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Effect of thermomechanical processing on the room-temperature tensile properties of an Fe3Al-based alloy
Scripta METALLURGICA et MATERIALIA
Vol. 29, pp. 589-594, 1993 Printed in the U.S.A.
Pergamon
Press Ltd
EFFECT OF THERMOMECHANICAL PROCESSING ON THE ROOM-TEMPERATURE TENSILE PROPERTIES OF AN Fe3Al-BASED ALLOY* S. Viswanathant, B. R. Sheltont, J. K. Wrighttt, and V. K. Sikkat tMetals and Ceramics Division Oak Ridge National Laboratory Oak Ridge, TN 37831-6083 ttMetals and Ceramics Idaho National Engineering Laboratory Idaho Falls, ID 83415-2218 (Received March 29, 1993) (Revised June i, 1993) Introduction The poor room-temperature ductility of Fe3Al-based iron aluminides has limited their use in structural applications (1-3). Increased ductilities are obtained in the presence of a partially recrystaUized microstructure in the specimen (4-6). In a previous study (5), samples of an Fe3Al-based alloy were subjected to final rolling at a temperature of 650°C and annealed at various temperatures; room-temperature tensile elongations of 15 to 20% were achieved as a result. This paper documents attempts to improve the room-temperature ductility of FA-129 alloy, designated for high-temperature use (1), through an optimization of the final rolling temperature. The composition of the alloy used in this study is given in Table 1. TABLE 1. Nominal Chemical Composition of FA-129 Alloy Element
Fe
A1
Cr
Nb
C
Atomic percent
Balance
28.0
5.0
0.5
0.2
Experimental Procedure The starting material for this study was a 406-mm-diam, 2300-kg electroslag-remelted ingot of FA-129 alloy. A cylindrical disk approximately 28 mm thick was cut from the top of the ingot. The disk was further cut with a band saw along one-third of its diameter, and two 28 × 76 × 102-mm specimens were obtained from the piece. The two specimens were hot forged at 1000*C in two steps, an initial 25% deformation and a subsequent 33% deformation, and then hot rolled at 800°C to 84% deformation so as to obtain a final thickness of 2.5 mm. Six 152-mm pieces were then obtained from the 2.5-mm sheet and roiled at six different temperatures (450, 500, 650, 700, 800, and 900°C) to a final thickness of 0.8 mm. Figure I shows a schematic of the processing schedule. All of the sheets were subjected to a stress-relief treatment for 1 h at 700*C and oil quenched in mineral oil to room temperature. Tensile specimens with a gauge length of 12.7 mm, a width of 3.2 mm, pin holes in the grip section, and with the tensile axis along the rolling direction were punched from the sheets. Specimens were annealed for 1 h at 700°C and oil quenched in mineral oil to room temperature. All specimens were tensile tested in air at room temperature at a strain rate of 0.003/s. *Research sponsored by the U.S. Department of Energy, Fossil Energy Advanced Research and Technology Development Materials Program, under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc., and contract DE-AC07-76ID01570 with the Idaho National Engineering Laboratory.
589 0956-716X/93 $6.00 + .00
590
Fe3AI-BASED
ALLOY
Vol.
29, No.
As Cast ..-., 27.8
E E
I Hot forged 25% @ I000° C
20.9 ~
\
I Hot forged 33% @ l(X)0° C
15.6 ~
"N
:~
Hot rolled 84% @ 800 ° C
2.5_
Warm Rolled 70% @ various temperatures
,~
0.8--
FIG. 1. Thermomechanical processing schedule used in this study. Results Room-temperature tensile properties for the FA- 129 alloy as a function of final rolling temperature are shown in Fig. 2. Ultimate tensile strength and total elongation are maximum in a final rolling temperature range of 500 to 700°C. Tensile properties decrease for final rolling temperatures below 500 and above 700°C. Microstructures observed through the sample thickness for each of the final rolling temperatures after annealing and tensile testing are shown in Fig. 3. Samples were obtained from tensile grips. The samples undergoing final rolling at 450 and 500°C are mostly recrystallized after heat treatment [Figs. 3(a) and (b)]. Samples processed at 650 to 800"C [Figs. 3(c-e)] show partially recrystallized grains, with more recrystallization occurring during heat treatment for lower final rolling temperatures. The sample processed at 900°C [Fig. 30')] contains comparatively large recrystallized grains that appear nearly identical to the as-rolled microstructure (Fig. 4), indicating dynamic stallization. 12" 800reel"
'~ 70O'
10'
i~, 600' "= ~
._o a UltimateTensileSmmgth "-'¢'-" YieldS~en~
o
500
30C 400
/
[]
4
500
600
700
800
900
Final Roiling Temperature (°C)
(a)
10O0
-
400
,
500
•
|
600
•
i
700
•
i
800
-
i
900
-
10O0
Final Rolling Temperature (*C)
(b)
FIG. 2. Tensile properties obtained for the various processing temperatures: (a) yield and ultimate tensile strength and (b) tensile elongation. Transmission electron microscopy (TEM) images of tensile specimens rolled at 500, 700, and 900°C are presented in Fig. 5. The TEM micrographs of the sample rolled at 500°C reveal significant dislocation densities and areas of recovery [Fig. 5(a)] after annealing. This detect structure is surprising since Fig. 3(b) indicates complete reerystallization, and binary Fe3A1 alloys tend to exhibit very low defect densities in the re,crystallized condition
5
Vo],
29,
No.
5
Fe3A1-BASED ALLOY
591
,r,<.
/
FIG. 3. Microstructures obtained from tensile specimens for the various final rolling temperatures: (a) 450°C, (b) 500°C, (c) 650°C, (d) 700°C, (e) 800°C, and (/') 900°C.
592
Fe3A1-BASED ALLOY
Vol.
29, No.
compared to metallic alloys. Samples rolled at 700°C [Fig. 5(b)] show extensive recovery after annealing, with dislocations arranged into low-angle boundaries. The TEM micrographs of samples rolled at 900°C [Fig. 5(c)] reveal that many dislocations are retained after annealing. The dynamic recrystallization during roiling results in grains with varying amounts of work (none with enough to statically recrystallize during heat treatment at 700°C, but some with enough to cause recovery).
FIG. 4. Microstructure obtained in the as-rolled condition for sample subjected to final rolling at 900"C. Discussion The results indicate a strong correlation between tensile-elongation values and extent of recrystallization during processing or heat treatment. The sample processed at the lowest final roiling temperature of 450"C is mostly recrystallized and also exhibits poor tensile elongation. Samples processed at the intermediate temperatures exhibit some recrystallization during heat treatment, with the majority of grains still elongated from the roiling process. The sample processed at 900"C shows large, dynamically recrystallized grains and correspondingly poor tensile properties. The extent of recrystallization in the samples during the annealing and stress-relieving treatments for 1 h at 700°C is clearly dependent on the retained dislocation structure in the sample at the end of the roiling operation. The TEM of as-rolled material verified that a lower roiling temperature produces more dislocation networks, which lead to greater recrystallization during annealing. The results show that the final rolling temperature controls the ductility of FA-129 alloy to the extent that it affects the grain size and the extent of stored work and, consequently, the extent of recrystallization during further processing. Conclusions The results obtained in this study show that the temperature of thermomechanical processing, and in particular the final rolling temperature and annealing treatments, control the ductility of the alloy by producing either partially recrystallized grains with recovery and retained dislocations, or large, dynamically recrystallized grains. They also agree with previous findings that showed that room-temperature ductility in these alloys can be
5
Vol.
29, No. 5
Fe3A1-BASED ALLOY
•
~
C
•
593
:l,a)
(c)
. . . . . .
FIG. 5. Transmission electron micrographs of tensile specimens roiled at: (a) 500°C, (b) 700°C, and (c) 900"C. Note the different magnification for Fig. 5(b) compared to 5(a) and 5(c).
594
Fe3AI-BASED ALLOY
Vol. 29, No. 5
maximized in the presence of highly elongated, unrecrystallized grains in the alloy sample (5,6). Based on the results of this study on FA-129 alloy, a processing temperature range of 500 to 700°C is recommended for maximizing the room-temperature ductility of Fe3Al-based alloys. Acknowledgments The authors thank P. J. Maziasz and T. K. Roche for reviewing the paper, K. Spence for editing, and M. L. Atchley for manuscript preparation. This research was supported in part by an appointment of the first author to the U.S. Department of Energy Postgraduate Research Program administered by the Oak Ridge Associated Universities. References 1. V. K. Sikka, C. G. McKamey, C. R. Howell, and R. H. Baldwin, Fabrication and Mechanical Properties of Fe3AI-BasedIron Aluminides, ORNI_/TM-11465, Martin Marietta Energy Systems, Inc., Oak Ridge Natl. Lab., March 1990. 2. V.K. Sikka, C. G. McKamey, C. R. Howell, and R. H. Baldwin, Properties of Large Heats of Fe3AI-Based Alloys, ORNIdTM-11796, Martin Marietta Energy Systems, Inc., Oak Ridge Nail. Lab., March 1991. 3. C.G. McKamey, J. H. DeVan, P. F. Tortorelli, and V. K. Sikka, J. Mater. Res. 6(8), 1779 (1991). 4. R. S. Diehm, M. P. Kemppainen, and D. E. Mikkola, Mater. Manuf. Proc. 4(1), 61 (1989). 5. P.G. Sanders, V. K. Sikka, C. R. Howell, and R. H. Baldwin, Scr. MetalL Mater. 25, 2365 (1991). 6. C.G. McKamey and D. H. Pierce, "Effect of Recrystallization on Room Temperature Tensile Properties of a Fe3A1-Based Alloy" (Paper to appear in Scr. MetaU. Mater. 28(10) (1993).
Vol. 29, pp. 589-594, 1993 Printed in the U.S.A.
Pergamon
Press Ltd
EFFECT OF THERMOMECHANICAL PROCESSING ON THE ROOM-TEMPERATURE TENSILE PROPERTIES OF AN Fe3Al-BASED ALLOY* S. Viswanathant, B. R. Sheltont, J. K. Wrighttt, and V. K. Sikkat tMetals and Ceramics Division Oak Ridge National Laboratory Oak Ridge, TN 37831-6083 ttMetals and Ceramics Idaho National Engineering Laboratory Idaho Falls, ID 83415-2218 (Received March 29, 1993) (Revised June i, 1993) Introduction The poor room-temperature ductility of Fe3Al-based iron aluminides has limited their use in structural applications (1-3). Increased ductilities are obtained in the presence of a partially recrystaUized microstructure in the specimen (4-6). In a previous study (5), samples of an Fe3Al-based alloy were subjected to final rolling at a temperature of 650°C and annealed at various temperatures; room-temperature tensile elongations of 15 to 20% were achieved as a result. This paper documents attempts to improve the room-temperature ductility of FA-129 alloy, designated for high-temperature use (1), through an optimization of the final rolling temperature. The composition of the alloy used in this study is given in Table 1. TABLE 1. Nominal Chemical Composition of FA-129 Alloy Element
Fe
A1
Cr
Nb
C
Atomic percent
Balance
28.0
5.0
0.5
0.2
Experimental Procedure The starting material for this study was a 406-mm-diam, 2300-kg electroslag-remelted ingot of FA-129 alloy. A cylindrical disk approximately 28 mm thick was cut from the top of the ingot. The disk was further cut with a band saw along one-third of its diameter, and two 28 × 76 × 102-mm specimens were obtained from the piece. The two specimens were hot forged at 1000*C in two steps, an initial 25% deformation and a subsequent 33% deformation, and then hot rolled at 800°C to 84% deformation so as to obtain a final thickness of 2.5 mm. Six 152-mm pieces were then obtained from the 2.5-mm sheet and roiled at six different temperatures (450, 500, 650, 700, 800, and 900°C) to a final thickness of 0.8 mm. Figure I shows a schematic of the processing schedule. All of the sheets were subjected to a stress-relief treatment for 1 h at 700*C and oil quenched in mineral oil to room temperature. Tensile specimens with a gauge length of 12.7 mm, a width of 3.2 mm, pin holes in the grip section, and with the tensile axis along the rolling direction were punched from the sheets. Specimens were annealed for 1 h at 700°C and oil quenched in mineral oil to room temperature. All specimens were tensile tested in air at room temperature at a strain rate of 0.003/s. *Research sponsored by the U.S. Department of Energy, Fossil Energy Advanced Research and Technology Development Materials Program, under contract DE-AC05-84OR21400 with Martin Marietta Energy Systems, Inc., and contract DE-AC07-76ID01570 with the Idaho National Engineering Laboratory.
589 0956-716X/93 $6.00 + .00
590
Fe3AI-BASED
ALLOY
Vol.
29, No.
As Cast ..-., 27.8
E E
I Hot forged 25% @ I000° C
20.9 ~
\
I Hot forged 33% @ l(X)0° C
15.6 ~
"N
:~
Hot rolled 84% @ 800 ° C
2.5_
Warm Rolled 70% @ various temperatures
,~
0.8--
FIG. 1. Thermomechanical processing schedule used in this study. Results Room-temperature tensile properties for the FA- 129 alloy as a function of final rolling temperature are shown in Fig. 2. Ultimate tensile strength and total elongation are maximum in a final rolling temperature range of 500 to 700°C. Tensile properties decrease for final rolling temperatures below 500 and above 700°C. Microstructures observed through the sample thickness for each of the final rolling temperatures after annealing and tensile testing are shown in Fig. 3. Samples were obtained from tensile grips. The samples undergoing final rolling at 450 and 500°C are mostly recrystallized after heat treatment [Figs. 3(a) and (b)]. Samples processed at 650 to 800"C [Figs. 3(c-e)] show partially recrystallized grains, with more recrystallization occurring during heat treatment for lower final rolling temperatures. The sample processed at 900°C [Fig. 30')] contains comparatively large recrystallized grains that appear nearly identical to the as-rolled microstructure (Fig. 4), indicating dynamic stallization. 12" 800reel"
'~ 70O'
10'
i~, 600' "= ~
._o a UltimateTensileSmmgth "-'¢'-" YieldS~en~
o
500
30C 400
/
[]
4
500
600
700
800
900
Final Roiling Temperature (°C)
(a)
10O0
-
400
,
500
•
|
600
•
i
700
•
i
800
-
i
900
-
10O0
Final Rolling Temperature (*C)
(b)
FIG. 2. Tensile properties obtained for the various processing temperatures: (a) yield and ultimate tensile strength and (b) tensile elongation. Transmission electron microscopy (TEM) images of tensile specimens rolled at 500, 700, and 900°C are presented in Fig. 5. The TEM micrographs of the sample rolled at 500°C reveal significant dislocation densities and areas of recovery [Fig. 5(a)] after annealing. This detect structure is surprising since Fig. 3(b) indicates complete reerystallization, and binary Fe3A1 alloys tend to exhibit very low defect densities in the re,crystallized condition
5
Vo],
29,
No.
5
Fe3A1-BASED ALLOY
591
,r,<.
/
FIG. 3. Microstructures obtained from tensile specimens for the various final rolling temperatures: (a) 450°C, (b) 500°C, (c) 650°C, (d) 700°C, (e) 800°C, and (/') 900°C.
592
Fe3A1-BASED ALLOY
Vol.
29, No.
compared to metallic alloys. Samples rolled at 700°C [Fig. 5(b)] show extensive recovery after annealing, with dislocations arranged into low-angle boundaries. The TEM micrographs of samples rolled at 900°C [Fig. 5(c)] reveal that many dislocations are retained after annealing. The dynamic recrystallization during roiling results in grains with varying amounts of work (none with enough to statically recrystallize during heat treatment at 700°C, but some with enough to cause recovery).
FIG. 4. Microstructure obtained in the as-rolled condition for sample subjected to final rolling at 900"C. Discussion The results indicate a strong correlation between tensile-elongation values and extent of recrystallization during processing or heat treatment. The sample processed at the lowest final roiling temperature of 450"C is mostly recrystallized and also exhibits poor tensile elongation. Samples processed at the intermediate temperatures exhibit some recrystallization during heat treatment, with the majority of grains still elongated from the roiling process. The sample processed at 900"C shows large, dynamically recrystallized grains and correspondingly poor tensile properties. The extent of recrystallization in the samples during the annealing and stress-relieving treatments for 1 h at 700°C is clearly dependent on the retained dislocation structure in the sample at the end of the roiling operation. The TEM of as-rolled material verified that a lower roiling temperature produces more dislocation networks, which lead to greater recrystallization during annealing. The results show that the final rolling temperature controls the ductility of FA-129 alloy to the extent that it affects the grain size and the extent of stored work and, consequently, the extent of recrystallization during further processing. Conclusions The results obtained in this study show that the temperature of thermomechanical processing, and in particular the final rolling temperature and annealing treatments, control the ductility of the alloy by producing either partially recrystallized grains with recovery and retained dislocations, or large, dynamically recrystallized grains. They also agree with previous findings that showed that room-temperature ductility in these alloys can be
5
Vol.
29, No. 5
Fe3A1-BASED ALLOY
•
~
C
•
593
:l,a)
(c)
. . . . . .
FIG. 5. Transmission electron micrographs of tensile specimens roiled at: (a) 500°C, (b) 700°C, and (c) 900"C. Note the different magnification for Fig. 5(b) compared to 5(a) and 5(c).
594
Fe3AI-BASED ALLOY
Vol. 29, No. 5
maximized in the presence of highly elongated, unrecrystallized grains in the alloy sample (5,6). Based on the results of this study on FA-129 alloy, a processing temperature range of 500 to 700°C is recommended for maximizing the room-temperature ductility of Fe3Al-based alloys. Acknowledgments The authors thank P. J. Maziasz and T. K. Roche for reviewing the paper, K. Spence for editing, and M. L. Atchley for manuscript preparation. This research was supported in part by an appointment of the first author to the U.S. Department of Energy Postgraduate Research Program administered by the Oak Ridge Associated Universities. References 1. V. K. Sikka, C. G. McKamey, C. R. Howell, and R. H. Baldwin, Fabrication and Mechanical Properties of Fe3AI-BasedIron Aluminides, ORNI_/TM-11465, Martin Marietta Energy Systems, Inc., Oak Ridge Natl. Lab., March 1990. 2. V.K. Sikka, C. G. McKamey, C. R. Howell, and R. H. Baldwin, Properties of Large Heats of Fe3AI-Based Alloys, ORNIdTM-11796, Martin Marietta Energy Systems, Inc., Oak Ridge Nail. Lab., March 1991. 3. C.G. McKamey, J. H. DeVan, P. F. Tortorelli, and V. K. Sikka, J. Mater. Res. 6(8), 1779 (1991). 4. R. S. Diehm, M. P. Kemppainen, and D. E. Mikkola, Mater. Manuf. Proc. 4(1), 61 (1989). 5. P.G. Sanders, V. K. Sikka, C. R. Howell, and R. H. Baldwin, Scr. MetalL Mater. 25, 2365 (1991). 6. C.G. McKamey and D. H. Pierce, "Effect of Recrystallization on Room Temperature Tensile Properties of a Fe3A1-Based Alloy" (Paper to appear in Scr. MetaU. Mater. 28(10) (1993).