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Secondary cyclic hardening and dislocation structures in type 316 stainless steel at 600°C
Materials Science and Engineering, 84 (1986) 23-33
Secondary
23
C y c l i c Hardening and Dislocation Structures in T y p e 3 1 6 Stainless
Steel at 6 0 0 °C M. GERLAND and P. VIOLAN
Laboratoire de M~canique et de Physique des Mat~riaux, Unit~ associ~e 863 au CNRS, Ecole Nationale Supdrieure de Mdcanique et d'Adrotechnique, 86034 Poitiers Cddex (France) (Received December 20, 1 9 8 5 ; i n revised form February 2t, 1986)
ABSTRACT
Polycrystalline type 316 stainless steel has been fatigued in air and in vacuum at 600 °C at two constant plastic strain amplitudes. Although in air the curve o f cyclic stress versus number o f cycles is prematurely interrupted by failure, in vacuum it is sufficiently extended to allow secondary cyclic hardening to occur. Transmission electron microscopy observations show dislocation configurations and evolutions o f configurations (during the fatigue life) which are similar for both strain amplitudes. During the saturation plateau, several configurations coexist: regular walls, a labyrinth structure and persistent slip bands (PSBs) with a ladder structure. A t failure (in vacuum) a cellular structure widely prevails while a labyrinth structure and PSBs with a ladder structure no longer exist. We propose different mechanisms to explain the evolution o f the dislocation configurations: the formation o f a labyrinth structure, the change o f walls into cells and the change o f PSBs into cells. We also explain the secondary cyclic hardening by the evolution o f microstructure.
1. INTRODUCTION
A great number of transmission electron microscopy (TEM) studies of dislocation configurations in f.c.c, metals fatigued to saturation have been reported in the literature. Most of these studies have been investigations of copper [1-14] or copper alloys [1, 1 5 , 1 6 ] , either on polycrystals [3-5, 11, 13, 15, 16] or on single crystals oriented for single slip [1, 2, 6-8, 12, 13] or multiple slip [9, 10]. More0025-5416/86/$3.50
over, all these studies have been performed at r o o m temperature. Among the most interesting features are the development of dipolar walls and labyrinth structure on the one hand and persistent slip bands (PSBs) on the other hand. PSBs have been observed in single crystals and polycrystals and have been connected either with the plateau region of the cyclic stress-strain curve [14, 1 7 - 2 0 ] , where they gradually increase throughout the fatigue life [5, 12], or with cyclic softening [11, 21]. These dislocation configurations have not been examined so frequently in austenitic stainless steels except for the work of Bernard et al. [22], Gorlier and coworkers [23, 24] and L'Esperance et al. [25]. The formation and change in these different structures are not y e t clearly known in spite of the various propositions made during the last few years. It is well known that the fatigue life is generally increased when tests are performed in vacuum instead of in air. This increase in fatigue life allows the microstructure to evolve in a significant way and thus makes it possible to study microstructural changes which could n o t occur during tests in air because of premature failure. At the same time, instead of cyclic softening which corresponds to the closeness of failure, secondary cyclic hardening can occur if the fatigue life is prolonged by fatiguing in vacuum [12, 13, 26]. In order to contribute to the understanding of the mechanisms of microstructural changes and the p h e n o m e n o n of secondary cyclic hardening, we report here the results of studies on t y p e 316 stainless steel fatigued at a constant plastic strain amplitude in air and in vacuum and we propose some mechanisms of microstructure evoIution. © Elsevier Sequoia/Printed in The Netherlands
24 TABLE 1 Chemical composition of the type 316SPH stainless steel
Element C Amount(wt.%) 0.022
Mn 1.69
Si 0.31
S 0.002
P 0.023
2. EXPERIMENTAL DETAILS
2.1. Specimens The t y p e 316SPH stainless steel investigated has the composition given in Table 1. PolycrystaUine specimens were prepared from sheet supplied by Creusot-Loire for the Super Phenix supergenerator. After machining, the grip ends had a diameter of 16 mm, a gauge length of 10 m m and a gauge diameter of 6 mm. Before fatigue, specimens were ground with very fine grinding paper, heat treated for I h at 1050 °C in high vacuum (about 5 X 10 -4 Pa) and water cooled; they were then electropolished in a solution of 10% perchloric acid and 90% acetic acid. 2.2. Cyclic deformation Cyclic deformation was performed in a symmetric uniaxial push-pull m o d e in an electromechanical machine at 600 °C in vacuu m (about 10 -3 Pa). Samples were tested at t w o constant plastic strain amplitudes (+ 2 X 10 -3 and + 5 X 10 -3) to failure or to a number of cycles corresponding to failure in air.* A triangular waveform c o m m a n d signal from a digital function generator was used for plastic strain control. The test frequency was 0.2 Hz for the amplitude of 2 X 1 0 - 3 and 0.1Hz for the amplitude of 5 X 10 -3, which corresponds to a constant plastic strain rate of 2 X 10 -3 s-z.
Ni 11.90
Cr 17.45
B 0.009
N2 0.069
Co 0.190
As 0.004
3. RESULTS
3.1. Fatigue hardening-softening curves Figure 1 shows the cyclic hardening-softening curves obtained by plotting the continuously recorded stress amplitude Aa/2 versus the number N of cycles. The two curves are obtained at 600 °C with plastic strain amplitudes of 2 X 10 -a and 5 X 10 -3. It should be noted that saturation starts earlier for the higher plastic strain amplitude. The numbers of cycles to failure in vacuum are 32 000 and 4800 respectively for plastic strain amplitudes of 2 X 10 -3 and 5 X 10 -3. The interrupted tests in vacuum were continued up to the number of cycles corresponding to failure in air, giving 8000 and 700 cycles at 2 X 10 -3 and 5 X 1 0 -3 respectively. These values fall within the dispersion band on the Manson-Coffin curve [27] and correspond to the end of the saturation plateau. For Ae, = 2 X 10 -3, after the saturation plateau the stress amplitude decreases conI
I
I
I
A 0"/2 (MPa)
/
/
°
I *Tests in air at the same temperature and with specimens from the same sheet have been carried o u t at Ecole Nationale Sup~rieure des Mines de SaintEtienne by C. Gorlier and J. H. Driver.
Cu 0.110
thinning to perforation using the double-jet technique in an acetic acid-perchloric acid solution. Observations were realized on a JEOL-100 B microscope at 100 kV with a double-tilt stage.
|
2.3. Transmission electron microscopy Fatigued specimens had been sectioned at a thickness of a b o u t 1 mm, normal and parallel to the applied strain axis with a spark cutter. All the sections were polished with fine paper d o w n to a thickness of 0.3 m m and then cut into discs b y spark machining. These discs were further milled with very fine paper to 0.1 mm, and the final thin foils obtained by
Mo 2.25
I
_
~Lo'
~,d
lO3
N (cycles)
1~
1~os
Fig. 1. Cyclic hardening-softening curves (T = 600 °C; vacuum).
25
tinuously until failure while, for Aep = 5 X 10 .3 , after the saturation plateau, secondary cyclic hardening occurs in vacuum. This type of secondary cyclic hardening has already been noted in various materials, e.g. ~-Fe [28] and copper single crystals and polycrystals [4, 11-13, 26]. This secondary cyclic hardening occurs earlier for the higher plastic strain amplitude and more easily in polycrystals than in single crystals for the same plastic strain amplitude [13]. Secondary cyclic hardening could not be observed in air because failure occurs, thus stopping the evolution of microstructure. However, tests in vacuum allow a sufficiently large n u m b e r of cycles to be reached that microstructural evolution takes place.
Fig. 2. Well-developed wall s t r u c t u r e o b s e r v e d o n a ( 1 1 0 ) s e c t i o n (walls parallel t o [ 1 1 0 ] ; A e p ] 2 = 2 x 10-3; N = 8 0 0 0 cycles).
3.2. Dislocation structures observed during cyclic saturation The dislocation arrangement in the specimen fatigued at Aep/2 = 2 X 10 .3 up to 8000 cycles is characterized by the coexistence of (i) a well-developed wall structure generally parallel to the [110] direction (Fig. 2), (ii) a two-system wall structure (the labyrinth
Fig. 3. T w o - s y s t e m wall s t r u c t u r e o b s e r v e d o n a ( 1 1 0 ) s e c t i o n ( A e p / 2 = 2 × 10-3; N = 8 0 0 0 cycles).
26
structure (Fig. 3)) and (iii) the well-known ( i 0 1 ) ladder structure of PSBs (Fig. 4). The few areas o f misoriented cell structure (on the right-hand side at the b o t t o m in Fig. 5) should also be noted. The wall spacing is a b o u t 0.5 pm in the regular wall structure and a b o u t 1.2 pm in the ladder structure. The dislocation arrangement of the specimen fatigued at A e p / 2 = 5 X 1 0 -3 up to 700 cycles is also characterized b y the coexistence of the three above-mentioned structures, the wall spacing being a b o u t 0.35 pm in the wall structure (Fig. 6) and a b o u t 1 pm in the ladder structure (Fig. 7). The very few observed cell structures are located near PSBs or at twin boundaries.
3.3. Dislocation structures observed after failure The dislocation structure formed after failure at Aep/2 = 2 X 10 -3 consists essentially of misoriented cells, either elongated cells elaborated from wall structure (Fig. 8, centre) or quasi-equiaxed cells (Fig. 8, left- and righthand sides). Some areas composed of wall structures are also present. The spacing between these walls is a b o u t 0.70 pm while the mean equiaxed cell diameter is a b o u t 0.75 pm. The length of the elongated cells is a b o u t 1.5-2 p m and their width is the same as the wall spacing. The interior of the cells or the channels is generally free of dislocations.
After failure at a plastic strain amplitude of 5 X 1 0 - 3 , the dislocation arrangement is rather similar to the structure obtained at a plastic strain amplitude of 2 X 10 -3 (Fig. 9). However, the wall spacing and the mean equiaxed
Fig..4. P S B with a ladder structure observed on a (121) section (Aep/2 = 2 x 10-3; N = 8000 cycles).
Fig. 5. Coexistence of walls, PSBs and cells o n a (121) s e c t i o n (Aep/2 = 2 x 10-3; N = 8000 cycles). The miso r i e n t a t i o n b e t w e e n cells c a n b e s e e n in t h e l o w e r right-hand corner.
27
Fig. 6. Walls and labyrinth structure observed on a (110) section (Aep/2 = 5 X 10-3; N = 700 cycles). Misoriented cells can be seen near the twin boundary.
28
Fig. 7. PSBs with a ladder structure and misoriented cells observed on a (110) section (Aep/2 = 5 × 10-3; N = 700 cycles).
cell diameter are 0.60 pm and 0.65 pm respectively while the length of the elongated cells is about 1 pm (Fig. 10). Contrary to the observations made at a plastic strain amplitude of 2 X 10 -3, the interior of the channels contains m a n y dislocations (Fig. 11).
4. DISCUSSION
4.1. Mechanical behaviour and dislocation configurations We have already mentioned that, in the plateau region and for both strain amplitudes, three structures coexist, as observed by Ackerman et al. [7] : wall or labyrinth structures, PSBs and cell structures, in the approximate amounts 70%, 20% and 10% respectively. Much later in the fatigue life, this microstructural configuration is changed into a structure composed of cells (elongated and equiaxed) and residual walls in the proportions of about 75% and 25% respectively. Therefore, if the cell structure is n o t characteristic of failure, it is at least typical of a stage close to failure.
I
Fig. 8. Elongated and equiaxed cells observed on a (013) section (Aep/2 = 2 × 10-3; Nf = 3.2 X 104). Marked misorientation can be seen between cells.
29 ~btained with a plastic strain amplitude of 5 X 10 -3 is possible because cycling in vacuum greatly increases the fatigue life as reported by Mendez [29] and Wang et al. [12] for copper and by Belamri [30] for type 316L stainless steel. This hardening can be associated with the reorganization of the dislocation structure from a wall structure to a cell structure. A wall structure allows very easy dislocation m o v e m e n t in the channels, while a cell structure does n o t permit dislocation movem e n t so easily and thus requires a higher stress to accommodate the applied plastic strain. The transformation of the wall structure into the cell structure can be seen in Fig. 11 (upper right-hand side and b o t t o m ) . From dislocation-free channels, first there is a quasiuniform repartition of dislocations in the channels, followed by the final formation of the cells. We t h i n k t h a t this transformation is responsible for secondary hardening.
4.2. Microstructural evolution during fatigue life Fig. 9. Elongated and equiaxed cells observed on a (110) section (Aep/2 = 5 X 10-3;Nf --- 4.8 X 103). Dislocations in the interior of elongated cells can be seen.
Fig. 10. Elongated cells with weak misorientation o n a (110) section (Aep/2 = 5 x 10-3; Nf = 4.8 x 103).
As can be expected, the greater the applied plastic strain amplitude, the smaller are the mean cell diameter and the wall spacing (cf. Section 3.3). The presence of secondary cyclic hardening
Numerous observations made on several thin foils and on grains with different orientations show that the labyrinth structure is derived from the wall structure (cf. Figs. 3 and 6), as shown in Fig. 12. Short and thick uncondensed walls form perpendicular to the channel direction from the primary wall dislocations (Fig. 6, A) and gradually cover several wall spacings. The configuration which appears in Fig. 6 and Fig. 12(b) is similar to t h a t observed by Charsley and Kuhlmann-Wilsdorf [ 15] and Charsley [16] in copper and reported in ref. 16, Fig. 2. Then the evolution continues until the usual labyrinth structure is formed, the wide walls becoming as narrow as the primary walls. As we have already stated, the dislocation arrangement was mostly composed of walls during the saturation stage, whereas at failure the cell structure strongly predominates. From the different observations performed on thin foils, we suggest four possible mechanisms of microstructure evolution to explain how this transformation occurs.
4.2.1. Direct transformation The first possibility shown schematically in Fig. 13 is the direct transformation from a wall structure into elongated cells by the
Fig. 11. C o e x i s t e n c e o f a m i s o r i e n t e d cell s t r u c t u r e a n d a wall s t r u c t u r e evolving in a cell s t r u c t u r e o b s e r v e d o n a ( 1 2 1 ) s e c t i o n ( A e p / 2 = 5 X 1 0 - 3 ; N f = 4.8 X 103). Th'e s t r o n g d e n s i t y o f d i s l o c a t i o n s in t h e f o r m e r c h a n n e l s c a n b e seen as cells in f o r m a t i o n A.
O
31
(c
Fig. 12. Schematic drawing showing the evolution from (a) a wall structure through (b) the formation of thick walls perpendicular to the channel direction to (c) a labyrinth structure.
Fig. 13. Schematic drawing showing the direct transformation from (a) a wall structure to (b) elongated cells. Fig. 15. Wall destruction and beginning of cell formation (Aep/2 = 2 x 10-3;N = 8000 cycles). :~:.-~.'~,.<,
(a)
b)
(c)
Fig. 14. Schematic drawing showing (b) the wall destruction and random dislocation distribution as the intermediate stage between (a) the wall structure and (c) the cell structure.
p a r t i t i o n i n g o f t h e c h a n n e l s as seen in Fig. 8. T h e w i d t h o f t h e s e cells is t h e n t h e s a m e as t h e wall spacing. T h e e l o n g a t e d cells c a n later t r a n s f o r m i n t o q u a s i - e q u i a x e d cells.
4.2.2. Wall destruction A s e c o n d w a y o f e v o l u t i o n f r o m a wall s t r u c t u r e t o cells is d e p i c t e d in Fig. 14. T h e walls are progressively d e s t r o y e d a n d t h e disl o c a t i o n s w h i c h c o m p o s e d t h e s e walls s p r e a d o u t u n i f o r m l y i n t o t h e c h a n n e l s a n d at t h e site o f t h e f o r m e r walls. T h e n t h e s e u n i f o r m l y d i s t r i b u t e d d i s l o c a t i o n s progressively f o r m q u a s i - e q u i a x e d cells, w i t h a m e a n d i a m e t e r slightly g r e a t e r t h a n t h e f o r m e r wall spacing. This s i t u a t i o n c a n be seen in Fig. 11, A, a n d in Fig. 15.
(c)
(d)
Fig. 16. Schematic drawing showing the evolution from (a) a wall structure to (d) a cell structure (all labyrinth walls have disappeared) with (b) the intermediate labyrinth structure ((c) the beginning of cell formation in the labyrinth structure).
4.2.3. Use o f the labyrinth structure The third possibility of transformation of walls a n d c h a n n e l s t o cell s t r u c t u r e uses t h e l a b y r i n t h s t r u c t u r e . This e v o l u t i o n is s h o w n d i a g r a m m a t i c a l l y in Fig. 16. A f t e r t h e labyr i n t h s t r u c t u r e has b e e n f o r m e d , as s h o w n in
32
Fig. 12, some cells begin to form in different corners of the labyrinth, first using the segments of the walls as cell boundaries. Then, around these first cells, dislocations become present in the neighbourhood, in walls as well as in channels, will form new cells and so on. The beginning of the cell formation in labyrinth structure can be seen in Fig. 6, B. In the same figure, in the twin, the formation of a cell structure is already complete, presumably because of a more favourable orientation of the channels. The mean cell diameter is in this case roughly equal to or slightly greater than the former wall spacing.
4.2.4. Transformation from the ladder structure o f persistent slip bands The wall structure (true walls and labyrinth) is not the only structure to transform into a cell structure, even if it is markedly more widespread than other structures (at least in our study). Cell formation can also be seen in the PSBs containing a ladder structure. This evolution has already been observed in copper single crystals by Wang et al. [12], Wang and Mughrabi [13] and Lepist5 et al. [10]. Jin and Winter [8] have also observed a regular labyrinth structure with PSBs in copper single crystals. The ladders of these PSBs are n o t very regular and the start of cell formation can be seen locally. The coexistence of cells and PSB ladder structures is also shown in their work reported in ref. 9. In Figs. 4, 5 and 7, some PSBs with ladders can be seen. Some of the ladders are n o t regular and have a t e n d e n c y to transform into cells. In several places, cell formation is quite complete, and this evolution may lead to the formation of new PSBs in the wall structure, as mentioned by Wang and Mughrabi [13].
5. CONCLUSIONS
The present study on t y p e 316 austenitic stainless steel deformed cyclically at a constant plastic strain amplitude at 600 °C in vacuum has led to the following main conclusions. (1) Secondary cyclic hardening can occur after the so~called cyclic saturation has been attained and this hardening is linked to a continuous microstructural evolution. This
secondary cyclic hardening occurs only after a great number of cycles which can generally only be obtained in vacuum. (2) During cyclic saturation, the microstructure is essentially composed of walls in either one or two directions (parallel walls or a labyrinth structure), while at failure the microstructure is to a large extent composed of misoriented cells. (3) The mean cell diameter and mean wall spacing decrease as the applied plastic strain amplitude increases. (4) The wall structure transforms into a labyrinth structure by the formation of thick walls perpendicular to the channel direction, which then thin down to the primary wall thickness. (5) The cell structure is derived from a wall structure by partitioning of the channels, by wall destruction and random distribution of dislocations or by the use of a labyrinth structure. (6) PSBs with a ladder structure can evolve into a cell structure.
REFERENCES 1 P. J. Woods, Philos. Mag., 28 (1973) 155. 2 H. Mughrabi, in P. Haasen, V. Gerold and G. Kostorz (eds.),Proc. 5th Int. Conf, on the Strength o f Metals and Alloys, Aachen, August 1979, Vol. 3, Pergamon, Oxford, 1980, p. 1615. 3 K. V. Rasmussen and O. B. Pedersen, Acta MetaIl., 28 (1980) 1467. 4 J. C. Figueroa, S. P. Bhat, R. Delaveaux, S. Murzenski and C. Laird, Acta Metall., 29 (1981) 1667. 5 A.T. Winter, O. B. Pedersen and K. V. Rasmussen, Aeta Metall., 29 (1981) 735. 6 T. Lepist6 and P. Kettunen, Scr. Metall., 16 (1982) 1145. 7 F. Ackermann, L. P. Kubin, J. Lepinoux and M. Mughrabi, Acta Metall., 32 (1984) 715. 8 N. Y. Jin and A. T. Winter, Acta Metall., 32 (1984) 1173. 9 N. Y. Jin and A. T. Winter, Acta Metall., 32 (1984) 989. 10 T. Lepist6, V. T. Kuokkala and P. Kettunen, Scr. Metall., 18 (1984) 245. 11 J. Pol~k and M. Klesnil, Mater. Sci. Eng., 63 (1984) 189. 12 R. Wang, H. Mughrabi, S. McGovern and M. Rapp, Mater. ScL Eng., 65 (1984) 219. 13 R. Wang and H. Mughrabi, Mater. Sci. Eng., 63 (1984) 147. 14 A. T. Winter, Philos. Mag. A, 37 (1978) 457. 15 P. Charsley and D. Kuhlmann-Wilsdorf, Philos. Mag. A, 44 (1981) 1351.
33 16 17 18 19 20 21 22
23
P. Charsley, Mater. Sci. Eng., 47 (1981) 181. H. Mughrabi, Mater. Sci. Eng., 33 (1978) 207. A. T. Winter, Philos. Mag., 30 (1974) 919. H. Mughrabi, F. Ackermann and K. Herz, in J. T. Fong (ed.), Fatigue Mechanisms, A S T M Spec. Teeh. Publ. 675, 1979, p. 69. C. Blochwitz and U. Veit, Cryst. Res. Technol., 17 (1982) 529. J. Pol~k and M. Klesnil, Scr. MetalI., 16 (1982) 1235. M. Bernard, J. B. Vogt, T. Bui-Quoc and J. I. Dickson, in C. J. Beevers (ed.), Fatigue 84, Proc. 2nd Int. Conf. on Fatigue and Fatigue Thresholds, London, 1984, Vol. II, Engineering Materials Advisory Services, Warley, 1984, p. 1029. C. Gorlier, Thesis D.I., St. Etienne, 1984.
24 C. Gorlier, C. Amzallag, P. Rieux and J. H. Driver, in C. J. Beevers (ed.), Fatigue 84, Proc. 2nd Int. Conf. on Fatigue and Fatigue Thresholds, London, 1984, Vol. I, Engineering Materials Advisory Services, Warley, 1984, p. 41. 25 G. L'Esperance, J. B. Vogt and J. I. Dickson, Mater. Sci. Eng., 79 (1986) 141. 26 A. Abel, Mater. Sci. Eng., 36 (1978) 117. 27 J. H. Driver, C. Gorlier, C. Belamri, P. Violan and C. Amzallag, A S T M S y m p . L o w Cycle Fatigue, Directions for the Future, October 1985, in the press. 28 O. K. Chopra and C. V. B. Gowda, Philos. Mag., 3 (1974) 583. 29 J. Mendez, Thesis, Poitiers, 1984. 30 C. Belamri, Thesis, Poitiers, 1986.
Secondary
23
C y c l i c Hardening and Dislocation Structures in T y p e 3 1 6 Stainless
Steel at 6 0 0 °C M. GERLAND and P. VIOLAN
Laboratoire de M~canique et de Physique des Mat~riaux, Unit~ associ~e 863 au CNRS, Ecole Nationale Supdrieure de Mdcanique et d'Adrotechnique, 86034 Poitiers Cddex (France) (Received December 20, 1 9 8 5 ; i n revised form February 2t, 1986)
ABSTRACT
Polycrystalline type 316 stainless steel has been fatigued in air and in vacuum at 600 °C at two constant plastic strain amplitudes. Although in air the curve o f cyclic stress versus number o f cycles is prematurely interrupted by failure, in vacuum it is sufficiently extended to allow secondary cyclic hardening to occur. Transmission electron microscopy observations show dislocation configurations and evolutions o f configurations (during the fatigue life) which are similar for both strain amplitudes. During the saturation plateau, several configurations coexist: regular walls, a labyrinth structure and persistent slip bands (PSBs) with a ladder structure. A t failure (in vacuum) a cellular structure widely prevails while a labyrinth structure and PSBs with a ladder structure no longer exist. We propose different mechanisms to explain the evolution o f the dislocation configurations: the formation o f a labyrinth structure, the change o f walls into cells and the change o f PSBs into cells. We also explain the secondary cyclic hardening by the evolution o f microstructure.
1. INTRODUCTION
A great number of transmission electron microscopy (TEM) studies of dislocation configurations in f.c.c, metals fatigued to saturation have been reported in the literature. Most of these studies have been investigations of copper [1-14] or copper alloys [1, 1 5 , 1 6 ] , either on polycrystals [3-5, 11, 13, 15, 16] or on single crystals oriented for single slip [1, 2, 6-8, 12, 13] or multiple slip [9, 10]. More0025-5416/86/$3.50
over, all these studies have been performed at r o o m temperature. Among the most interesting features are the development of dipolar walls and labyrinth structure on the one hand and persistent slip bands (PSBs) on the other hand. PSBs have been observed in single crystals and polycrystals and have been connected either with the plateau region of the cyclic stress-strain curve [14, 1 7 - 2 0 ] , where they gradually increase throughout the fatigue life [5, 12], or with cyclic softening [11, 21]. These dislocation configurations have not been examined so frequently in austenitic stainless steels except for the work of Bernard et al. [22], Gorlier and coworkers [23, 24] and L'Esperance et al. [25]. The formation and change in these different structures are not y e t clearly known in spite of the various propositions made during the last few years. It is well known that the fatigue life is generally increased when tests are performed in vacuum instead of in air. This increase in fatigue life allows the microstructure to evolve in a significant way and thus makes it possible to study microstructural changes which could n o t occur during tests in air because of premature failure. At the same time, instead of cyclic softening which corresponds to the closeness of failure, secondary cyclic hardening can occur if the fatigue life is prolonged by fatiguing in vacuum [12, 13, 26]. In order to contribute to the understanding of the mechanisms of microstructural changes and the p h e n o m e n o n of secondary cyclic hardening, we report here the results of studies on t y p e 316 stainless steel fatigued at a constant plastic strain amplitude in air and in vacuum and we propose some mechanisms of microstructure evoIution. © Elsevier Sequoia/Printed in The Netherlands
24 TABLE 1 Chemical composition of the type 316SPH stainless steel
Element C Amount(wt.%) 0.022
Mn 1.69
Si 0.31
S 0.002
P 0.023
2. EXPERIMENTAL DETAILS
2.1. Specimens The t y p e 316SPH stainless steel investigated has the composition given in Table 1. PolycrystaUine specimens were prepared from sheet supplied by Creusot-Loire for the Super Phenix supergenerator. After machining, the grip ends had a diameter of 16 mm, a gauge length of 10 m m and a gauge diameter of 6 mm. Before fatigue, specimens were ground with very fine grinding paper, heat treated for I h at 1050 °C in high vacuum (about 5 X 10 -4 Pa) and water cooled; they were then electropolished in a solution of 10% perchloric acid and 90% acetic acid. 2.2. Cyclic deformation Cyclic deformation was performed in a symmetric uniaxial push-pull m o d e in an electromechanical machine at 600 °C in vacuu m (about 10 -3 Pa). Samples were tested at t w o constant plastic strain amplitudes (+ 2 X 10 -3 and + 5 X 10 -3) to failure or to a number of cycles corresponding to failure in air.* A triangular waveform c o m m a n d signal from a digital function generator was used for plastic strain control. The test frequency was 0.2 Hz for the amplitude of 2 X 1 0 - 3 and 0.1Hz for the amplitude of 5 X 10 -3, which corresponds to a constant plastic strain rate of 2 X 10 -3 s-z.
Ni 11.90
Cr 17.45
B 0.009
N2 0.069
Co 0.190
As 0.004
3. RESULTS
3.1. Fatigue hardening-softening curves Figure 1 shows the cyclic hardening-softening curves obtained by plotting the continuously recorded stress amplitude Aa/2 versus the number N of cycles. The two curves are obtained at 600 °C with plastic strain amplitudes of 2 X 10 -a and 5 X 10 -3. It should be noted that saturation starts earlier for the higher plastic strain amplitude. The numbers of cycles to failure in vacuum are 32 000 and 4800 respectively for plastic strain amplitudes of 2 X 10 -3 and 5 X 10 -3. The interrupted tests in vacuum were continued up to the number of cycles corresponding to failure in air, giving 8000 and 700 cycles at 2 X 10 -3 and 5 X 1 0 -3 respectively. These values fall within the dispersion band on the Manson-Coffin curve [27] and correspond to the end of the saturation plateau. For Ae, = 2 X 10 -3, after the saturation plateau the stress amplitude decreases conI
I
I
I
A 0"/2 (MPa)
/
/
°
I *Tests in air at the same temperature and with specimens from the same sheet have been carried o u t at Ecole Nationale Sup~rieure des Mines de SaintEtienne by C. Gorlier and J. H. Driver.
Cu 0.110
thinning to perforation using the double-jet technique in an acetic acid-perchloric acid solution. Observations were realized on a JEOL-100 B microscope at 100 kV with a double-tilt stage.
|
2.3. Transmission electron microscopy Fatigued specimens had been sectioned at a thickness of a b o u t 1 mm, normal and parallel to the applied strain axis with a spark cutter. All the sections were polished with fine paper d o w n to a thickness of 0.3 m m and then cut into discs b y spark machining. These discs were further milled with very fine paper to 0.1 mm, and the final thin foils obtained by
Mo 2.25
I
_
~Lo'
~,d
lO3
N (cycles)
1~
1~os
Fig. 1. Cyclic hardening-softening curves (T = 600 °C; vacuum).
25
tinuously until failure while, for Aep = 5 X 10 .3 , after the saturation plateau, secondary cyclic hardening occurs in vacuum. This type of secondary cyclic hardening has already been noted in various materials, e.g. ~-Fe [28] and copper single crystals and polycrystals [4, 11-13, 26]. This secondary cyclic hardening occurs earlier for the higher plastic strain amplitude and more easily in polycrystals than in single crystals for the same plastic strain amplitude [13]. Secondary cyclic hardening could not be observed in air because failure occurs, thus stopping the evolution of microstructure. However, tests in vacuum allow a sufficiently large n u m b e r of cycles to be reached that microstructural evolution takes place.
Fig. 2. Well-developed wall s t r u c t u r e o b s e r v e d o n a ( 1 1 0 ) s e c t i o n (walls parallel t o [ 1 1 0 ] ; A e p ] 2 = 2 x 10-3; N = 8 0 0 0 cycles).
3.2. Dislocation structures observed during cyclic saturation The dislocation arrangement in the specimen fatigued at Aep/2 = 2 X 10 .3 up to 8000 cycles is characterized by the coexistence of (i) a well-developed wall structure generally parallel to the [110] direction (Fig. 2), (ii) a two-system wall structure (the labyrinth
Fig. 3. T w o - s y s t e m wall s t r u c t u r e o b s e r v e d o n a ( 1 1 0 ) s e c t i o n ( A e p / 2 = 2 × 10-3; N = 8 0 0 0 cycles).
26
structure (Fig. 3)) and (iii) the well-known ( i 0 1 ) ladder structure of PSBs (Fig. 4). The few areas o f misoriented cell structure (on the right-hand side at the b o t t o m in Fig. 5) should also be noted. The wall spacing is a b o u t 0.5 pm in the regular wall structure and a b o u t 1.2 pm in the ladder structure. The dislocation arrangement of the specimen fatigued at A e p / 2 = 5 X 1 0 -3 up to 700 cycles is also characterized b y the coexistence of the three above-mentioned structures, the wall spacing being a b o u t 0.35 pm in the wall structure (Fig. 6) and a b o u t 1 pm in the ladder structure (Fig. 7). The very few observed cell structures are located near PSBs or at twin boundaries.
3.3. Dislocation structures observed after failure The dislocation structure formed after failure at Aep/2 = 2 X 10 -3 consists essentially of misoriented cells, either elongated cells elaborated from wall structure (Fig. 8, centre) or quasi-equiaxed cells (Fig. 8, left- and righthand sides). Some areas composed of wall structures are also present. The spacing between these walls is a b o u t 0.70 pm while the mean equiaxed cell diameter is a b o u t 0.75 pm. The length of the elongated cells is a b o u t 1.5-2 p m and their width is the same as the wall spacing. The interior of the cells or the channels is generally free of dislocations.
After failure at a plastic strain amplitude of 5 X 1 0 - 3 , the dislocation arrangement is rather similar to the structure obtained at a plastic strain amplitude of 2 X 10 -3 (Fig. 9). However, the wall spacing and the mean equiaxed
Fig..4. P S B with a ladder structure observed on a (121) section (Aep/2 = 2 x 10-3; N = 8000 cycles).
Fig. 5. Coexistence of walls, PSBs and cells o n a (121) s e c t i o n (Aep/2 = 2 x 10-3; N = 8000 cycles). The miso r i e n t a t i o n b e t w e e n cells c a n b e s e e n in t h e l o w e r right-hand corner.
27
Fig. 6. Walls and labyrinth structure observed on a (110) section (Aep/2 = 5 X 10-3; N = 700 cycles). Misoriented cells can be seen near the twin boundary.
28
Fig. 7. PSBs with a ladder structure and misoriented cells observed on a (110) section (Aep/2 = 5 × 10-3; N = 700 cycles).
cell diameter are 0.60 pm and 0.65 pm respectively while the length of the elongated cells is about 1 pm (Fig. 10). Contrary to the observations made at a plastic strain amplitude of 2 X 10 -3, the interior of the channels contains m a n y dislocations (Fig. 11).
4. DISCUSSION
4.1. Mechanical behaviour and dislocation configurations We have already mentioned that, in the plateau region and for both strain amplitudes, three structures coexist, as observed by Ackerman et al. [7] : wall or labyrinth structures, PSBs and cell structures, in the approximate amounts 70%, 20% and 10% respectively. Much later in the fatigue life, this microstructural configuration is changed into a structure composed of cells (elongated and equiaxed) and residual walls in the proportions of about 75% and 25% respectively. Therefore, if the cell structure is n o t characteristic of failure, it is at least typical of a stage close to failure.
I
Fig. 8. Elongated and equiaxed cells observed on a (013) section (Aep/2 = 2 × 10-3; Nf = 3.2 X 104). Marked misorientation can be seen between cells.
29 ~btained with a plastic strain amplitude of 5 X 10 -3 is possible because cycling in vacuum greatly increases the fatigue life as reported by Mendez [29] and Wang et al. [12] for copper and by Belamri [30] for type 316L stainless steel. This hardening can be associated with the reorganization of the dislocation structure from a wall structure to a cell structure. A wall structure allows very easy dislocation m o v e m e n t in the channels, while a cell structure does n o t permit dislocation movem e n t so easily and thus requires a higher stress to accommodate the applied plastic strain. The transformation of the wall structure into the cell structure can be seen in Fig. 11 (upper right-hand side and b o t t o m ) . From dislocation-free channels, first there is a quasiuniform repartition of dislocations in the channels, followed by the final formation of the cells. We t h i n k t h a t this transformation is responsible for secondary hardening.
4.2. Microstructural evolution during fatigue life Fig. 9. Elongated and equiaxed cells observed on a (110) section (Aep/2 = 5 X 10-3;Nf --- 4.8 X 103). Dislocations in the interior of elongated cells can be seen.
Fig. 10. Elongated cells with weak misorientation o n a (110) section (Aep/2 = 5 x 10-3; Nf = 4.8 x 103).
As can be expected, the greater the applied plastic strain amplitude, the smaller are the mean cell diameter and the wall spacing (cf. Section 3.3). The presence of secondary cyclic hardening
Numerous observations made on several thin foils and on grains with different orientations show that the labyrinth structure is derived from the wall structure (cf. Figs. 3 and 6), as shown in Fig. 12. Short and thick uncondensed walls form perpendicular to the channel direction from the primary wall dislocations (Fig. 6, A) and gradually cover several wall spacings. The configuration which appears in Fig. 6 and Fig. 12(b) is similar to t h a t observed by Charsley and Kuhlmann-Wilsdorf [ 15] and Charsley [16] in copper and reported in ref. 16, Fig. 2. Then the evolution continues until the usual labyrinth structure is formed, the wide walls becoming as narrow as the primary walls. As we have already stated, the dislocation arrangement was mostly composed of walls during the saturation stage, whereas at failure the cell structure strongly predominates. From the different observations performed on thin foils, we suggest four possible mechanisms of microstructure evolution to explain how this transformation occurs.
4.2.1. Direct transformation The first possibility shown schematically in Fig. 13 is the direct transformation from a wall structure into elongated cells by the
Fig. 11. C o e x i s t e n c e o f a m i s o r i e n t e d cell s t r u c t u r e a n d a wall s t r u c t u r e evolving in a cell s t r u c t u r e o b s e r v e d o n a ( 1 2 1 ) s e c t i o n ( A e p / 2 = 5 X 1 0 - 3 ; N f = 4.8 X 103). Th'e s t r o n g d e n s i t y o f d i s l o c a t i o n s in t h e f o r m e r c h a n n e l s c a n b e seen as cells in f o r m a t i o n A.
O
31
(c
Fig. 12. Schematic drawing showing the evolution from (a) a wall structure through (b) the formation of thick walls perpendicular to the channel direction to (c) a labyrinth structure.
Fig. 13. Schematic drawing showing the direct transformation from (a) a wall structure to (b) elongated cells. Fig. 15. Wall destruction and beginning of cell formation (Aep/2 = 2 x 10-3;N = 8000 cycles). :~:.-~.'~,.<,
(a)
b)
(c)
Fig. 14. Schematic drawing showing (b) the wall destruction and random dislocation distribution as the intermediate stage between (a) the wall structure and (c) the cell structure.
p a r t i t i o n i n g o f t h e c h a n n e l s as seen in Fig. 8. T h e w i d t h o f t h e s e cells is t h e n t h e s a m e as t h e wall spacing. T h e e l o n g a t e d cells c a n later t r a n s f o r m i n t o q u a s i - e q u i a x e d cells.
4.2.2. Wall destruction A s e c o n d w a y o f e v o l u t i o n f r o m a wall s t r u c t u r e t o cells is d e p i c t e d in Fig. 14. T h e walls are progressively d e s t r o y e d a n d t h e disl o c a t i o n s w h i c h c o m p o s e d t h e s e walls s p r e a d o u t u n i f o r m l y i n t o t h e c h a n n e l s a n d at t h e site o f t h e f o r m e r walls. T h e n t h e s e u n i f o r m l y d i s t r i b u t e d d i s l o c a t i o n s progressively f o r m q u a s i - e q u i a x e d cells, w i t h a m e a n d i a m e t e r slightly g r e a t e r t h a n t h e f o r m e r wall spacing. This s i t u a t i o n c a n be seen in Fig. 11, A, a n d in Fig. 15.
(c)
(d)
Fig. 16. Schematic drawing showing the evolution from (a) a wall structure to (d) a cell structure (all labyrinth walls have disappeared) with (b) the intermediate labyrinth structure ((c) the beginning of cell formation in the labyrinth structure).
4.2.3. Use o f the labyrinth structure The third possibility of transformation of walls a n d c h a n n e l s t o cell s t r u c t u r e uses t h e l a b y r i n t h s t r u c t u r e . This e v o l u t i o n is s h o w n d i a g r a m m a t i c a l l y in Fig. 16. A f t e r t h e labyr i n t h s t r u c t u r e has b e e n f o r m e d , as s h o w n in
32
Fig. 12, some cells begin to form in different corners of the labyrinth, first using the segments of the walls as cell boundaries. Then, around these first cells, dislocations become present in the neighbourhood, in walls as well as in channels, will form new cells and so on. The beginning of the cell formation in labyrinth structure can be seen in Fig. 6, B. In the same figure, in the twin, the formation of a cell structure is already complete, presumably because of a more favourable orientation of the channels. The mean cell diameter is in this case roughly equal to or slightly greater than the former wall spacing.
4.2.4. Transformation from the ladder structure o f persistent slip bands The wall structure (true walls and labyrinth) is not the only structure to transform into a cell structure, even if it is markedly more widespread than other structures (at least in our study). Cell formation can also be seen in the PSBs containing a ladder structure. This evolution has already been observed in copper single crystals by Wang et al. [12], Wang and Mughrabi [13] and Lepist5 et al. [10]. Jin and Winter [8] have also observed a regular labyrinth structure with PSBs in copper single crystals. The ladders of these PSBs are n o t very regular and the start of cell formation can be seen locally. The coexistence of cells and PSB ladder structures is also shown in their work reported in ref. 9. In Figs. 4, 5 and 7, some PSBs with ladders can be seen. Some of the ladders are n o t regular and have a t e n d e n c y to transform into cells. In several places, cell formation is quite complete, and this evolution may lead to the formation of new PSBs in the wall structure, as mentioned by Wang and Mughrabi [13].
5. CONCLUSIONS
The present study on t y p e 316 austenitic stainless steel deformed cyclically at a constant plastic strain amplitude at 600 °C in vacuum has led to the following main conclusions. (1) Secondary cyclic hardening can occur after the so~called cyclic saturation has been attained and this hardening is linked to a continuous microstructural evolution. This
secondary cyclic hardening occurs only after a great number of cycles which can generally only be obtained in vacuum. (2) During cyclic saturation, the microstructure is essentially composed of walls in either one or two directions (parallel walls or a labyrinth structure), while at failure the microstructure is to a large extent composed of misoriented cells. (3) The mean cell diameter and mean wall spacing decrease as the applied plastic strain amplitude increases. (4) The wall structure transforms into a labyrinth structure by the formation of thick walls perpendicular to the channel direction, which then thin down to the primary wall thickness. (5) The cell structure is derived from a wall structure by partitioning of the channels, by wall destruction and random distribution of dislocations or by the use of a labyrinth structure. (6) PSBs with a ladder structure can evolve into a cell structure.
REFERENCES 1 P. J. Woods, Philos. Mag., 28 (1973) 155. 2 H. Mughrabi, in P. Haasen, V. Gerold and G. Kostorz (eds.),Proc. 5th Int. Conf, on the Strength o f Metals and Alloys, Aachen, August 1979, Vol. 3, Pergamon, Oxford, 1980, p. 1615. 3 K. V. Rasmussen and O. B. Pedersen, Acta MetaIl., 28 (1980) 1467. 4 J. C. Figueroa, S. P. Bhat, R. Delaveaux, S. Murzenski and C. Laird, Acta Metall., 29 (1981) 1667. 5 A.T. Winter, O. B. Pedersen and K. V. Rasmussen, Aeta Metall., 29 (1981) 735. 6 T. Lepist6 and P. Kettunen, Scr. Metall., 16 (1982) 1145. 7 F. Ackermann, L. P. Kubin, J. Lepinoux and M. Mughrabi, Acta Metall., 32 (1984) 715. 8 N. Y. Jin and A. T. Winter, Acta Metall., 32 (1984) 1173. 9 N. Y. Jin and A. T. Winter, Acta Metall., 32 (1984) 989. 10 T. Lepist6, V. T. Kuokkala and P. Kettunen, Scr. Metall., 18 (1984) 245. 11 J. Pol~k and M. Klesnil, Mater. Sci. Eng., 63 (1984) 189. 12 R. Wang, H. Mughrabi, S. McGovern and M. Rapp, Mater. ScL Eng., 65 (1984) 219. 13 R. Wang and H. Mughrabi, Mater. Sci. Eng., 63 (1984) 147. 14 A. T. Winter, Philos. Mag. A, 37 (1978) 457. 15 P. Charsley and D. Kuhlmann-Wilsdorf, Philos. Mag. A, 44 (1981) 1351.
33 16 17 18 19 20 21 22
23
P. Charsley, Mater. Sci. Eng., 47 (1981) 181. H. Mughrabi, Mater. Sci. Eng., 33 (1978) 207. A. T. Winter, Philos. Mag., 30 (1974) 919. H. Mughrabi, F. Ackermann and K. Herz, in J. T. Fong (ed.), Fatigue Mechanisms, A S T M Spec. Teeh. Publ. 675, 1979, p. 69. C. Blochwitz and U. Veit, Cryst. Res. Technol., 17 (1982) 529. J. Pol~k and M. Klesnil, Scr. MetalI., 16 (1982) 1235. M. Bernard, J. B. Vogt, T. Bui-Quoc and J. I. Dickson, in C. J. Beevers (ed.), Fatigue 84, Proc. 2nd Int. Conf. on Fatigue and Fatigue Thresholds, London, 1984, Vol. II, Engineering Materials Advisory Services, Warley, 1984, p. 1029. C. Gorlier, Thesis D.I., St. Etienne, 1984.
24 C. Gorlier, C. Amzallag, P. Rieux and J. H. Driver, in C. J. Beevers (ed.), Fatigue 84, Proc. 2nd Int. Conf. on Fatigue and Fatigue Thresholds, London, 1984, Vol. I, Engineering Materials Advisory Services, Warley, 1984, p. 41. 25 G. L'Esperance, J. B. Vogt and J. I. Dickson, Mater. Sci. Eng., 79 (1986) 141. 26 A. Abel, Mater. Sci. Eng., 36 (1978) 117. 27 J. H. Driver, C. Gorlier, C. Belamri, P. Violan and C. Amzallag, A S T M S y m p . L o w Cycle Fatigue, Directions for the Future, October 1985, in the press. 28 O. K. Chopra and C. V. B. Gowda, Philos. Mag., 3 (1974) 583. 29 J. Mendez, Thesis, Poitiers, 1984. 30 C. Belamri, Thesis, Poitiers, 1986.