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On the flow localization concepts in the processing maps of IN718
Materials Science and Engineering A267 (1999) 159 – 161
Letter
On the flow localization concepts in the processing maps of IN718 S.V.S. Narayana Murty a, B. Nageswara Rao b,* a
Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Tri6andrum 695 022, India b Structural Engineering Group, Vikram Sarabhai Space Centre, Tri6andum 695 022, India Received 22 September 1998; received in revised form 25 February 1999
Abstract On the basis of microstructural observations in titanium and its alloys, a limit on the workability parameters has been proposed in the literature for flow localization or fracture to occur during hot deformation of materials. In this letter, the fixed limit on the workability parameters is verified by considering the flow stress data and the microstructural observations of IN718. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Efficiency parameter; Normalised flow softening rate; Strain rate sensitivity parameter; Processing map; IN718
1. Introduction Hot forged nickel-base superalloy IN718 is used for several critical gas turbine components. To obtain the required low cycle fatigue and fracture properties, it is essential that the microstructure be controlled during the processing stage. One of the requirement for forge process modeling is a knowledge of the alloy flow behavior in order to define deformation maps that delineate ‘safe’ and ‘nonsafe’ hotworking conditions. These maps show on axes of temperature and strain rate (processing space) the processing conditions for stable and unstable deformation [1], where the mechanical behaviour of materials is characterized by relating the flow stress (s) to the strain (o), strain rate (o; ) and temperature (T). Workability problem may arise when metal deformation is localized to a narrow zone in the work piece. This results in regions of different structures and properties that can be the site of failure in service. Localization of deformation can also be so severe that it leads to failure during the deformation process. Flow localization may also occur during hot working during the absence of frictional or chilling effects. In * Corresponding author. Tel.: +91-471-563640; fax: + 91-471461795.
this case, localization results from flow softening (negative strain hardening) as a result of structural instabilities such as adiabatic heating, generation of a softer texture during deformation, grain coarsening or spheroidisation. Flow softening has been correlated with material properties by the parameter, a, for plane strain compression [2–4]: a=
−g m
(1)
where the normalised flow softening rate: g=
1 ds , s do
dlogs .On dlogo; the basis of microstructural observations in titanium and its alloys, a limit on workability parameter: and the strain rate sensitivity parameter:m =
a\ 5
(2)
has been fixed for flow localization or fracture to occur during hot deformation of materials. The purpose of this letter is to examine the fixed limit on the workability parameter (a) by considering the flow stress data [5] and microstructural observations of IN718 from different sources [1,5–7]. For comparison, the simple instability condition:
0921-5093/99/$ - see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 9 9 ) 0 0 1 2 2 - 7
S.V.S.N. Murty, B.N. Rao / Materials Science and Engineering A267 (1999) 159–161
160
2mBh 5 0
(3)
which has been recently developed for delineating the regions of unstable metal flow during hot deformation [8], is also considered. Here, the efficiency of power dissipation: h = 2−
1 so;
&
o;
s do; .
0
2. Results and discussion The workability parameters, such as strain rate sensitivity parameter (m), and the normalized flow softening rate (g) were calculated from the flow stress data of IN718 [5] over the temperature range of 900 –1200°C and strain rate range of 0.001 – 100 s − 1 using a cubic spline fit where the flow stress s= f(o, o; , T). Once values of a are computed from Eq. (1) at a constant strain, regions of instabilities can be identified through Eq. (2) and graphically shown in o; – T space (Fig. 1). Eq. (2) is valid for m \ 0. For the case m 5 0, dynamic strain aging occurs, and the flow becomes unstable. This condition (m B0) has also been used to delineate the regions of flow instability in Fig. 1. Regarding the microstructural observations on IN718, only limited data are available in the open literature due to the complex chemical composition of IN718 alloy and, especially, the intermetallic phase transformation which occurs during the high temperature deformation. Srinivasan and Prasad [5] have stud-
ied the hot working characteristics of the material in the temperature range 900–1200°C and strain rate range 0.001–100 s − 1. It is noted from their investigation that two safe domains representing dynamic recrystallization (DRX), one centered around 950°C and 0.001 s − 1 and another 1200°C and 0.1 s − 1 are present. Flow instabilities in the form of adiabatic shear bands were reported at temperatures lower than 1000°C and strain rates higher than 1 s − 1. Intercrystalline cracking was reported at temperatures higher than 1150°C and strain rates higher than 1 s − 1. It can be seen from Fig. 1 that the instability criterion (Eq. (2)) shows three areas and the other criterion (Eq. (3)) indicates unstable flow in five regions. All the three unstable regions identified by Eq. (2) are found to be inside the larger regions shown by Eq. (3). In addition, Eq. (3) identifies two more unstable regions which are not identified by Eq. (2). It is very interesting to note that, all the microstructural observations reported in [5] shown in Fig. 1 fall in the appropriate locations indicating the validity of both the conditions. The microstructural observations reported by Howson and Couts [1] are also marked in Fig. 1. It may be noted from Fig. 1 that the specimen deformed at 982°C and 10 s − 1 shows stable microstructure which falls in the unstable region predicted by Eq. (3). This indicates conservative identification of the boundaries of ‘safe’ and ‘unsafe’ regions by Eq. (3). Guimaraes and Jonas [6] observed that dynamic recrystallization occurs in the temperature range of 975–1090°C and strain rate range of 9.3× 10 − 4 –9.3×10 − 2 s − 1 during compression testing of
Fig. 1. Instability map for IN718 superalloy:— Eq. (2); --- Eq. (3). Microstructural observations: X unstable [5]; stable [5]; stable [1].
S.V.S.N. Murty, B.N. Rao / Materials Science and Engineering A267 (1999) 159–161
solid cylinders at 0.4 strains, which is well within the ‘safe’ regions identified by both criteria (Eq. (2) and Eq. (3)). It is also noted from the microstructural observations of Zhou and Baker [7] that the specimens deformed at the temperature of 950, 1000 and 1050°C and strain rate of 0.1 s − 1 exhibited a dynamically recrystallized microstructures. These observations also support the predictions of both criteria. Based on the microstructural observations available in the open literature, the predictions of Eq. (3) are found to be conservative compared to those of Eq. (3).
3. Summary Unstable flow during hot deformation of IN718 was analyzed using a parameter as suggested by Semiatin and Lahoti [4] and compared with the other simple instability condition given by Eq. (3). Although, the value of a fixed in [4] worked well for IN718, due to empirical nature of this criterion, its applicability for other materials will be known only after examining the microstructural observations during hot deformation. Since the instability condition given by Eq. (3) is based
.
161
on continuum principles for large plastic deformation, it is not empirical and, is valid for any type of flow stress versus strain rate curve, and has no such restrictions in identifying the unstable regions in the processing maps.
References [1] T.E. Howson, W.J. Couts, Jr., Metallurgy and Applications of Superalloy 718, TMS-AMIE, Warrandale, PA, 1989, pp. 685– 694. [2] George E. Dieter, in: Metals Handbook, 9th ed., vol. 14, American Society for Metals, Metals Park, OH, 1989, pp. 363–372. [3] S.L. Semiatin, J.J. Jonas, Formability and Workability of Metals: Plastic Instability and Flow Localization, American Society for Metals, Metal Park, OH, 1984. [4] S.L. Semiatin, G.D. Lahoti, Metall. Trans. A 13A (1982) 275– 288. [5] N. Srinivasan, Y.V.R.K. Prasad, Metall. Trans. A 25A (1994) 2275 – 2284. [6] A.A. Guimaraes, J.J. Jonas, Metall. Trans. A 12A (1981) 1655– 1666. [7] L.X. Zhou, T.N. Baker, Mater.Sci. Eng. A 177 (1994) 1–9. [8] S.V.S. Narayana Murty, B. Nageswara Rao, Mater. Sci. Eng. A 254 (1998) 76 – 82.
Letter
On the flow localization concepts in the processing maps of IN718 S.V.S. Narayana Murty a, B. Nageswara Rao b,* a
Materials and Metallurgy Group, Vikram Sarabhai Space Centre, Tri6andrum 695 022, India b Structural Engineering Group, Vikram Sarabhai Space Centre, Tri6andum 695 022, India Received 22 September 1998; received in revised form 25 February 1999
Abstract On the basis of microstructural observations in titanium and its alloys, a limit on the workability parameters has been proposed in the literature for flow localization or fracture to occur during hot deformation of materials. In this letter, the fixed limit on the workability parameters is verified by considering the flow stress data and the microstructural observations of IN718. © 1999 Published by Elsevier Science S.A. All rights reserved. Keywords: Efficiency parameter; Normalised flow softening rate; Strain rate sensitivity parameter; Processing map; IN718
1. Introduction Hot forged nickel-base superalloy IN718 is used for several critical gas turbine components. To obtain the required low cycle fatigue and fracture properties, it is essential that the microstructure be controlled during the processing stage. One of the requirement for forge process modeling is a knowledge of the alloy flow behavior in order to define deformation maps that delineate ‘safe’ and ‘nonsafe’ hotworking conditions. These maps show on axes of temperature and strain rate (processing space) the processing conditions for stable and unstable deformation [1], where the mechanical behaviour of materials is characterized by relating the flow stress (s) to the strain (o), strain rate (o; ) and temperature (T). Workability problem may arise when metal deformation is localized to a narrow zone in the work piece. This results in regions of different structures and properties that can be the site of failure in service. Localization of deformation can also be so severe that it leads to failure during the deformation process. Flow localization may also occur during hot working during the absence of frictional or chilling effects. In * Corresponding author. Tel.: +91-471-563640; fax: + 91-471461795.
this case, localization results from flow softening (negative strain hardening) as a result of structural instabilities such as adiabatic heating, generation of a softer texture during deformation, grain coarsening or spheroidisation. Flow softening has been correlated with material properties by the parameter, a, for plane strain compression [2–4]: a=
−g m
(1)
where the normalised flow softening rate: g=
1 ds , s do
dlogs .On dlogo; the basis of microstructural observations in titanium and its alloys, a limit on workability parameter: and the strain rate sensitivity parameter:m =
a\ 5
(2)
has been fixed for flow localization or fracture to occur during hot deformation of materials. The purpose of this letter is to examine the fixed limit on the workability parameter (a) by considering the flow stress data [5] and microstructural observations of IN718 from different sources [1,5–7]. For comparison, the simple instability condition:
0921-5093/99/$ - see front matter © 1999 Published by Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 9 9 ) 0 0 1 2 2 - 7
S.V.S.N. Murty, B.N. Rao / Materials Science and Engineering A267 (1999) 159–161
160
2mBh 5 0
(3)
which has been recently developed for delineating the regions of unstable metal flow during hot deformation [8], is also considered. Here, the efficiency of power dissipation: h = 2−
1 so;
&
o;
s do; .
0
2. Results and discussion The workability parameters, such as strain rate sensitivity parameter (m), and the normalized flow softening rate (g) were calculated from the flow stress data of IN718 [5] over the temperature range of 900 –1200°C and strain rate range of 0.001 – 100 s − 1 using a cubic spline fit where the flow stress s= f(o, o; , T). Once values of a are computed from Eq. (1) at a constant strain, regions of instabilities can be identified through Eq. (2) and graphically shown in o; – T space (Fig. 1). Eq. (2) is valid for m \ 0. For the case m 5 0, dynamic strain aging occurs, and the flow becomes unstable. This condition (m B0) has also been used to delineate the regions of flow instability in Fig. 1. Regarding the microstructural observations on IN718, only limited data are available in the open literature due to the complex chemical composition of IN718 alloy and, especially, the intermetallic phase transformation which occurs during the high temperature deformation. Srinivasan and Prasad [5] have stud-
ied the hot working characteristics of the material in the temperature range 900–1200°C and strain rate range 0.001–100 s − 1. It is noted from their investigation that two safe domains representing dynamic recrystallization (DRX), one centered around 950°C and 0.001 s − 1 and another 1200°C and 0.1 s − 1 are present. Flow instabilities in the form of adiabatic shear bands were reported at temperatures lower than 1000°C and strain rates higher than 1 s − 1. Intercrystalline cracking was reported at temperatures higher than 1150°C and strain rates higher than 1 s − 1. It can be seen from Fig. 1 that the instability criterion (Eq. (2)) shows three areas and the other criterion (Eq. (3)) indicates unstable flow in five regions. All the three unstable regions identified by Eq. (2) are found to be inside the larger regions shown by Eq. (3). In addition, Eq. (3) identifies two more unstable regions which are not identified by Eq. (2). It is very interesting to note that, all the microstructural observations reported in [5] shown in Fig. 1 fall in the appropriate locations indicating the validity of both the conditions. The microstructural observations reported by Howson and Couts [1] are also marked in Fig. 1. It may be noted from Fig. 1 that the specimen deformed at 982°C and 10 s − 1 shows stable microstructure which falls in the unstable region predicted by Eq. (3). This indicates conservative identification of the boundaries of ‘safe’ and ‘unsafe’ regions by Eq. (3). Guimaraes and Jonas [6] observed that dynamic recrystallization occurs in the temperature range of 975–1090°C and strain rate range of 9.3× 10 − 4 –9.3×10 − 2 s − 1 during compression testing of
Fig. 1. Instability map for IN718 superalloy:— Eq. (2); --- Eq. (3). Microstructural observations: X unstable [5]; stable [5]; stable [1].
S.V.S.N. Murty, B.N. Rao / Materials Science and Engineering A267 (1999) 159–161
solid cylinders at 0.4 strains, which is well within the ‘safe’ regions identified by both criteria (Eq. (2) and Eq. (3)). It is also noted from the microstructural observations of Zhou and Baker [7] that the specimens deformed at the temperature of 950, 1000 and 1050°C and strain rate of 0.1 s − 1 exhibited a dynamically recrystallized microstructures. These observations also support the predictions of both criteria. Based on the microstructural observations available in the open literature, the predictions of Eq. (3) are found to be conservative compared to those of Eq. (3).
3. Summary Unstable flow during hot deformation of IN718 was analyzed using a parameter as suggested by Semiatin and Lahoti [4] and compared with the other simple instability condition given by Eq. (3). Although, the value of a fixed in [4] worked well for IN718, due to empirical nature of this criterion, its applicability for other materials will be known only after examining the microstructural observations during hot deformation. Since the instability condition given by Eq. (3) is based
.
161
on continuum principles for large plastic deformation, it is not empirical and, is valid for any type of flow stress versus strain rate curve, and has no such restrictions in identifying the unstable regions in the processing maps.
References [1] T.E. Howson, W.J. Couts, Jr., Metallurgy and Applications of Superalloy 718, TMS-AMIE, Warrandale, PA, 1989, pp. 685– 694. [2] George E. Dieter, in: Metals Handbook, 9th ed., vol. 14, American Society for Metals, Metals Park, OH, 1989, pp. 363–372. [3] S.L. Semiatin, J.J. Jonas, Formability and Workability of Metals: Plastic Instability and Flow Localization, American Society for Metals, Metal Park, OH, 1984. [4] S.L. Semiatin, G.D. Lahoti, Metall. Trans. A 13A (1982) 275– 288. [5] N. Srinivasan, Y.V.R.K. Prasad, Metall. Trans. A 25A (1994) 2275 – 2284. [6] A.A. Guimaraes, J.J. Jonas, Metall. Trans. A 12A (1981) 1655– 1666. [7] L.X. Zhou, T.N. Baker, Mater.Sci. Eng. A 177 (1994) 1–9. [8] S.V.S. Narayana Murty, B. Nageswara Rao, Mater. Sci. Eng. A 254 (1998) 76 – 82.