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Transmission electron microscopy study of fatigue deformation in Fe-25Cr-4Al alloy
Materials Science and Engineering, 95 (1987) 145-150
145
Transmission Electron Microscopy Study of Fatigue Deformation in Fe-25Cr-4AI Alloy S. C. TJONG, L. T. WU and N. J. HO Institute of Materials Science and Engineering, National Sun Yat-sen University, Kaohsiung (Taiwan) (Received December 22, 1986; in revised form March 16, 1987)
ABSTRACT A ferritic F e - 2 5 C r - 4 A l alloy (where the composition is in weight per cent) was cyclically deformed at room temperature. Transmission electron microscopy (TEM) was used to investigate the dislocation structures developed in this alloy after cycling at various total strain amplitudes. TEM observations o f a specimen deformed at a low strain amplitude (0.2%) revealed the presence o f jogged screw dislocations, dislocation tangles and loop debris. A s the total strain amplitude increased to 0.3%, there was a greater tendency for dis. location bundles to form in the alloy. These bundles were condensed into walls o f a ragged cell structure as the applied strain amplitudes were subsequently increased. On further cycling o f the alloy at a higher strain amplitude (2.0%), the dislocation density increased markedly, leading to the formation o f a highly elongated and banded dislocation structure.
1. INTRODUCTION Ferritic Fe-Cr-A1 stainless alloys are increasingly being used in high temperature applications such as heating elements, nuclear reactors and petroleum refineries [ 1]. These alloys generally contain about 12-30 wt.% Cr, and they have good resistance to oxidizing environments, as they form protective oxide barriers which inhibit subsequent reactions between the alloys and environments. The oxidation of Fe-Cr-A1 alloys depends on the selective oxidation of aluminium, leading to the formation of a protective ~-A1203 scale. In actual practice, good resistance to oxidation requires that the protective ~-A120 a scale maintains good adherence to the alloy sub0025-5416/87/$3.50
strate during cyclic oxidation. However, the ~-A1203 scale usually tends to spall from the substrate surface during thermal cycling. The adherence of oxide scales can be improved by the addition of either reactive elements, such as yttrium [2 ], or Y203 dispersoid particles [ 3]. It is generally known that the addition of aluminium (more than 8 wt.%) to iron-base alloys can lead to the occurrence of high temperature embrittlement [ 4]. Fe-Cr alloys with aluminium contents of 4-6 wt.% were found to have good oxidation resistance [2, 5 , 6 ] and mechanical properties [ 7 ]. It is well known that the yield strength of b.c.c, metals is strongly dependent on temperature [ 8 , 9 ]. This temperature dependence is more pronounced at temperatures below a critical temperature (223 °C) and is considered to be caused by the effects on the mobility of screw dislocations in the b.c.c, lattice [8]. Furthermore, dislocation gliding in b.c.c. metals usually occurs on different slip systems in tension and in compression [10-12]. Such slip asymmetry can lead to stress asymmetries and shape changes of specimens when cyclically deformed in a symmetric tension-compression mode [13-16]. In the past, many studies have been carried o u t on the dislocation substructures formed in cyclic deformation and cyclic deformation behaviours of f.c.c. metals and alloys [17-24]. In contrast, fewer studies have been made on the microstructures of cyclically deformed b.c.c, metals and alloys [25,26]. This study is a preliminary investigation of dislocation microstructures developed in a polycrystalline Fe-25Cr-4A1 alloy (where the composition is in weight per cent) after cycling at r o o m temperature and at various total strain amplitudes. In this paper, we shall report some results obtained in the investigation. These results will pro@)Elsevier Sequoia/Printed in The Netherlands
146
vide useful information which will benefit understanding of cyclic deformation behaviour of Fe-Cr-A1 alloys containing yttrium oxide dispersoids at elevated temperatures.
rinsed in a reagent-grade methanol and distilled water and then examined with a JEOL 200CX scanning transmission electron microscope, operated at 200 kV.
2. E X P E R I M E N T A L
3. R E S U L T S A N D D I S C U S S I O N
DETAILS
The ferritic Fe-25Cr-4A1 alloy was prepared from electrolytic iron, high purity chromium and aluminium in a vacuum induction furnace (Leybold-Heraus IS8/III) under a controlled protective argon atmosphere. The cast ingot was then h o t forged into plates of 16 mm thickness. The alloy was solution treated at 1050 °C for 3 h and subsequently quenched into oil. Then the solution-treated plates were machined for fatigue testing. These specimens were wet ground down to 600 grit emery paper and then polished with alumina powder down to 0.05 pm grade alumina. Fatigue tests were conducted at r o o m temperature under fully reversed strain control by using an Instron (model 1332) closed-loop servohydraulic testing machine. A clip-on extensometer of 10.0 mm gauge length was used to measure and control total strain amplitudes. Sine wave signals were employed for strain control at various frequencies ranging from 0.05 to 1 Hz. Woods' metal was used to aid alignment of specimens. Stress-strain hysteresis loops were plotted on an x - y recorder during fatigue testing. There are two different types of development of dislocation structures in cyclic deformation. One is the development of dislocation structures which occur when cyclic strain hardening is taking place at a given total strain amplitude. The other is the development of dislocation structures which occur when the strain amplitude is changed after a saturation stage is reached at a given strain amplitude. The microstructures in this paper are related to the former case. This work was performed at a constant-total-straincontrolled fatigue testing. The specimens were then sectioned beneath the fracture surface for transmission electron microscopy (TEM) studies, and disc specimens of 3 mm diameter were punched o u t and thinned by jet polishing. A polishing solution containing 10% perchloric acid and 90% acetic acid was used. Following perforation, disc specimens were
Figure 1 shows the cyclic stress responses of the Fe-25Cr-4A1 alloy fatigued at various total strain amplitudes Aet/2. It is apparent that the alloy investigated exhibited an initial stage of fatigue cyclic hardening which is followed by the saturation state, as was observed in other b.c.c, single crystals [26]. The cyclic hardening rates increase with increasing total strain amplitudes. The initial hardening rate increased markedly when fatigue tests were carried o u t at a high total strain amplitude of 2%, and negligible hardening was observed at a total strain amplitude of 0~2%. The cyclic stress amplitudes which were measured at the saturation stage from the hysteresis loops of the companion specimens were plotted vs. the total strain amplitudes, as shown in Fig. 2. Figure 3 consists of TEM micrographs of the Fe-25Cr-4A1 alloy cycled at a total strain amplitude of 0.2%. By careful tilting, it was possible to make the primary screw dislocations o u t of contrast, and only secondaries and some edge dislocations remained. As can be seen in Fig. 3(a), long and heavily jogged screw dislocations developed in the alloy after cycling at this strain amplitude. Edge dipole dislocations (shown in Fig. 3(b)) which may be produced by the dragging of screw dislocation jogs [25], dislocation tangles and loop
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Fig. 1. C y c l i c h a r d e n i n g c u r v e s o f t h e F e - 2 5 C r - 4 A I a l l o y c y c l e d at v a r i o u s t o t a l s t r a i n a m p l i t u d e s .
147 600
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Fig. 2. Cyclic (-- -) and monotonic ( strain curves of the Fe-25Cr-4A1 alloy.
) stress-
debris {shown in Fig. 3(c)) were also observed. The formation of jogs, tangles and loop debris are general features that dislocation structures have when produced in b.c.c, single crystals cycled at low strain amplitudes [26,27]. These features correspond to the low cyclic hardening behaviour during cycling. At a total strain amplitude of 0.3%, a substantial change in dislocation structures occurred in the present alloy. In this case, dislocation bundles were observed, as shown in Fig. 4. These bundles were considered to be formed from the edge multipoles and debris [28]. With a further increase in the strain amplitude to 0.4%, the dislocation bundles seem to have moved together to form dense dislocation walls (Fig. 5). This figure also shows an early stage of the development of a ragged two-dimensional cell structure, as reported by Mori e t al. [26]. The cyclic strain hardening gradually becomes prominent as the cell structures are developed. A well-developed ragged cell structure was observed in a specimen fatigued at a total strain amplitude of 0.6% (Fig. 6(a)). In this figure, it can be seen that the ragged cell structure appears to have a dislocation configuration similar to the so-called labyrinth structure in fatigued f.c.c. metals [29, 30]. The labyrinth structure in f.c.c, metals can be formed through the operation of two slip systems [30]. Figure 6(b) is a TEM micrograph showing a ragged cell
Fig. 3. D i s l o c a t i o n a r r a n g e m e n t s in t h e F e - 2 5 C r - 4 A l alloy cycled to f r a c t u r e at A e t / 2 = 0.2%, s h o w i n g (a) jogged screw dislocations, (b) dipoles a n d (c) disl o c a t i o n tangles a n d l o o p debris.
structure developed in a specimen fatigued at a total strain amplitude of 0.6%. This figure was taken at the foil normal of [ i 1 3 ] orientation. A large number of dislocations are also seen between cell walls. These dislocations are probably emitted from cell walls during
148
Fig. 4. Dislocation bundles observed at A e t / 2 = 0.3%.
Fig. 5. Ill-defined dislocation walls in the alloy cycled to fracture at A e t / 2 = 0.4%.
Fig. 6. Well-developed dislocation walls and ragged cell structures in the F e - 2 5 C r - 4 A l alloy cycled to fracture at ~ e t / 2 = 0.6%: (a) zone axis, [001]; (b) zone axis, [113].
Fig. 7. F o r m a t i o n o f t h e elongated cell in the F e - 2 5 C r - 4 A I alloy cycled to fracture at A e J 2 = 1% (zone axis, [123]).
149
Fig. 8. TEM micrograph of the Fe-25Cr-4A1 alloy cycled to fracture at Act/2 = 2%, showing an elongated and banded dislocation structure (zone axis, [011]).
cycling. T h a t is, the plastic strain applied is c o n s i d e r e d t o have m a i n l y been a c c o m m o d a t e d b y the t o - a n d - f r o m o t i o n o f the screw dislocations t h a t are c r e a t e d at cell walls [26]. Figure 7 shows a d i s l o c a t i o n s t r u c t u r e dev e l o p e d in a specimen fatigued at a strain a m p l i t u d e o f 1%. The p r i m a r y features to n o t e in this figure are the e l o n g a t e d cell structure, i n d i c a t e d b y the a r r o w in the micrograph. The interiors o f t h e e l o n g a t e d cells are relatively free o f dislocations. TEM examination o f a specimen cyclically d e f o r m e d at a higher strain a m p l i t u d e o f 2% revealed t h a t highly e l o n g a t e d bands w h i c h consist o f high d e n s i t y dislocations were f o r m e d , as s h o w n in Fig. 8. The length o f the d i s l o c a t i o n bands m e a s u r e d was m o r e t h a n 12.2 pm.
2 J. K. Tien and F, S. Pettit, Metall. Trans.. 3 (1972) 1587. 3 T. A. Ramanarayanan, M. Rahgavan and R. Petkovic-Luton, J. Electrochem. Soc., 131 (1984) 923. 4 D. Hardwick and G. R. Wallwork, Rev. HighTemp. Mater., 4 (1978) 48. 5 G. C. Wood and F. H. Stott, in R. A. Rapp (ed.), Proc. 6th Int. Conf. on High Temperature Corrosion, San Diego, CA, 1981, National Association
6 7 8 9 10 11
4. CONCLUSION T h e dislocation s t r u c t u r e s o f the F e - 2 5 C r 4A1 alloy c y c l e d at a low strain a m p l i t u d e o f 0.2% consist o f j o g g e d screw dislocations and l o o p debris. H o w e v e r , at a strain a m p l i t u d e o f 0.6%, the d i s l o c a t i o n s t r u c t u r e s consist o f well-developed d i s l o c a t i o n walls and ragged cells. F u r t h e r m o r e , cycling o f the alloy at a higher strain a m p l i t u d e o f 2% results in the f o r m a t i o n o f a highly e l o n g a t e d and b a n d e d dislocation structure.
12 13 14 15 16 17 18 19 20
REFERENCES
21 22
1 S. D. Sastry, P. K. Rohatgi, K. P. Abraham and Y. V. R. K. Prasay, Scr. Metall., 13 (1979) 817.
23
of Corrosion Engineers, Houston, TX, 1981, p. 227. I. Kvernes, M. Oliveira and P. Kofstad, Corros. Sci., 17 (1977) 237. S. D. Sastry, P. K. Rohatgi, K. P. Abraham and Y. V. R. K. Prasay, Met. Technol., 7 (1980) 393. A. Luft and L. Kaun, Phys. Status Solidi, 3 7 (1970) 781. V. Vitek, Cryst. Lattice Defects, 5 (1974) 1. L. P. Kubin, Rev. Deform. Behav. Mater., 1 (1977) 244. L.P. Kubin, Rev. Deform. Behav. Mater., 3 (1982) 181. H. Mughrabi and Ch. Wurthrich, Philos. Mag., 33 (1976) 963. R. Neumann, Z. Metallkd., 66 (1975) 26. T. Magnin and J. H. Driver, Mater. Sci. Eng., 39 (1979) 175. M. Anglada, B. Etemad, J. A. Planell and F. Guiu, Scr. Metall., 14 (1980) 1319. M. Anglada and F. Guiu,Philos. Mag., 44 (1981) 499. K. D. Challenger and J. Moteff, Metall. Trans., 3 (1972) 675. H. Nahm, J. Moteff and D. R. Dierks, Acta Metall., 25 (1977) 107. R. Wang and H. Mughrabi, Mater. Sci. Eng., 63 (1984) 147. F. Ackermann, L. P. Kubin, J. Lepinoux and H. Mughrabi, Acta Metall., 32 (1984) 715. B. D. Yan, A. Hunsche, P. Neumann and C. Laird, Mater. Sci. Eng., 79 (1986) 9. B. D. Yan, G. C. Farrington and C. Laird, Acta Metall., 33 (1985) 1533. C. Laird, S. Stanzl, R. De La Veaux and L. Buch-
150 inger, Mater. Sci. Eng., 80 (1986) 143. 24 C. Laird, P. Charsley and H. Mughrabi, Mater. Sci, Eng., 81 (1986) 433. 25 G. Taylor and J. W. Christian, Philos. Mag., 15 (1967) 893. 26 H. Mori, M. Tokuwame and T. Miyazaki, Philos. Mag., 40 (1979) 409.
27 J. Planell and F. Guiu, Philos. Mag., 54 (1986) 343. 28 T. Magnin, A. Fourdeux and J. H. Driver, Phys. Status Solidi, 65 (1981) 301. 29 P. Charsley, Mater. Sci. Eng., 47 (1981) 181. 30 P. Charsley and D. Kuhlmann-Wilsdorf, Philos. Mag., 44 (1981) 1351.
145
Transmission Electron Microscopy Study of Fatigue Deformation in Fe-25Cr-4AI Alloy S. C. TJONG, L. T. WU and N. J. HO Institute of Materials Science and Engineering, National Sun Yat-sen University, Kaohsiung (Taiwan) (Received December 22, 1986; in revised form March 16, 1987)
ABSTRACT A ferritic F e - 2 5 C r - 4 A l alloy (where the composition is in weight per cent) was cyclically deformed at room temperature. Transmission electron microscopy (TEM) was used to investigate the dislocation structures developed in this alloy after cycling at various total strain amplitudes. TEM observations o f a specimen deformed at a low strain amplitude (0.2%) revealed the presence o f jogged screw dislocations, dislocation tangles and loop debris. A s the total strain amplitude increased to 0.3%, there was a greater tendency for dis. location bundles to form in the alloy. These bundles were condensed into walls o f a ragged cell structure as the applied strain amplitudes were subsequently increased. On further cycling o f the alloy at a higher strain amplitude (2.0%), the dislocation density increased markedly, leading to the formation o f a highly elongated and banded dislocation structure.
1. INTRODUCTION Ferritic Fe-Cr-A1 stainless alloys are increasingly being used in high temperature applications such as heating elements, nuclear reactors and petroleum refineries [ 1]. These alloys generally contain about 12-30 wt.% Cr, and they have good resistance to oxidizing environments, as they form protective oxide barriers which inhibit subsequent reactions between the alloys and environments. The oxidation of Fe-Cr-A1 alloys depends on the selective oxidation of aluminium, leading to the formation of a protective ~-A1203 scale. In actual practice, good resistance to oxidation requires that the protective ~-A120 a scale maintains good adherence to the alloy sub0025-5416/87/$3.50
strate during cyclic oxidation. However, the ~-A1203 scale usually tends to spall from the substrate surface during thermal cycling. The adherence of oxide scales can be improved by the addition of either reactive elements, such as yttrium [2 ], or Y203 dispersoid particles [ 3]. It is generally known that the addition of aluminium (more than 8 wt.%) to iron-base alloys can lead to the occurrence of high temperature embrittlement [ 4]. Fe-Cr alloys with aluminium contents of 4-6 wt.% were found to have good oxidation resistance [2, 5 , 6 ] and mechanical properties [ 7 ]. It is well known that the yield strength of b.c.c, metals is strongly dependent on temperature [ 8 , 9 ]. This temperature dependence is more pronounced at temperatures below a critical temperature (223 °C) and is considered to be caused by the effects on the mobility of screw dislocations in the b.c.c, lattice [8]. Furthermore, dislocation gliding in b.c.c. metals usually occurs on different slip systems in tension and in compression [10-12]. Such slip asymmetry can lead to stress asymmetries and shape changes of specimens when cyclically deformed in a symmetric tension-compression mode [13-16]. In the past, many studies have been carried o u t on the dislocation substructures formed in cyclic deformation and cyclic deformation behaviours of f.c.c. metals and alloys [17-24]. In contrast, fewer studies have been made on the microstructures of cyclically deformed b.c.c, metals and alloys [25,26]. This study is a preliminary investigation of dislocation microstructures developed in a polycrystalline Fe-25Cr-4A1 alloy (where the composition is in weight per cent) after cycling at r o o m temperature and at various total strain amplitudes. In this paper, we shall report some results obtained in the investigation. These results will pro@)Elsevier Sequoia/Printed in The Netherlands
146
vide useful information which will benefit understanding of cyclic deformation behaviour of Fe-Cr-A1 alloys containing yttrium oxide dispersoids at elevated temperatures.
rinsed in a reagent-grade methanol and distilled water and then examined with a JEOL 200CX scanning transmission electron microscope, operated at 200 kV.
2. E X P E R I M E N T A L
3. R E S U L T S A N D D I S C U S S I O N
DETAILS
The ferritic Fe-25Cr-4A1 alloy was prepared from electrolytic iron, high purity chromium and aluminium in a vacuum induction furnace (Leybold-Heraus IS8/III) under a controlled protective argon atmosphere. The cast ingot was then h o t forged into plates of 16 mm thickness. The alloy was solution treated at 1050 °C for 3 h and subsequently quenched into oil. Then the solution-treated plates were machined for fatigue testing. These specimens were wet ground down to 600 grit emery paper and then polished with alumina powder down to 0.05 pm grade alumina. Fatigue tests were conducted at r o o m temperature under fully reversed strain control by using an Instron (model 1332) closed-loop servohydraulic testing machine. A clip-on extensometer of 10.0 mm gauge length was used to measure and control total strain amplitudes. Sine wave signals were employed for strain control at various frequencies ranging from 0.05 to 1 Hz. Woods' metal was used to aid alignment of specimens. Stress-strain hysteresis loops were plotted on an x - y recorder during fatigue testing. There are two different types of development of dislocation structures in cyclic deformation. One is the development of dislocation structures which occur when cyclic strain hardening is taking place at a given total strain amplitude. The other is the development of dislocation structures which occur when the strain amplitude is changed after a saturation stage is reached at a given strain amplitude. The microstructures in this paper are related to the former case. This work was performed at a constant-total-straincontrolled fatigue testing. The specimens were then sectioned beneath the fracture surface for transmission electron microscopy (TEM) studies, and disc specimens of 3 mm diameter were punched o u t and thinned by jet polishing. A polishing solution containing 10% perchloric acid and 90% acetic acid was used. Following perforation, disc specimens were
Figure 1 shows the cyclic stress responses of the Fe-25Cr-4A1 alloy fatigued at various total strain amplitudes Aet/2. It is apparent that the alloy investigated exhibited an initial stage of fatigue cyclic hardening which is followed by the saturation state, as was observed in other b.c.c, single crystals [26]. The cyclic hardening rates increase with increasing total strain amplitudes. The initial hardening rate increased markedly when fatigue tests were carried o u t at a high total strain amplitude of 2%, and negligible hardening was observed at a total strain amplitude of 0~2%. The cyclic stress amplitudes which were measured at the saturation stage from the hysteresis loops of the companion specimens were plotted vs. the total strain amplitudes, as shown in Fig. 2. Figure 3 consists of TEM micrographs of the Fe-25Cr-4A1 alloy cycled at a total strain amplitude of 0.2%. By careful tilting, it was possible to make the primary screw dislocations o u t of contrast, and only secondaries and some edge dislocations remained. As can be seen in Fig. 3(a), long and heavily jogged screw dislocations developed in the alloy after cycling at this strain amplitude. Edge dipole dislocations (shown in Fig. 3(b)) which may be produced by the dragging of screw dislocation jogs [25], dislocation tangles and loop
800
........
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........
i
........
i
........
,
........
=
2.0I
5O0 I . OX
~
O. BZ 0.4X
• -----" 400
<
<
~ 3oo
4o
5 2OO
2D lO0
loj
lo~ Number
oF
1o3 Reversolso
loW 2N
Fig. 1. C y c l i c h a r d e n i n g c u r v e s o f t h e F e - 2 5 C r - 4 A I a l l o y c y c l e d at v a r i o u s t o t a l s t r a i n a m p l i t u d e s .
147 600
•
•
•
,
•
•
•
8O 0
5OO
f
7O
IF" 0
~-4oo
6O V
5O O3 tD 300 4O
bq
W O~ L~
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200 I--
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100 l0 •
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.
•
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TRUE STRAIN (X)
Fig. 2. Cyclic (-- -) and monotonic ( strain curves of the Fe-25Cr-4A1 alloy.
) stress-
debris {shown in Fig. 3(c)) were also observed. The formation of jogs, tangles and loop debris are general features that dislocation structures have when produced in b.c.c, single crystals cycled at low strain amplitudes [26,27]. These features correspond to the low cyclic hardening behaviour during cycling. At a total strain amplitude of 0.3%, a substantial change in dislocation structures occurred in the present alloy. In this case, dislocation bundles were observed, as shown in Fig. 4. These bundles were considered to be formed from the edge multipoles and debris [28]. With a further increase in the strain amplitude to 0.4%, the dislocation bundles seem to have moved together to form dense dislocation walls (Fig. 5). This figure also shows an early stage of the development of a ragged two-dimensional cell structure, as reported by Mori e t al. [26]. The cyclic strain hardening gradually becomes prominent as the cell structures are developed. A well-developed ragged cell structure was observed in a specimen fatigued at a total strain amplitude of 0.6% (Fig. 6(a)). In this figure, it can be seen that the ragged cell structure appears to have a dislocation configuration similar to the so-called labyrinth structure in fatigued f.c.c. metals [29, 30]. The labyrinth structure in f.c.c, metals can be formed through the operation of two slip systems [30]. Figure 6(b) is a TEM micrograph showing a ragged cell
Fig. 3. D i s l o c a t i o n a r r a n g e m e n t s in t h e F e - 2 5 C r - 4 A l alloy cycled to f r a c t u r e at A e t / 2 = 0.2%, s h o w i n g (a) jogged screw dislocations, (b) dipoles a n d (c) disl o c a t i o n tangles a n d l o o p debris.
structure developed in a specimen fatigued at a total strain amplitude of 0.6%. This figure was taken at the foil normal of [ i 1 3 ] orientation. A large number of dislocations are also seen between cell walls. These dislocations are probably emitted from cell walls during
148
Fig. 4. Dislocation bundles observed at A e t / 2 = 0.3%.
Fig. 5. Ill-defined dislocation walls in the alloy cycled to fracture at A e t / 2 = 0.4%.
Fig. 6. Well-developed dislocation walls and ragged cell structures in the F e - 2 5 C r - 4 A l alloy cycled to fracture at ~ e t / 2 = 0.6%: (a) zone axis, [001]; (b) zone axis, [113].
Fig. 7. F o r m a t i o n o f t h e elongated cell in the F e - 2 5 C r - 4 A I alloy cycled to fracture at A e J 2 = 1% (zone axis, [123]).
149
Fig. 8. TEM micrograph of the Fe-25Cr-4A1 alloy cycled to fracture at Act/2 = 2%, showing an elongated and banded dislocation structure (zone axis, [011]).
cycling. T h a t is, the plastic strain applied is c o n s i d e r e d t o have m a i n l y been a c c o m m o d a t e d b y the t o - a n d - f r o m o t i o n o f the screw dislocations t h a t are c r e a t e d at cell walls [26]. Figure 7 shows a d i s l o c a t i o n s t r u c t u r e dev e l o p e d in a specimen fatigued at a strain a m p l i t u d e o f 1%. The p r i m a r y features to n o t e in this figure are the e l o n g a t e d cell structure, i n d i c a t e d b y the a r r o w in the micrograph. The interiors o f t h e e l o n g a t e d cells are relatively free o f dislocations. TEM examination o f a specimen cyclically d e f o r m e d at a higher strain a m p l i t u d e o f 2% revealed t h a t highly e l o n g a t e d bands w h i c h consist o f high d e n s i t y dislocations were f o r m e d , as s h o w n in Fig. 8. The length o f the d i s l o c a t i o n bands m e a s u r e d was m o r e t h a n 12.2 pm.
2 J. K. Tien and F, S. Pettit, Metall. Trans.. 3 (1972) 1587. 3 T. A. Ramanarayanan, M. Rahgavan and R. Petkovic-Luton, J. Electrochem. Soc., 131 (1984) 923. 4 D. Hardwick and G. R. Wallwork, Rev. HighTemp. Mater., 4 (1978) 48. 5 G. C. Wood and F. H. Stott, in R. A. Rapp (ed.), Proc. 6th Int. Conf. on High Temperature Corrosion, San Diego, CA, 1981, National Association
6 7 8 9 10 11
4. CONCLUSION T h e dislocation s t r u c t u r e s o f the F e - 2 5 C r 4A1 alloy c y c l e d at a low strain a m p l i t u d e o f 0.2% consist o f j o g g e d screw dislocations and l o o p debris. H o w e v e r , at a strain a m p l i t u d e o f 0.6%, the d i s l o c a t i o n s t r u c t u r e s consist o f well-developed d i s l o c a t i o n walls and ragged cells. F u r t h e r m o r e , cycling o f the alloy at a higher strain a m p l i t u d e o f 2% results in the f o r m a t i o n o f a highly e l o n g a t e d and b a n d e d dislocation structure.
12 13 14 15 16 17 18 19 20
REFERENCES
21 22
1 S. D. Sastry, P. K. Rohatgi, K. P. Abraham and Y. V. R. K. Prasay, Scr. Metall., 13 (1979) 817.
23
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