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Dynamic Recrystallization Mechanism of Inconel 690Superalloy During Hot deformation at High Strain Rate

Bin Wang, Shi-Hong Zhang, Ming Cheng, and Hong-Wu Song

(Submitted December 10, 2012; in revised form February 24, 2013; published online March 19, 2013)

The hot deformation characteristics of Inconel 690 superalloy were investigated on the Gleeble-3800thermal-mechanical simulator. The testing temperatures were in the range of 1000-1200 �C, the strain ratewas 10 s21, and the maximum true strain was 0.9. Optical microscopy, transmission electron microscopy,and electron backscatter diffraction techniques were employed to analyze the microstructure evolution andnucleation mechanisms of dynamic recrystallization (DRX). The results show that multiple-cycle discon-tinuous dynamic recrystallization (DDRX) occurs in the process of hot deformation under the conditionsabove. DRX grain size decreases with decreasing temperature and increasing strain. DDRX with sub-grainsdirectly transforming into grains is the dominating nucleation mechanism of DRX. And, the nucleationmechanism of bulging of the original grain boundaries can only be considered as an assistant nucleationmechanism of DRX, which mainly occurs in the beginning of the deformation.

Keywords DRX, hot deformation, Inconel 690 superalloy,microstructure evolution

1. Introduction

Inside steam generators of nuclear power plants, fluids bothinside and outside the thin tubes flow differently and heat isexchanged through the thin tubes. During the process of heatexchange, the thin tubes work at elevated temperatures, highstress, and complex environmental conditions (Ref 1).

Inconel 690, a solid solution strengthened nickel-basedsuperalloy known for its good formability, high strength, andexcellent corrosion resistance (Ref 2, 3) has been used incorrosive and high temperature environments and nuclearapplications, such as in pressurized water reactor operatingconditions (Ref 4, 5). The components in these applications aresubjected to a complex combination of elevated temperatures,high stress, and terrible environmental conditions (Ref 6, 7).

As a kind of effective method to produce nickel-based alloytubes, the hot extrusion process is used in the industry. Byimproving the extrusion speed, billets can be deformed intotubes at high temperatures, which make the extrusion processeasier because the alloy has better plastic formability at hightemperatures. On the other hand, it can avoid the extrusionmandril melting because of the temperature rising in a shorttime. Therefore, it comes to the concept of high strain rate hotextrusion.

It is very important to control the hot extrusion process toobtain a uniform microstructure (Ref 8). Numerous approacheshave been proposed over the past few years for developing

constitutive relationships describing the stress-strain behaviorof metallic materials (Ref 9). Many studies have been carriedout to investigate the microstructural influences on corrosionproperties (Ref 10-13). But, there exists limited information inthe published literature on the mechanism of DRX of Inconel690 superalloy, especially the mechanism of DRX during hightemperature and high speed deformation.

In this paper, a series of hot compression tests were carriedout to investigate the hot deformation behavior and dynamicrecrystallization (DRX) mechanism and microstructure evolu-tion of the Inconel 690 superalloy during high temperature andhigh strain rate deformation. These results will provideimportant support for the optimization of the hot workingparameters and the prediction of microstructure.

2. Experimental

2.1 Material

A forged bar of Inconel 690 superalloy with the diameter of16 mm was used as the investigated material; the chemicalcompositions are presented in Table 1. In order to obtain asingle austenite phase, the forged bar were treated at 1150 �Cfor 5 min followed by water quenching. The equiaxed grainswith the average size of about 89 lm and some annealing twinswere observed in the Inconel 690 superalloy, and the solution-treated microstructure is shown in Fig. 1.

2.2 Experimental Procedure

Cylindrical specimens of 6 mm diameter and 9 mm heightwere machined from the forged bar for compression testing. Toreduce the effects of friction, both ends of the specimen werecoated with tantalum plates as a lubricant. Isothermal hotcompression tests were conducted using the Gleeble-3800thermal-mechanical simulator. The tests were carried out atconstant strain rate of 10 s�1, the temperature ranged from

Bin Wang, Shi-Hong Zhang, Ming Cheng and Hong-Wu Song,Institute of Metal Research Chinese Academy of Sciences, Shenyang110016, China. Contact e-mail: [email protected].

JMEPEG (2013) 22:2382–2388 �ASM InternationalDOI: 10.1007/s11665-013-0520-4 1059-9495/$19.00

2382—Volume 22(8) August 2013 Journal of Materials Engineering and Performance

1000 to 1200 �C at an interval of 50 �C, and the true strainswere 0.1, 0.3, 0.5, 0.7, and 0.9. As illustrated in Fig. 2, thespecimens were heated to test temperatures at a heating rate of5 �C/s, held for 180 s, compressed, and then water quenched toroom temperature. The true stress-strain data were obtained inthe compression tests.

These specimens were prepared by sectioning them parallelto the load axis. The metallographic specimens were preparedusing mechanical polishing and electro-etching with 10%oxalic acid for 30 s at 15 V and examined on an Axiovert 200MAT optical microscope (OM); the transmission electronmicroscopy (TEM) specimens were prepared by mechanicalpolishing and twin-jet electro-polishing with a polishing

solution consisting of 10% HClO4 and 90% CH3OH underthe conditions of 200 mA, 160 kV, and �293 K. The micro-structural analysis was performed by a TEM-2000EX electronmicroscope. The electron backscatter diffraction (EBSD)specimens were prepared by mechanical polishing and elec-tro-etching with 20% H2SO4 + 80% CH3OH for 30-40 s at15-20 V; the microstructural analysis was performed by aChannel 5 system of LEO-1450 scanning electron microscope.

3. Results and Discussion

3.1 Flow Behavior

Typical experimental curves of flow stress versus strain forthe Inconel 690 superalloy at different temperatures areillustrated in Fig. 3. It can be observed that the influences oftemperature on the flow stress are significant for all the testedconditions. The flow stress decreases significantly with increas-ing temperature. The flow stress curves exhibit similar features,i.e., multipeaks at the low strain zone followed by the strainsoftening stage and then a steady stage at the high strain zone.The characteristics of the flow stress curves are the typical onesobserved in low stacking-fault energy alloys. However, mul-tipeak stress-strain curves occur at the strain rate of 10 s�1,which are not found at low strain rates and in other Nickle-based alloys.

3.2 Microstructure

Figure 4 shows the typical microstructures at the tempera-ture of 1150 �C and at the strain rate of 10 s�1. At thebeginning of the deformation, fine grains are observed insidethe original coarse grain, and some new grains appear at grainboundaries, which are shown in Fig. 4(a). The percentage ofthe DRX increases with the increasing strain. Multiple-cycle

Table 1 Chemical compositions of Inconel 690 superalloy (wt.%)

Cr Fe Si C Al Ti Mn S P Ni

28.80 10.05 0.37 0.040 0.31 0.29 0.30 0.0021 0.0018 Bal.

Fig. 1 Microstructure of solution-treated Inconel 690 superalloy

Fig. 2 Schematic diagram illustrating sequence of hot compressiontests

Fig. 3 True stress-true strain curves of Inconel 690 alloy at differ-ent temperatures

Journal of Materials Engineering and Performance Volume 22(8) August 2013—2383

DRX occurs in this process and so the new grains are finer thanever. The recrystallization process is completed basically whenthe strain increases to 0.9; only very few original coarse grainsexist and the mean size of the grains is finer than that of theformer, which is shown Fig. 4(e).

The TEM photographs of the Inconel 690 superalloydeformed at T = 1150 �C, _e = 10 s�1, and e = 0.7 are shownin Fig. 5. A large number of dislocations appear inside therecrystallized grains when the true strain is 0.7 (Fig. 5a).Further investigation indicates that these dislocations make upmany cellular structures (Fig. 5b) which will transform into realgrains in the process of hot deformation. This will refine thegrains of the Inconel 690 superalloy and accounts for much ofthe DRX.

3.3 Mechanism of Dynamic Recrystallization

The EBSD orientation maps of the Inconel 690 superalloyunder different strains are shown in Fig. 6; the black thick lines

Fig. 4 The deformed microstructure of Inconel 690 superalloy under 1150 �C and 10 s�1 at different strains: (a) 0.1, (b) 0.3, (c) 0.5, (d) 0.7,and (e) 0.9

Fig. 5 TEM photograph of Inconel 690 superalloy deformed under1150 �C and 10 s�1 at strain of 0.7: (a) recrystallized grain and(b) the internal structure of recrystallized grain


indicate the high angle grain boundaries (HAGB) and thegray thin lines stand for the low angle grain boundaries(LAGB). Fine recrystallized grains appear inside the originalcoarse grains, which means that sub-grains transform intograins, which is shown in Fig. 6(a). The bulging of someoriginal grain boundaries indicates that discontinuousdynamic recrystallization (DDRX) occurs in this process.The HAGB content increases significantly with increasingstrain.

The distribution of the misorientation angle calculated by theresults of EBSD is shown in Fig. 7. The vertical axis shows thefractionofmisorientation in each interval.ManyLAGBsexistwhenthe true strain is 0.1 (Fig. 7a). Then, the LAGBs decrease with theincreasing strain and the HAGBs increase at the same time.The value of increasing HAGBs is equal to that of decreasingLAGBs.

The experiments carried out in this paper confirmed that theboundary misorientation strongly increases with increasingstrain. The transformation of LAGBs into HAGBs wasdemonstrated.

3.4 Discussion

DRX includes DDRX and continuous dynamic recrystalli-zation (CDRX). CDRX is generally caused by the formation ofHAGBs from an increase in the misorientation angle throughaccumulation and rearrangement of the dislocations. It has beenreported that this process results from the combination of threeelementary mechanisms (Ref 14): (1) LAGBs with a very lowmisorientation angle are created in the deformed matrix as aresult of dynamic recovery. (2) The LAGBs gradually trans-form into grain boundaries. The increase in the misorientationis more or less rapid. (3) The LAGBs and HAGBs are partlyeliminated through grain boundary migration.

The results mentioned above support the fact that the CDRXmechanism also works as a nucleation mechanism duringDDRX (Ref 15). And, it is one of the reasons why a duplexgrain structure occurs in the process of high temperature andhigh strain rate deformation of the Inconel 690 superalloy.

So, the dominating nucleation mechanism of the Inconel 690superalloy is DDRX by sub-grains transforming into grains

Fig. 6 EBSD orientation maps of Inconel 690 alloy deformed under 1150 �C and 10 s�1 at different strains: (a) 0.1, (b) 0.3, (c) 0.5, (d) 0.7,and (e) 0.9


directly instead of transforming gradually.And, some approachesmay be proposed to improve the uniformity of microstructure ofthe Inconel 690 superalloy after hot deformation according tothese investigation results in the near future.

Peng et al. (Ref 16) investigated the mechanism of DRX ofGH690 alloy at a constant strain rate ranging from 0.001 to1 s�1. The true stress-strain curves are shown in Fig. 8; theflow stress curves exhibit similar features: a single peak at acritical strain followed by a strain softening stage and a steadystage at the high strain zone.

The results show that DRX occurs under the deformationconditions above. Figure 9 shows the OIM photograph of thegrain boundary (Fig. 9a) and TEM micrograph (Fig. 9b); theoptical microstructures of GH690 alloy deformed at 1050 �Cand a strain rate of 0.1 s�1 are shown in Fig. 10. The DRX

Fig. 7 EBSD misorientation angle maps (deformed under 1150 �C and 10 s�1)

Fig. 8 True stress-true strain curves of GH690 alloy at the strainrate of 1.0 s�1


grains retain the same size with increasing strain; the necklacemicrostructure and low dislocation density inside the DRXgrain indicate that single-cycle DRX occurs in this process, andDDRX with a nucleation mechanism of bulging of the originalgrain boundaries is the dominating nucleation mechanism ofDRX of the GH690 alloy. A CDRX with sub-grain rotation,which can only be considered as an assistant nucleationmechanism of DRX, occurs simultaneously with the DDRX.

4. Conclusions

(1) The mean size of the grains decreases with increasing strain,and the percentage of DRX increases with increasing strain.

(2) The recrystallization process is completed basically

when the strain is 0.9, except for a few original coarsegrains. The microstructure consists of a few unrecrystal-lized coarse grains and recrystallized fine grains.

(3) The dominating nucleation mechanism of the Inconel690 superalloy at a high strain rate is the DDRX of sub-grains transforming into grains directly, which is one ofthe reasons why the duplex grain structure occurs in theprocess of high temperature and high strain rate defor-mation.

Acknowledgment

This work has been supported by the National Natural ScienceFoundation of China (No. 50834008).

Fig. 9 OIM photograph of GH690 alloy deformed under 1.0 s�1 and 950 �C at strain of 0.7 (Ref 16): (a) OIM photograph of grain boundaryand (b) TEM micrograph

Fig. 10 Optical microstructures of GH690 alloy deformed under 1050 �C and 0.1 s�1 at different strains (Ref 16): (a) 0.1, (b) 0.3, (c) 0.5, and(d) 0.7


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