from 45.5 HRC to 46 HRC. Although the volume fraction of the primary γ ′ phase and secondary γ ′ phase is small when the sample is solution treated at 1080°C, the number of γ ′ phase is large, the precipitation strengthening is obvious, and the Rockwell hardness of the sample is higher. When the solution temperature increases to 1100°C, the grain length of the sample is greatly smaller, and the content of the primary γ ′ phase and secondary γ ′ phase is reasonable, so the Rockwell hardness of the sample increases to 47 HRC. This is because the primary γ ′ phase at this time is mainly distributed at the grain boundary, which hinders grains' growth and the grain boundary's strengthening effect is more obvious. When the solution temperature of the sample increases to 1120°C, the solution effect is more sufficient. At this time, the volume fraction and number of primary γ ′ phases are smaller than those of the sample with a solid solution temperature of 1100°C, and the growth of grains is large, so the Rockwell hardness of the sample will decrease. Figure 7 shows the microstructure of samples after primary aging treatment at different temperatures. When the primary aging treatment temperature is 650°C, the grain size of the samples is fine and uniform, the average grain size is about 4.5 μm, and the primary γ ′ phase with a larger size is precipitated around the grain boundary. With the increase of primary aging treatment temperature, the grain growth of the samples is slow, and the average grain size of the samples was 6.5 μm when the primary aging treatment temperature reached 770°C. Figures 8 and 9 show the morphology and volume fraction of γ ′ phases after primary aging treatment at different temperatures. As the primary aging temperature increases from 650°C to 770°C, the volume fraction of the primary γ ′ phase increases from 28.7% to 31.6%, and the volume fraction of the secondary γ ′ phase gradually decreases. When the primary aging temperature exceeds 730°C, the γ ′ phases of samples begin coarsening. The pinning effect of the primary γ ′ phase on grain boundary is reduced so grains grow up. At the same time, the small-sized γ ′ phase particles gradually dissolve, and the large-sized secondary γ ′ phase particles gradually grow, resulting in a larger spacing between the secondary γ ′ phase and a reduction in their number.  Studies have shown that the tertiary γ ′ phase will completely dissolve and disappear below 700°C. The higher the aging temperature, the shorter the time required for complete dissolution [22]. Figure 10 shows the relationship between the Rockwell hardness of samples and primary aging temperatures. The Rockwell hardness of the samples gradually decreases as the primary aging treatment temperature gradually increases. This is because the primary γ ′ phase of the samples grows gradually in the process of increasing the primary aging treatment temperature, and the pinning effect on the grain boundaries is weakened, resulting in the growth of grains, so the fine grain strengthening effect of GH4720Li high-temperature alloy is reduced; the uniform dense spherical secondary γ ′ phase of the samples coarsens in the aging treatment process, and the smaller γ ′ phases dissolve, and the number of the γ ′ phases that hinder the dislocation movement is reduced. As the size of the γ ′ phases increases, the distance between particles increases, and the ability to impede dislocation motion decreases, so the hardness of the samples decreases. Figures 11 and 12 show the samples' microstructure and γ ′ phases morphology after secondary aging treatment at different temperatures. The average grain size did not change significantly with the increase of secondary aging temperature from 760°C to 800°C. Studies have shown that the secondary γ ′ phase is more susceptible to Ostwald ripening at higher temperatures [23,24]. The experimental results show that the volume fraction of the primary γ ′ phase increases from 28.7% to 31.3%, while the secondary γ ′ phase is coarse. When the secondary aging treatment temperature increases to 880°C, the grains gradually grow, and the average grain size increases to 6.7μm. The volume fraction of the primary γ ′ phase also increases to 32.6%, and the secondary γ ′ phase coarsening is more obvious. This is consistent with the results obtained by Helm and Keefe et al in longterm aging at 760°C and 845°C, where the higher the temperature at which coarsening of the secondary γ ′ phase particles occurred, the shorter the time required for significant coarsening to occur.    figure 13, the volume fraction of the primary γ ′ phase gradually increases as the secondary aging temperature increases, while the volume fraction of the secondary γ ′    phase gradually decreases. Combined with figure 12, it can be obtained that as the secondary aging temperature increases, the smaller the size and lower the content of the secondary γ ′ phase, the higher the hardness of the secondary γ ′ phase when the content and size of the primary γ ′ phase are close. When the secondary aging treatment temperature of the samples exceeds 840°C, the secondary γ ′ phase grows to a certain extent, the solute elements around its particles become scarce, the diffusion of solute changes from short-range diffusion to longrange diffusion, and the coarsening rate of the γ ′ phases slow down, resulting in the overall hardness change of the samples tends to be flat. Studies have shown that with the increase of aging temperature, the larger solute diffusion coefficient promotes the Ostwald ripening mechanism controlled by the diffusion interface so that the larger size of the γ ′ phase in the sample grows faster and the smaller size of the γ ′ phase dissolves faster [25,26]. The experimental results show that with the increase of the second aging treatment temperature, the average size and spacing of the γ ′ phase in the sample increase, and the number decreases gradually, which weakens the ability of the phase to hinder dislocations and reduces the hardness of the sample. Figure 15 shows the TEM images of the forgings, where there are large-sized primary γ ′ phase and smallsized secondary γ ′ phase in the grain inner. Figure 16 is a TEM photograph of the alloy after heat treatment, in which the size of the primary γ ′ phase is reduced, the size of the secondary γ ′ phase is also reduced, and is  uniformly distributed in the matrix. After heat treatment, the volume of large size γ ′ phases in the grain inner has been reduced, and more spherical small size γ ′ phases precipitated in the grain inner, thus improving the material's mechanical properties after heat treatment.

Conclusions
(1)The hardness of the alloy sample increases first and then decreases at the solution treatment temperature of 1060°C ∼ 1120°C. When the solution treatment was carried out at 1100°C, the grain length of the sample was small, the content of the primary γ ′ phase and the secondary γ ′ phase was more reasonable, and the hardness reached the peak. When the solution treatment temperature exceeds 1100°C, the primary γ ′ phase of the sample dissolves in large quantities, the grains grow rapidly, and the hardness decreases rapidly.
(2)The microstructure and mechanical properties of the alloy after the two-stage aging treatment show a similar rule with the change in aging temperature. That is, with the increase of aging temperature, the primary γ ′ phase of the sample gradually grows, the ability to pin the grain boundary is weakened, and the volume fraction of the secondary γ ′ phase of the sample gradually decreases and coarsens, resulting in grain growth and a gradual decrease in the hardness of the sample. When the sample was subjected to primary aging treatment, the higher the content of the secondary γ ′ phase, the better the strengthening effect. When the secondary aging temperature of the sample exceeds 840°C, the coarsening rate of the γ ′ phase slows down, resulting in a flat hardness of the sample.
(3)Based on the final microstructure and mechanical properties, the optimum process parameters were determined to be 1100°C/OC × 4h + 650°C/AC × 8h + 760°C/AC × 8h. At this time, the grain size of the sample was the smallest, the average grain size reached 4.5μm, the γ ′ phase distribution was the most uniform, and the hardness was the highest, reaching 47 HRC.