- Email: [email protected]
Effect of Deformation Conditions on the Dynamic Recrystallization of GH690 Alloy
Rare Metal Materials and Engineering Volume 41, Issue 8, August 2012 Online English edition of the Chinese language journal
Cite this article as: Rare Metal Materials and Engineering, 2012, 41(8): 1317-1322.
ARTICLE
Effect of Deformation Conditions on the Dynamic Recrystallization of GH690 Alloy Peng Haijian,
Li Defu,
Guo Qingmiao,
Guo Shengli,
Xu Xiaojing,
Hu Jie
General Research Institute for Non-ferrous Metals, Beijing 100088, China
Abstract: Hot compression tests were conducted using a Gleeble–3500 thermomechanical simulator to investigate the effect of hot deformation conditions on dynamic recrystallization (DRX) of GH690 alloy. The results show that the DRX process of GH690 alloy is controlled by deformation temperature and strain rate. Under the investigated condition at a constant strain rate ranging from 0.001 to 1 s-1, the temperature needed for fully dynamic recrystallization increases with the increases of strain rate, and the size of dynamically recrystallized grain is greatly affected by the increase of deformation temperature. The values of critical strain for the initiation of DRX can be determined from the strain-stress curves, and a equation related to the Zener-Hollomon Parameter is as follows: εc =1.135×10-3Z0.14233. A discontinuous dynamic recrystallization (DDRX) mechanism with nucleation of bulging of the original grain boundaries is the operating nucleation mechanism of DRX of GH690 alloy. A continuous dynamic recrystallization (CDRX) with subgrain rotation, which can only be considered as an assistant nucleation mechanism of DRX, occurs simultaneously with the DDRX. Key words: GH690 alloy; hot deformation; dynamic recrystallization
GH690 is a nickel-based wrought superalloy with high chromium content (around 30wt%). It not only possesses the good intergranular corrosion resistance of other nickel-based alloys, such as Inconel-600, but also appears to be less vulnerable to stress corrosion cracking. Therefore, it is suitable for manufacturing the heat exchanger tubes of steam generators in the nuclear power plant industries. In present, the heat exchanger tubes of the steam generators in the Chinese nuclear power plant industries are mainly dependent on import. Along with the growing requirement for the localization of GH690 tubes, much research work about GH690 alloy has been done at home. For example, Liu Sue investigated the influence of chemical composition and microstructure on corrosion behavior of GH690 alloy[1]. Zhang Song-chuang and Zhu Hong studied the effect of cold deformation and solid-solution on structure and mechanical behavior of inconel 690 alloy[2,3]. Wang Huailiu researched the hot extrusion process of
GH690 alloy[4]. However, information on the basic research of the hot deformation behavior of GH690 alloy was still limited. Nickel is a low stacking fault energy material and is characterized by difficult dislocation climb and cross slip during deformation. Thus the recovery process of nickel-base alloys are slow, and the dislocation density can increase easily to the critical value necessary for dynamic recrystallization (DRX) to occur, therefore, DRX is the main softening mechanism of nickel-base alloys. So the study of the DRX behavior of GH690 alloy is of theoretical and practical importance, which can help us to deeply understand the hot deformation behavior of GH690 alloy. In this work, the effect of hot deformation conditions such as temperature, strain rate and deformation degree on DRX of GH690 alloy was studied by isothermal compression tests on Gleeble 3500 thermomechanical simulator.
1 Experiment
Received date: August 08, 2011 Foundation item: Project supported by National Nature Science Foundation of China (50834008) and Baoshan Iron and Steel Co., Ltd Corresponding author: Peng Haijian, Candidate for Ph. D., Business Unit for Non-ferrous Metals Forming, General Research Institute for Non-ferrous Metals, Beijing 100088, P. R. China, Tel: 0086-10-82241289, E-mail: [email protected] Copyright © 2012, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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Peng Haijian et al. / Rare Metal Materials and Engineering, 2012, 41(8): 1317-1322
Table 1
A forged bar of GH690 superalloy with a nominal diameter of 15 mm was used as the investigated material, its chemical composition is listed in Table 1. In order to obtain a fine homogeneous microstructure, the forged bar was annealed at 1060 ºC for 30 min followed by water quenching. The equiaxed grains with the average size of about 45 μm and some annealing twins were observed in the annealed GH690 superalloy, as seen in Fig. 1. Cylindrical specimens with the dimension of Φ 8 mm×12 mm were machined taken from the center part of the annealed forged bar. Hot compression tests were carried out on a Gleeble-3500 simulator at temperature ranging from 950 to 1200 ºC at constant strain rates of 0.001̚1 s-1. The specific strains were 0.1,0.3,0.5 and 0.7. Each specimen was heated to the set temperatures at a rate of 10 ºC/min and held in the chamber for 3 min to ensure the temperature uniformity. After hot compression, the specimens were water quenched so as to preserve the hot-deformed microstructures. The deformed specimens were sectioned parallel to the compression axis for microstructure analysis. A LEICA-2100 optical microscope was utilized to observe the morphologies of specimens. Electron back-scattered diffractometry (EBSD) measurements were carried out using a LEO-1450 electron probe equipped with Channel 5 software provided by HKL technology to investigate the misorientations of grains and subgrains. Metallographic specimens were ground, polished and etched in a solution consisting of 10 mL H2SO4+100 mL HCl+10 g CuSO4 solid. The specimens for EBSD investigation were machined and then polished electrolytically with 20% solution of H2SO4 in methanol.
2
C
True Stress/MPa
200
Fig.1
950 ºC 1000 ºC 1050 ºC 1100 ºC 1150 ºC
50
0.2
0.4
0.6
True Strain Fig.2
0.8
Fe
Al
Ti
950 ºC 1000 ºC 1050 ºC 1100 ºC 1150 ºC 1200 ºC
50 0.2
0.4
0.6
Ni
Optical microstructure of the annealed GH690 alloy forged
300
c
d 300
100
0 0.0
Cr
stacking-fault energy alloys, which implies the happening of DRX phenomenon during hot deformation [5]. The stress continues to increase until the softening due to the progress of DRX balances the continuing strain hardening in the uncrystallized parts of the material. This balance is manifested by the peak stress (σp) attained at the strain (εp). Therefore, DRX is initiated before the strain that corresponds to the stress peak. The true stress-true strain data can be used to calculate the values of the strain hardening rate(θ=dσ/dε), and then the strain hardening rate values can be plotted as a function of the flow stress (Fig.3a), According to the approach of Poliak and Jonas [6,7], in the curve of θ-σ the point at which the work hardening rate equals zero (θ=0) represents the peak stress (σp). The inflection point of θ-σ curves indicates the critical stress (σc) for the initiation of DRX. From the data given in Fig.3a, the derivative of the strain hardening rate (əθ/əσ) as a function of the flow stress was calculated. The əθ/əσ-σ plot is given in Fig.3b. The minimum point in these plots represents the critical stress. The critical strain (εc) can be defined by mapping the critical stress back into stress-strain curve, and the value of εc is 0.115 when the sample deformed at 1050 °C at a strain rate of 0.1 s –1 . The critical strains of other deformation b
1200 ºC
0 0.0
S
bar
150
100
P
50 µm
2.1 Effect of deformation degree on DRX True stress–true strain curves of GH690 alloy obtained at different strain rates and various temperatures from 950 to 1200 ºC are shown in Fig.2. The flow stress curves exhibit the similar features, i.e. a single peak at a critical strain followed by a strain softening stage and then sometimes a steady stage at high strain zone. The characteristics of the flow stress curves are the typical ones observed in low a
Mn
0.038 0.36 0.30 0.0017 0.0021 28.72 10.05 0.33 0.29 Bal.
Results and Discussion
150
Si
Chemical composition of GH690 (wt%)
0.8
950 ºC
200 100 0 0.0
True Strain
950 ºC
0.2
0.4
0.6
True Strain
1000 ºC
1000 ºC
200
1050 ºC 1100 ºC 1150 ºC 1200 ºC
100
0.8
0 0.0
1050 ºC 1100 ºC 1150 ºC 1200 ºC
0.2
0.4
0.6
0.8
True Strain
True stress-strain curves of GH690 alloy obtained from hot compression test at different pre-set temperatures and strain rates: (a) -1 -1 -1 -1 ε =0.001 s , (b) ε =0.01 s , (c) ε =0.1 s , and (d) ε =1.0 s
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Table 2
θ/MPa
GH690 alloy at different deformation conditions
a
600
Critical strains (εc) for the initiation of DRX of
–1 950 ºC 1000 ºC 1050 ºC 1100 ºC 1150 ºC 1200 ºC ε /s
0.001
450
0.064
0.062
0.044
0.037
0.040
0.030
0.01
0.097
0.091
0.080
0.050
0.047
0.040
300
0.1
0.103
0.121
0.115
0.104
0.100
0.055
150
1
0.188
0.150
0.131
0.128
0.120
0.111
σc
0 130
140
150
160
50
170
a
b
c
d
b
–∂θ/∂σ
40 30 20 0 130
140
150
160
170
180
σ/MPa Fig.3
The θ-σ curve (a) and the ∂θ/∂σ-σ curve (b) of GH690 al-
50 µm
loy deformed at 1050 °C at a strain rate of 0.1 s-1
conditions can be also calculated by this method, and the results are given in table 2. From the data given in Table 2, the relationship between critical strain (εc) for the onset of DRX of GH690 alloy and the Zener-Hollomon parameter can be established by multiple regression: εc=1.135× 10-3Z0.14233. In order to investigate the effect of deformation degree on DRX, the samples of GH690 alloy deformed to different strains were analyzed. The typical microstructure evolution for GH690 alloy obtained at temperature of 1050 °C a strain rate of 0.1 s-1 is shown in Fig. 4. It is noted that at low strain (ε=0.1), the microstructure changes little compared to the original microstructure, for the strain does not reach the critical strain for the initiation of DRXˈas seen in Fig.1 and Fig.4a. When the nominal strain reaches 0.3, a basically full recrystallization has taken place in the examined area of the specimen, as seen in Fig.4b. After that, however, the continued deformation will raise the dislocation density in the recrystallized grains, when it again reaches the critical value for the initiation of DRX, the nucleation of grains and subsequent migration of grain boundaries will leave new dislocation-free grains. Overlapping of the DRX process occurs, therefore, there is almost no change in the size of the DRX grains with increasing of strain when the microstructure turns to be dynamic recrystallization grains, as seen in Fig.4c and Fig.4d. 2.2 Effect of strain rate on DRX The strain rate is one of main influence factors on DRX, it affects not only recrystallization nucleation, but also the growth of DRX grain. Orientation imaging microscopy
Fig.4
Optical microstructures of GH690 alloy deformed at 1050 ć and strain rate of 0.1 s-1: (a) ε=0.1, (b) ε=0.3, (c) ε=0.5, and (d) ε=0.7
(OIM) photographs of GH690 alloy deformed at different strain rates are shown in Fig.5. From these figures, the effect of strain rate on DRX of GH690 alloy can be observed clearly. At a deformation temperature of 950 ºC, a strain rate of ε =1.0 s-1, and a strain of ε=0.7, partially DRX has taken place, as seen in Fig.5a. With the decrease of strain rate, the volume fraction of DRX grains augments. When strain rate decrease to 0.001 s-1, the structure features of the original grains almost disappear and the microstructure consists of new equiaxed recrystallized grains (Fig.5d), indicating that the deformed austenite is in the state of fully DRX. When at a constant deformation temperature (950 ºC) and a constant nominal strain, the DRX grains do not have enough time to grow up, therefore, the volume fraction of DRX grains gradually decreases with the increase of strain rate. If we want to acquire fully DRX microstructure at high strain rate, the deformation temperature should be heightened to accelerate the growth speed of DRX grains. Table 3 shows the temperature values of GH690 alloy for achieving fully DRX at different strain rates when the nominal strain is 0.7. Under the investigated conditions with a constant strain rate ranging from 0.001 to 1 s-1, the temperature needed for fully dynamic recrystallization increases from 950 to 1050 ºC. Therefore, the DRX process of GH690 alloy is dependent on deformation temperature and strain rate.
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Peng Haijian et al. / Rare Metal Materials and Engineering, 2012, 41(8): 1317-1322
b
10 µm
10 µm
c
d
20 µm
20 µm
Orientation imaging microscopy (OIM) photographs of GH690 alloy deformed to a nominal strain of 0.7 at temperatures of 950 ºC and different strain rates: (a) ε =1.0 s-1, (b) ε =0.1 s-1, (c) ε =0.01 s-1, and (d) ε =0.001 s-1
Table 3
The temperature values of GH690 alloy for achieving fully DRX at different strain rates when ε is 0.7
Strain rate, ε /s-1 The temperature needed for fully DRX/ºC
0.001
0.01
0.1
950
950˘Tİ1000
1.0
1000 1050
b
a DRX grain
B A
Original grain and intragranular subgrain
20 µm
1 µm
Misorientation Angle/(º)
A typical OIM photograph of grain boundary for GH690 alloy deformed to a nominal strain of 0.7 at 950 ºC with a stain rate of 0.1 s-1 is shown in Fig. 6a. In the photograph, high angle boundaries (>15°) are represented by black lines, and low angle boundaries(<15°) are represented by blue lines, red lines and black lines. It may be noted that the original grain boundaries extensively become serrated and bulging, which is closely related to the strain-induced grain boundary migration. The TEM micrographs of GH690 alloy deformed on this condition is shown in Fig. 6b. From this figure, bulging of the DRX grain boundary can be observed clearly. These phenomenons imply that the recrystallization 10
c
8 6 4 2 0 0
6
12
18
24
Distance/µm Fig.6
30
Misorientation Angle/(º)
Fig.5
a
mechanism of GH690 alloy belongs to the discontinuous dynamic recrystallization (DDRX). Fig.6c and Fig.6d show the point-to-point misorientation developed along the line A and the line B in Fig.6a. Many subgrains with low angle boundaries can be observed in the large original grains, and we can see that some point-to-point misorientations have reached 6°̚8° whether near the original high angle grain boundary (line A) or from grain boundary to grain interior (line B). In addition, a spot of new grains with high angle boundaries appears in the large original grain interiorsˈas seen in Fig.6a. These phenomena support the viewpoint that continuous dynamic recrystallization (CDRX) also plays a role in the nucleation of DRX in GH690 alloy. In order to give a further study to the nucleation mechanism of DRX of GH690 alloy, the misorientation angle distributions at different strain rates for GH690 alloy were calculated according to the results of EBSD test, as seen in Fig.7. Low angle boundaries (<15°) are presented in large numbers when the sample of GH690 alloy is on the condition of partially DRX, but the percentage of subgrain boundary possessing the misorientation angles between 10° and 15° are all low on the conditions with different volume fraction of DRX grain (Fig.5a, 5b, 5c, 7a, 7b and 7c). These results indicate that only a spot of new grain is formed by subgrain rotation. The measured grain boundaries are mainly high angle boundaries when the sample of GH690 alloy is in the condition of fully DRX (Fig.7d). Therefore, the DDRX with nucleation mechanism of bulging of the original grain boundaries is the operating nucleation mechanism of DRX of GH690 alloy, and the CDRX with subgrain rotation, which can only be considered as an assistant nucleation mechanism of DRX, occurs simultaneously with the DDRX. 2.3 Effect of deformation temperature on DRX Fig.8 presents typical microstructures of GH690 alloy developed at various temperatures at a strain rate of 0.1 s-1 and a nominal strain of 0.7. It can be seen that the evolution of microstructures is strongly dependent on temperature. At the deformation temperature of 950 ć (Fig. 8a), some large 10
d
8 6 4 2 0
0
3
6
9
12
Distance/µm
OIM photograph and orientation analysis of GH690 alloy deformed at ε =1 s-1, T=950 ºC and ε=0.7: (a) OIM photograph of grain boundary, (b) TEM micrograph, (c) and (d) misorientations measured along the lines A and B marked in Fig.6a, respectively
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Peng Haijian et al. / Rare Metal Materials and Engineering, 2012, 41(8): 1317-1322
Rel. Frequency
0.25 0.20
a 0.15 0.12
0.15
0.09
0.10
0.06
0.05
0.03
b 0.18 0.15 0.12
c
0.09 0.06
0.03 0.00 0.00 0 10 20 30 40 50 60 0.000 10 20 30 40 50 60 0 10 20 30 40 50 60 Misorientation Angle/(º)
Fig.7
Misorientation Angle/(º)
Misorientation Angle/(º)
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
d
0 10 20 30 40 50 60 Misorientation Angle/(º)
Corresponding misorientation angle distribution maps of GH690 alloy deformed to a nominal strain of 0.7 at temperatures of 950 ºC and different strain rates: (a) ε =1.0 s-1, (b) ε =0.1 s-1, (c) ε =0.01 s-1 , and (d) ε =0.001 s-1 a
b
c
d
e
f
50 µm
Fig.8
Effect of temperature on microstructures of GH690 alloy deformed to a nominal strain of 0.7 at a strain rates of 1 s-1 and different temperatures: (a) 950 ºC, (b) 1000 ºC, (c) 1050 ºC, (d) 1100 ºC, (e) 1150 ºC, and (f) 1200 ºC
pre-existing grains are elongated in the direction perpendicular to the compression axis and many small recrystallized grains are developed, indicating that the deformed austenite is in the state of partially DRX. The increase of deformation temperature brings about an increase in mobility of grain boundaries and dislocations, which leads to the increase in the volume fraction of DRX grains, as seen in Fig.8b. When the deformation temperature is up to 1050 ºC, the examined area of the specimen has been fully recrystallized. With the continuous increase of deformation temperature, the size of DRX grains increases gradually. When the deformation temperature rises to 1150 ºC, the mean size of the DRX grains is 15 μm, while when the deformation temperature is up to 1200 ºC, an average grain size of about 25 μm is reached, as seen in Fig.8e and Fig.8f.
3
Conclusions
1) The equation of the critical strain of GH690 alloy for the initiation of DRX can be obtained: εc=1.135×10-3Z0.14233˗The value of critical strain is 0.115
when the samples deformed at 1050 ºC and a strain rate of 0.1 s-1, and a full recrystallization will take place when the nominal strain reaches 0.3. After that, however, there is almost no change in the size of the DRX grains with increasing of strain, and the microstructure turns that with dynamic recrystallization grains. 2) Under the investigated condition with a nominal strain of 0.7 and a constant strain rate ranging from 0.001 to 1 s-1, the temperature needed for fully dynamic recrystallization increases with the increases of strain rate, The temperature value needed for fully DRX is 950 ºC when the strain rate is 0.001 s-1, while it increases to 1050 ºC when the strain rate is 1.0 s-1. 3) A discontinuous dynamic recrystallization (DDRX) with nucleation mechanism of bulging of the original grain boundaries is the operating nucleation mechanism of DRX of GH690 alloy. A continuous dynamic recrystallization (CDRX) with subgrain rotation, which can only be considered as an assistant nucleation mechanism of DRX, occurs simultaneously with the DDRX. 4) The size of the DRX grains is greatly affected by de-
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Peng Haijian et al. / Rare Metal Materials and Engineering, 2012, 41(8): 1317-1322
formation temperature which increases gradually with the increase of deformation temperature. The mean size of the DRX grains can be controlled in the range of 15 to 25 μm, when ε =1 s-1ˈε=0.7 and deformation temperature is controlled in the range of 1150 to 1200 ºC.
References 1 2
search[J], 2009, 21(12): 49 3
Zhu Hong et al. Journal of University of Science and Technology Beijing[J], 2002, 24(5): 511
4
Wang Huailiu et al. Special Steel Technology[J], 2008, 14(55): 31
5
Wang Yan et al. Materials Science and Engineering A[J], 2008, 486: 321
Liu Sue et al. Corrosion Science and Protection Tec- hnology
6
Poliak E I et al. Acta Material[J], 1996, 44(1): 127
[J], 1995, 7(2): 146
7
Shaban M et al. Materials Science and Engineering A[J],
Zhang Songchuang et al. Journal of Iron and Steel Re-
2010, 527: 4320
1322
Cite this article as: Rare Metal Materials and Engineering, 2012, 41(8): 1317-1322.
ARTICLE
Effect of Deformation Conditions on the Dynamic Recrystallization of GH690 Alloy Peng Haijian,
Li Defu,
Guo Qingmiao,
Guo Shengli,
Xu Xiaojing,
Hu Jie
General Research Institute for Non-ferrous Metals, Beijing 100088, China
Abstract: Hot compression tests were conducted using a Gleeble–3500 thermomechanical simulator to investigate the effect of hot deformation conditions on dynamic recrystallization (DRX) of GH690 alloy. The results show that the DRX process of GH690 alloy is controlled by deformation temperature and strain rate. Under the investigated condition at a constant strain rate ranging from 0.001 to 1 s-1, the temperature needed for fully dynamic recrystallization increases with the increases of strain rate, and the size of dynamically recrystallized grain is greatly affected by the increase of deformation temperature. The values of critical strain for the initiation of DRX can be determined from the strain-stress curves, and a equation related to the Zener-Hollomon Parameter is as follows: εc =1.135×10-3Z0.14233. A discontinuous dynamic recrystallization (DDRX) mechanism with nucleation of bulging of the original grain boundaries is the operating nucleation mechanism of DRX of GH690 alloy. A continuous dynamic recrystallization (CDRX) with subgrain rotation, which can only be considered as an assistant nucleation mechanism of DRX, occurs simultaneously with the DDRX. Key words: GH690 alloy; hot deformation; dynamic recrystallization
GH690 is a nickel-based wrought superalloy with high chromium content (around 30wt%). It not only possesses the good intergranular corrosion resistance of other nickel-based alloys, such as Inconel-600, but also appears to be less vulnerable to stress corrosion cracking. Therefore, it is suitable for manufacturing the heat exchanger tubes of steam generators in the nuclear power plant industries. In present, the heat exchanger tubes of the steam generators in the Chinese nuclear power plant industries are mainly dependent on import. Along with the growing requirement for the localization of GH690 tubes, much research work about GH690 alloy has been done at home. For example, Liu Sue investigated the influence of chemical composition and microstructure on corrosion behavior of GH690 alloy[1]. Zhang Song-chuang and Zhu Hong studied the effect of cold deformation and solid-solution on structure and mechanical behavior of inconel 690 alloy[2,3]. Wang Huailiu researched the hot extrusion process of
GH690 alloy[4]. However, information on the basic research of the hot deformation behavior of GH690 alloy was still limited. Nickel is a low stacking fault energy material and is characterized by difficult dislocation climb and cross slip during deformation. Thus the recovery process of nickel-base alloys are slow, and the dislocation density can increase easily to the critical value necessary for dynamic recrystallization (DRX) to occur, therefore, DRX is the main softening mechanism of nickel-base alloys. So the study of the DRX behavior of GH690 alloy is of theoretical and practical importance, which can help us to deeply understand the hot deformation behavior of GH690 alloy. In this work, the effect of hot deformation conditions such as temperature, strain rate and deformation degree on DRX of GH690 alloy was studied by isothermal compression tests on Gleeble 3500 thermomechanical simulator.
1 Experiment
Received date: August 08, 2011 Foundation item: Project supported by National Nature Science Foundation of China (50834008) and Baoshan Iron and Steel Co., Ltd Corresponding author: Peng Haijian, Candidate for Ph. D., Business Unit for Non-ferrous Metals Forming, General Research Institute for Non-ferrous Metals, Beijing 100088, P. R. China, Tel: 0086-10-82241289, E-mail: [email protected] Copyright © 2012, Northwest Institute for Nonferrous Metal Research. Published by Elsevier BV. All rights reserved.
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Peng Haijian et al. / Rare Metal Materials and Engineering, 2012, 41(8): 1317-1322
Table 1
A forged bar of GH690 superalloy with a nominal diameter of 15 mm was used as the investigated material, its chemical composition is listed in Table 1. In order to obtain a fine homogeneous microstructure, the forged bar was annealed at 1060 ºC for 30 min followed by water quenching. The equiaxed grains with the average size of about 45 μm and some annealing twins were observed in the annealed GH690 superalloy, as seen in Fig. 1. Cylindrical specimens with the dimension of Φ 8 mm×12 mm were machined taken from the center part of the annealed forged bar. Hot compression tests were carried out on a Gleeble-3500 simulator at temperature ranging from 950 to 1200 ºC at constant strain rates of 0.001̚1 s-1. The specific strains were 0.1,0.3,0.5 and 0.7. Each specimen was heated to the set temperatures at a rate of 10 ºC/min and held in the chamber for 3 min to ensure the temperature uniformity. After hot compression, the specimens were water quenched so as to preserve the hot-deformed microstructures. The deformed specimens were sectioned parallel to the compression axis for microstructure analysis. A LEICA-2100 optical microscope was utilized to observe the morphologies of specimens. Electron back-scattered diffractometry (EBSD) measurements were carried out using a LEO-1450 electron probe equipped with Channel 5 software provided by HKL technology to investigate the misorientations of grains and subgrains. Metallographic specimens were ground, polished and etched in a solution consisting of 10 mL H2SO4+100 mL HCl+10 g CuSO4 solid. The specimens for EBSD investigation were machined and then polished electrolytically with 20% solution of H2SO4 in methanol.
2
C
True Stress/MPa
200
Fig.1
950 ºC 1000 ºC 1050 ºC 1100 ºC 1150 ºC
50
0.2
0.4
0.6
True Strain Fig.2
0.8
Fe
Al
Ti
950 ºC 1000 ºC 1050 ºC 1100 ºC 1150 ºC 1200 ºC
50 0.2
0.4
0.6
Ni
Optical microstructure of the annealed GH690 alloy forged
300
c
d 300
100
0 0.0
Cr
stacking-fault energy alloys, which implies the happening of DRX phenomenon during hot deformation [5]. The stress continues to increase until the softening due to the progress of DRX balances the continuing strain hardening in the uncrystallized parts of the material. This balance is manifested by the peak stress (σp) attained at the strain (εp). Therefore, DRX is initiated before the strain that corresponds to the stress peak. The true stress-true strain data can be used to calculate the values of the strain hardening rate(θ=dσ/dε), and then the strain hardening rate values can be plotted as a function of the flow stress (Fig.3a), According to the approach of Poliak and Jonas [6,7], in the curve of θ-σ the point at which the work hardening rate equals zero (θ=0) represents the peak stress (σp). The inflection point of θ-σ curves indicates the critical stress (σc) for the initiation of DRX. From the data given in Fig.3a, the derivative of the strain hardening rate (əθ/əσ) as a function of the flow stress was calculated. The əθ/əσ-σ plot is given in Fig.3b. The minimum point in these plots represents the critical stress. The critical strain (εc) can be defined by mapping the critical stress back into stress-strain curve, and the value of εc is 0.115 when the sample deformed at 1050 °C at a strain rate of 0.1 s –1 . The critical strains of other deformation b
1200 ºC
0 0.0
S
bar
150
100
P
50 µm
2.1 Effect of deformation degree on DRX True stress–true strain curves of GH690 alloy obtained at different strain rates and various temperatures from 950 to 1200 ºC are shown in Fig.2. The flow stress curves exhibit the similar features, i.e. a single peak at a critical strain followed by a strain softening stage and then sometimes a steady stage at high strain zone. The characteristics of the flow stress curves are the typical ones observed in low a
Mn
0.038 0.36 0.30 0.0017 0.0021 28.72 10.05 0.33 0.29 Bal.
Results and Discussion
150
Si
Chemical composition of GH690 (wt%)
0.8
950 ºC
200 100 0 0.0
True Strain
950 ºC
0.2
0.4
0.6
True Strain
1000 ºC
1000 ºC
200
1050 ºC 1100 ºC 1150 ºC 1200 ºC
100
0.8
0 0.0
1050 ºC 1100 ºC 1150 ºC 1200 ºC
0.2
0.4
0.6
0.8
True Strain
True stress-strain curves of GH690 alloy obtained from hot compression test at different pre-set temperatures and strain rates: (a) -1 -1 -1 -1 ε =0.001 s , (b) ε =0.01 s , (c) ε =0.1 s , and (d) ε =1.0 s
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Peng Haijian et al. / Rare Metal Materials and Engineering, 2012, 41(8): 1317-1322
Table 2
θ/MPa
GH690 alloy at different deformation conditions
a
600
Critical strains (εc) for the initiation of DRX of
–1 950 ºC 1000 ºC 1050 ºC 1100 ºC 1150 ºC 1200 ºC ε /s
0.001
450
0.064
0.062
0.044
0.037
0.040
0.030
0.01
0.097
0.091
0.080
0.050
0.047
0.040
300
0.1
0.103
0.121
0.115
0.104
0.100
0.055
150
1
0.188
0.150
0.131
0.128
0.120
0.111
σc
0 130
140
150
160
50
170
a
b
c
d
b
–∂θ/∂σ
40 30 20 0 130
140
150
160
170
180
σ/MPa Fig.3
The θ-σ curve (a) and the ∂θ/∂σ-σ curve (b) of GH690 al-
50 µm
loy deformed at 1050 °C at a strain rate of 0.1 s-1
conditions can be also calculated by this method, and the results are given in table 2. From the data given in Table 2, the relationship between critical strain (εc) for the onset of DRX of GH690 alloy and the Zener-Hollomon parameter can be established by multiple regression: εc=1.135× 10-3Z0.14233. In order to investigate the effect of deformation degree on DRX, the samples of GH690 alloy deformed to different strains were analyzed. The typical microstructure evolution for GH690 alloy obtained at temperature of 1050 °C a strain rate of 0.1 s-1 is shown in Fig. 4. It is noted that at low strain (ε=0.1), the microstructure changes little compared to the original microstructure, for the strain does not reach the critical strain for the initiation of DRXˈas seen in Fig.1 and Fig.4a. When the nominal strain reaches 0.3, a basically full recrystallization has taken place in the examined area of the specimen, as seen in Fig.4b. After that, however, the continued deformation will raise the dislocation density in the recrystallized grains, when it again reaches the critical value for the initiation of DRX, the nucleation of grains and subsequent migration of grain boundaries will leave new dislocation-free grains. Overlapping of the DRX process occurs, therefore, there is almost no change in the size of the DRX grains with increasing of strain when the microstructure turns to be dynamic recrystallization grains, as seen in Fig.4c and Fig.4d. 2.2 Effect of strain rate on DRX The strain rate is one of main influence factors on DRX, it affects not only recrystallization nucleation, but also the growth of DRX grain. Orientation imaging microscopy
Fig.4
Optical microstructures of GH690 alloy deformed at 1050 ć and strain rate of 0.1 s-1: (a) ε=0.1, (b) ε=0.3, (c) ε=0.5, and (d) ε=0.7
(OIM) photographs of GH690 alloy deformed at different strain rates are shown in Fig.5. From these figures, the effect of strain rate on DRX of GH690 alloy can be observed clearly. At a deformation temperature of 950 ºC, a strain rate of ε =1.0 s-1, and a strain of ε=0.7, partially DRX has taken place, as seen in Fig.5a. With the decrease of strain rate, the volume fraction of DRX grains augments. When strain rate decrease to 0.001 s-1, the structure features of the original grains almost disappear and the microstructure consists of new equiaxed recrystallized grains (Fig.5d), indicating that the deformed austenite is in the state of fully DRX. When at a constant deformation temperature (950 ºC) and a constant nominal strain, the DRX grains do not have enough time to grow up, therefore, the volume fraction of DRX grains gradually decreases with the increase of strain rate. If we want to acquire fully DRX microstructure at high strain rate, the deformation temperature should be heightened to accelerate the growth speed of DRX grains. Table 3 shows the temperature values of GH690 alloy for achieving fully DRX at different strain rates when the nominal strain is 0.7. Under the investigated conditions with a constant strain rate ranging from 0.001 to 1 s-1, the temperature needed for fully dynamic recrystallization increases from 950 to 1050 ºC. Therefore, the DRX process of GH690 alloy is dependent on deformation temperature and strain rate.
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b
10 µm
10 µm
c
d
20 µm
20 µm
Orientation imaging microscopy (OIM) photographs of GH690 alloy deformed to a nominal strain of 0.7 at temperatures of 950 ºC and different strain rates: (a) ε =1.0 s-1, (b) ε =0.1 s-1, (c) ε =0.01 s-1, and (d) ε =0.001 s-1
Table 3
The temperature values of GH690 alloy for achieving fully DRX at different strain rates when ε is 0.7
Strain rate, ε /s-1 The temperature needed for fully DRX/ºC
0.001
0.01
0.1
950
950˘Tİ1000
1.0
1000 1050
b
a DRX grain
B A
Original grain and intragranular subgrain
20 µm
1 µm
Misorientation Angle/(º)
A typical OIM photograph of grain boundary for GH690 alloy deformed to a nominal strain of 0.7 at 950 ºC with a stain rate of 0.1 s-1 is shown in Fig. 6a. In the photograph, high angle boundaries (>15°) are represented by black lines, and low angle boundaries(<15°) are represented by blue lines, red lines and black lines. It may be noted that the original grain boundaries extensively become serrated and bulging, which is closely related to the strain-induced grain boundary migration. The TEM micrographs of GH690 alloy deformed on this condition is shown in Fig. 6b. From this figure, bulging of the DRX grain boundary can be observed clearly. These phenomenons imply that the recrystallization 10
c
8 6 4 2 0 0
6
12
18
24
Distance/µm Fig.6
30
Misorientation Angle/(º)
Fig.5
a
mechanism of GH690 alloy belongs to the discontinuous dynamic recrystallization (DDRX). Fig.6c and Fig.6d show the point-to-point misorientation developed along the line A and the line B in Fig.6a. Many subgrains with low angle boundaries can be observed in the large original grains, and we can see that some point-to-point misorientations have reached 6°̚8° whether near the original high angle grain boundary (line A) or from grain boundary to grain interior (line B). In addition, a spot of new grains with high angle boundaries appears in the large original grain interiorsˈas seen in Fig.6a. These phenomena support the viewpoint that continuous dynamic recrystallization (CDRX) also plays a role in the nucleation of DRX in GH690 alloy. In order to give a further study to the nucleation mechanism of DRX of GH690 alloy, the misorientation angle distributions at different strain rates for GH690 alloy were calculated according to the results of EBSD test, as seen in Fig.7. Low angle boundaries (<15°) are presented in large numbers when the sample of GH690 alloy is on the condition of partially DRX, but the percentage of subgrain boundary possessing the misorientation angles between 10° and 15° are all low on the conditions with different volume fraction of DRX grain (Fig.5a, 5b, 5c, 7a, 7b and 7c). These results indicate that only a spot of new grain is formed by subgrain rotation. The measured grain boundaries are mainly high angle boundaries when the sample of GH690 alloy is in the condition of fully DRX (Fig.7d). Therefore, the DDRX with nucleation mechanism of bulging of the original grain boundaries is the operating nucleation mechanism of DRX of GH690 alloy, and the CDRX with subgrain rotation, which can only be considered as an assistant nucleation mechanism of DRX, occurs simultaneously with the DDRX. 2.3 Effect of deformation temperature on DRX Fig.8 presents typical microstructures of GH690 alloy developed at various temperatures at a strain rate of 0.1 s-1 and a nominal strain of 0.7. It can be seen that the evolution of microstructures is strongly dependent on temperature. At the deformation temperature of 950 ć (Fig. 8a), some large 10
d
8 6 4 2 0
0
3
6
9
12
Distance/µm
OIM photograph and orientation analysis of GH690 alloy deformed at ε =1 s-1, T=950 ºC and ε=0.7: (a) OIM photograph of grain boundary, (b) TEM micrograph, (c) and (d) misorientations measured along the lines A and B marked in Fig.6a, respectively
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Rel. Frequency
0.25 0.20
a 0.15 0.12
0.15
0.09
0.10
0.06
0.05
0.03
b 0.18 0.15 0.12
c
0.09 0.06
0.03 0.00 0.00 0 10 20 30 40 50 60 0.000 10 20 30 40 50 60 0 10 20 30 40 50 60 Misorientation Angle/(º)
Fig.7
Misorientation Angle/(º)
Misorientation Angle/(º)
0.35 0.30 0.25 0.20 0.15 0.10 0.05 0.00
d
0 10 20 30 40 50 60 Misorientation Angle/(º)
Corresponding misorientation angle distribution maps of GH690 alloy deformed to a nominal strain of 0.7 at temperatures of 950 ºC and different strain rates: (a) ε =1.0 s-1, (b) ε =0.1 s-1, (c) ε =0.01 s-1 , and (d) ε =0.001 s-1 a
b
c
d
e
f
50 µm
Fig.8
Effect of temperature on microstructures of GH690 alloy deformed to a nominal strain of 0.7 at a strain rates of 1 s-1 and different temperatures: (a) 950 ºC, (b) 1000 ºC, (c) 1050 ºC, (d) 1100 ºC, (e) 1150 ºC, and (f) 1200 ºC
pre-existing grains are elongated in the direction perpendicular to the compression axis and many small recrystallized grains are developed, indicating that the deformed austenite is in the state of partially DRX. The increase of deformation temperature brings about an increase in mobility of grain boundaries and dislocations, which leads to the increase in the volume fraction of DRX grains, as seen in Fig.8b. When the deformation temperature is up to 1050 ºC, the examined area of the specimen has been fully recrystallized. With the continuous increase of deformation temperature, the size of DRX grains increases gradually. When the deformation temperature rises to 1150 ºC, the mean size of the DRX grains is 15 μm, while when the deformation temperature is up to 1200 ºC, an average grain size of about 25 μm is reached, as seen in Fig.8e and Fig.8f.
3
Conclusions
1) The equation of the critical strain of GH690 alloy for the initiation of DRX can be obtained: εc=1.135×10-3Z0.14233˗The value of critical strain is 0.115
when the samples deformed at 1050 ºC and a strain rate of 0.1 s-1, and a full recrystallization will take place when the nominal strain reaches 0.3. After that, however, there is almost no change in the size of the DRX grains with increasing of strain, and the microstructure turns that with dynamic recrystallization grains. 2) Under the investigated condition with a nominal strain of 0.7 and a constant strain rate ranging from 0.001 to 1 s-1, the temperature needed for fully dynamic recrystallization increases with the increases of strain rate, The temperature value needed for fully DRX is 950 ºC when the strain rate is 0.001 s-1, while it increases to 1050 ºC when the strain rate is 1.0 s-1. 3) A discontinuous dynamic recrystallization (DDRX) with nucleation mechanism of bulging of the original grain boundaries is the operating nucleation mechanism of DRX of GH690 alloy. A continuous dynamic recrystallization (CDRX) with subgrain rotation, which can only be considered as an assistant nucleation mechanism of DRX, occurs simultaneously with the DDRX. 4) The size of the DRX grains is greatly affected by de-
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Peng Haijian et al. / Rare Metal Materials and Engineering, 2012, 41(8): 1317-1322
formation temperature which increases gradually with the increase of deformation temperature. The mean size of the DRX grains can be controlled in the range of 15 to 25 μm, when ε =1 s-1ˈε=0.7 and deformation temperature is controlled in the range of 1150 to 1200 ºC.
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