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Superplastic properties of Inconel 718
Journal of Materials Processing Technology 137 (2003) 17–20
Superplastic properties of Inconel 718 Han Xuea,*, Wu Lijuna, Xia Huia, Liu Runguangb, Wang Shaogangc, Chen Zhonglinc a
b
General Research Institute for Non-ferrous Metals, Harbin 100088, PR China Harbin Institute of Technology, Harbin 150001, Heilongiang Province, PR China c Shenyang Liming Engine Manufacturing Company, Harbin 110043, PR China
Abstract The superplastic tensile test of Inconel 718 was carried out using cylinder-like specimens, the flow stress, the strain rate and the value of m under different strain rates and different deformation temperature being examined. Excellent elongation (467%) and high flow stress (41.3 MPa) were gained in the superplastic tensile test of Inconel 718 refined by pre-treatment, i.e., at a temperature of 1000 8C and strain rate of 1:14 104 s1. At the same time, Inconel 718 showed good superplasticity when the strain rate and deformation temperature were changed in the range 104 to 101 s1 and 940–1020 8C, respectively. Additionally, the superplastic properties were explained through observation of the microstructure. # 2002 Published by Elsevier Science B.V. Keywords: Inconel 718; Superplasticity; Flow stress; Strain rate; Value of m
1. Introduction Inconel alloy 718 has been used widely in the aviation, space navigation and shipping industries because of its outstanding multi-properties. However, it is very hard to form Inconel 718 precisely, such as for blades, by using routine forging methods because of its narrow forging temperature range. Some experiments [1,2] have been carried out, for example the superplastic tensile test with a sheet specimen, the superplastic forming test, etc. However, there are some limitations in the research of the conditions of superplastic deformation in these tests, such as the determination of the deformation temperature. The task of this work is to secure an overall view about the conditions of superplastic deformation and the mechanical properties of Inconel 718, and to explain these phenomena through the observation of the microstructure.
ASTM was achieved through multi-heat-treatment. An optical micrograph is shown in Fig. 1 and the tensile test specimen is shown in Fig. 2. Some un-pretreated specimens were prepared to compare with the above specimen. To prevent oxidation, the surface of the specimens was coated with a defending lubricant of glass and the experiments were carried out in argon gas. The value of strain-rate sensitivity was measured by the method of Backfen [3]. Then, the variation of the microstructure, i.e., the grain size and the distribution of the second phase under different tensile conditions, was examined by SEM, and the dislocation structure in the deformation areas was observed by TEM.
3. Results and discussion 3.1. The change of flow stress s, elongation d and strain rate sensitivity m with strain rate
2. Experiment The chemical composition of Inconel 718 is as follows: C 0.20%, Cr 17.00%, Mo 2.80%, Nb þ Te 5.00%, Al 0.70%, Si 0:35%, Co 1:00%. The fine grain (12 grade) of * Corresponding author. Present address: Harbin Institute of Technology, Harbin 150001, Heilongiang Province, PR China.
0924-0136/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 1 0 5 5 - 5
In the test, temperature T was set at 1000 8C and the specimen was tensioned at various selected strain rates. The results are shown in Fig. 3. The elongation decreases monotonously as the strain rate increases. In the curve of tensile strain rate, the phenomenon of a low at two sides and a high at the middle did not appear, and the curve also looked smoother than that of high-plasticity materials.
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Fig. 1. The grain state of the tensile specimen.
Fig. 2. The tensile specimen (dimensions: mm).
The changing tendency of the value of m was similar to that of the elongation d when the strain rate was limited to within 104 to 103 s1. The m value of Inconel 718 was lower than 0.4, which was smaller than that of superplastic materials [4]. However, Inconel 718 still showed good superplasticity. In the TEM photo of 130% deformed specimen, there were some bands of grain boundaries (Fig. 4(a)). This meant that at the beginning of the superplastic deformation the slip
Fig. 4. TEM micrograph of deformed Inconel 718: (a) bands of grain boundary, d ¼ 130%; (b) dislocations near to the precipitation phase, fracture (e ¼ 2:78 104 s1, T ¼ 980 8C).
of grain boundaries and diffusion made some effects, while as the deformation progressed, the dislocation density increased, and produced a great deal of dislocations in the crystal. The tangle of dislocations near to the second phase shown in Fig. 4(b) proved the emergence and the movement of the dislocations. The continuous proliferation and slip towards the grain boundaries of the dislocations resulted in the distinct slip of crystals. If the strain rate was increased, the resistance of the dislocation movement would be much greater, so that the flow stress would increase. Dislocation proliferation and slip acted as an important mechanism in the superplastic deformation of Inconel 718 alloy, which might be the answer to why the value of m was very small in the superplastic deformation [2]. 3.2. The change of flow stress s and elongation d with deformation temperature
Fig. 3. The change of flow stress, elongation and m value with strain rate (~, ~) elongation; (*, *) flow stress; (solid line) superplastic state; (dashed line) original state.
In the test, the strain rate was 2:78 104 s1 and the specimens were tensioned at a series of temperatures from 940 to 1020 8C (Fig. 5).As shown in Fig. 5, the flow stress declined monotonously as the temperature increased, which resulted from reduction of the austenite’s strength. However, after 980 8C the declining speed of the flow stress became slow as the deformation temperature increased, which was just at the solutionizing temperature of the d phase of Inconel 718. The phenomenon probably related to the soluting of phase d at high temperature, which accelerated the growth of grain by decreasing the resistance of the
H. Xue et al. / Journal of Materials Processing Technology 137 (2003) 17–20
19
Fig. 5. The change of flow stress and elongation with deformation temperature (~) elongation; (*) flow stress, e ¼ 2:78 104 s1. Fig. 8. The tensile specimens of GH4169 alloy after superplastic elongation: (a) before tension; (b) in the original state; (c) in the pre-treated state.
Inconel 718 showed a good plasticity over a broad temperature range from 940 to 1020 8C. Through the grain size stability test of Inconel 718, Ma and Langdon [1] selected 954 8C as the deformation temperature of Alloy 718 to study its superplasticity. However, from the tensile test, the optimum temperature of superplastic deformation was 990 8C, at the turning point for grains to grow rapidly.
4 1
Fig. 6. Relationship between grain size and T (e ¼ 2:78 10 T ¼ 1000 8C).
s ,
migration of the grain boundary. This could be confirmed from Figs. 6 and 7. Elongation d increased as the temperature increased and the top elongation d was close to 400%. The top elongation took place at 990 8C, with the strain rate of 2:67 104 s1.
3.3. Effect of pre-treatment on elongation d and flow stress s It can be seen from Fig. 3 that under the same deformation conditions, the elongation d of Inconel 718 tensioned in the non-pre-treated state was far less than that of the pre-treated specimen, whereas the difference of the flow stress between the two specimens was not as large. This could mean that it was not the slip of the grain boundaries that acts as the main mechanism in the superplastic deformation of Inconel 718. Fig. 8 shows that at the initial stage of the tensile test of Inconel 718 in the original state, as the strain increased, shrinking developed quickly at a certain places, where the specimen ultimately broke. Thus, its elongation was below 150%. However, the pre-treated tensile specimen exhibited the ability to resist shrinking during the whole deformation course. The specimen could be deformed uniformly to a certain degree (200–300%) and good elongation was obtained, depending on the sluggish evolution of shrinking at certain places. From this it can be concluded that, no matter which deformation mechanism acted as the main rule, fine grains were advantageous in achieving homogeneous deformation.
4. Conclusions
Fig. 7. SEM micrograph of Inconel 718 under different conditions: (a) 940 8C; (b)1240 8C (e ¼ 2:78 104 s1).
1. Inconel 718 shows good superplasticity when the strain rate and deformation temperature change in the range 104 to 101 s1 and 940–1020 8C, respectively. An elongation of 467% and a stress of 41.3 MPa were obtained in a tensile test at 1000 8C and a strain rate of 1:14 104 s1.
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2. The strain rate curve of Inconel 718 is different from that of typical superplastic materials. This is brought about by the deformation mechanics based on dislocation proliferation and movement. 3. In the superplastic deformation, either it is the slip of the grain boundary or the dislocation proliferation that dominates. Fine grains can decrease the flow stress, enhance the homogeneous deformation ability of materials, and increase the plasticity.
References [1] Y. Ma, T.G. Langdon, Superalloy 1992 (1992) 43–52. [2] M.W. Mahoney, R. Crookds, Superplasticity in Aerospace (1988) 331–344. [3] H. Jingsu, W. Yanwen, Superplasticity of Metal, The Press of Science, Beijing, 1986. [4] O.A. Kaibyshev, Elasticity and Superplasticity of Metal, The Press of Mechanic Industry, 1982 translated by Wang Yanwen.
Superplastic properties of Inconel 718 Han Xuea,*, Wu Lijuna, Xia Huia, Liu Runguangb, Wang Shaogangc, Chen Zhonglinc a
b
General Research Institute for Non-ferrous Metals, Harbin 100088, PR China Harbin Institute of Technology, Harbin 150001, Heilongiang Province, PR China c Shenyang Liming Engine Manufacturing Company, Harbin 110043, PR China
Abstract The superplastic tensile test of Inconel 718 was carried out using cylinder-like specimens, the flow stress, the strain rate and the value of m under different strain rates and different deformation temperature being examined. Excellent elongation (467%) and high flow stress (41.3 MPa) were gained in the superplastic tensile test of Inconel 718 refined by pre-treatment, i.e., at a temperature of 1000 8C and strain rate of 1:14 104 s1. At the same time, Inconel 718 showed good superplasticity when the strain rate and deformation temperature were changed in the range 104 to 101 s1 and 940–1020 8C, respectively. Additionally, the superplastic properties were explained through observation of the microstructure. # 2002 Published by Elsevier Science B.V. Keywords: Inconel 718; Superplasticity; Flow stress; Strain rate; Value of m
1. Introduction Inconel alloy 718 has been used widely in the aviation, space navigation and shipping industries because of its outstanding multi-properties. However, it is very hard to form Inconel 718 precisely, such as for blades, by using routine forging methods because of its narrow forging temperature range. Some experiments [1,2] have been carried out, for example the superplastic tensile test with a sheet specimen, the superplastic forming test, etc. However, there are some limitations in the research of the conditions of superplastic deformation in these tests, such as the determination of the deformation temperature. The task of this work is to secure an overall view about the conditions of superplastic deformation and the mechanical properties of Inconel 718, and to explain these phenomena through the observation of the microstructure.
ASTM was achieved through multi-heat-treatment. An optical micrograph is shown in Fig. 1 and the tensile test specimen is shown in Fig. 2. Some un-pretreated specimens were prepared to compare with the above specimen. To prevent oxidation, the surface of the specimens was coated with a defending lubricant of glass and the experiments were carried out in argon gas. The value of strain-rate sensitivity was measured by the method of Backfen [3]. Then, the variation of the microstructure, i.e., the grain size and the distribution of the second phase under different tensile conditions, was examined by SEM, and the dislocation structure in the deformation areas was observed by TEM.
3. Results and discussion 3.1. The change of flow stress s, elongation d and strain rate sensitivity m with strain rate
2. Experiment The chemical composition of Inconel 718 is as follows: C 0.20%, Cr 17.00%, Mo 2.80%, Nb þ Te 5.00%, Al 0.70%, Si 0:35%, Co 1:00%. The fine grain (12 grade) of * Corresponding author. Present address: Harbin Institute of Technology, Harbin 150001, Heilongiang Province, PR China.
0924-0136/02/$ – see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 1 0 5 5 - 5
In the test, temperature T was set at 1000 8C and the specimen was tensioned at various selected strain rates. The results are shown in Fig. 3. The elongation decreases monotonously as the strain rate increases. In the curve of tensile strain rate, the phenomenon of a low at two sides and a high at the middle did not appear, and the curve also looked smoother than that of high-plasticity materials.
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Fig. 1. The grain state of the tensile specimen.
Fig. 2. The tensile specimen (dimensions: mm).
The changing tendency of the value of m was similar to that of the elongation d when the strain rate was limited to within 104 to 103 s1. The m value of Inconel 718 was lower than 0.4, which was smaller than that of superplastic materials [4]. However, Inconel 718 still showed good superplasticity. In the TEM photo of 130% deformed specimen, there were some bands of grain boundaries (Fig. 4(a)). This meant that at the beginning of the superplastic deformation the slip
Fig. 4. TEM micrograph of deformed Inconel 718: (a) bands of grain boundary, d ¼ 130%; (b) dislocations near to the precipitation phase, fracture (e ¼ 2:78 104 s1, T ¼ 980 8C).
of grain boundaries and diffusion made some effects, while as the deformation progressed, the dislocation density increased, and produced a great deal of dislocations in the crystal. The tangle of dislocations near to the second phase shown in Fig. 4(b) proved the emergence and the movement of the dislocations. The continuous proliferation and slip towards the grain boundaries of the dislocations resulted in the distinct slip of crystals. If the strain rate was increased, the resistance of the dislocation movement would be much greater, so that the flow stress would increase. Dislocation proliferation and slip acted as an important mechanism in the superplastic deformation of Inconel 718 alloy, which might be the answer to why the value of m was very small in the superplastic deformation [2]. 3.2. The change of flow stress s and elongation d with deformation temperature
Fig. 3. The change of flow stress, elongation and m value with strain rate (~, ~) elongation; (*, *) flow stress; (solid line) superplastic state; (dashed line) original state.
In the test, the strain rate was 2:78 104 s1 and the specimens were tensioned at a series of temperatures from 940 to 1020 8C (Fig. 5).As shown in Fig. 5, the flow stress declined monotonously as the temperature increased, which resulted from reduction of the austenite’s strength. However, after 980 8C the declining speed of the flow stress became slow as the deformation temperature increased, which was just at the solutionizing temperature of the d phase of Inconel 718. The phenomenon probably related to the soluting of phase d at high temperature, which accelerated the growth of grain by decreasing the resistance of the
H. Xue et al. / Journal of Materials Processing Technology 137 (2003) 17–20
19
Fig. 5. The change of flow stress and elongation with deformation temperature (~) elongation; (*) flow stress, e ¼ 2:78 104 s1. Fig. 8. The tensile specimens of GH4169 alloy after superplastic elongation: (a) before tension; (b) in the original state; (c) in the pre-treated state.
Inconel 718 showed a good plasticity over a broad temperature range from 940 to 1020 8C. Through the grain size stability test of Inconel 718, Ma and Langdon [1] selected 954 8C as the deformation temperature of Alloy 718 to study its superplasticity. However, from the tensile test, the optimum temperature of superplastic deformation was 990 8C, at the turning point for grains to grow rapidly.
4 1
Fig. 6. Relationship between grain size and T (e ¼ 2:78 10 T ¼ 1000 8C).
s ,
migration of the grain boundary. This could be confirmed from Figs. 6 and 7. Elongation d increased as the temperature increased and the top elongation d was close to 400%. The top elongation took place at 990 8C, with the strain rate of 2:67 104 s1.
3.3. Effect of pre-treatment on elongation d and flow stress s It can be seen from Fig. 3 that under the same deformation conditions, the elongation d of Inconel 718 tensioned in the non-pre-treated state was far less than that of the pre-treated specimen, whereas the difference of the flow stress between the two specimens was not as large. This could mean that it was not the slip of the grain boundaries that acts as the main mechanism in the superplastic deformation of Inconel 718. Fig. 8 shows that at the initial stage of the tensile test of Inconel 718 in the original state, as the strain increased, shrinking developed quickly at a certain places, where the specimen ultimately broke. Thus, its elongation was below 150%. However, the pre-treated tensile specimen exhibited the ability to resist shrinking during the whole deformation course. The specimen could be deformed uniformly to a certain degree (200–300%) and good elongation was obtained, depending on the sluggish evolution of shrinking at certain places. From this it can be concluded that, no matter which deformation mechanism acted as the main rule, fine grains were advantageous in achieving homogeneous deformation.
4. Conclusions
Fig. 7. SEM micrograph of Inconel 718 under different conditions: (a) 940 8C; (b)1240 8C (e ¼ 2:78 104 s1).
1. Inconel 718 shows good superplasticity when the strain rate and deformation temperature change in the range 104 to 101 s1 and 940–1020 8C, respectively. An elongation of 467% and a stress of 41.3 MPa were obtained in a tensile test at 1000 8C and a strain rate of 1:14 104 s1.
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2. The strain rate curve of Inconel 718 is different from that of typical superplastic materials. This is brought about by the deformation mechanics based on dislocation proliferation and movement. 3. In the superplastic deformation, either it is the slip of the grain boundary or the dislocation proliferation that dominates. Fine grains can decrease the flow stress, enhance the homogeneous deformation ability of materials, and increase the plasticity.
References [1] Y. Ma, T.G. Langdon, Superalloy 1992 (1992) 43–52. [2] M.W. Mahoney, R. Crookds, Superplasticity in Aerospace (1988) 331–344. [3] H. Jingsu, W. Yanwen, Superplasticity of Metal, The Press of Science, Beijing, 1986. [4] O.A. Kaibyshev, Elasticity and Superplasticity of Metal, The Press of Mechanic Industry, 1982 translated by Wang Yanwen.