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Effect of the δ phase on the deformation behavior in isothermal compression of superalloy GH4169
Materials Science and Engineering A 528 (2011) 4723–4731
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Effect of the ı phase on the deformation behavior in isothermal compression of superalloy GH4169 K. Wang, M.Q. Li ∗ , J. Luo, C. Li School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, PR China
a r t i c l e
i n f o
Article history: Received 18 January 2011 Received in revised form 22 February 2011 Accepted 23 February 2011 Available online 2 March 2011 Keywords: Flow stress Strain rate sensitivity exponent Strain hardening exponent Kinetic analysis Dynamic recrystallization
a b s t r a c t The effect of the ı phase on the deformation behavior, including flow stress–strain curve, strain rate sensitivity exponent, strain hardening exponent and kinetic analysis, was investigated by isothermal compression of superalloy GH4169 with two kinds of solution treatment. The experimental results show that the existence of ı phase results in the decreasing of flow stress, and makes the flow stress reach a peak value at small strain. The strain rate sensitivity exponent (m) increases with the increasing of deformation temperature. The existence of ı phase leads to the increasing of strain rate sensitivity exponent (m) at a certain deformation temperature and strain. Moreover, the strain hardening exponent (n) has a close relationship with the deformation temperature, strain rate and strain, especially the strain affects n more significantly. The existence of ı phase results in the decreasing of n values, and makes the n values reach a negative value at small strain. It was observed that the changes in strain rate sensitivity exponent and strain hardening exponent were closely related to the microstructural evolution in the deformation process. Based on the kinetic analysis of superalloy GH4169 with two kinds of solution treatment, the apparent activation energy of superalloy GH4169 containing ı phase was calculated to be 476.136 kJ mol−1 , which was slightly higher than that of superalloy GH4169 (455.434 kJ mol−1 ) without ı phase. And the peak flow stress in the isothermal compression of superalloy GH4169 was observed to increase with the increasing of parameter Z. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Superalloy GH4169 is a precipitation strengthened nickedbased superalloy, containing significant amounts of Fe, Nb and Mo, with minor contents of Al and Ti. The metastable bodycentered tetragonal coherent precipitate (Ni3 Nb) phase and the face-centered cubic coherent precipitate [Ni3 (Al, Ti)] phase are strengthening phase, and phase is the major strengthening phase. The equilibrium phase corresponding to phase is the orthorhombic incoherent ␦ (Ni3 Nb) phase [1,2]. Because of its excellent mechanical properties at elevated temperatures and good corrosion resistance, superalloy GH4169 is extensively used in gas turbines, rocket engines, turbine blades [3,4]. In order to meet the present requirements of favorable microstructure and properties, better control of forging processes for superalloy GH4169 is rather important. It is well known that the deformation behavior is strongly dependent on the processing parameters such as temperature, strain rate and strain. In the past decades, a number of hot torsion and hot compression experiments
∗ Corresponding author. Tel.: +86 29 88460328; fax: +86 29 88492642. E-mail address: [email protected] (M.Q. Li). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.02.081
were carried out to investigate the deformation behavior of Inconel 718 [5–7]. To describe the plastic flow performance of superalloy GH4169 in a form that can be used in computer code to model the forging process, some models describing the relationship between the flow stress and the processing parameters have been proposed. Moreover, it has been reported that dynamic recrystallization is the critical process in the high temperature deformation of Inconel 718 [8,9]. The ı phase presented in superalloy GH4169 plays an important role in controlling the microstructure and mechanical properties [10], which have been paid much attention. Recently, the effect of the ı phase on the high temperature notch sensitivity and impact toughness and grain size control in delta processed Inconel 718 have been studied [3], in which the results indicated that ␦ phase could form in the alloy in processing and in service processes. Wang et al. [11] also investigated the effect of ı phase on the deformation behavior of delta processed Inconel 718, in which the results showed that the ı phase could stimulate the occurrence of dynamic recrystallization. Cone [12] observed the evolution of ı phase in different ı phase precipitation cycles and rolling. Zhang et al. [13] studied the deformation characteristics of ı phase in the delta processed Inconel 718, in which the results showed that the dissolution of plate-like ı phase and the precipitation of spheri-
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Table 1 Chemical composition of superalloy GH4169 (wt %). Ni
Cr
Nb + Ta
Mo
Al
Ti
Fe
54.10
19.39
5.21–5.81
3.05
0.53
1.02
Bal.
cal ı phase particles coexisted in deformation process. Yuan and Liu [14] researched the effect of ı phase on the high temperature deformation behavior of Inconel 718, in which the results showed that ı phase affected the peak flow stress, strain corresponding to the peak flow stress (εp ) and apparent activation energy. Azadian et al. [15] studied the precipitation and dissolution kinetics of the ı phase. Cai et al. [16] investigated the dissolution kinetic of ı phase and the effect of ı phase on the notch sensitivity of Inconel 718. However, there are few systematical reports dealing with the effect of ı phase on the deformation behavior, including flow stress–strain curve, strain rate sensitivity exponent, strain hardening exponent and kinetic analysis. The shapes of stress–strain curves show some features that help in identifying the mechanisms of high temperature deformation, although not in a conclusive fashion. The strain rate sensitivity exponent (m) is related to the deformation mechanisms of material. On the other hand, the strain hardening exponent (n) reflecting the work-hardening of material is also an important parameter in plastic deformation. Therefore, it is necessary to carry out more work and deeper analysis to investigate the effect of ı phase on the hot deformation behavior. In this paper, two kinds of solution treated superalloy GH4169 were isothermally compressed. And the effect of ı phase on the flow stress–strain curve, strain rate sensitivity exponent, strain hardening exponent and kinetic analysis was studied systematically.
not be observed in the other specimens. Because the temperature of solution treatment is higher than the temperature for the or phase to be dissolved completely, two kinds of solution treated specimens almost contain no strengthening phase and . That is to say the effect of the strengthening phase and on the deformation behavior of superalloy GH4169 was eliminated in the experiments, approximately. So the effect of ı phase on the flow stress–strain curve, strain rate sensitivity exponent, strain hardening exponent and kinetic analysis could be investigated more clearly. 2.2. Isothermal compression A series of isothermal compressions were conducted in a Thermecmaster-Z simulator at deformation temperatures of 1123, 1223 and 1288 K, strain rates of 0.1, 1.0 and 10.0 s−1 , and height reductions of 20% and 60%. The specimens were heated and held for 3 min at the deformation temperature to establish a uniform temperature in the specimens. The flow stress–strain curves were recorded automatically in isothermal compression. After the compression, the specimens were cooled in air to room temperature. To observe the microstructural evolution, the isothermal compressed specimens were axially sectioned and prepared using standard metallographic techniques. 3. Results and discussion 3.1. Flow stress
2.1. Material and solution treatment
Figs. 2 and 3 show the flow stress–strain curves at deformation temperatures ranging from 1123 to 1288 K and strain rates ranging from 0.1 to 10.0 s−1 for superalloy GH4169 treated by procedure A and procedure B, respectively. Figs. 2 and 3 show some similar characteristics as follows.
The chemical composition of superalloy GH4169 used in the present investigation is shown in Table 1. The cylindrical compression specimens are 8.0 mm in diameter and 12.0 mm in height, and the cylindrical ends were grooved with 0.2 mm for retention of glass lubricants in the isothermal compression. In order to study the effect of ı phase on the deformation behavior of superalloy GH4169, the solution treatment prior to isothermal compression was conducted in the following two procedures: (A) heated to 1233 K and held for 30 min, (B) heated to 1293 K and held for 60 min. After the solution treatment, the specimens were cooled in water to room temperature, and the corresponding SEM micrographs of the two kinds of solution treated superalloy GH4169 are shown in Fig. 1. It can be seen from Fig. 1 that the specimens treated by procedure A have a large amount of ı phase in the austenitic matrix, which can
(1) The flow stress increases to a peak value with the increasing of strain, and then decreases with the further increasing of strain. The initial rapid rise of stress is associated with the increasing of dislocation density. In the alloys with low or intermediate stacking fault energy, e.g. nickel or stainless steel, the dynamic recovery proceeded slowly. This characteristic would induce a high dislocation density that stimulates the occurrence of dynamic recrystallization once a critical strain was exceeded [17], and meanwhile the flow stress decreased. Under some conditions, a steady state region is achieved in which the flow stress was observed to remain nearly constant with the increasing of strain. The steady flow stress occurs as the dynamic softening effect is sufficient to counteract the work-hardening effect of this alloy in the isothermal compression.
2. Experimental
Fig. 1. SEM micrographs of the solution treated superalloy GH4169 (a) procedure A, (b) procedure B.
K. Wang et al. / Materials Science and Engineering A 528 (2011) 4723–4731
700
a
10.0 s
Flow stress/MPa
800
1.0 s
700 600
b
600
Flow stress/MPa
900
4725
0.1 s
500 400 300
10.0 s
500 1.0 s
400 0.1 s
300 200
200 100
100 0 0.0
0.2
0.4
0.6
0 0.0
0.8
0.2
0.4
0.6
0.8
Strain
Strain 600
c
500
Flow stress/MPa
10.0 s
400 1.0 s
300 200
0.1 s
100 0.0
0.2
0.4
0.6
0.8
Strain Fig. 2. Flow stress–strain curves in the isothermal compression of superalloy GH4169 treated by procedure A at the deformation temperature of: (a) 1123 K, (b) 1223 K, (c) 1288 K.
800
700
a
b
600
Flow stress /MPa
Flow stress /MPa
700 600 500 400 300
500 400 300 200
200 100 100 0.0
0.2
0.4
0.6
0.0
0.8
0.2
0.4
600
0.6
0.8
strain
Strain
c
Flow stress /MPa
500 400 300 200 100 0 0.0
0.2
0.4
0.6
0.8
Strain Fig. 3. Flow stress–strain curves in the isothermal compression of superalloy GH4169 treated by procedure B at the deformation temperature of: (a) 1123 K, (b) 1223 K, (c) 1288 K.
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Peak flow stress/MPa
800
the nucleation rates of dynamic recrystallization. And the interfacial energy between the ı phase and the matrix could serve as the driving force for the dynamic recrystallization. However, the peak flow stress of superalloy GH4169 treated by procedure A is higher than that from procedure B at a deformation temperature of 1123 K and the strain rates of 0.1 and 10.0 s−1 . This phenomenon is possibly related to the fact that the strengthening phase or , which cannot be dissolved completely at the temperature of 1123 K, was precipitated in the heating process. Based on the above analysis, it can be concluded that the ı phase presented in the superalloy GH4169 promotes the flow softening.
procedure A procedure B
700 600
10.0 s
-1
500 1.0 s
400 300
-1
0.1 s
-1
200
3.2. Strain rate sensitivity exponent 1120
1160
1200
1240
1280
1320
Deformation temperature/K Fig. 4. Peak flow stresses in the isothermal compression of superalloy GH4169.
(2) The flow stress increases obviously with the increasing of strain rate at a certain deformation temperature, and decreases with the increasing of deformation temperature at a certain strain rate. The peak flow stress, strain corresponding to the peak flow stress (εp ) and steady flow stress show a decrease with the increasing of deformation temperature or decreasing of strain rate. This phenomenon is due to that the increase of deformation temperature stimulates the occurrence of the dynamic recrystallization which could accelerate the dislocation annihilation, inducing the decrease of deformation resisting force. The decrease of strain rate extends the deformation period which induces the dynamic recovery and recrystallization to proceed more sufficiently, meanwhile the increase of strain rate accelerates the dislocation moving speed, which increases the stress. (3) When the strain rate is up to 10.0 s−1 , the flow stress decreases sharply after a peak flow stress during the deformation of the solution treated superalloy GH4169, which possibly results from the increase of deformation temperature during hot deformation processes.
0.24
a
0.20
proceduce A proceduce B
0.16 0.12 0.08 0.04 0.00
1120
1160
∂ ln m= ∂ ln ε˙
1200
1240
1280
Deformation temperature/K
(1) ˙ ε,T
where m is the strain rate sensitivity exponent, is the flow stress (MPa), ε˙ is the strain rate (s−1 ), ε is the strain and T is the absolute deformation temperature (K). As seen from the Fig. 5, the strain rate sensitivity exponents have a similar characteristic between the two kinds of solution treated superalloy GH4169 at the strain of p and 0.5, i.e. the strain rate sensitivity exponents increase with the increasing of deformation temperature. Fig. 5 also shows that the effect of the ı phase on the strain rate sensitivity exponents at the strain of p and 0.5. The existence of ı phase makes the strain rate sensitivity exponents of the superalloy GH4169 treated by procedure A higher than that from procedure B at every deformation temperature. del Valle and Ruano [18] and Luo et al. [19] had respectively analyzed the effect of microstructure on the strain rate sensitivity exponent of Mg–Al–Zn alloy and Ti–6Al–4V alloy. Similarly, it is observed that the strain rate sensitivity exponent of superalloy GH4169 is closely related to the microstructure evolution in the isothermal compression of superalloy GH4169. As seen from the Figs. 6 and 7, the grain size of the two kinds of solution treated superalloy GH4169 decreases obviously with the increasing of deformation temperature. And the microstructure of the superalloy GH4169 treated by procedure A is much finer than that from procedure B at the same deformation temperature. Strain rate sensitivity exponent (m) represents the capacity of material to resist necking and affects the overall deformation and stability in high temperature deformation. It is believed that the finer microstructure can result in a more uniform deforma-
Strain rate sensitivity exponent
Strain rate sensitivity exponent
Because of the existence of ı phase, there are some different characteristics between the flow stress–strain curves of the two kinds of solution treated superalloy GH4169. Comparing the flow stress–strain curves in Figs. 2 and 3, it reveals that the flow stress of superalloy GH4169 treated by procedure A drops much earlier than that from procedure B. Fig. 4 suggests that the ı phase presented in superalloy GH4169 results in the decrease of the peak flow stress. This phenomenon indicates that the ı phase promotes the flow softening. The ı phase existing in isothermal compression could serve as the crystal nucleus of dynamic recrystallization which increases
Based on the flow stress data in isothermal compression of superalloy GH4169, the strain rate sensitivity exponent (m) at given strain and deformation temperature could be calculated in the following form.
0.24
b procedure A procedure B
0.20 0.16 0.12 0.08 0.04 0.00
1120
1160
1200
1240
1280
Deformation temperature/K
Fig. 5. Strain rate sensitivity exponent (m) in the isothermal compression of superalloy GH4169 at the strains of: (a) εp , (b) 0.5.
K. Wang et al. / Materials Science and Engineering A 528 (2011) 4723–4731
4727
Fig. 6. Typical microstructure in the isothermal compression of superalloy GH4169 treated by procedure A: (a) 1123 K, 1.0 s−1 , 60%, (b) 1223 K, 1.0 s−1 , 60%, (c) 1288 K, 1.0 s−1 , 60%.
Fig. 7. Typical microstructure in the isothermal compression of superalloy GH4169 treated by procedure B: (a) 1123 K, 1.0 s−1 , 60%, (b) 1223 K, 1.0 s−1 , 60%, (c) 1288 K, 1.0 s−1 , 60%.
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tion. Therefore, the decrease of grain size strengthens the stability of high temperature deformation and results in the increase of m value of superalloy GH4169. Some investigators [20] suggest that the strain rate sensitivity exponent of material is supposed to be determined by thermal activation of dislocation glide. It is well known that the cluster of dislocation could result in the stress concentration, which could reduce the stability of high temperature deformation. Figs. 6 and 7 show that the increase of deformation temperature and the existence of ı phase result in the decrease of grain size of superalloy GH4169. It reveals that the increase of deformation temperature and the existence of ı phase promote the dynamic recrystallization, which accelerates the dislocation annihilation and reduces the stress concentration. So the strain rate sensitivity exponent increases with the decreasing of grain size of superalloy GH4169. Some investigators [18,21] studied the effect of grain boundary sliding with the fine grain on the increasing of m value. This investigation observes the decreasing of grain size, however, further evidence is required to confirm the occurrence of grain boundary sliding of superalloy GH4169. 3.3. Strain hardening exponent
3.4. Kinetic analysis
The strain hardening exponent (n) results from a balance between hardening mechanisms and softening mechanisms, which depend mainly on strain and time respectively [20]. In present work, the strain hardening exponent is calculated in the following form n=
d log d log ε ε,T ˙
at the large strain. It can be concluded that, with the increasing of strain, the softening effect becomes more remarkable at a higher strain rate in the isothermal compression. There is a remarkable different characteristic of strain hardening exponents between the two kinds of solution treated superalloy GH4169. Fig. 8 shows that the n values of superalloy GH4169 treated by procedure A are lower than that from procedure B. And the n values of superalloy GH4169 treated by procedure A reach a negative value at small strain. This phenomenon suggests that the existence of ı phase can strengthen the softening effect in the isothermal compression of superalloy GH4169. Figs. 6 and 7 and Figs. 9 and 10 show that the grain size of superalloy GH4169 treated by procedure A is smaller than that from procedure B, which indicates that the ı phase promotes the occurrence of dynamic recrystallization. Therefore, the existence of ı phase strengthens the softening effect. However, the n values of superalloy GH4169 solution treated by procedure A are higher than that from procedure B in some deformation conditions. It is possibly because that the fine grain causes a strengthening effect on the superalloy GH4169 solution treated by procedure A.
(2)
where is the flow stress (MPa), ε is the strain, ε is the strain rate (s−1 ), and T is the absolute deformation temperature (K). Based on the flow stress data in the isothermal compression of superalloy GH4169, the strain hardening exponent (n) is calculated at a constant strain rate and deformation temperature. As seen from the Fig. 8, there is a same characteristic of the variation of strain hardening exponent with the strain of two kinds of solution treated superalloy GH4169. The n value decreases with the increasing of strain, and it reaches a negative value at small strain and high deformation temperature. It is can be concluded that the softening effect becomes stronger with the increasing of strain. And the high deformation temperature makes the softening mechanism a predominant mechanism at small strain in the isothermal compression. This softening phenomenon can be reflected in the flow stress–strain curves. As seen from the Figs. 2 and 3, the flow stress deceases with the increasing of strain, and the high deformation temperature makes the flow stress reach a peak value at small strain. Figs. 9 and 10 show the typical microstructure of the two kinds of solution treated superalloy GH4169 at different height reduction, which could reveal the effect of strain on the strain hardening exponent. The grain size of the two kinds of solution treated superalloy GH4169 deceases with the increasing of height reduction from 20% to 60%, which indicates that the dynamic recrystallization occurs in the isothermal compression. It is well known that the dynamic recrystallization plays an important role in softening mechanism during the hot deformation. The dynamic recrystallization can accelerate the dislocation annihilation, which could result in the dynamic softening. So the n value decreases with the increasing of strain. On the other hand, Fig. 8 also shows the variation of n values with the strain rate in the isothermal compression of the two kinds of solution treated superalloy GH4169. The n value increases with the increasing of strain rate during the early deformation period, i.e. at the small strain. However, the n value decreases more sharply at high strain rate, and becomes lower than that at lower strain rate during the latter deformation period, i.e.
The peak flow stress can be selected as the representative stress of each curve. Figs. 11 and 12 show the dependence of the peak flow stress on experimental conditions of the two kinds of solution treated superalloy GH4169, respectively. The relationship between the peak flow stress, strain rate and deformation temperature is generally expressed in the form of a constitutive equations derived by Sellars and Tegart [22]:
Q
ε˙ = A n1 exp −
(3)
RT
where ε˙ is the strain rate (s−1 ), Q is the apparent activation energy for deformation (J mol−1 ), is the peak flow stress (MPa), n1 is the stress exponent, T is the absolute deformation temperature (K), R is the gas constant (8.3145 J mol−1 K−1 ). According to the experimental result, the values of A, n1 , Q of superalloy GH4169 treated by procedure A can be calculated as: A = 5.165 × 10−4 , n1 = 8.91027, Q = 476136 J mol−1 . So the Eq. (3) can be expressed as follows.
476136
ε˙ = 5.165 × 10−4 8.91027 exp −
RT
(4)
and the values of A, n1 , Q of superalloy GH4169 treated by procedure B can be calculated as: A = 2.042 × 10−7 , n1 = 9.802, Q = 455434 J mol−1 . So the Eq. (3) can be expressed as follows.
455434
ε˙ = 2.042 × 10−7 9.802 exp −
RT
(5)
As seen from the Eq. (4) and (5), the apparent activation energy for deformation of the present investigation is similar with other previous investigations, such as 443.2 kJ mol−1 by Wang et al. [23], 485 kJ mol−1 by Garcia et al. [24]. And the apparent activation energy for deformation of superalloy GH4169 treated by procedure A is slightly higher than that from procedure B, well consistent with the work of Yuan and Liu [14]. The effect of deformation temperature and strain rate on the peak flow stress can be expressed by the Zener–Hollomon parameter (Z = ε˙ exp Q/RT ). The variations of peak flow stress with Zener–Hollomon parameter for the two kinds of solution treated superalloy GH4169 are plotted in Fig. 13. It can be seen that, for the two kinds of solution treated superalloy GH4169, the peak flow stress increases with the increasing of parameter Z.
0.3
a
0.3
Procedure A Procedure B
0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 0.2
0.3
0.4
0.5
0.6
Strain hardening exponent
Strain hardening exponent
K. Wang et al. / Materials Science and Engineering A 528 (2011) 4723–4731
b
Procedure A Procedure B
0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6
0.2
0.3
Strain hardening exponent
Strain 0.2
4729
0.4
0.5
0.6
Strain
c
Procedure A Procedure B
0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 0.2
0.3
0.4
0.5
0.6
Strain Fig. 8. Strain hardening exponent (n) in the isothermal compression of superalloy GH4169 at the deformation temperatures of: (a) 1123 K, (b) 1223 K, (c) 1288 K.
Fig. 9. Typical microstructure in the isothermal of superalloy GH4169 treated by procedure A: (a) 1288 K, 1.0 s−1 , 20.0%, (b) 1288 K, 1.0 s−1 , 60.0%.
Fig. 10. Typical microstructure in the isothermal of superalloy GH4169 treated by procedure B: (a) 1288 K, 1.0 s−1 , 20.0%, (b) 1288 K, 1.0 s−1 , 60.0%.
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6.8
ln(Peak flow stress/MPa)
ln(Peak flow stress/MPa)
6.6 6.4 6.2 6.0 5.8 1123K 1223K 1288K
5.6 5.4 -3
-2
-1
0
1
2
6.6 6.4 6.2 6.0 5.8 5.6 5.4 5.2 0.76
3
0.78
0.80
-1
0.82
0.84
0.86
0.88
0.90
1000/T/K
ln(Strain rate/s )
6.6
ln(Peak flow stress/MPa)
ln(Peak flow stress/MPa)
Fig. 11. Relationship between the peak flow stress and the deformation conditions in the isothermal compression of superalloy GH4169 treated by procedure A.
6.4 6.2 6.0 5.8
1123K 1223K 1288K
5.6 5.4 -3
-2
-1
0
1
2
6.6 6.4 6.2 6.0 5.8 5.6 5.4 0.76
3
0.78
0.80
0.82
-1
0.84
0.86
0.88
0.90
1000/T/K
ln(Strain rate/s )
Fig. 12. Relationship between the peak flow stress and the deformation conditions in the isothermal compression of superalloy GH4169 treated by procedure B.
6.8
6.6
a
ln(Peak flow stress/MPa)
ln(Peak flow stress/MPa)
6.8
6.4 6.2 6.0 5.8 5.6 5.4 5.2
42
44
46
48
50
52
54
6.6
b
6.4 6.2 6.0 5.8 5.6 5.4
40
42
44
46
48
50
52
lnZ
lnZ
Fig. 13. Relationship between the peak flow stress and the parameter Z for superalloy GH4169: (a) procedure A, (b) procedure B.
4. Conclusions The effect of ı phase on hot deformation behavior, including flow stress–strain curve, strain rate sensitivity exponent, strain hardening exponent and kinetic analysis, is investigated by isothermal compression of two kinds of solution treated superalloy GH4169. The following conclusions have been obtained from the present investigation. The flow stress–strain curves of superalloy GH4169 solution treated by procedure A and procedure B have some similar characteristics. The existence of ı phase results in the decrease of the flow stress, and makes the flow stress reach a peak value at small strain. The strain rate sensitivity exponents increase with the increasing of deformation temperature. The existence of ı phase results in the increase of the strain rate sensitivity exponent at a certain deformation temperature and strain.
The analysis of the strain hardening exponent (n) suggests that the dynamic softening is related to the deformation temperature, strain rate, and strain. Moreover, the existence of ı phase results in the decreasing of n values, and makes the n values reach a negative value at small strain. The apparent activation energy of superalloy GH4169 treated by procedure A was calculated as 476.136 kJ mol−1 , which was slightly higher than that of superalloy GH4169 (455.434 kJ mol−1 ) treated by procedure B. And the peak flow stress of the two kinds of solution treated superalloy GH4169 was observed to increase with the increasing of parameter Z. Acknowledgement This work was supported by the National Natural Science Foundation of China with grant no. 50975234.
K. Wang et al. / Materials Science and Engineering A 528 (2011) 4723–4731
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Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of the ı phase on the deformation behavior in isothermal compression of superalloy GH4169 K. Wang, M.Q. Li ∗ , J. Luo, C. Li School of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, PR China
a r t i c l e
i n f o
Article history: Received 18 January 2011 Received in revised form 22 February 2011 Accepted 23 February 2011 Available online 2 March 2011 Keywords: Flow stress Strain rate sensitivity exponent Strain hardening exponent Kinetic analysis Dynamic recrystallization
a b s t r a c t The effect of the ı phase on the deformation behavior, including flow stress–strain curve, strain rate sensitivity exponent, strain hardening exponent and kinetic analysis, was investigated by isothermal compression of superalloy GH4169 with two kinds of solution treatment. The experimental results show that the existence of ı phase results in the decreasing of flow stress, and makes the flow stress reach a peak value at small strain. The strain rate sensitivity exponent (m) increases with the increasing of deformation temperature. The existence of ı phase leads to the increasing of strain rate sensitivity exponent (m) at a certain deformation temperature and strain. Moreover, the strain hardening exponent (n) has a close relationship with the deformation temperature, strain rate and strain, especially the strain affects n more significantly. The existence of ı phase results in the decreasing of n values, and makes the n values reach a negative value at small strain. It was observed that the changes in strain rate sensitivity exponent and strain hardening exponent were closely related to the microstructural evolution in the deformation process. Based on the kinetic analysis of superalloy GH4169 with two kinds of solution treatment, the apparent activation energy of superalloy GH4169 containing ı phase was calculated to be 476.136 kJ mol−1 , which was slightly higher than that of superalloy GH4169 (455.434 kJ mol−1 ) without ı phase. And the peak flow stress in the isothermal compression of superalloy GH4169 was observed to increase with the increasing of parameter Z. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Superalloy GH4169 is a precipitation strengthened nickedbased superalloy, containing significant amounts of Fe, Nb and Mo, with minor contents of Al and Ti. The metastable bodycentered tetragonal coherent precipitate (Ni3 Nb) phase and the face-centered cubic coherent precipitate [Ni3 (Al, Ti)] phase are strengthening phase, and phase is the major strengthening phase. The equilibrium phase corresponding to phase is the orthorhombic incoherent ␦ (Ni3 Nb) phase [1,2]. Because of its excellent mechanical properties at elevated temperatures and good corrosion resistance, superalloy GH4169 is extensively used in gas turbines, rocket engines, turbine blades [3,4]. In order to meet the present requirements of favorable microstructure and properties, better control of forging processes for superalloy GH4169 is rather important. It is well known that the deformation behavior is strongly dependent on the processing parameters such as temperature, strain rate and strain. In the past decades, a number of hot torsion and hot compression experiments
∗ Corresponding author. Tel.: +86 29 88460328; fax: +86 29 88492642. E-mail address: [email protected] (M.Q. Li). 0921-5093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2011.02.081
were carried out to investigate the deformation behavior of Inconel 718 [5–7]. To describe the plastic flow performance of superalloy GH4169 in a form that can be used in computer code to model the forging process, some models describing the relationship between the flow stress and the processing parameters have been proposed. Moreover, it has been reported that dynamic recrystallization is the critical process in the high temperature deformation of Inconel 718 [8,9]. The ı phase presented in superalloy GH4169 plays an important role in controlling the microstructure and mechanical properties [10], which have been paid much attention. Recently, the effect of the ı phase on the high temperature notch sensitivity and impact toughness and grain size control in delta processed Inconel 718 have been studied [3], in which the results indicated that ␦ phase could form in the alloy in processing and in service processes. Wang et al. [11] also investigated the effect of ı phase on the deformation behavior of delta processed Inconel 718, in which the results showed that the ı phase could stimulate the occurrence of dynamic recrystallization. Cone [12] observed the evolution of ı phase in different ı phase precipitation cycles and rolling. Zhang et al. [13] studied the deformation characteristics of ı phase in the delta processed Inconel 718, in which the results showed that the dissolution of plate-like ı phase and the precipitation of spheri-
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Table 1 Chemical composition of superalloy GH4169 (wt %). Ni
Cr
Nb + Ta
Mo
Al
Ti
Fe
54.10
19.39
5.21–5.81
3.05
0.53
1.02
Bal.
cal ı phase particles coexisted in deformation process. Yuan and Liu [14] researched the effect of ı phase on the high temperature deformation behavior of Inconel 718, in which the results showed that ı phase affected the peak flow stress, strain corresponding to the peak flow stress (εp ) and apparent activation energy. Azadian et al. [15] studied the precipitation and dissolution kinetics of the ı phase. Cai et al. [16] investigated the dissolution kinetic of ı phase and the effect of ı phase on the notch sensitivity of Inconel 718. However, there are few systematical reports dealing with the effect of ı phase on the deformation behavior, including flow stress–strain curve, strain rate sensitivity exponent, strain hardening exponent and kinetic analysis. The shapes of stress–strain curves show some features that help in identifying the mechanisms of high temperature deformation, although not in a conclusive fashion. The strain rate sensitivity exponent (m) is related to the deformation mechanisms of material. On the other hand, the strain hardening exponent (n) reflecting the work-hardening of material is also an important parameter in plastic deformation. Therefore, it is necessary to carry out more work and deeper analysis to investigate the effect of ı phase on the hot deformation behavior. In this paper, two kinds of solution treated superalloy GH4169 were isothermally compressed. And the effect of ı phase on the flow stress–strain curve, strain rate sensitivity exponent, strain hardening exponent and kinetic analysis was studied systematically.
not be observed in the other specimens. Because the temperature of solution treatment is higher than the temperature for the or phase to be dissolved completely, two kinds of solution treated specimens almost contain no strengthening phase and . That is to say the effect of the strengthening phase and on the deformation behavior of superalloy GH4169 was eliminated in the experiments, approximately. So the effect of ı phase on the flow stress–strain curve, strain rate sensitivity exponent, strain hardening exponent and kinetic analysis could be investigated more clearly. 2.2. Isothermal compression A series of isothermal compressions were conducted in a Thermecmaster-Z simulator at deformation temperatures of 1123, 1223 and 1288 K, strain rates of 0.1, 1.0 and 10.0 s−1 , and height reductions of 20% and 60%. The specimens were heated and held for 3 min at the deformation temperature to establish a uniform temperature in the specimens. The flow stress–strain curves were recorded automatically in isothermal compression. After the compression, the specimens were cooled in air to room temperature. To observe the microstructural evolution, the isothermal compressed specimens were axially sectioned and prepared using standard metallographic techniques. 3. Results and discussion 3.1. Flow stress
2.1. Material and solution treatment
Figs. 2 and 3 show the flow stress–strain curves at deformation temperatures ranging from 1123 to 1288 K and strain rates ranging from 0.1 to 10.0 s−1 for superalloy GH4169 treated by procedure A and procedure B, respectively. Figs. 2 and 3 show some similar characteristics as follows.
The chemical composition of superalloy GH4169 used in the present investigation is shown in Table 1. The cylindrical compression specimens are 8.0 mm in diameter and 12.0 mm in height, and the cylindrical ends were grooved with 0.2 mm for retention of glass lubricants in the isothermal compression. In order to study the effect of ı phase on the deformation behavior of superalloy GH4169, the solution treatment prior to isothermal compression was conducted in the following two procedures: (A) heated to 1233 K and held for 30 min, (B) heated to 1293 K and held for 60 min. After the solution treatment, the specimens were cooled in water to room temperature, and the corresponding SEM micrographs of the two kinds of solution treated superalloy GH4169 are shown in Fig. 1. It can be seen from Fig. 1 that the specimens treated by procedure A have a large amount of ı phase in the austenitic matrix, which can
(1) The flow stress increases to a peak value with the increasing of strain, and then decreases with the further increasing of strain. The initial rapid rise of stress is associated with the increasing of dislocation density. In the alloys with low or intermediate stacking fault energy, e.g. nickel or stainless steel, the dynamic recovery proceeded slowly. This characteristic would induce a high dislocation density that stimulates the occurrence of dynamic recrystallization once a critical strain was exceeded [17], and meanwhile the flow stress decreased. Under some conditions, a steady state region is achieved in which the flow stress was observed to remain nearly constant with the increasing of strain. The steady flow stress occurs as the dynamic softening effect is sufficient to counteract the work-hardening effect of this alloy in the isothermal compression.
2. Experimental
Fig. 1. SEM micrographs of the solution treated superalloy GH4169 (a) procedure A, (b) procedure B.
K. Wang et al. / Materials Science and Engineering A 528 (2011) 4723–4731
700
a
10.0 s
Flow stress/MPa
800
1.0 s
700 600
b
600
Flow stress/MPa
900
4725
0.1 s
500 400 300
10.0 s
500 1.0 s
400 0.1 s
300 200
200 100
100 0 0.0
0.2
0.4
0.6
0 0.0
0.8
0.2
0.4
0.6
0.8
Strain
Strain 600
c
500
Flow stress/MPa
10.0 s
400 1.0 s
300 200
0.1 s
100 0.0
0.2
0.4
0.6
0.8
Strain Fig. 2. Flow stress–strain curves in the isothermal compression of superalloy GH4169 treated by procedure A at the deformation temperature of: (a) 1123 K, (b) 1223 K, (c) 1288 K.
800
700
a
b
600
Flow stress /MPa
Flow stress /MPa
700 600 500 400 300
500 400 300 200
200 100 100 0.0
0.2
0.4
0.6
0.0
0.8
0.2
0.4
600
0.6
0.8
strain
Strain
c
Flow stress /MPa
500 400 300 200 100 0 0.0
0.2
0.4
0.6
0.8
Strain Fig. 3. Flow stress–strain curves in the isothermal compression of superalloy GH4169 treated by procedure B at the deformation temperature of: (a) 1123 K, (b) 1223 K, (c) 1288 K.
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Peak flow stress/MPa
800
the nucleation rates of dynamic recrystallization. And the interfacial energy between the ı phase and the matrix could serve as the driving force for the dynamic recrystallization. However, the peak flow stress of superalloy GH4169 treated by procedure A is higher than that from procedure B at a deformation temperature of 1123 K and the strain rates of 0.1 and 10.0 s−1 . This phenomenon is possibly related to the fact that the strengthening phase or , which cannot be dissolved completely at the temperature of 1123 K, was precipitated in the heating process. Based on the above analysis, it can be concluded that the ı phase presented in the superalloy GH4169 promotes the flow softening.
procedure A procedure B
700 600
10.0 s
-1
500 1.0 s
400 300
-1
0.1 s
-1
200
3.2. Strain rate sensitivity exponent 1120
1160
1200
1240
1280
1320
Deformation temperature/K Fig. 4. Peak flow stresses in the isothermal compression of superalloy GH4169.
(2) The flow stress increases obviously with the increasing of strain rate at a certain deformation temperature, and decreases with the increasing of deformation temperature at a certain strain rate. The peak flow stress, strain corresponding to the peak flow stress (εp ) and steady flow stress show a decrease with the increasing of deformation temperature or decreasing of strain rate. This phenomenon is due to that the increase of deformation temperature stimulates the occurrence of the dynamic recrystallization which could accelerate the dislocation annihilation, inducing the decrease of deformation resisting force. The decrease of strain rate extends the deformation period which induces the dynamic recovery and recrystallization to proceed more sufficiently, meanwhile the increase of strain rate accelerates the dislocation moving speed, which increases the stress. (3) When the strain rate is up to 10.0 s−1 , the flow stress decreases sharply after a peak flow stress during the deformation of the solution treated superalloy GH4169, which possibly results from the increase of deformation temperature during hot deformation processes.
0.24
a
0.20
proceduce A proceduce B
0.16 0.12 0.08 0.04 0.00
1120
1160
∂ ln m= ∂ ln ε˙
1200
1240
1280
Deformation temperature/K
(1) ˙ ε,T
where m is the strain rate sensitivity exponent, is the flow stress (MPa), ε˙ is the strain rate (s−1 ), ε is the strain and T is the absolute deformation temperature (K). As seen from the Fig. 5, the strain rate sensitivity exponents have a similar characteristic between the two kinds of solution treated superalloy GH4169 at the strain of p and 0.5, i.e. the strain rate sensitivity exponents increase with the increasing of deformation temperature. Fig. 5 also shows that the effect of the ı phase on the strain rate sensitivity exponents at the strain of p and 0.5. The existence of ı phase makes the strain rate sensitivity exponents of the superalloy GH4169 treated by procedure A higher than that from procedure B at every deformation temperature. del Valle and Ruano [18] and Luo et al. [19] had respectively analyzed the effect of microstructure on the strain rate sensitivity exponent of Mg–Al–Zn alloy and Ti–6Al–4V alloy. Similarly, it is observed that the strain rate sensitivity exponent of superalloy GH4169 is closely related to the microstructure evolution in the isothermal compression of superalloy GH4169. As seen from the Figs. 6 and 7, the grain size of the two kinds of solution treated superalloy GH4169 decreases obviously with the increasing of deformation temperature. And the microstructure of the superalloy GH4169 treated by procedure A is much finer than that from procedure B at the same deformation temperature. Strain rate sensitivity exponent (m) represents the capacity of material to resist necking and affects the overall deformation and stability in high temperature deformation. It is believed that the finer microstructure can result in a more uniform deforma-
Strain rate sensitivity exponent
Strain rate sensitivity exponent
Because of the existence of ı phase, there are some different characteristics between the flow stress–strain curves of the two kinds of solution treated superalloy GH4169. Comparing the flow stress–strain curves in Figs. 2 and 3, it reveals that the flow stress of superalloy GH4169 treated by procedure A drops much earlier than that from procedure B. Fig. 4 suggests that the ı phase presented in superalloy GH4169 results in the decrease of the peak flow stress. This phenomenon indicates that the ı phase promotes the flow softening. The ı phase existing in isothermal compression could serve as the crystal nucleus of dynamic recrystallization which increases
Based on the flow stress data in isothermal compression of superalloy GH4169, the strain rate sensitivity exponent (m) at given strain and deformation temperature could be calculated in the following form.
0.24
b procedure A procedure B
0.20 0.16 0.12 0.08 0.04 0.00
1120
1160
1200
1240
1280
Deformation temperature/K
Fig. 5. Strain rate sensitivity exponent (m) in the isothermal compression of superalloy GH4169 at the strains of: (a) εp , (b) 0.5.
K. Wang et al. / Materials Science and Engineering A 528 (2011) 4723–4731
4727
Fig. 6. Typical microstructure in the isothermal compression of superalloy GH4169 treated by procedure A: (a) 1123 K, 1.0 s−1 , 60%, (b) 1223 K, 1.0 s−1 , 60%, (c) 1288 K, 1.0 s−1 , 60%.
Fig. 7. Typical microstructure in the isothermal compression of superalloy GH4169 treated by procedure B: (a) 1123 K, 1.0 s−1 , 60%, (b) 1223 K, 1.0 s−1 , 60%, (c) 1288 K, 1.0 s−1 , 60%.
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tion. Therefore, the decrease of grain size strengthens the stability of high temperature deformation and results in the increase of m value of superalloy GH4169. Some investigators [20] suggest that the strain rate sensitivity exponent of material is supposed to be determined by thermal activation of dislocation glide. It is well known that the cluster of dislocation could result in the stress concentration, which could reduce the stability of high temperature deformation. Figs. 6 and 7 show that the increase of deformation temperature and the existence of ı phase result in the decrease of grain size of superalloy GH4169. It reveals that the increase of deformation temperature and the existence of ı phase promote the dynamic recrystallization, which accelerates the dislocation annihilation and reduces the stress concentration. So the strain rate sensitivity exponent increases with the decreasing of grain size of superalloy GH4169. Some investigators [18,21] studied the effect of grain boundary sliding with the fine grain on the increasing of m value. This investigation observes the decreasing of grain size, however, further evidence is required to confirm the occurrence of grain boundary sliding of superalloy GH4169. 3.3. Strain hardening exponent
3.4. Kinetic analysis
The strain hardening exponent (n) results from a balance between hardening mechanisms and softening mechanisms, which depend mainly on strain and time respectively [20]. In present work, the strain hardening exponent is calculated in the following form n=
d log d log ε ε,T ˙
at the large strain. It can be concluded that, with the increasing of strain, the softening effect becomes more remarkable at a higher strain rate in the isothermal compression. There is a remarkable different characteristic of strain hardening exponents between the two kinds of solution treated superalloy GH4169. Fig. 8 shows that the n values of superalloy GH4169 treated by procedure A are lower than that from procedure B. And the n values of superalloy GH4169 treated by procedure A reach a negative value at small strain. This phenomenon suggests that the existence of ı phase can strengthen the softening effect in the isothermal compression of superalloy GH4169. Figs. 6 and 7 and Figs. 9 and 10 show that the grain size of superalloy GH4169 treated by procedure A is smaller than that from procedure B, which indicates that the ı phase promotes the occurrence of dynamic recrystallization. Therefore, the existence of ı phase strengthens the softening effect. However, the n values of superalloy GH4169 solution treated by procedure A are higher than that from procedure B in some deformation conditions. It is possibly because that the fine grain causes a strengthening effect on the superalloy GH4169 solution treated by procedure A.
(2)
where is the flow stress (MPa), ε is the strain, ε is the strain rate (s−1 ), and T is the absolute deformation temperature (K). Based on the flow stress data in the isothermal compression of superalloy GH4169, the strain hardening exponent (n) is calculated at a constant strain rate and deformation temperature. As seen from the Fig. 8, there is a same characteristic of the variation of strain hardening exponent with the strain of two kinds of solution treated superalloy GH4169. The n value decreases with the increasing of strain, and it reaches a negative value at small strain and high deformation temperature. It is can be concluded that the softening effect becomes stronger with the increasing of strain. And the high deformation temperature makes the softening mechanism a predominant mechanism at small strain in the isothermal compression. This softening phenomenon can be reflected in the flow stress–strain curves. As seen from the Figs. 2 and 3, the flow stress deceases with the increasing of strain, and the high deformation temperature makes the flow stress reach a peak value at small strain. Figs. 9 and 10 show the typical microstructure of the two kinds of solution treated superalloy GH4169 at different height reduction, which could reveal the effect of strain on the strain hardening exponent. The grain size of the two kinds of solution treated superalloy GH4169 deceases with the increasing of height reduction from 20% to 60%, which indicates that the dynamic recrystallization occurs in the isothermal compression. It is well known that the dynamic recrystallization plays an important role in softening mechanism during the hot deformation. The dynamic recrystallization can accelerate the dislocation annihilation, which could result in the dynamic softening. So the n value decreases with the increasing of strain. On the other hand, Fig. 8 also shows the variation of n values with the strain rate in the isothermal compression of the two kinds of solution treated superalloy GH4169. The n value increases with the increasing of strain rate during the early deformation period, i.e. at the small strain. However, the n value decreases more sharply at high strain rate, and becomes lower than that at lower strain rate during the latter deformation period, i.e.
The peak flow stress can be selected as the representative stress of each curve. Figs. 11 and 12 show the dependence of the peak flow stress on experimental conditions of the two kinds of solution treated superalloy GH4169, respectively. The relationship between the peak flow stress, strain rate and deformation temperature is generally expressed in the form of a constitutive equations derived by Sellars and Tegart [22]:
Q
ε˙ = A n1 exp −
(3)
RT
where ε˙ is the strain rate (s−1 ), Q is the apparent activation energy for deformation (J mol−1 ), is the peak flow stress (MPa), n1 is the stress exponent, T is the absolute deformation temperature (K), R is the gas constant (8.3145 J mol−1 K−1 ). According to the experimental result, the values of A, n1 , Q of superalloy GH4169 treated by procedure A can be calculated as: A = 5.165 × 10−4 , n1 = 8.91027, Q = 476136 J mol−1 . So the Eq. (3) can be expressed as follows.
476136
ε˙ = 5.165 × 10−4 8.91027 exp −
RT
(4)
and the values of A, n1 , Q of superalloy GH4169 treated by procedure B can be calculated as: A = 2.042 × 10−7 , n1 = 9.802, Q = 455434 J mol−1 . So the Eq. (3) can be expressed as follows.
455434
ε˙ = 2.042 × 10−7 9.802 exp −
RT
(5)
As seen from the Eq. (4) and (5), the apparent activation energy for deformation of the present investigation is similar with other previous investigations, such as 443.2 kJ mol−1 by Wang et al. [23], 485 kJ mol−1 by Garcia et al. [24]. And the apparent activation energy for deformation of superalloy GH4169 treated by procedure A is slightly higher than that from procedure B, well consistent with the work of Yuan and Liu [14]. The effect of deformation temperature and strain rate on the peak flow stress can be expressed by the Zener–Hollomon parameter (Z = ε˙ exp Q/RT ). The variations of peak flow stress with Zener–Hollomon parameter for the two kinds of solution treated superalloy GH4169 are plotted in Fig. 13. It can be seen that, for the two kinds of solution treated superalloy GH4169, the peak flow stress increases with the increasing of parameter Z.
0.3
a
0.3
Procedure A Procedure B
0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 0.2
0.3
0.4
0.5
0.6
Strain hardening exponent
Strain hardening exponent
K. Wang et al. / Materials Science and Engineering A 528 (2011) 4723–4731
b
Procedure A Procedure B
0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -0.6
0.2
0.3
Strain hardening exponent
Strain 0.2
4729
0.4
0.5
0.6
Strain
c
Procedure A Procedure B
0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 0.2
0.3
0.4
0.5
0.6
Strain Fig. 8. Strain hardening exponent (n) in the isothermal compression of superalloy GH4169 at the deformation temperatures of: (a) 1123 K, (b) 1223 K, (c) 1288 K.
Fig. 9. Typical microstructure in the isothermal of superalloy GH4169 treated by procedure A: (a) 1288 K, 1.0 s−1 , 20.0%, (b) 1288 K, 1.0 s−1 , 60.0%.
Fig. 10. Typical microstructure in the isothermal of superalloy GH4169 treated by procedure B: (a) 1288 K, 1.0 s−1 , 20.0%, (b) 1288 K, 1.0 s−1 , 60.0%.
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6.8
ln(Peak flow stress/MPa)
ln(Peak flow stress/MPa)
6.6 6.4 6.2 6.0 5.8 1123K 1223K 1288K
5.6 5.4 -3
-2
-1
0
1
2
6.6 6.4 6.2 6.0 5.8 5.6 5.4 5.2 0.76
3
0.78
0.80
-1
0.82
0.84
0.86
0.88
0.90
1000/T/K
ln(Strain rate/s )
6.6
ln(Peak flow stress/MPa)
ln(Peak flow stress/MPa)
Fig. 11. Relationship between the peak flow stress and the deformation conditions in the isothermal compression of superalloy GH4169 treated by procedure A.
6.4 6.2 6.0 5.8
1123K 1223K 1288K
5.6 5.4 -3
-2
-1
0
1
2
6.6 6.4 6.2 6.0 5.8 5.6 5.4 0.76
3
0.78
0.80
0.82
-1
0.84
0.86
0.88
0.90
1000/T/K
ln(Strain rate/s )
Fig. 12. Relationship between the peak flow stress and the deformation conditions in the isothermal compression of superalloy GH4169 treated by procedure B.
6.8
6.6
a
ln(Peak flow stress/MPa)
ln(Peak flow stress/MPa)
6.8
6.4 6.2 6.0 5.8 5.6 5.4 5.2
42
44
46
48
50
52
54
6.6
b
6.4 6.2 6.0 5.8 5.6 5.4
40
42
44
46
48
50
52
lnZ
lnZ
Fig. 13. Relationship between the peak flow stress and the parameter Z for superalloy GH4169: (a) procedure A, (b) procedure B.
4. Conclusions The effect of ı phase on hot deformation behavior, including flow stress–strain curve, strain rate sensitivity exponent, strain hardening exponent and kinetic analysis, is investigated by isothermal compression of two kinds of solution treated superalloy GH4169. The following conclusions have been obtained from the present investigation. The flow stress–strain curves of superalloy GH4169 solution treated by procedure A and procedure B have some similar characteristics. The existence of ı phase results in the decrease of the flow stress, and makes the flow stress reach a peak value at small strain. The strain rate sensitivity exponents increase with the increasing of deformation temperature. The existence of ı phase results in the increase of the strain rate sensitivity exponent at a certain deformation temperature and strain.
The analysis of the strain hardening exponent (n) suggests that the dynamic softening is related to the deformation temperature, strain rate, and strain. Moreover, the existence of ı phase results in the decreasing of n values, and makes the n values reach a negative value at small strain. The apparent activation energy of superalloy GH4169 treated by procedure A was calculated as 476.136 kJ mol−1 , which was slightly higher than that of superalloy GH4169 (455.434 kJ mol−1 ) treated by procedure B. And the peak flow stress of the two kinds of solution treated superalloy GH4169 was observed to increase with the increasing of parameter Z. Acknowledgement This work was supported by the National Natural Science Foundation of China with grant no. 50975234.
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