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Power-law breakdown and the dislocation microstructure in type 304 stainless steel
Volume 2, number 5B
August 1984
MATERIALS LETTERS
POWER-LAW BREAKDOWN AND THE DISLOCATION
MICROSTRUCTURE
IN TYPE 304 STAINLESS STEEL M.E. KASSNER * Lawrence Livermore National Laboratory,
Livermore,
CA 94550,
USA
Received 16 April 1984;in final form 30 May 1984
The dislocation microstructure was examined in type 304 stainless-steel specimens torsionally deformed to large strains (0.92) at temperatures corresponding to the power-law and power-law-breakdown regimes. New evidence is presented that shows that while subgrain boundaries readily form at higher (power-law) temperatures, they are not generally stable at lower (power-law-breakdown) temperatures.
1. Introduction 1028
on the high-temperature creep of stainless steels show that the dependence of the steady-state creep rate, iss, on the steady-state flow stress, uss, follows a power law described by Studies
4s=N%,lW ,
A
Young
etk.
[9]
A 1023 K l
1138 K
0
1338'K I
Thisstudy
1024
(1)
where A is a temperature-dependent constant. At lower steady-state stresses or temperaturecompensated strain-rate values (&/A), the exponent is independent of temperature, stress, and strain rate and is equal to ~5. Creep at these low stresses or high temperatures (above x(0.5-0.6)Tm) is sometimes termed “power-law” creep. At lower temperatures or higher stresses, the exponent increases with increasing stress (see fig. 1) and creep within this regime is described as “power-law breakdown”. In this study, the dislocation microstructure of type 304 stainless steel was examined at various strains for deformation at strain-rate-temperature combinations corresponding to the power-law and power-lawbreakdown regimes. The degree of perfection of subgrain boundaries was determined as a function of strain over a relatively wide strain range (o-0.92) using torsional deformation. Bhargava et al. [ 1,2] per* Presently on leave as a Fulbright Senior Scholar at the Metaalinstituut TNO, Apeldoorn, The Netherlands.
0 167-557x/84/$ 03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
cc k
1020
0" .w
d
10'6
10'2
108 10-5
10-S
10-4
10-z
US/E
Fig. 1. The lattice-diffusion-compensated strain rate (t/DL) as a function of the modulus-compensated stress (ass/E) for type 304 stainless steel during steadystate flow.
4.51
Volume 2, number SB
MATERIALS LE’ITERS
formed similar studies on type 304 stainless steel. They found that, within the power-law regime, lowenergy dislocation configurations consisted of welldefined subgrain boundaries. However, for temperature-strain-rate combinations corresponding to powerlaw breakdown (modulus-compensated stresses (us,,/,!?) above ~2 X 10e3), the dislocation configurations were either cellular or mixed (both subgrain and cellular) in nature. They suggested that this transition was the result of a change in the mechanism of plastic deformation - from dislocation creep within the power-law regime to dislocation glide in the power-law-breakdown regime. The observations of Monteiro and Kestenbach [3] on the same material are consistent with these findings. The microstructural observations of type 3 16 stainless steel by Kestenbach et al. [4] at power-law-breakdown creep temperatures and by Moteff and co-workers [5,6] at both power-law and power-law-breakdown creep temperatures suggest an identical transition in the degree of perfection of subgrain boundaries across power-law breakdown in this alloy. Pharr [7] suggested that this transition may be general to all subgrain-forming materials. Because the mechanical tests by Bhargava et al. (and also those performed in the type 316 studies cited above) were performed in tension, the strain to rupture was only 0.25-0.45. Recent torsional work on 304 stainless steel [8] reported that the total plastic strain required to achieve steady state within the power-law-breakdown regime is ~0.50 or greater. This raises a question about the Bhargava et al. study: if the deformation could have been continued to larger strains into the steady-state or “equilibrium” condition, would well-defined subgrains have eventually formed in specimens tested in the power-law-breakdown regime? In the present study, large-strain torsion tests were performed at three conditions: (1) 1023 K (75O’C) and ; = 5.04 X lOA s-l ; (2) 1138 K (865°C) and i = 3.22 X 10e5 s-l ; and (3) 1338 K (1065’C) and i = 3.22 X 10T5 s-l . The first temperature-strain-rate combination corresponds to a condition that might be regarded as a transition from the power-law to the power-law-breakdown regime. The steady-state stress exponent, n, associated with this combination is =7 (see fig. 1). Within the power-law regime (e.g., conditions (2) and (3)), n has a value very close to 5. Two solid torsion specimens were deformed at each of the 452
August 1984
three conditions; one was deformed to the strain that corresponds to the onset of steady state (i.e., ess = 0.52 for condition (l), 0.38 for condition (2), and 0.17 for condition (3)) and the other to E = 0.92. The dislocation microstructure of the six specimens was examined under a transmission electron microscope (TEM); the observations answered the question about the propensity of subgrain boundaries to form during deformation within the power-law-breakdown regime. In addition, a special annealing study at 1023 K provided insight into the stability of subgrains in the power-law-breakdown regime.
2. Experimental The 304 stainless steel used was provided in the form of rolled 25 mm thick plate and was taken from the standard heat 9T2796 (the same heat as in refs. [ 1,2]). Solid tension specimens with a 25 mm gage length were machined from the plate and subsequently solution-annealed in vacuum at 1323 K for 1 h. The specimens were tested in torsion to the desired strains and waterquenched. At 1023 K the torsional strain to failure for this material is =l .l and the flow stress is maximum at ~0 92. For this reason tests above thi: latter value of strain were not conducted. TEM foils were sparkcut from the 3/4-radius position with the plane of the foil parallel to the axis of rotation of the torsion specimen. Specimens were electrolytically thinned using a Fischione jet polisher operating at 16 V and 1.3 A/cm2 and using a solution of 90% acetic acid and 10% perchloric acid. The thinned foils were examined in a Phillips EM 400 and a Jeol2OOCX TEM. At least two foils were examined from each torsion specimen.
3. Results As expected, the dislocation configurations observed in the four specimens deformed in the power-law regime always consisted of well-defined subgrains. Fig. 2a is a TEM micrograph of a typical region of a specimen deformed at 1138 K and i = 3.22 X 1O-5 s-l (condition (2)) to a strain of 0.92. Many well-defined subgrain boundaries are illustrated in this micrograph. However, in the specimen deformed to a steady-state strain of
MATERIALS LETTERS
Volume 2, number SB
Fig. 2. TEM micrographs of specimens deformed to a relatively large steady-state strain of 0.92: (a) 1138 K and h = 3.22 x lo-’ s-l (power-law regime); (b) 1023 K and E’= 5.04 x 10e4 s-l (power-law-breakdown regime).
0.52 at 1023 K (the onset of power-law welldefined
subgrains
were observed
breakdown),
in only ~40%
of the observed grains. In the remaining grains of this inhomogeneous substructure, subgrain boundaries are either nonexistent (in ~20% of the grains), cellular, or diffuse. The diffuse boundaries possess order greater than that classically attributed to cell walls, but less than that of the well-defined subgrain walls. Such ambiguous structures (and also well-defined subgrain boundaries) corresponding to this strain and deformation condition are illustrated by the author in ref. [8]. The ambiguous structures seem to be very similar in character to those described as “mixed” in nature by Bhargava et al. In the specimen deformed to a strain of 0.92, the substructure appeared somewhat more homogeneous. About the same fraction of grains contained well-
August 1984
defined subgrain boundaries as in the specimen deformed to 0.52 strain. Most of the grains, however, consisted of the “ambiguous” boundaries just described. That is, at the large strain of E = 2ess, welldefined subgrain boundaries still do not usually form at this power-law-breakdown condition. Fig. 2b is a TEM micrograph that is representative of the specimen. Some of the dislocation configurations are cell boundaries, while others are the “ambiguous” type. Presumably, as the temperature is lowered and the deformation conditions are well within the power-law-breakdown regime, subgrain boundary formation would be even less common. If, however, the deformation is terminated at the onset of steady state, and the specimen is statically held (in the unloaded condition) at temperature for 10 min, all of the observed boundaries are welldefined subgrain boundaries, such as those illustrated in fig. 2a. This suggests that all of the “ambiguous” structures readily transform to well-defined subgrain boundaries with the static anneal. The temperature associated with this power-law-breakdown deformation condition appears, then, sufficient to recover the dislocations into low-angle subgrain boundaries. The applied strain rate, however, opposes this action by “breaking up” the structure. Therefore, for 304 stainless steel (and perhaps for other subgrain-forming materials) it appears that subgrain boundaries are not generally stable at powerlaw-breakdown deformation conditions. However, it should be emphasized that this lack of stability is merely an effect or manifestation of the underlying cause of power-law breakdown. As described in earlier work by the author [8], subgrain-free or “structureless” 304 stainless steel exhibits power-law-breakdown behavior in a manner very analogous to the steady-state or subgraincontaining steel. A subsequent study by the author is determining the dependence of the subgrain size and the misorientation angle of adjacent subgrains on the strain from the onset of steady state to the 0.92 strain level. This further study should provide new insight as to the microstructural features responsible for creep resistance.
Acknowledgement Most of this research was supported by the U.S. 453
Volume 2. number 5B
MATERIALS LETTERS
Department of Energy, Office of Basic Energy Sciences, under Contract DE-AM03-76SF00326, Project Agreement DE-AT03-76ER 70057. Later stages of the work were supported by Lawrence Livermore National Laboratory under the auspices of the U.S. Department of Energy, Contract W-7405 -Eng48, and the Netherlands-America Commission for Educational Exchange.
VI R. Bhargava, J. Moteff and R.W. Swindeman, Symposium
[31
[41 [51 [61
References [ 11 R. Bhargava, J. Moteff and R.W. Swindeman, Metall. Trans. 7A (1976) 879.
454
August 1984
[71 [aI [91
on the Structural Materials for Service at Elevated Temperatures in Nuclear Power Generation, ASME (1975) pp. 31-54. S.N. Monteiro and H.-J. Kestenbach, Metall. Trans. 6A (1975) 938. H.-J. Kestenbach, T.L. da Silveira and S.N. Monteiro, Metall. Trans. 7A (1976) 155. K.D. Challenger and J. Moteff, Metall. Trans. 4 (1973) 749. D.J. Michel, J. Moteff and A.J. Love& Acta Metall. 21 (1973) 1269. G.M. Pharr, Scripta MetaJl. 15 (1981) 713. M.E. Kassner, A.K. Miller and O.D. Sherby, MetaJl. Trans. 13A (1982) 1977. C.M. Young, E.M. Cady and O.D. Sherby, U.S. Army Materials and Mechanical Research Center, Watertown, MA, Report CTR-72-27 (1972).
August 1984
MATERIALS LETTERS
POWER-LAW BREAKDOWN AND THE DISLOCATION
MICROSTRUCTURE
IN TYPE 304 STAINLESS STEEL M.E. KASSNER * Lawrence Livermore National Laboratory,
Livermore,
CA 94550,
USA
Received 16 April 1984;in final form 30 May 1984
The dislocation microstructure was examined in type 304 stainless-steel specimens torsionally deformed to large strains (0.92) at temperatures corresponding to the power-law and power-law-breakdown regimes. New evidence is presented that shows that while subgrain boundaries readily form at higher (power-law) temperatures, they are not generally stable at lower (power-law-breakdown) temperatures.
1. Introduction 1028
on the high-temperature creep of stainless steels show that the dependence of the steady-state creep rate, iss, on the steady-state flow stress, uss, follows a power law described by Studies
4s=N%,lW ,
A
Young
etk.
[9]
A 1023 K l
1138 K
0
1338'K I
Thisstudy
1024
(1)
where A is a temperature-dependent constant. At lower steady-state stresses or temperaturecompensated strain-rate values (&/A), the exponent is independent of temperature, stress, and strain rate and is equal to ~5. Creep at these low stresses or high temperatures (above x(0.5-0.6)Tm) is sometimes termed “power-law” creep. At lower temperatures or higher stresses, the exponent increases with increasing stress (see fig. 1) and creep within this regime is described as “power-law breakdown”. In this study, the dislocation microstructure of type 304 stainless steel was examined at various strains for deformation at strain-rate-temperature combinations corresponding to the power-law and power-lawbreakdown regimes. The degree of perfection of subgrain boundaries was determined as a function of strain over a relatively wide strain range (o-0.92) using torsional deformation. Bhargava et al. [ 1,2] per* Presently on leave as a Fulbright Senior Scholar at the Metaalinstituut TNO, Apeldoorn, The Netherlands.
0 167-557x/84/$ 03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
cc k
1020
0" .w
d
10'6
10'2
108 10-5
10-S
10-4
10-z
US/E
Fig. 1. The lattice-diffusion-compensated strain rate (t/DL) as a function of the modulus-compensated stress (ass/E) for type 304 stainless steel during steadystate flow.
4.51
Volume 2, number SB
MATERIALS LE’ITERS
formed similar studies on type 304 stainless steel. They found that, within the power-law regime, lowenergy dislocation configurations consisted of welldefined subgrain boundaries. However, for temperature-strain-rate combinations corresponding to powerlaw breakdown (modulus-compensated stresses (us,,/,!?) above ~2 X 10e3), the dislocation configurations were either cellular or mixed (both subgrain and cellular) in nature. They suggested that this transition was the result of a change in the mechanism of plastic deformation - from dislocation creep within the power-law regime to dislocation glide in the power-law-breakdown regime. The observations of Monteiro and Kestenbach [3] on the same material are consistent with these findings. The microstructural observations of type 3 16 stainless steel by Kestenbach et al. [4] at power-law-breakdown creep temperatures and by Moteff and co-workers [5,6] at both power-law and power-law-breakdown creep temperatures suggest an identical transition in the degree of perfection of subgrain boundaries across power-law breakdown in this alloy. Pharr [7] suggested that this transition may be general to all subgrain-forming materials. Because the mechanical tests by Bhargava et al. (and also those performed in the type 316 studies cited above) were performed in tension, the strain to rupture was only 0.25-0.45. Recent torsional work on 304 stainless steel [8] reported that the total plastic strain required to achieve steady state within the power-law-breakdown regime is ~0.50 or greater. This raises a question about the Bhargava et al. study: if the deformation could have been continued to larger strains into the steady-state or “equilibrium” condition, would well-defined subgrains have eventually formed in specimens tested in the power-law-breakdown regime? In the present study, large-strain torsion tests were performed at three conditions: (1) 1023 K (75O’C) and ; = 5.04 X lOA s-l ; (2) 1138 K (865°C) and i = 3.22 X 10e5 s-l ; and (3) 1338 K (1065’C) and i = 3.22 X 10T5 s-l . The first temperature-strain-rate combination corresponds to a condition that might be regarded as a transition from the power-law to the power-law-breakdown regime. The steady-state stress exponent, n, associated with this combination is =7 (see fig. 1). Within the power-law regime (e.g., conditions (2) and (3)), n has a value very close to 5. Two solid torsion specimens were deformed at each of the 452
August 1984
three conditions; one was deformed to the strain that corresponds to the onset of steady state (i.e., ess = 0.52 for condition (l), 0.38 for condition (2), and 0.17 for condition (3)) and the other to E = 0.92. The dislocation microstructure of the six specimens was examined under a transmission electron microscope (TEM); the observations answered the question about the propensity of subgrain boundaries to form during deformation within the power-law-breakdown regime. In addition, a special annealing study at 1023 K provided insight into the stability of subgrains in the power-law-breakdown regime.
2. Experimental The 304 stainless steel used was provided in the form of rolled 25 mm thick plate and was taken from the standard heat 9T2796 (the same heat as in refs. [ 1,2]). Solid tension specimens with a 25 mm gage length were machined from the plate and subsequently solution-annealed in vacuum at 1323 K for 1 h. The specimens were tested in torsion to the desired strains and waterquenched. At 1023 K the torsional strain to failure for this material is =l .l and the flow stress is maximum at ~0 92. For this reason tests above thi: latter value of strain were not conducted. TEM foils were sparkcut from the 3/4-radius position with the plane of the foil parallel to the axis of rotation of the torsion specimen. Specimens were electrolytically thinned using a Fischione jet polisher operating at 16 V and 1.3 A/cm2 and using a solution of 90% acetic acid and 10% perchloric acid. The thinned foils were examined in a Phillips EM 400 and a Jeol2OOCX TEM. At least two foils were examined from each torsion specimen.
3. Results As expected, the dislocation configurations observed in the four specimens deformed in the power-law regime always consisted of well-defined subgrains. Fig. 2a is a TEM micrograph of a typical region of a specimen deformed at 1138 K and i = 3.22 X 1O-5 s-l (condition (2)) to a strain of 0.92. Many well-defined subgrain boundaries are illustrated in this micrograph. However, in the specimen deformed to a steady-state strain of
MATERIALS LETTERS
Volume 2, number SB
Fig. 2. TEM micrographs of specimens deformed to a relatively large steady-state strain of 0.92: (a) 1138 K and h = 3.22 x lo-’ s-l (power-law regime); (b) 1023 K and E’= 5.04 x 10e4 s-l (power-law-breakdown regime).
0.52 at 1023 K (the onset of power-law welldefined
subgrains
were observed
breakdown),
in only ~40%
of the observed grains. In the remaining grains of this inhomogeneous substructure, subgrain boundaries are either nonexistent (in ~20% of the grains), cellular, or diffuse. The diffuse boundaries possess order greater than that classically attributed to cell walls, but less than that of the well-defined subgrain walls. Such ambiguous structures (and also well-defined subgrain boundaries) corresponding to this strain and deformation condition are illustrated by the author in ref. [8]. The ambiguous structures seem to be very similar in character to those described as “mixed” in nature by Bhargava et al. In the specimen deformed to a strain of 0.92, the substructure appeared somewhat more homogeneous. About the same fraction of grains contained well-
August 1984
defined subgrain boundaries as in the specimen deformed to 0.52 strain. Most of the grains, however, consisted of the “ambiguous” boundaries just described. That is, at the large strain of E = 2ess, welldefined subgrain boundaries still do not usually form at this power-law-breakdown condition. Fig. 2b is a TEM micrograph that is representative of the specimen. Some of the dislocation configurations are cell boundaries, while others are the “ambiguous” type. Presumably, as the temperature is lowered and the deformation conditions are well within the power-law-breakdown regime, subgrain boundary formation would be even less common. If, however, the deformation is terminated at the onset of steady state, and the specimen is statically held (in the unloaded condition) at temperature for 10 min, all of the observed boundaries are welldefined subgrain boundaries, such as those illustrated in fig. 2a. This suggests that all of the “ambiguous” structures readily transform to well-defined subgrain boundaries with the static anneal. The temperature associated with this power-law-breakdown deformation condition appears, then, sufficient to recover the dislocations into low-angle subgrain boundaries. The applied strain rate, however, opposes this action by “breaking up” the structure. Therefore, for 304 stainless steel (and perhaps for other subgrain-forming materials) it appears that subgrain boundaries are not generally stable at powerlaw-breakdown deformation conditions. However, it should be emphasized that this lack of stability is merely an effect or manifestation of the underlying cause of power-law breakdown. As described in earlier work by the author [8], subgrain-free or “structureless” 304 stainless steel exhibits power-law-breakdown behavior in a manner very analogous to the steady-state or subgraincontaining steel. A subsequent study by the author is determining the dependence of the subgrain size and the misorientation angle of adjacent subgrains on the strain from the onset of steady state to the 0.92 strain level. This further study should provide new insight as to the microstructural features responsible for creep resistance.
Acknowledgement Most of this research was supported by the U.S. 453
Volume 2. number 5B
MATERIALS LETTERS
Department of Energy, Office of Basic Energy Sciences, under Contract DE-AM03-76SF00326, Project Agreement DE-AT03-76ER 70057. Later stages of the work were supported by Lawrence Livermore National Laboratory under the auspices of the U.S. Department of Energy, Contract W-7405 -Eng48, and the Netherlands-America Commission for Educational Exchange.
VI R. Bhargava, J. Moteff and R.W. Swindeman, Symposium
[31
[41 [51 [61
References [ 11 R. Bhargava, J. Moteff and R.W. Swindeman, Metall. Trans. 7A (1976) 879.
454
August 1984
[71 [aI [91
on the Structural Materials for Service at Elevated Temperatures in Nuclear Power Generation, ASME (1975) pp. 31-54. S.N. Monteiro and H.-J. Kestenbach, Metall. Trans. 6A (1975) 938. H.-J. Kestenbach, T.L. da Silveira and S.N. Monteiro, Metall. Trans. 7A (1976) 155. K.D. Challenger and J. Moteff, Metall. Trans. 4 (1973) 749. D.J. Michel, J. Moteff and A.J. Love& Acta Metall. 21 (1973) 1269. G.M. Pharr, Scripta MetaJl. 15 (1981) 713. M.E. Kassner, A.K. Miller and O.D. Sherby, MetaJl. Trans. 13A (1982) 1977. C.M. Young, E.M. Cady and O.D. Sherby, U.S. Army Materials and Mechanical Research Center, Watertown, MA, Report CTR-72-27 (1972).