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Study of Mechanical Properties and Microstructural Analysis for Inconel Alloy sheet at Elevated Temperature
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ScienceDirect Materials Today: Proceedings 5 (2018) 18016–18023
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ICMPC_2018
Study of Mechanical Properties and Microstructural Analysis for Inconel Alloy sheet at Elevated Temperature Gauri Mahallea, Nitin Kotkundea*, Amit Kumar Guptaa, Swadesh Kumar Singhb a
b
Department of Mechanical Engineering, BITS Pilani, Hyderabad, Telangana, 500078, India Department of Mechanical Engineering, Gokaraju Rangaraju Institute of Engineering and Technology, Hyderabad, Telangana, 500090, India
Abstract A reliable and accurate prediction of material properties and flow behavior of metals considering the coupled effects of strain, strain rate and temperature is a crucial parameter for optimizing the workability. In this study, hot uniaxial tensile tests have been performed on Inconel 718 alloy from Room Temperature (RT) to 6000C at an interval of 2000C and strain rates from 0.1 s-1 to 0.0001 s-1. Generally, the anisotropy of rolled sheet influences the material properties, material properties have been determined with respect to rolling direction (RD), Normal direction (ND) and transverse direction (TD). The various material properties such as yield stress (σy), ultimate tensile strength (σu), total elongation and strain hardening exponent (n) have been evaluated over the range of temperatures and strain rates. Considerable variation in the material properties have been noticed with respect to temperature than strain rate. Furthermore, microstructure analysis has been carried out using optical microscopy and Scanning Electron Microscope (SEM). SEM micrographs with EDS analysis of fracture specimens revealed carbides with the nonhomogeneous distribution of large number of δ precipitates. SEM study confirms dimples and shear band which indicate predominantly ductile fracture in all the cases. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords: Inconel 718 alloy, Uniaxial tensile test, Material Properties, Microstructure Analysis
* Corresponding author. Tel.: +91 9010451444; fax: +91 40 66303998. E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.
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1. Introduction Inconel 718 is a precipitation hardening nickel chromium iron-based superalloy developed by H. L. Eiselstein of International Nickel Company [1]. Inconel 718 has a FCC microstructure and is a polycrystalline Ni-base wrought superalloy containing significant amount of Fe, Nb and Mb with lesser amount of Ti and Al that is manufactured by conventional melting and casting techniques. Inconel 718 exhibits of excellent mechanical properties, corrosion resistance and high strength with outstanding weldability, including resistance to post-weld cracking at elevated temperatures up to 700°C. The alloys have excellent creep-rupture strength and oxidation resistant to 982 0C because of Chromium and molybdenum contents [2]. As a result, Inconel 718 is suitable for manufacturing pressure vessel to store high pressure oxygen for aerospace missions [3] and also used for outer casings of Ni-H2 cells to store energy in satellite power system [4]. Moreover, Inconel 718 alloy is significantly used in aerospace applications include rotor blades, wheel and bolts for gas turbines, jet and rocket engine thrust chambers, hot air ducting systems, thrust reversers for airframes. Even many forming tools, extrusion dies and fixtures used in heattreating are made of Inconel alloy [5]. Due to continuous work hardening ability of ϒ’ strengthening precipitate like nickel, columbium, titanium and aluminum in IN718 alloys, ductility is very high at low strains also during uniaxial deformation. The deformation mechanism is completely controlled by glide and climb of dislocation until the start of precipitation and governed by self-diffusion of nickel contents [6]. Hall [7] studied the complex interdependence of microstructure, composition, heat treatment and their effect on mechanical properties of Inconel 718 alloy. The major outcome from the study is with increase in the Ti-content (by weight %) in alloy composition, increase the yield strength but decrease the % elongation which intern decrease in stress rupture properties also. Thomas et al. [8] stated that the secondary phases γ’ and γ” can be found in less extension below 700ºC, possible reason for drop in ultimate stress while studying the high temperature deformation behavior of Inconel 718 alloy. Satheesh Kumar et al. [9], studied about microstructural evolution during hot deformation of a hot isostatically processed nickel base superalloy. They reported that flow softening behavior is influenced by strain rate variations. K. Sajun Prasad et al. [10], observed two stage work hardening behavior and minor variation in the material properties with respect to different direction of sheet at room temperature.
Recently, few studies reported on the warm deformation behavior, phase
precipitation, work hardening behavior and fracture characteristics of Inconel 718 alloy [11-13]. However, it need to give more insight on systematic investigation about the material properties, flow behavior and microstructure studies of Inconel 718 alloy at various temperatures and strain rates. In the present work, the hot deformation behavior of a cold-rolled Inconel 718 alloy was investigated using tensile tests from RT to 600 °C and strain rate range from 0.0001- 0.1 s-1. Additionally, microstructure studies have been carried out using optical microscope, SEM and EDS.
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2. Material properties and Microstructure study In the present study, Inconel 718 alloy sheet of thickness 1 mm was used. The chemical composition evaluated by optical emission spectrometry is mentioned in Table 1. Table 1: Chemical composition of Inconel 718 alloy Element
Ni
Cr
Nb
Mo
Ti
Al
Cu
B
wt.%
51.463
18.279
5.0122
2.87
1.09
0.5611
0.0306
0.0024
Element
C
Si
Mn
P
S
Co
Zr
Fe
wt.%
0.0271
0.0505
0.0616
0.001
0.002
0.0925
0.0091
REM
The tensile test samples were prepared at rolling direction (RD), Normal direction (ND) and transverse direction (TD) as shown in Fig.1 (a). The dimensions of the tensile test specimens are as per sub sized ASTM E08/E8M-11 standard as shown in Fig. 1(b). The specimens were prepared using wire cut Electric Discharge Machining (EDM) process for good finish and least distortion in the thin sheet sample. Isothermal tensile tests are carried out on a computer controlled universal testing machine (UTM), as shown in Fig. 2, which has a maximum load capacity of 100 kN.
Fig. 1: Schematic of tensile test specimen (a) with different orientation (b) sub sized ASTM E08/E8M-11 standard
Fig. 2: Computerized UTM with magnified view of high temperature contact type extensometer
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UTM is equipped with a feedback control system to impose exponential increase of the actuator speed to obtain constant true strain rates. The cross-head speed is varied with respect to time as per Equation (1). =
( )
(1)
where, v is the cross-head speed, έ is the constant strain rate, L0 is the gauge length of the specimen (30 mm) and t is time. Software modifications have been done to have exponentially increasing crosshead speed for constant strain rate. A high temperature contact type extensometer is used to measure the extension of the specimen which is shown in the magnified view of Fig. 2. The pull rods for the high temperature testing are made of nickel base super alloy CM-247. Machine is attached with a 3-zone split furnace for high temperature testing as shown in the magnified view of Fig. 2. It has uniform distribution of heating coils, which are arranged in three zones to achieve temperature up to 1000°C with ± 3°C accuracy. The experiments were performed from RT to 6000C at an interval of 2000C and wide range of slow strain rates from 0.1 s-1 to 0.0001 s-1. The true stress vs true strain data is obtained from all the test. 3. Results and Discussion 3.1. Flow stress behavior The stress strain curve is mentioned for two respective setting in fig. 3, one is variation of temperature at strain rate (0.001 s-1) and another is variation of strain rates at room temperature. It is observed from the stress strain curve that variation in flow behaviour is considerable with temperature change. However, strain rate influence is negligible in all three rolling directions. As expected, flow stress decreases with increase in the strain rate. However, slight increases in flow behaviour has seen with respect to higher strain rate values. The various calculated material properties are mentioned in Table 2. As expected, yield stress and ultimate tensile stress decreases with increasing in temperature. However, the decrease in yield stress found around 40% and ultimate stress around 20%. The similar trend observed in all three orientations. Conversely, the percentage elongation is increases with temperature increases. The increase in elongation is recorded from 35% to 55%. The common observation found in all the cases are yield stress, ultimate tensile stress and % elongation is higher in RD than ND and TD as shown in Fig.4. The main reason could be grains are elongated in RD direction which shows better material properties. The decrease in yield stress and ultimate tensile strength indicate that material become more flowable at high temperature so suitable for complex part forming processes. Strain-hardening coefficient (n) is determined by the dependence of the flow (yield) stress on the level of strain. In materials with a high n value, the flow stress increases rapidly with strain. This tends to distribute further strain to regions of lower strain and flow stress. A high n value leads to a large difference between yield strength and ultimate tensile strength which is an indication of good workability of the material. The n value is calculated based on the Hollomon power law. Generally, for many structural materials, it has been seen that n value is largely dependent on temperature and strain rate change. Fig. 4 (d) shows the variation of n value with respect to
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temperature. It has been noticed from Fig. 4(d) and Table 2 that n value increases as temperature increases. The similar trend has been seen in all three orientations. In this case also the RD values are slightly higher than the other two directions of the sheet. Thus, based on the above material properties variations, it can be concluded that workability of the Inconel 718 alloy is improved at higher temperature and slow strain rate. 1000
1200 20 ºC 200 ºC 400 ºC 600 ºC
Stress ϭ (MPa)
True Stress ϭ (MPa)
800
0.1/s 0.01/s 0.001/s 0.0001/s
1000
600
400
800
600
400
200
200
0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.0
0.7
0.1
0.2
0.3
0.4
0.5
0.6
0.7
True Strain ɛ
True Strain ɛ (a)
(b)
Fig. 3: Representative true stress-strain curve of Inconel 718 alloy (a) with variation of temperatures at strain rate of 0.001 s-1 in RD (b) with variation of strain rates at RT in RD 1000
Yield Stress (MPa)
450
400
RD ND TD
950
Ultimate Stress (MPa)
RD ND TD
500
350
900 850 800 750 700 650
0
100
200
300
400
500
600
600
0
100
200
Temperature (0C) (a)
400
500
600
700
(b)
60
0.75
Strength Hardening Exponent n
RD ND TD
55
% elongation
300
Temperature (0C)
50
45
40
35
RD ND TD
0.70
0.65
0.60
0.55 0.50
0.45
30
0
100
200
300
400
500
600
0
100
Temperature (0C) (c)
200
300
400
500
600
Temperature (0C)
(d)
Fig. 4: Various material properties of Inconel 718 alloy at strain rate of 0.001 s-1: (a) Yield stress (σy) (b) Ultimate stress (σut) (c) Elongation (%) (d) Strength hardening exponent (n)
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Table 2: Material properties of Inconel 718 alloy at various temperatures Temp. (0C) RT 200 400 600
Orientation RD ND TD RD ND TD RD ND TD RD ND TD
Yield Strength (MPa) 506.57 500.64 494.06 456.29 448.92 434.19 413.91 401.19 388.30 364.56 347.23 344.34
Ultimate tensile strength (MPa) 951.14 931.91 882.23 877.57 864.40 845.29 833.35 823.62 802.09 781.32 760.88 713.96
Elongation (%) 37.85 35.22 33.67 46.20 45.05 43.60 51.73 49.22 48.68 54.62 53.31 51.46
Strain hardening exponent (n) 0.488 0.462 0.448 0.591 0.585 0.573 0.638 0.627 0.612 0.686 0.675 0.642
3.2. Microstructural analysis The as-received sample has been hot mounted by using commercially available hot setting compound and then wet ground on progressively finer grades of silicon carbide impregnated emery paper using copious amounts of water both as a lubricant and as a coolant. Subsequently, the ground samples have been mechanically polished using five-micron diamond solution. Fine polishing to a perfect mirror-like finish of the surface has been achieved using one-micron diamond solution as the lubricant. The samples were electrically etched/polished using a mixture of oxalic acid and water in the ratio of 1:10. The polished and etched surface of the sample has been observed under an optical microscope and photographed using standard bright field illumination technique. The metallographic observations were obtained by light microscopy having an average grain size of 14.1µm with a difference in morphology in rolling (RD), normal (ND) and transverse (TD) direction of sheet shown in Fig. 5. The tested specimen is with mostly fine elongated and compressed grain size in ND and RD directional planes with considerable delta phase and large MC-type primary carbides. The fracture surface of the fully deformed tensile test samples is comprehensively examined using a scanning electron microscope (SEM) of make Hitachi, S-3400N accelerating voltage 15kV. The samples for observation are sectioned parallel to the fracture surface. The fracture surfaces are observed at different magnifications to determine the macroscopic fracture mode and to concurrently characterize the intrinsic features on the tensile fracture surface during uniaxial tensile deformation. Representative fractographs of the tensile fracture surface of Inconel 718 alloy at 400 0C is shown in Fig. 6. The Overall morphology of the tensile fracture surface is appeared to be rough and uneven as shown in Fig. 6 (a). Observation of fracture surface at higher magnification (2000X) revealed a healthy population of shallow type dimples of varying size and shapes as shown in Fig. 6 (b). When overload is the principal cause of fracture, Inconel 718 alloy fail by a process known as microvoid coalescence. The microvoids nucleate at regions of localized strain discontinuity, such as that associated with second phase particles, inclusions, grain boundaries, and dislocation pile-ups. As the strain in the material increases, the microvoids grow, coalesce, and eventually form a continuous fracture surface. The cuplike depressions are referred to as dimples, and the fracture mode is known as dimple rupture.
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(a)
(b)
(c)
Fig. 5: Optical micrographs showing the key micro-constituents in Inconel 718 alloy at 200X magnification (a) RD (b) ND (c) TD
(b)
(a) 0
Fig.6: Representative fractography images of fracture tensile specimens at 400 C in RD (a) 30X magnification (b) 2000X magnification
The representative Energy-Dispersive X-ray Spectroscopy (EDS) analysis is taken at spectrum 1 and spectrum 2 (Refer Fig. 6 (b)) as shown in Fig.7. The SEM micrograph shown in Fig. 6 reveals carbides as well as the nonhomogeneous distribution of large number of δ precipitates. The alloy has a carbon content of 0.0271%, and it combined with active elements such as niobium and titanium to form MC-type carbides. These MC carbides decomposed into lower carbides during solution annealing and traces of brittle intermetallics were found at the grain boundaries from EDS spectrum analysis. This irregularly shaped carbide may offer both oxidation and hot corrosion resistance to the material. Further, the presence of these carbides at the grain boundaries increases the rupture strength at high temperature. Mixture of dimples and shear bands can be seen in fractography images. Tear ridges are visible in tensile test specimen. Micro voids can easily be identified in tensile test specimen at higher magnification. Thus, presence of dimples and shear bands indicate predominately ductile failure.
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(b)
(a)
Fig.7: Representative Energy-Dispersive X-ray Spectroscopy (EDS) analysis of fracture tensile specimens at 4000C in RD (a) Spectrum 1 (b) Spectrum 2
4. Conclusion This work involves study of material properties and microstructure analysis of Inconel 718 alloy at elevated temperatures. Based on the above discussion, the important findings are: I. Flow stress behavior of Inconel 718 alloy is significantly dependent on temperature variation. However, flow stress behavior is less sensitive with respect to strain rate variation. The higher temperature influences the yield strength significantly than the ultimate tensile strength. Additionally, there is substantial increase in elongation and strain hardening exponent which indicate better workability at higher temperature. II. The average grain size is 14.1 µm with different morphology with respect to the orientation of the sheet. The grains are elongated and compressed in ND and RD directional planes with considerable delta phase and large MC-type primary carbides. The SEM micrograph reveals carbides as well as the nonhomogeneous distribution of large no of δ precipitates. Mixture of shallow dimples and shear bands have been observed in fractogrphy which indicate predominately ductile failure. Future work involves further detail studies of flow behavior and material properties variation using EBSD and TEM analysis. Acknowledgement The financial support received for this research work from Science and Engineering Research Board (SERB – DST ECR) Government of India, ECR/2016/001402 is gratefully acknowledged. References [1] H. L. Eiselstein, Age-Hardenable Nickel Alloy, U. S. Patent 3,046, (1962)108. [2]S. M. Corporation, Inconel Alloys 718, Publication Number SMC-045, 2007. [3]S.C. Krishna, S.K. Singh, S.V.S.N. Murty, G.V. Narayana, A.K. Jha, B. Pant, and K.M. George, J. Metall.,(2014). [4]L.H. Thaller and A.H. Zimmerman, NASA Tech. Rep., 2003, (NASA TP—2003-211905). [5]D W Douglas, E.S.Robert, The Minerals, Metals & Material Society, 2001. [6]D. R. Roamer, C J Tyne, The Minerals, Metals & Materials Society, 1997. [7]R.C.Hall, Journal of Basic Engineering, ASM, (1967) 511-516. [8]A. Thomas, M. El-Wahabi , J.M. Cabrera, J.M. Prado, J. Mat. Pro. Tech. 177, (2006) 469–472. [9] S.S.Kumar, S.Raghu, Bhattacharjee, P.Prasad, A.Rao, Borah,J. Mat. Sci.: Mat. in Electronics, 50 (19), (2015) 6444-6456,ISSN 0022-2461. [10] K. S.Prasad, S. K. Panda, S. K. Kar, M. Sen, S. V. S. N. Murty, S. C. Sharma, J. Mat. Eng. Perf. 26:4, (2017) 1513–1530. [11] K.V.U. Praveen, G.V.S. Sastry, V.Singh, , Metallurgical And Materials Transactions A, Volume 39A, ( 2008) 65-75. [12]H.Zhang, C.Li, Y. Liu, Q. Guo, Y. Huang, H. Li, J. Yu, Journal of Alloys and Compounds 716, (2017) 65-72. [13]S.H. Zhang, H.Y.Zhang, M.Cheng, Materials Science and Engineering A 528, (2011) 6253–6258.
ScienceDirect Materials Today: Proceedings 5 (2018) 18016–18023
www.materialstoday.com/proceedings
ICMPC_2018
Study of Mechanical Properties and Microstructural Analysis for Inconel Alloy sheet at Elevated Temperature Gauri Mahallea, Nitin Kotkundea*, Amit Kumar Guptaa, Swadesh Kumar Singhb a
b
Department of Mechanical Engineering, BITS Pilani, Hyderabad, Telangana, 500078, India Department of Mechanical Engineering, Gokaraju Rangaraju Institute of Engineering and Technology, Hyderabad, Telangana, 500090, India
Abstract A reliable and accurate prediction of material properties and flow behavior of metals considering the coupled effects of strain, strain rate and temperature is a crucial parameter for optimizing the workability. In this study, hot uniaxial tensile tests have been performed on Inconel 718 alloy from Room Temperature (RT) to 6000C at an interval of 2000C and strain rates from 0.1 s-1 to 0.0001 s-1. Generally, the anisotropy of rolled sheet influences the material properties, material properties have been determined with respect to rolling direction (RD), Normal direction (ND) and transverse direction (TD). The various material properties such as yield stress (σy), ultimate tensile strength (σu), total elongation and strain hardening exponent (n) have been evaluated over the range of temperatures and strain rates. Considerable variation in the material properties have been noticed with respect to temperature than strain rate. Furthermore, microstructure analysis has been carried out using optical microscopy and Scanning Electron Microscope (SEM). SEM micrographs with EDS analysis of fracture specimens revealed carbides with the nonhomogeneous distribution of large number of δ precipitates. SEM study confirms dimples and shear band which indicate predominantly ductile fracture in all the cases. © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization. Keywords: Inconel 718 alloy, Uniaxial tensile test, Material Properties, Microstructure Analysis
* Corresponding author. Tel.: +91 9010451444; fax: +91 40 66303998. E-mail address: [email protected] 2214-7853 © 2018 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of Materials Processing and characterization.
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1. Introduction Inconel 718 is a precipitation hardening nickel chromium iron-based superalloy developed by H. L. Eiselstein of International Nickel Company [1]. Inconel 718 has a FCC microstructure and is a polycrystalline Ni-base wrought superalloy containing significant amount of Fe, Nb and Mb with lesser amount of Ti and Al that is manufactured by conventional melting and casting techniques. Inconel 718 exhibits of excellent mechanical properties, corrosion resistance and high strength with outstanding weldability, including resistance to post-weld cracking at elevated temperatures up to 700°C. The alloys have excellent creep-rupture strength and oxidation resistant to 982 0C because of Chromium and molybdenum contents [2]. As a result, Inconel 718 is suitable for manufacturing pressure vessel to store high pressure oxygen for aerospace missions [3] and also used for outer casings of Ni-H2 cells to store energy in satellite power system [4]. Moreover, Inconel 718 alloy is significantly used in aerospace applications include rotor blades, wheel and bolts for gas turbines, jet and rocket engine thrust chambers, hot air ducting systems, thrust reversers for airframes. Even many forming tools, extrusion dies and fixtures used in heattreating are made of Inconel alloy [5]. Due to continuous work hardening ability of ϒ’ strengthening precipitate like nickel, columbium, titanium and aluminum in IN718 alloys, ductility is very high at low strains also during uniaxial deformation. The deformation mechanism is completely controlled by glide and climb of dislocation until the start of precipitation and governed by self-diffusion of nickel contents [6]. Hall [7] studied the complex interdependence of microstructure, composition, heat treatment and their effect on mechanical properties of Inconel 718 alloy. The major outcome from the study is with increase in the Ti-content (by weight %) in alloy composition, increase the yield strength but decrease the % elongation which intern decrease in stress rupture properties also. Thomas et al. [8] stated that the secondary phases γ’ and γ” can be found in less extension below 700ºC, possible reason for drop in ultimate stress while studying the high temperature deformation behavior of Inconel 718 alloy. Satheesh Kumar et al. [9], studied about microstructural evolution during hot deformation of a hot isostatically processed nickel base superalloy. They reported that flow softening behavior is influenced by strain rate variations. K. Sajun Prasad et al. [10], observed two stage work hardening behavior and minor variation in the material properties with respect to different direction of sheet at room temperature.
Recently, few studies reported on the warm deformation behavior, phase
precipitation, work hardening behavior and fracture characteristics of Inconel 718 alloy [11-13]. However, it need to give more insight on systematic investigation about the material properties, flow behavior and microstructure studies of Inconel 718 alloy at various temperatures and strain rates. In the present work, the hot deformation behavior of a cold-rolled Inconel 718 alloy was investigated using tensile tests from RT to 600 °C and strain rate range from 0.0001- 0.1 s-1. Additionally, microstructure studies have been carried out using optical microscope, SEM and EDS.
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2. Material properties and Microstructure study In the present study, Inconel 718 alloy sheet of thickness 1 mm was used. The chemical composition evaluated by optical emission spectrometry is mentioned in Table 1. Table 1: Chemical composition of Inconel 718 alloy Element
Ni
Cr
Nb
Mo
Ti
Al
Cu
B
wt.%
51.463
18.279
5.0122
2.87
1.09
0.5611
0.0306
0.0024
Element
C
Si
Mn
P
S
Co
Zr
Fe
wt.%
0.0271
0.0505
0.0616
0.001
0.002
0.0925
0.0091
REM
The tensile test samples were prepared at rolling direction (RD), Normal direction (ND) and transverse direction (TD) as shown in Fig.1 (a). The dimensions of the tensile test specimens are as per sub sized ASTM E08/E8M-11 standard as shown in Fig. 1(b). The specimens were prepared using wire cut Electric Discharge Machining (EDM) process for good finish and least distortion in the thin sheet sample. Isothermal tensile tests are carried out on a computer controlled universal testing machine (UTM), as shown in Fig. 2, which has a maximum load capacity of 100 kN.
Fig. 1: Schematic of tensile test specimen (a) with different orientation (b) sub sized ASTM E08/E8M-11 standard
Fig. 2: Computerized UTM with magnified view of high temperature contact type extensometer
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UTM is equipped with a feedback control system to impose exponential increase of the actuator speed to obtain constant true strain rates. The cross-head speed is varied with respect to time as per Equation (1). =
( )
(1)
where, v is the cross-head speed, έ is the constant strain rate, L0 is the gauge length of the specimen (30 mm) and t is time. Software modifications have been done to have exponentially increasing crosshead speed for constant strain rate. A high temperature contact type extensometer is used to measure the extension of the specimen which is shown in the magnified view of Fig. 2. The pull rods for the high temperature testing are made of nickel base super alloy CM-247. Machine is attached with a 3-zone split furnace for high temperature testing as shown in the magnified view of Fig. 2. It has uniform distribution of heating coils, which are arranged in three zones to achieve temperature up to 1000°C with ± 3°C accuracy. The experiments were performed from RT to 6000C at an interval of 2000C and wide range of slow strain rates from 0.1 s-1 to 0.0001 s-1. The true stress vs true strain data is obtained from all the test. 3. Results and Discussion 3.1. Flow stress behavior The stress strain curve is mentioned for two respective setting in fig. 3, one is variation of temperature at strain rate (0.001 s-1) and another is variation of strain rates at room temperature. It is observed from the stress strain curve that variation in flow behaviour is considerable with temperature change. However, strain rate influence is negligible in all three rolling directions. As expected, flow stress decreases with increase in the strain rate. However, slight increases in flow behaviour has seen with respect to higher strain rate values. The various calculated material properties are mentioned in Table 2. As expected, yield stress and ultimate tensile stress decreases with increasing in temperature. However, the decrease in yield stress found around 40% and ultimate stress around 20%. The similar trend observed in all three orientations. Conversely, the percentage elongation is increases with temperature increases. The increase in elongation is recorded from 35% to 55%. The common observation found in all the cases are yield stress, ultimate tensile stress and % elongation is higher in RD than ND and TD as shown in Fig.4. The main reason could be grains are elongated in RD direction which shows better material properties. The decrease in yield stress and ultimate tensile strength indicate that material become more flowable at high temperature so suitable for complex part forming processes. Strain-hardening coefficient (n) is determined by the dependence of the flow (yield) stress on the level of strain. In materials with a high n value, the flow stress increases rapidly with strain. This tends to distribute further strain to regions of lower strain and flow stress. A high n value leads to a large difference between yield strength and ultimate tensile strength which is an indication of good workability of the material. The n value is calculated based on the Hollomon power law. Generally, for many structural materials, it has been seen that n value is largely dependent on temperature and strain rate change. Fig. 4 (d) shows the variation of n value with respect to
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temperature. It has been noticed from Fig. 4(d) and Table 2 that n value increases as temperature increases. The similar trend has been seen in all three orientations. In this case also the RD values are slightly higher than the other two directions of the sheet. Thus, based on the above material properties variations, it can be concluded that workability of the Inconel 718 alloy is improved at higher temperature and slow strain rate. 1000
1200 20 ºC 200 ºC 400 ºC 600 ºC
Stress ϭ (MPa)
True Stress ϭ (MPa)
800
0.1/s 0.01/s 0.001/s 0.0001/s
1000
600
400
800
600
400
200
200
0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0 0.0
0.7
0.1
0.2
0.3
0.4
0.5
0.6
0.7
True Strain ɛ
True Strain ɛ (a)
(b)
Fig. 3: Representative true stress-strain curve of Inconel 718 alloy (a) with variation of temperatures at strain rate of 0.001 s-1 in RD (b) with variation of strain rates at RT in RD 1000
Yield Stress (MPa)
450
400
RD ND TD
950
Ultimate Stress (MPa)
RD ND TD
500
350
900 850 800 750 700 650
0
100
200
300
400
500
600
600
0
100
200
Temperature (0C) (a)
400
500
600
700
(b)
60
0.75
Strength Hardening Exponent n
RD ND TD
55
% elongation
300
Temperature (0C)
50
45
40
35
RD ND TD
0.70
0.65
0.60
0.55 0.50
0.45
30
0
100
200
300
400
500
600
0
100
Temperature (0C) (c)
200
300
400
500
600
Temperature (0C)
(d)
Fig. 4: Various material properties of Inconel 718 alloy at strain rate of 0.001 s-1: (a) Yield stress (σy) (b) Ultimate stress (σut) (c) Elongation (%) (d) Strength hardening exponent (n)
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Table 2: Material properties of Inconel 718 alloy at various temperatures Temp. (0C) RT 200 400 600
Orientation RD ND TD RD ND TD RD ND TD RD ND TD
Yield Strength (MPa) 506.57 500.64 494.06 456.29 448.92 434.19 413.91 401.19 388.30 364.56 347.23 344.34
Ultimate tensile strength (MPa) 951.14 931.91 882.23 877.57 864.40 845.29 833.35 823.62 802.09 781.32 760.88 713.96
Elongation (%) 37.85 35.22 33.67 46.20 45.05 43.60 51.73 49.22 48.68 54.62 53.31 51.46
Strain hardening exponent (n) 0.488 0.462 0.448 0.591 0.585 0.573 0.638 0.627 0.612 0.686 0.675 0.642
3.2. Microstructural analysis The as-received sample has been hot mounted by using commercially available hot setting compound and then wet ground on progressively finer grades of silicon carbide impregnated emery paper using copious amounts of water both as a lubricant and as a coolant. Subsequently, the ground samples have been mechanically polished using five-micron diamond solution. Fine polishing to a perfect mirror-like finish of the surface has been achieved using one-micron diamond solution as the lubricant. The samples were electrically etched/polished using a mixture of oxalic acid and water in the ratio of 1:10. The polished and etched surface of the sample has been observed under an optical microscope and photographed using standard bright field illumination technique. The metallographic observations were obtained by light microscopy having an average grain size of 14.1µm with a difference in morphology in rolling (RD), normal (ND) and transverse (TD) direction of sheet shown in Fig. 5. The tested specimen is with mostly fine elongated and compressed grain size in ND and RD directional planes with considerable delta phase and large MC-type primary carbides. The fracture surface of the fully deformed tensile test samples is comprehensively examined using a scanning electron microscope (SEM) of make Hitachi, S-3400N accelerating voltage 15kV. The samples for observation are sectioned parallel to the fracture surface. The fracture surfaces are observed at different magnifications to determine the macroscopic fracture mode and to concurrently characterize the intrinsic features on the tensile fracture surface during uniaxial tensile deformation. Representative fractographs of the tensile fracture surface of Inconel 718 alloy at 400 0C is shown in Fig. 6. The Overall morphology of the tensile fracture surface is appeared to be rough and uneven as shown in Fig. 6 (a). Observation of fracture surface at higher magnification (2000X) revealed a healthy population of shallow type dimples of varying size and shapes as shown in Fig. 6 (b). When overload is the principal cause of fracture, Inconel 718 alloy fail by a process known as microvoid coalescence. The microvoids nucleate at regions of localized strain discontinuity, such as that associated with second phase particles, inclusions, grain boundaries, and dislocation pile-ups. As the strain in the material increases, the microvoids grow, coalesce, and eventually form a continuous fracture surface. The cuplike depressions are referred to as dimples, and the fracture mode is known as dimple rupture.
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Fig. 5: Optical micrographs showing the key micro-constituents in Inconel 718 alloy at 200X magnification (a) RD (b) ND (c) TD
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Fig.6: Representative fractography images of fracture tensile specimens at 400 C in RD (a) 30X magnification (b) 2000X magnification
The representative Energy-Dispersive X-ray Spectroscopy (EDS) analysis is taken at spectrum 1 and spectrum 2 (Refer Fig. 6 (b)) as shown in Fig.7. The SEM micrograph shown in Fig. 6 reveals carbides as well as the nonhomogeneous distribution of large number of δ precipitates. The alloy has a carbon content of 0.0271%, and it combined with active elements such as niobium and titanium to form MC-type carbides. These MC carbides decomposed into lower carbides during solution annealing and traces of brittle intermetallics were found at the grain boundaries from EDS spectrum analysis. This irregularly shaped carbide may offer both oxidation and hot corrosion resistance to the material. Further, the presence of these carbides at the grain boundaries increases the rupture strength at high temperature. Mixture of dimples and shear bands can be seen in fractography images. Tear ridges are visible in tensile test specimen. Micro voids can easily be identified in tensile test specimen at higher magnification. Thus, presence of dimples and shear bands indicate predominately ductile failure.
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Fig.7: Representative Energy-Dispersive X-ray Spectroscopy (EDS) analysis of fracture tensile specimens at 4000C in RD (a) Spectrum 1 (b) Spectrum 2
4. Conclusion This work involves study of material properties and microstructure analysis of Inconel 718 alloy at elevated temperatures. Based on the above discussion, the important findings are: I. Flow stress behavior of Inconel 718 alloy is significantly dependent on temperature variation. However, flow stress behavior is less sensitive with respect to strain rate variation. The higher temperature influences the yield strength significantly than the ultimate tensile strength. Additionally, there is substantial increase in elongation and strain hardening exponent which indicate better workability at higher temperature. II. The average grain size is 14.1 µm with different morphology with respect to the orientation of the sheet. The grains are elongated and compressed in ND and RD directional planes with considerable delta phase and large MC-type primary carbides. The SEM micrograph reveals carbides as well as the nonhomogeneous distribution of large no of δ precipitates. Mixture of shallow dimples and shear bands have been observed in fractogrphy which indicate predominately ductile failure. Future work involves further detail studies of flow behavior and material properties variation using EBSD and TEM analysis. Acknowledgement The financial support received for this research work from Science and Engineering Research Board (SERB – DST ECR) Government of India, ECR/2016/001402 is gratefully acknowledged. References [1] H. L. Eiselstein, Age-Hardenable Nickel Alloy, U. S. Patent 3,046, (1962)108. [2]S. M. Corporation, Inconel Alloys 718, Publication Number SMC-045, 2007. [3]S.C. Krishna, S.K. Singh, S.V.S.N. Murty, G.V. Narayana, A.K. Jha, B. Pant, and K.M. George, J. Metall.,(2014). [4]L.H. Thaller and A.H. Zimmerman, NASA Tech. Rep., 2003, (NASA TP—2003-211905). [5]D W Douglas, E.S.Robert, The Minerals, Metals & Material Society, 2001. [6]D. R. Roamer, C J Tyne, The Minerals, Metals & Materials Society, 1997. [7]R.C.Hall, Journal of Basic Engineering, ASM, (1967) 511-516. [8]A. Thomas, M. El-Wahabi , J.M. Cabrera, J.M. Prado, J. Mat. Pro. Tech. 177, (2006) 469–472. [9] S.S.Kumar, S.Raghu, Bhattacharjee, P.Prasad, A.Rao, Borah,J. Mat. Sci.: Mat. in Electronics, 50 (19), (2015) 6444-6456,ISSN 0022-2461. [10] K. S.Prasad, S. K. Panda, S. K. Kar, M. Sen, S. V. S. N. Murty, S. C. Sharma, J. Mat. Eng. Perf. 26:4, (2017) 1513–1530. [11] K.V.U. Praveen, G.V.S. Sastry, V.Singh, , Metallurgical And Materials Transactions A, Volume 39A, ( 2008) 65-75. [12]H.Zhang, C.Li, Y. Liu, Q. Guo, Y. Huang, H. Li, J. Yu, Journal of Alloys and Compounds 716, (2017) 65-72. [13]S.H. Zhang, H.Y.Zhang, M.Cheng, Materials Science and Engineering A 528, (2011) 6253–6258.