Study on superplastic behaviour of cold rolled Inconel 718 alloy with δ phase dispersion distribution

ABSTRACT Cold-rolled Inconel 718 with δ phase dispersion distribution was prepared and exhibited excellent superplasticity at 950°C with a strain rate of 6 × 10−4 ~4 × 10−3 s−1. The elongation to failure of 561% was obtained at a strain rate of 1 × 10−3 s−1. Superplastic deformation accelerates the recrystallization of cold-rolled deformed microstructure, thus improving grain uniformity. The dispersed δ phase promotes the nucleation of recrystallized grains and pin grain boundaries to limit grain size. Diffusion-coordinated grain boundary slip is the main deformation mechanism of the alloy sheet during superplastic stretching. The dense dislocations in the deformed grains act as the channel of atomic diffusion in the early deformation stage. Subsequently, the material conversion between grain boundaries promotes atomic diffusion. The present results have certain reference significance for preparing and forming superplastic Inconel 718 alloy.


Introduction
Superplastic forming technology has the advantages of large deformation, low forming pressure, and precise dimensions.The superplasticity of alloys at high temperatures makes it possible to manufacture complex parts by gas blowing.Inconel 718 is a nickel-based superalloy with high-temperature strength, excellent fatigue resistance, and good corrosion resistance.On the other hand, due to its high deformation resistance and severe work-hardening, the traditional forming process of the alloy has high requirements for moulds and equipment.Therefore, the research on the superplasticity of the alloy is beneficial to reduce production costs and expand its application range.The combination of deformation and heat treatment to improve the microstructure is the main method to enhance the superplasticity of the alloy.
Jafarian et al. [1] found that superplasticity occurs in metals with a grain size of less than 10 µm and good structural stability.Conditions for realizing superplasticity include high deformation temperature (T ≥0.5 T m ) and low strain rate (10 −4 ~10 −1 s −1 ).Moreover, Sato and Kuribayashi [2] pointed out that since dynamic recrystallization (DRX) occurs within the temperature range of superplastic deformation, nonannealed alloys can exhibit superplasticity by refining grains through DRX at the initial stage of superplastic deformation.Therefore, it is feasible to achieve superplasticity in some deformed alloys.Yang et al. [3], through analysing the microscopic superplastic deformation mechanism of cold-rolled Inconel 718, proved that atomic diffusion, grain rotation, dislocation creep, and continuous DRX occur during superplastic deformation.Lv et al. [4] carried out a superplastic tensile test on a hot extruded nickel-based superalloy.It was pointed out that the extruded alloy has a highdensity dislocation, γ' phase plays the role of pinning grain boundaries, and grain boundary slip (GBS) controlled by grain boundary diffusion and lattice diffusion is the superplastic deformation mechanism.Yakovtseva et al. [5] reported that DRX is beneficial for grain refinement through PSN and Zener pinning mechanisms, providing a large m value and sufficient GBS for the high strain rate superplastic deformation of aluminium-based alloy.Hidalgo-Manrique [6] obtained equiaxed grains with an average size of less than 500 nm through the multi-pass rolling process and proved that the stored energy and the pinning effect brought by rolling are beneficial to promote recrystallization and refine grains in the preheating stage of superplastic deformation.
Many studies have shown that the δ phase in Inconel 718 alloy achieves grain refinement by both pinning grain boundaries and promoting nucleation.For example, Páramo-Kañetas et al. [7] studied that the δ phase inhibits grain growth and promotes nucleation of recrystallized grains during thermal deformation.Xu et al. [8]  increased the nucleation of δ phase and decreased the critical nucleation work, thus promoting the precipitation of δ phase.
In this paper, a rolled Inconel 718 alloy with a dispersed distribution of δ was prepared by twostage rolling and intermediate annealing.The superplasticity of the alloy prepared by the above process was characterized.The microstructure evolution and related mechanism during deformation were studied in detail.In addition, we focus on the role of the δ phase in the superplastic deformation.

Preparation process
The initial microstructure with an average grain size of 5.1 μm is indicated in Figure 1a.The alloy was solutiontreated at 1050°C for 1 h and cut into 5 mm thickness plates.Subsequently, the samples were cold-rolled (CR) to a thickness of 1.6 mm with a reduction of 68%.The samples were heated in a tube furnace for 5 h at 890°C to precipitate δ phase.The microstructure of plates after δ phase precipitation treatment (CD plates) is shown in Figure 1b.Then, the CD plates were CR with a reduction of 16% to obtain δ phase dispersed coldrolled sheets (CDC sheets, Figure 1c).The volume fraction of the δ phase was calculated as 10.72%.The detailed preparation method and subsequent superplastic tensile test are indicated in Figure 2.

Superplastic tensile test
The CDC sheets were heated to 950°C and maintained for 5 min.Then, the alloy was stretched at a rate of 6 × 10 −4 to 4 × 10 −3 s −1 using an AG-X plus 100 N. To understand the microstructural evolution, a few samples were stretched to an elongation of 50%, 150%, and 350% at a strain rate of 1 × 10 −3 s −1 and 950°C.Fractured and interrupted specimens from the above stretching process were cooled to room temperature by water quenching.

Microstructure detection
The specimens for OM and SEM observation were ground and polished, followed by chemical etching using a solution of 20 mL HCL +20 mL C 2 H 5 OH +5 g CuSO 4 .The content of the δ phase in 4000 magnification images was counted by Image-Pro Plus.Electron backscatter diffraction (EBSD) measurements were executed using a NordlysMax3 equipped with HKL Channel 5 software.The samples for EBSD investigation were electrolytically polished with a solution of 90% CH 3 CH 2 OH +10% HClO 4 at 30 V for 15 s.Transmission electron microscope (TEM) was employed to examine the morphology of the δ phase and microstructure on a JEOL JEM-F200 at 200 kV.

Characterization of superplastic mechanical properties
It is shown in Figure 3a that the stress increases at first and then decreases with the increase of strain.The maximum flow stress is below 140 MPa, which proves low flow stress in superplastic forming.Figure 3b shows that the elongation exceeds 430% at all strain rates.It proves that the rolled sheets with a dispersed distribution of the δ phase exhibit excellent superplasticity.At a strain of 0.3, the strain-rate sensitivity index (m value) is 0.368, calculated from the true stress and true strain, as shown in Figure 3c.This indicates GBS is the main deformation mechanism.As depicted in Figure 3d, the true stress-strain curves with strains of 0.41, 0.92, and 1.88 coincide, which proves the reliability of the intermediate data.

Evolution of microstructure
Figure 4 shows the evolution of the δ phase under different conditions.Figure 4a shows that δ phase content of the tensile specimen at strain e = 0.41 shows a marked drop in comparison with initial CDC plates (Figure 1c).It illustrates that during superplastic tensile, the δ phase is significantly dissolved at a higher temperature.With the increase of strain, as shown in Figure 4a-c, the coarsening of the δ phase indicates that the deformation promotes the accumulation of the fine δ phase.In Figure 4d, during the static annealing, the δ phase is dissolved without obvious growth.
Figure 5 shows the volume fraction and average size of the δ phase for different treatment conditions.It can be quantified from Figure 5a that comparing the CDC sheets, the volume fraction of the δ phase within the tensile specimens all decreases significantly due to the high temperature.In the deformation area, the content of the δ phase grew slightly with the increase of deformation, while δ phase content for the static annealing decreases more apparently.Figure 5b shows that the size of the δ phase gradually increases with the increase of deformation, which maximumly reached 0.54 μm at a true strain of 1.88.However, under the static annealing condition, the size of the δ phase is still 0.28 μm, which means that deformation promotes the aggregation of the δ phase while also inhibiting its dissolution.Figure 6a-d shows the microstructure of the CRC sheets under different deformation conditions.Figure 6a shows that most of the grains in the CRC sheet have recrystallized after 5 min of preheating.High-density dislocations generated during the two stages of cold rolling provide a large amount of stored energy, which incr6a, bs the number and rate of recrystallization nucleation.The red box shows that a few coarse deformed grains are still unannealed.
Comparing Figure 6a,b, it is found that the deformed grain undergoes DRX during superplastic deformation and transforms into an annealed structure without lattice distortion.Figure 6c-d shows the gradual growth of grains during the superplastic stretching.For the specimen with a strain of e = 1.88, the size of the grain in the deformation zone is larger than that in the static annealing zone.This suggests that deformation promotes grain growth.
Figure 7 shows the distribution of grain boundaries and the average grain size under different deformation conditions.Figure 7a shows that the average misorientation angle increases and then tends to be stable with an increased strain rate.DRX occurs at the early stage of deformation, resulting in the transformation of low-angle grain boundaries (LAGBs) to high-angle grain boundaries (HAGBs).Figure 7b shows that the grain size increases gradually with  increasing deformation and reaches a maximum of 2.56 μm at strain e = 1.88.However, in the region of static annealing, the grains grew slightly.The results suggest that the deformation promotes grain enlargement.
Figure 8a shows that after 5 min of preheating, a small amount of deformed structure and the granular δ phase precipitated during the CDC process are present in the alloy and that some of the grains have undergone DRX.For the specimen with a strain of 0.41, it is observed in Figure 8b,c that the dense δ phase particles are distributed around the grain boundaries of the recrystallized grains, thus acting as pinning of the grain boundaries and limiting grain growth.Moreover, the matrix has been completely transformed into a recrystallized structure without lattice distortion.

The effect of the δ phase
The shear bands and dislocations formed in the alloy during the first cold rolling process provide a position for   the nucleation of the particles, thus promoting δ phase precipitation [9].The acicular δ phase in the CD plate (Figure 1b) is transformed from the pre-precipitated γ″ phase in the matrix at 890°C.In the second cold rolling process with a reduced rate of 16%, the fully precipitated δ phase particles are crushed and uniformly distributed in the matrix [10] (Figure 1c).
In the early stage of stretching, most of the alloy structure is transformed into recrystallized grains (Figure 6b).Compared to the preheat structure of Inconel 718 sheets obtained by direct large deformation cold rolling [3], the preheat structure obtained in the process designed in this paper has a significantly higher proportion of recrystallized grains.It is suggested that the homogeneous and fine δ phase which precipitated in the previous process promotes recrystallization nucleation during the preheating [11].During a brief period of warming and preheating, most of the deformed structures are transformed into equiaxed grains (Figure 6a).This state of structure facilitates the GBS during subsequent deformation [12].
Grain growth during superplastic deformation includes both static and dynamic growth.As a result of the homogeneous and fine δ phase pins the grain boundaries and limits the largescale migration of the grain boundaries (Figures 4a and 8a), the grain size does not change significantly at the initial stage of deformation (Figure 7b).In the later stages of superplastic deformation, a significant increase in grain size occurs (Figure 7b), because of the decrease in the pinning effect due to the increase in size and decrease in the amount of the δ phase (Figures 5b  and 4a-d) [13].During superplastic tensile processes, the δ phase coarsens as a result of interfacial plasticity-induced processes, which rapidly reduces the cohesiveness of the second phase and the substrate [14].

Mechanism of superplastic deformation
In the early stage of deformation, most of the grains have completed DRX, and relatively small equiaxed crystals are formed (Figure 7b).The redistribution of the δ phase reflects grain rotation and GBS.Kim [15] pointed out that when the grain size is small, the main mechanism of superplastic deformation shifts to a GBS mechanism regulated by diffusion.The m value greater than 0.3 obtained in this study also reflects the occurrence of GBS.It is difficult for GBS to carry out all deformation alone, and a coordination mechanism is needed [12].It is observed that there are still a few deformed grains that have not been recrystallized (Figure 6a).A large number of dislocations exist in the deformed grains (Figure 8a), which provides a channel for atomic diffusion [16].The diffusion of atoms in grains serves as a way of superplastic deformation.In addition, LAGBs are consumed by recrystallized grains as deformed substructures (Figure 6a,b), DRX occurs in the alloy [17], while the nature of DRX is a thermal diffusion process [11].Li et al. [18] pointed out that relatively large size grains have reduced grain boundary density and the contribution of GBS to the deformation mechanism is not significant.As a result, the coarse deformed structure with irregular grain boundary shape achieves superplasticity of the alloy by atomic diffusion.
In the late stage of superplastic stretching, all the grains have completed recrystallization during the deformation process, which provides convenient conditions for GBS.Moreover, the obvious grain growth phenomenon (Figure 7b) proves that the exchange of substances between grain boundaries is more frequent.The additional channel of grain boundary diffusion accelerates the transport of material between two adjacent grains, thus promoting grain growth [19].The average grain size in the deformation zone is significantly larger than that in the static annealing zone (Figure 7b).This indicates that the superplastic deformation accelerates the grain boundary migration [20] and the material exchange between grain boundaries [21].Therefore, the late deformation mechanism of superplastic deformation is the GBS regulated by grain boundary diffusion.

Conclusion
In this paper, a rolled Inconel 718 alloy plate with a dispersed distribution of δ was prepared and the superplasticity of the plate is characterized.Several important conclusions are as follows.
(1) The superplastic elongation of the CDC plate exceeds 400% as strain rates of 6 × 10 −4 to 4 × 10 −3 s −1 at 950°C.The maximum elongation is 561% at a strain rate of 1 × 10 −3 s −1 and the m value is about 0.367.(2) At the early stage of deformation, the alloy grains do not grow significantly due to the pinning effect of the δ phase.At the later stage, the coarsening of the δ phase is caused by grain rotation during deformation induction and leads to grain growth, which was the main reason for the fracture of the plate.(3) The deformed grains at the early stage realize the superplasticity through atomic diffusion, and the fine equiaxed grains realize the superplasticity through GBS.During the superplastic tensile process, DRX occurs in the deformed grains, and the deformation mechanism at a later stage transforms into GBS with a grain boundary diffusion coordination.

Figure 2 .
Figure 2. Flow chart of the preparation process and superplastic tensile test.

Figure 3 .
Figure 3. (a) True stress-strain curves at 950°C and different strain rates.(b) Relationship between the elongation and strain rate (c) Plot of logarithmic stress-logarithmic strain rate.(d) True stress-strain curves at different strains at a strain rate of 10 −3 s −1 .

Figure 4 .
Figure 4. SEM image of the deformation zone of the alloy sheet with a strain of (a) 0.41, (b) 0.92, and (c)1.88 at 950°C and a strain rate of 1 × 10 −3 s −1 .(d) SEM image of the gripping zone.

Figure 5 .
Figure 5. (a) Volume fraction and (b) average size of the δ phase at different deformation conditions.

Figure 7 .
Figure 7. (a) the average misorientation angle and (b) the average size of grain in different conditions.

Figure 8 .
Figure 8. TEM bright-field images with strains of (a) 0, and (b) (c) 0.41 of the deformation zone.