Effect of annealing temperature on microstructure and properties of the microtubes for GH4169 alloy

The effects of annealing temperature on the recrystallization and grain boundary distribution characteristic for the drawn microtube of GH4169 alloy were studied by scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) analysis system and transmission electron microscopy (TEM). The results demonstrated that at an annealing temperature of 850 °C, only a small proportion of recrystallization for the microtube underwent. As the annealing temperature increased, the proportion of recrystallization also increased. Specifically, at 900 °C, the proportion reached 95.8%, indicating complete recrystallization. Fine recrystallization grains were obtained with an average grain size of 4.73 μm. As the annealing temperature is above 900 °C, the microtube also undergo complete recrystallization, and the grains grow up gradually. At 1000 °C, the average grain size grows to 7.42 μm. The results also revealed that with the annealing temperature increasing, the proportion of Σ 3 grain boundary of the microtube was rising. The proportion of Σ 3 grain boundary increased from 30.1% to 34.2% with the annealing temperature increased from 900 °C to 1000 °C. Moreover, as the annealing temperature increased from 850 °C to 1000 °C, the room temperature tensile strength of the tube decreased from 1323 MPa to 965 MPa, the yield strength decreased from 1054 MPa to 523 MPa, and the fracture elongation increased from 10.2% to 31.2%.


Introduction
Nickel-based superalloys exhibit remarkable high strength, strong oxidation resistance and good corrosion ability within the temperature range of 650-1000 °C [1].GH4169 (Incone1718) alloy is precipitation hardened Ni-base superalloy with γ' phase of body-centered cubic structure as the main strengthening phase and γ' phase of face-centered cubic structure as auxiliary strengthening [2].Due to its high yield strength, superior plasticity and large coherent distortion between precipitates and the matrix at 650 °C, GH4169 alloy has become one of the most widely used superalloys.The microtube of GH4169 alloy is a key component of the new generation hypersonic aero-engine heat transfer system.Owing to its special service conditions, including high temperature gradient, high pressure and so on, high dimensional accuracy, microstructures and properties of the microtube are required [3].
The microtubes of GH4169 alloy are usually prepared by multi-pass cold-drawn deformation combined with intermediate annealing.They have high strength and poor plasticity.It must be subjected to annealing to obtain the requirements properties.It is widely known that annealing has great influence on mechanical properties and microstructure of metal materials.The annealing heat treatment significantly affects in the recrystallization and grain size of the alloy.The refined grain plays an important role in improving strength and plasticity.In addition, proper annealing heat treatment increases the increasing of the proportion of low Σ coincidence site lattice (CSL) grain boundaries are help to improve the properties of poly crystalline materials [4].Yang Xu [5] studied the fine grain process and superplastic deformation behavior of GH4169 alloy sheet during cold rolling and heat treatment.The optimal rolling and heat treatment process parameters were determined and the process was simplified.Xue Hao [6] used cold rolling and heat treatment processes to refine the grains of GH4169 alloy plate, studying the effects of cold rolling reduction rate and heat treatment time on the precipitation of the δ phase.Chen Ming-Song [7] adopted a new two-stage heat treatment method, including aging treatment and cooling recrystallization heat treatment.The proposed two-stage annealing heat treatment could effectively improve the uniformity and refinement of coarse grains in deformed GH4169 alloy, and enhance the hardness and strength of the alloy.Yang Xu [8] designed an 84% reduction rate through cold rolling, followed by a two-step annealing process to refine and homogenize the grains.Compared with cold rolling and single-stage annealing, superplasticity with a finer grain size of 1.21 microns (elongation of up to 657%) was obtained at 950 °C under strain rates of 6×10 -4 to 1 × 10 -2 s −1 .Liu Feng [9] studied the effects of thermomechanical treatment on the microstructure and grain boundary characteristic distribution of the GH4169 alloy bar.It is found that the small deformation has the most obvious promoting effect on the formation of special grain boundaries, and the proportion of low Σ CSL grain boundaries is the highest when the deformation is 9.8%.
The above studies find that heat treatment can significantly change the microstructure and properties of GH4169 alloy.Annealing affects the degree of grain homogenization and ultimately the alloy properties.However, for GH4169 alloy, most studies of heat treatment are focused on sheet or foil, while research on the heat treatment process of microtube is still a gap.Similarly, annealing impacts the recrystallization, grain boundary character distribution, and mechanical properties of the microtubes produced.Thus, it is crucial to study the effect of annealing temperature on the microtube evolution and mechanical properties of the microtube.

Preparation of the microtube
The GH4169 alloy tubes with an outer diameter of 14 mm and a wall thickness of 1.5 mm were received firstly.The chemical composition of GH4169 alloy is presented in table 1.And then those as-received tubes were subjected to multi-pass cold-drawn deformation combined with intermediate annealing until the microtubes with an outer diameter of 2 mm and a wall thickness of 0.08 mm were obtained.The deformation of each cold drawing process is about 10%.Once the cumulative cold-drawn deformation reached approximately 55%, an intermediate annealing treatment was carried out at a temperature of 1050 °C.
Figure 1 shows the inverse pole figure (IPF) of the as-received original tubes and the final cold-drawing microtubes.Equiaxed grains are presented in the original tubes with an average grain size of 32.55 μm, as shown in figure 1(a) Some annealing twins also exist in the original tubes.Typical deformation microstructures are  presented in the cold-drawn microtubes, including deformation bands, subgrains and dislocation structures, and the original grains were flattened and elongated.

Heat treatment process
After the final cold-drawing process, the microtubes were subsequently annealed at temperature of 850 °C, 900 °C, 950 °C and 1000 °C for 60 min each in an argon atmosphere.This was followed by air cooling to room temperature.The objective of this study was to investigate the impact of various annealing temperatures on the microstructure and mechanical properties of the microtubes.

Microstructure characterization and mechanical properties test
The microstructure of microtubes after annealing at various temperatures was characterized using a scanning electron microscope (SEM) equipped with an electron backscatter diffraction (EBSD) analysis system and transmission electron microscopy (TEM).The grain refinement and special grain boundary changes of the microtubes were analyzed by EBSD after annealing, with an index rate exceeding 95% for each scan.The qualitative assessment of the deformation structure, substructure and recrystallization structure of the different state microtubes are determined using the DefRex diagram.Specifically, grains with an average mismatch angle less than 1.5°were considered recrystallized grain, those with angles between 1.5°and 5°were deemed substructures, and those exceeding 5°were identified as deformed structures [10][11][12].The recrystallized grains of the annealed microtubes were identified using grain orientation spread (GOS) [13].TEM analysis was performed using a JEM2010 microscope at an accelerated voltage of 200 kV TEM samples were ground to 30 μm and twin-jet electropolishing at −21 °C and 25 V.The tensile specimens for the microtubes, measuring 220 mm in length, 0.08 mm in wall thickness, and 2 mm in outer diameter, were prepared.The tensile tests were carried out at a strain rate of 1.5 × 10 -3 s −1 on a CMT5205 electronic universal testing machine.Additionally, the fracture morphology of the microtubes was observed using SEM.

Effect of annealing temperature on grain size of microtubes
The annealing heat treatment process can effectively control the grain size [14][15][16].Figure 2 shows the IPF of the microtubes after annealing at the different temperatures.Microstructural observations reveal a noteworthy impact of annealing temperatures on microstructural characterization.With increasing the annealing temperature, the fraction of recrystallized grains increases.As can be seen from the figure 2(a), at an annealing temperature of 850 °C, fine recrystallized grains are observed along the grain boundaries of deformed grains.
The results show that static recovery occurs mainly and a little recrystallization occurs in the microtube.With the annealing temperature increases to 900 °C, the deformed grains are primarily replaced by fine recrystallized grains.With further increase of the annealing temperature, the size of recrystallized grains exhibits a slight increase, albeit not significantly.Compared with the microstructure of original tube (figure 1(a)), after complete recrystallization annealing, the microstructure of the microtube underwent significant refinement.
The grain size distribution corresponding to various annealing temperatures was depicted in figure 3.At an annealing temperature of 850 °C, the grain size distribution is relatively divergent, with a significant difference between the coarsest and smallest grains of approximately 43.60 μm.The average grain size is about 21.54 μm.Notably, at an annealing temperature of 900 °C, the grain size distribution narrows significantly, with the majority of grains being fine and equiaxed.The fraction of fine grains ranging from 0.5-9 μm reaches 91.02%.The average grain size is about 4.73 μm, and the difference of the size between the individual relatively large grain and individual small grain is about 14.61 μm.As the annealing temperature increases further, the grain size distribution tends to be more dispersed.At annealing temperature of 950 °C and 1000 °C, the differences in grain size between the largest grains and smallest grains are around 18.04 μm and 24.52 μm, and the average grain size is about 6.23 μm and 7.42 μm, respectively.When the annealing temperature is between 900 °C and 1000 °C, there is minimal change in grain size, indicating that grain refinement and uniform distribution are archived.
The influence of annealing temperature on the average grain size of the microtubes was graphically represented in figure 4.Among them, the smallest average grain size was observed at 900 °C.After 900 °C, the grains grew gradually.Typically, the continuous migration of grain boundaries towards the curvature center leads to the disappearance of smaller grains, resulting in the amalgamation of larger grains.This migration process results in the flattening of grain boundaries, causing the concave surfaces to flatten out an increase in size.The grain boundary migration necessitates certain energy conditions, primarily driven by the reduction of total interfacial energy.Once these energy conditions are met, the migration process can commence, regarded as a thermally activated mechanism.Generally, during the annealing process, the migration of grain boundaries is primarily influenced by the annealing temperature and time.During the growth process, the average migration rate of grain boundaries exhibits a positive correlation with temperature [17].The increase in annealing temperature accelerates the rate of grain boundary migration.In the present study, the annealing time was maintained at a content duration of 60 min.Given the prolonged annealing duration for the microtubes, the grain boundaries had sufficient time to migrate.Consequently, grain growth within the microtubes occurred with the elevation of annealing temperature above 900 °C.

Effect of annealing temperature on recrystallization of the microtubes
The distribution of recrystallized regions is shown in figure 5, with the blue region representing the fully recrystallized region, the yellow region representing the substructure region, the red region representing the deformed structure region.The precise percentages of completely recrystallized, substructures, and deformed structures regions are outlined in table 2. It can be seen from figure 5(a) that the microtubes annealed at 850 °C comprised a mixture of recrystallized, substructured, elongated deformed grains.Notably, the recrystallization fraction was limited to just 17%, as shown in table 2. The recrystallized grains are mainly distributed in the deformed grain boundaries (figure 5(a)).Interestingly, microtubes annealed at or above 900 °C exhibit a distinct microstructure, consisting primarily of recrystallized and substructured grains.Almost no deformed grains were observed, as shown in figures 5(b)-(d).The recrystallization fractions exceeded 95% (table 2), indicating the occurrence of near-complete recrystallization.The misorientation angle distributions of the annealed microtubes are shown in figure 6. Figure 6(a) reveals a significant proportion of low-angle boundaries and a limited number of high-angle boundaries within the microtubes.This observation suggests limited recrystallization at an annealing temperature of 850 °C [18].Consequently, the grains primarily maintain their cold-drawn deformation microstructures.At the annealing temperature of 900 °C or higher, a small number of low-angle boundaries and a large number of the high-angle grain boundaries, as shown in figures 6(b)-(d).This shift indicates complete recrystallization of the microtubes during the annealing process.
Recrystallization leads to the formation of new distortion-free grains in the previously deformed structure.It involves nucleation and growth, and there are two common ways for nucleation: grain boundary bulging and sub crystalline nucleation [19,20].Figure 7 presents the TEM morphology of both cold-drawn and annealed the microtubes.In the cold-drawn microtubes, a considerable amount of dislocation networks, dislocation cells and deformation bands are readily observable as shown figure 7(a).Additionally, figures 6(b)-(d) reveals the grain boundary bulging, the emergence of recrystallization grains, and the formation of subgrains are also observed.It is clear from figures 7(b)-(d) that, with the increase of annealing temperature, dislocation density decreases, the recrystallization is more developed, the grains grow up, and the grain boundaries become straight.The acceleration of recrystallization is related to the increase of annealing temperature, which leads to a reduced dislocation density.Disparities in dislocation density are observed across high-angle grain boundaries.Additionally, microstructural observations revealed that the primary nucleation mechanism of static recrystallization during annealing process for the microtubes is the discontinuous recrystallization, which is characterized by the bulging of the grain boundaries [21].

Effect of annealing temperature on special grain boundaries of the microtubes
Figure 8 illustrates the distribution of grain boundaries in the microtubes following annealing within the temperature range of 850 °C to 1000 °C.Table 3 presents the grain boundary character distributions of the microtubes at different annealing temperatures.At an annealing temperature of 850 °C, a trace amount of Σ 3 grain boundaries is observed, accounting for a relatively low percentage.As the annealing temperature reaches 900 °C, a large number of flat Σ 3 grain boundaries appear.Additionally, a small number of Σ 9 grain boundaries are generated at the meeting of Σ 3 grain boundaries, while the ratio of other low Σ CSL grain boundaries is low.Upon further increasing the temperature to 950 °C and 1000 °C, a slight increase in the Σ 3 grain boundaries is noted compared to that at 900 °C.It is generally believed that the ratio of Σ 9 + Σ 27 grain boundaries can be used to measure the degree of grain boundary structure optimization.Therefore, changes in the ratio of Σ 9 + Σ 27 grain boundaries reflect alterations in the optimization level of the grain boundary structure.A higher ratio of Σ 9 + Σ 27 grain boundaries is observed at 900 °C, indicating a corresponding increase in the degree of grain boundary structure optimization.This optimization closely correlates with the proportion of complete recrystallization achieved.
For metals with a face-centered cubic crystal structure, during the grain growth process, under the condition of satisfying the energy potential barrier, atomic layer stacking faults tend to occur on the (111) faces within the crystal structure [13].These faults generate twinned grain boundaries, resulting in the formation of annealing twins at the grain boundary corners.Metals with low stacking fault energy are more prone to the formation of these annealing twins [22].The bent Σ 3 grain boundaries can be either non-congruent grain boundaries or other asymmetric Σ 3 grain boundaries tilted side grain boundaries.These bent grain boundaries possess significantly higher energy than congruent twinned [23], thus facilitating their migration.When continuously generated twinned crystals encounter each other, the twinned crystals located in the different {111} surfaces have a 60°orientation difference, leading to the formation of Σ 3 n grain boundaries [24], specifically Σ 9 grain  boundaries.These derived Σ 9 grain boundaries can further evolve from Σ 3 grain boundaries to form Σ 27 grain boundaries.Improving the proportion of low Σ grain boundaries can effectively improve the intergranular corrosion resistance of the alloy [25].Therefore, the integrating recrystallization region and the proportion of low Σ grain boundaries, the annealing temperature is controlled at about 900 °C, which is conducive to improving the corrosion resistance of the alloy and optimizing the crystal structure.

Effect of annealing temperature on mechanical properties of the microtubes
The microtube's tensile test results across various annealing temperatures, ranging from 850 °C to 1000 °C, are presented in figure 9.The tensile properties of the microtubes were investigated over a range of annealing temperatures.It can be seen from figure 9. that the annealing temperature has a notable impact on the mechanical properties of the microtubes.As shown in figure 9(a), the microtubes first undergoes elastic deformation and then enters yield plastic deformation, followed by a longer period of plastic deformation.Last, the fracture occurs and the tensile stress reaches its maximum value.It is clear from figure 9(b) that the tensile strength showed a gradual decrease from 1323 MPa at 850 °C to 965 MPa at 1000 °C.Similarly, the yield strength exhibited a decrease from 1054 MPa at 850 °C to 523 MPa at 1000 °C.Interestingly, the fracture elongation demonstrated an opposite trend, increasing steadily with the annealing temperature.Specifically, the fracture elongation rose from 10.2% at 850 °C to 31.2% at 1000 °C.The most significant increase occurred between 850 °C and 900 °C.
The fracture morphology of each specimen after pulling off is shown in figure 10.The fracture characteristics exhibit a transition from brittle river-like patterns to ductile fossa patterns, indicating an increasing toughness [26][27][28][29].This transformation is closely associated with grain size, which varies with temperature and refinement.Additionally, an increase in air holes count affects the ductility of the material, although above 900 °C, the elongation changes minimal.The results of the tensile test align with those of the GH4169 microtube, indicating that room temperature drawing induces a localized loading mode.This localized loading results in plastic strain within the material, causing dislocation movement, which further triggers additional dislocations.Drawing, as a local loading mode of plastic strain, prompts dislocation movement within the material.This movement generates new dislocations, and as deformation increases, grains gradually elongate, break, and rearrange in a fibrous manner.Upon completion of recrystallization, the alloy undergoes re-nucleation and the growth of new grains without distortion.The phenomenon of work hardening is eliminated, and the primary factor influencing the mechanical properties shifts from the original work hardening to the organization of the alloy itself.Notably, the fracture characteristics lack obvious second-phase particles and inclusions.The size and uniformity of grains significantly influence the mechanical properties of the alloy.The relationship between grain size and alloy strength can be effectively described using the Hall-Petch [30] relationship equation: where σ s is the yield strength, σ 0 is the friction force that prevents dislocation slip, k is the coefficient of influence of neighboring grains' orientation difference on dislocation motion, which can also be called grain boundary resistance, and d is the grain size.In crystal structures, grains exert mutual constraints on each other.As grains refine during deformation, they generate more grain boundaries and impede each other's motion, necessitating a higher external force to induce dislocation plugging.Consequently, the room temperature strength of the alloy increases with grain refinement or an increase in the total area of grain boundaries.Conversely, as grains coarsen, the room temperature strength of the alloy decreases.This trend is consistent with the results obtained from tensile testing, where the room temperature strength of the alloy gradually diminishes as the grain size increases at annealing temperatures exceeding 900 °C.

Conclusions
(1) The annealing treatment of the microtubes for GH4169 alloy was conducted in this work, and the microstructure evolution and mechanical behaviors were symmetrically analyzed.When the annealing temperature increases from 850 °C to 1000 °C, the average grain size of the alloy decreases at first and then increases slightly, reaching the minimum value of 4.73 μm at 900 °C.Additionally, the grains are more uniform at 900 °C.
(2) The proportion of recrystallization increases during the annealing treatment.the recrystallization ratio reaches 95.8% and it has been fully recrystallized when the annealing temperature is 900 °C.The ratio of Σ 3 grain boundary increases continuously with the increase of annealing temperature, up to 34.6% at 1000 °C.
The ratio of Σ 9 + Σ 27 grain boundary reaches the highest at 900 °C, and then decreases slightly with the increase of annealing temperature.
(3) As the annealing temperature increased from 850 °C to 1000 °C, the tensile strength of the microtubes at room temperature decreased from 1323 MPa to 965 MPa, while the yield strength decreased from 1054 MPa to 523 MPa.Concurrently, the elongation increased from 10.2% to 31.2%.The fracture morphology underwent a gradual transformation, evolving from a distinct fluvial pattern to a dimple morphology.Besides, the grains within the fracture were refined with the increase of annealing temperature, and the stomata increased at the same time.

Figure 4 .
Figure 4. Variation of the average grain size of the microtubes as a function of the annealing temperature.

Figure 9 .
Figure 9. Mechanical properties of microtubes in different states: (a) engineering stress-strain curves: (b) the value of engineering stress and elongation.

Table 2 .
Effect of annealing temperature on recrystallization volume fraction.

Table 3 .
Effect of annealing temperature on the percentage of special grain boundaries of microtubes (Length fraction, %).