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Evolution of dislocation structures and cyclic behaviour of a 316L-type austenitic stainless steel cycled in vacuo at room temperature
Materials &'ience and Engineering, A l l 8 (1989) 83-95
83
Evolution of Dislocation Structures and Cyclic Behaviour of a 316L-type Austenitic Stainless Steel Cycled in vacuo at Room Temperature M. GERLAND, J. MENDEZ, P. VIOLAN and B. AIT SAADI Laboratoire de Mkcanique et de Physique des Mat~riaux, URA CNRS 863, 86034 Poitiers Cedex (France)
(Received January 23, 1989; in revised form April 17, 1989)
Abstract
Dislocation structures formed during cycfic deformation at room temperature in vacuo of a 316L-type austenitic stainless steel are presented. It has been shown that for this material having a rather low stacking fault energy of about 28 mJ m - 2, dislocation structures exhibit a planar slip or a wavy slip character depending on the cyclic plastic strain amplitude. In particular the existence of wall and channel, labyrinth or ladder structures has been shown; these structures have been observed to evolve progressively into cells during extensive cycling in vacuo. The volume fraction of each type of structure in the specimen has been evaluated quantitatively as a function of the cyclic plastic strain level and the number of cycles throughout the fatigue life in vacuo.
1. Introduction
During the last 10 years the numerous studies which have been made on the cyclic behaviour of metals and alloys have considerably improved our understanding of the internal dislocation arrangements in relation with the cyclic strain-stress behaviour and the crack initiation processes. Most of these investigations have concerned f.c.c, materials and in particular monocrystalline and polycrystalline pure copper [1-17] or copper alloys [1, 18-21] cycled at room temperature, i.e. in the low homologous temperature range ( T< 0.25 Tr). One of the most interesting findings from these studies on cyclic deformation of copper, obtained in particular on single crystals, concerns the existence of well defined "soft" structures such as the wall and channel structure or the ladder structure which clearly differ from a surrounding much less 0921-5093/89/$3.50
deformable matrix. The analysis of the dislocation motion into these zones of high localized slip activity and the characterization of the associated surface deformation features as extrusion-intrusion pairs, led to a better understanding of the surface crack initiation phenomena in copper. Such detailed work has not been conducted so far for other f.c.c, materials, in particular for industrial materials, although some studies conducted on anstenitic stainless steels have revealed structures very similar to those observed in copper [22-30]. An important aspect of cyclic deformation processes which has received little attention, with the exception of a few works [26, 27], concerns the stability of the different dislocation structures with cycling. This question was often ignored because most of the fatigue tests were conducted in air; now, it has been clearly shown that in this environment the fatigue lives can be considerably reduced compared with those observed in vacuo or in inert atmospheres (even in the low cycle fatigue range and for low temperatures) [26, 28, 31-33]. As a consequence, dislocation structures appear without appreciable change during cycling at the scale of the fatigue life in air; prolonging considerably the fatigue life and therefore the accumulated plastic strain by performing the fatigue tests in vacuo, in some cases reveals that some types of dislocation structures which would appear stable in air evolve in fact progressively in a more stable structure. Examples of such a transformation are given in the work of Wang and Mughrabi [13] on monocrystalline copper cycled at room temperatm'e in vacuo or by the work of Gerland and Violan [26] on an austenitic stainless steel cycled in vacuo at 600 °C. In both studies these authors have observed that the wall and channel structure corresponding to persistent slip bands progress© Elsevier Sequoia/Printed in The Netherlands
84 ively transforms into a cell structure; such an evolution is accompanied in some cases by a modification of cyclic behaviour (secondary cyclic hardening). In this paper a study is presented of the internal dislocation structure induced by cyclic deformation in vacuo at room temperature in a 316L-type austenitic stainless steel. The respective roles of the cyclic plastic strain amplitude and the number of cycles have been considered. In particular, we have compared, at a fixed plastic strain amplitude, the microstructural states associated with the cumulative plastic strains reached at failure in air and in vacuo; the differences in fatigue damage (crack initiation processes or crack propagation kinetics) in both environments are not considered in this paper but will be published separately [34].
2. Experimental conditions The austenitic stainless steel studied in this work is a 316L-type steel denoted by 17-12 SPH whose composition has been indicated elsewhere [26, 28]. Cylindrical specimens with a diameter of 6 mm and a gauge length of 13 mm were used. They were annealed before fatigue by heating for 1 h at 1050 °C in vacuo and water quenched. The specimens were carefully electropolished in a solution of 10% perchloric acid and 90% acetic acid. Cyclic deformation tests were performed in a symmetrical uniaxial push-pull mode on an electromechanical machine at 25 °C in laboratory air or in vacuo (pressure lower than 10 -3 Pa). Tests were performed in plastic strain controlled mode with constant plastic strain rate (2 x 1 0 - 3 s-l). The cyclic plastic strain amplitudes Aep/2 investigated were in the range 6 x 10 - 4 10 - 2 The fatigue tests were performed to failure in each environment or stopped at a fixed number of cycles. The specimens were cut by spark machining, then mechanically polished to 0.1 mm; the final thin foils were then obtained using the electrolytic double-jet technique with a solution of acetic acid and perchloric acid solution. Observations were performed on a JEOL-100 B microscope at 100 kV with a double-tilt stage. For each sample, 6-8 thin foils were studied with at least two of them perpendicular to the stress axis. Each foil generally has 15-20 observable grains, which corresponds to a minimum of 100 grains observed for each specimen.
The volume fractions of the different structures described in Section 3 are obtained by averaging the whole observations realized on all the observed grains in a given sample.
3. Experimental results 3.1. Cyclichardening
Figure 1 shows the curves giving the evolution of the stress amplitude A a/2 as a function of the number of cycles in a semilogarithmic diagram, for the different strain amplitudes investigated. These curves correspond to the vacuum-tested specimens; several of them are marked by an arrow which indicates the number of cycles leading to failure in air; details of the air and vacuum test data will be given in a separate paper [34]. The cyclic behaviour of the 17-12 SPH stainless steel is characterized by a primary hardening over a number of cycles which increases with the cyclic strain amplitude but which always remains limited; it varies from 5 to 500 cycles when Aep/2 varies from 6 x 10 - 4 to 10- 2. This short period of hardening is followed by a period of softening, except for the highest strain amplitude of 10 -2 . This softening period is longer at low Aep/2 values and is much more extended, in all cases, than the previous hardening stage; however, it remains small compared to the total fatigue life. For all the strain amplitudes the stress level is stabilized during most of the test and no trace of secondary hardening is observed even when the test in vacuo is significantly prolonged beyond the fatigue life in air.
'
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. . . . . . .
t
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~0[
30[ 20C
10C
.......
J
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Jill
10 z
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103
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10 ~
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106
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Fig. 1. Cyclic hardening curves of 17-12 SPH stainless steel cycled at 20 °C in vacuo. NfA indicates the number of cycles at failure in air.
85
Figure 2 gives the cyclic stress-strain curve (CSSC) and the corresponding monotonic curve established from the first quarter-cycle of each fatigue test. Up to an amplitude of about 2 x 10 -3 the softening period balances the initial hardening and no significant cyclic hardening is observed on the CSSC; however, beyond 2 x 10 -3 the stabilized cyclic stress amplitude is up to 50% higher than the corresponding monotonic stress. 3.2. Types and proportions of the different dislocation structures at failure in vacuo: influence of the cyclic plastic strain amplitude Depending on the cyclic plastic strain amplitude, different types of dislocation structures have been observed in the specimens cycled up to failure in vacuo. These different types can be
classified as: cells; labyrinths; walls and channels; ladder structures; microtwins; stacking faults; planar slip bands; dipoles and multipoles; tangles. The existence of each structure and its volumic fraction depend on the plastic strain amplitude: the higher Aep/2 the higher the proportion of cells; the lower Aep/2 the greater the tendency of tangles to prevail. Table 1 gives all the details concerning the dimensions of each structure and its proportion in the bulk of the specimens cycled to failure in vacuo, for the different cyclic plastic strain amplitudes. 3.2.1. Cells Although the cellular structure is very rarely observed at Aep/2=6 X 10 -4, it occupies 20%-30% in volume at 8 x 10-4; in this case, the
A(~2MPa 5OO
400 300
200
10 -4
/ Cgctic //~~j
l -3
10
Monotonic
I -2
a~P/2
~_
10
Fig. 2. C y c l i c s t r e s s - s t r a i n curve and corresponding m o n o t o n i c c u r v e of t h e 1 7 - 1 2 S P H steel at 2 0 °C.
Fig. 3. C e l l u l a r s t r u c t u r e for Ae_/2 = 8 x 10 -4 at Ntv = 380 0 0 0 cycles. Foil n o r m a l c l o s e to [12 [].
Proportions and dimensions of different structures of dislocations observed in 1 7 - 1 2 S P H stainless steel cycled up to failure in v a c u o for several plastic strain amplitudes
TABLE 1
Structures
Cells
Walls-channels Labyrinths PSBs with ladder Microtwins P l a n a r slip b a n d s Stacking faults Dipoles and multipoles Tangles a M a j o r d i m e n s i o n o f cell. b M i n o r d i m e n s i o n of cell.
Aep/2 10 2
5 x 1 0 -3
2xlO 3
8.3×10-4
6x10-4
70% L a= 0.6-2.5/~m l b = 0.2-0.6/~m 10% 0.2-0.3/,m 5%- 10% 0.2-0.4/~m 2% 10%- 15% -----
60% L a = 0.4-2.5/~m l b = 0.2-0.4 pm 15% 0.3-0.5/,m 2% 0.2-0.3/~m 2% 5%- 10% ---10%
50% L a = 0.8-3.5/~m Ib = 0.3-0.6/~m 20% 0.5-0.8 pm 4%
20%-30%
Very r a r e
2% 2% 5% --15%
20%-30%
-Very r a r e 10% Very r a r e 5% 20%
-Very r a r e -Very r a r e 5% 90%
86
Fig. 4. Cellular structure for Ae_/2 = 2 × cycles. Foil normal close to [011~.
1 0 -3
at Nfv = 50 000
Fig. 6. Cellular structure for Aep/2 = 5 × 10 -3 at Nfv= 8100 cycles. Foil normal close to [111].
Fig. 5. UncondensedceUsforAep/2=5x10-3atNfv=8100 cycles. Foil normal close to [011].
Fig. 7. Elongated cells for A e J 2 = 5 x l 0 cycles. Foil normal close to [112].
cells are badly formed with poor uncondensed walls (Fig. 3). At 2 x 10-3 the proportion of cells is twice as high (about 50%), but the cells still remain badly formed (Fig. 4); for this amplitude two types of cells are observed, either equiaxed cells with a mean diameter of about 0.4-0.8/~m, or elongated cells of length up to 3/~m. For the amplitude of 5 x 10 -3 uncondensed cells (Fig. 5) as well as cells in a more advanced stage with fine condensed walls (Fig. 6) can be observed. The majority of the cells is equiaxed with a mean diameter which varies between 0.3 and 0.7/~m; however, one can also observe elongated cells (Fig. 7) up to 2.5/~m in length located in channels 0.2-0.4/~m in width. At the highest amplitude of 1 0 - 2 the cells are the predominant structure (about 70%) and have, in general, fine condensed walls (Fig. 8); the
geometrical features remain very similar to those observed at 5 x 10-3.
-3 at Nfv=8100
3.2.2. Walls and channels, ladder structures and labyrinths Walls and channels are observed from the amplitude of 8 x 10- 4, but even for amplitudes of 2 x 10- 3 they remain badly formed although they cover about 20% in volume; the walls are thick and wavy and their width varies between 0.5 and 0.8/~m. Beyond this amplitude a decrease of the volume covered by this type of structure is observed when Aep/2 increases (about 15%); the walls become regular but remain thick and badly formed, and the interior of the channels 0.3-0.5 ~m in width, contains numerous dislocations (Fig. 9). At 1 0 - 2 the wall and channel structure covers only 10% of the bulk and the wails are badly aligned, wavy and thick; the width of the
87
Fig. 8. Condensed cells for Aep/2 = 10 -2 at Nrv= 950 cycles. Foil normal close to [011].
Fig. 9. Wall and channel structure for A%/2=5 ×10 -3 at Nfv= 8100 cycles.Foil normal close to [011]. channels varies between 0.2 and 0.3 ktm, the dislocation density is rather important inside and a high tendency to transformation of this structure into cells is observed (Fig. 10). A ladder-type persistent slip band (PSB) structure (regular arranged walls in dislocation free channels) was observed for the amplitudes 2 x 10 -3, 5 × 10 -3 and 10 .2 (Fig. 11). This structure, which only covers about 2% of the total area of the observed foils, presents some differences for the classical ladder structure in copper; in the case of this stainless steel the surrounding structure is c o m p o s e d of badly formed cells or other channels instead of the classical structure of veins in copper. T h e labyrinth structure was observed at the same plastic strain amplitudes as the wall and channel or the ladder structures; it has been estimated to cover about 4 % in volume for A~p/2 =
Fig. 10. Beginning of transformation from wall structure into cells for Aep/2= 10 2 at Nfv=950 cycles. Foil normal close to [012].
Fig. 11. Ladder-type PSB structure for Aep/2= 10 2 at Nw= 950 cycles.Foil normal close to [123]. 2 x 10 -3 and near to 7% for the highest amplitude of 10 -2 (Fig. 12). T h e channel width varies little with Aep/2 and is about 0.3 #m. For high amplitudes numerous traces of local transformation of this structure into cells were found. 3.2.3. Microtwins T h e existence of mechanical microtwinning was noted from the lowest amplitudes but with very low p r o p o r t i o n at 6 x 10 -4 or 8 × 10 -4. Microtwins cover about 2% of the foil area at A%/2 = 2 x 10 -3, 7% at 5 × 10 -3 and about 12% at 10 -2. At 10 -2 microtwins cover up to 50% of certain grains (Fig. 13). 3.2.4. Other structures Less well-organized structures, such as tangles, are rarely observed in specimens tested at high
88
Fig. 12. Labyrinth structure for Aep/2= 10 -2 at cycles. Foil normal close to [011].
Nfv =
Fig. 13. Extended microtwinning for Aep/2= 1 0 - 2 Nfv = 950 cycles. Foil normal close to [110]; dark field.
950
at
amplitudes, but represent the main part of the dislocation structure for low plastic amplitudes (about 90% at Aep/2 = 6 x 10-4). In the specimens cycled at 6 × 1 0 -4 or 8 × 10-4 some planar slip bands, stacking faults or dipoles and multipoles have also been observed.
3.3. Microstructural evolution during cycling 3.3.1. Dislocation structures after failure in air Observations were made o n samples fatigued in air up to failure for three plastic strain amplitudes Aep/2 = 7 x 10 -4, 2 x 10 -3 and 5 x 10 -3. T h e same structures as those observed on samples cycled up to failure in vacuo were seen but with slightly different proportions (Figs. 14-17). Table 2 shows that m o r e walls and channels are observed at failure in air but less cells, microtwins or labyrinth structures.
Fig. 14. Ladder-type PSB structure for Aep/2=5 x 10 -3 at NfA=4200 cycles.Foil normal close to [112].
Fig. 15. Labyrinth structure for AeJ2=5xl0-3 NfA=4200 cycles.Foil normal close to [112].
at
Although the dimensions of these structures are quite similar to those in samples cycled in vacuo, we have noted that the structures are less well developed such that the cell boundaries are rather thick and loose and the majority of the cells are not completely closed with numerous dislocations inside. Also, the walls are thick and often appear to be in the process of being transformed into cell structure especially for the amplitudes 2 x 1 0 - 3 and 5 x 10-3.
3.3.2. Microstructural evolution during saturation stage for Aep/2 = 5 x 10 -3 Comparison between structures after failure in air (generally corresponding to half of the saturation stage in vacuo) and after failure in vacuo, shows an evolution of some of these structures though the stress remains stable. A detailed study of the evolution of structures was carried out for
89
K
Fig. 16. Badly formed cells for A%/2=7× 10 -4 at NeA= 145 100 cycles. Foil normal close to [111].
Fig. 17. Wall and channel structure for Aep/2 = 5 × NfA = 4200
10 -3
at
cycles. Foil normal close to [114].
TABLE 2 Proportions and dimensions of different structures of dislocations observed in 17-12 SPH stainless steel cycled up to failure in air for several plastic strain amplitudes Structures
Cells
Walls-channels Labyrinths PSBs with ladder Microtwins Planar slip bands Stacking faults Dipoles and multipoles Tangles
A ep/2 5 × 1 0 -.1
2 x l O -,~
7×10 ~
50%-60% 0.5-0.7/~m La = 0.6-2.5/am l b = 0.3-0.5/~m 20% 0.4-0.6 pm 4% 0.2-0.4 ktm 2% Very rare Very rare Very rare
50% 0.6-0.8 pm La= 1-4.5/~m l b= 0.3-0.8 pm 25% 0.4-0.7 #m
5%- 10%
10%
25%
20%
Very rare 10% Very rare 50/0- 10% 5O%
"Major dimension of cell. bMinor dimension of cell.
the amplitude 5 × 1 0 - 3. Samples were thus cycled up to different n u m b e r s of cycles: (i) 25 cycles corresponding to the end of the primary hardening stage; (ii) 300 cycles corresponding to the end of the softening stage and to the beginning of the saturation stage; (iii) 4 2 0 0 cycles corresponding to failure in air; (iv) 8100 cycles corresponding to failure in vacuo;
(v) 1 0 5 4 0 cycles; this sample was cycled b e y o n d the fatigue life in v a c u o by performing several polishings during the test. Table 3 gives the characteristics of each structure and its p r o p o r t i o n inside the sample accord-
ing to the n u m b e r of cycles (or the cumulative plastic strain). After 25 cycles cellular and wall structures are already developing (Figs. 18 and 19) although the distribution of the dislocations is very heterogeneous f r o m grain to grain and inside the same grain. T h e interiors of these structures contain n u m e r o u s dislocations, and the walls or boundaries are thick and badly formed. T h e proportion occupied by cells is about 5 0 % and an evaluation of the m e a n cell diameter give a value of 0.5-0.7 p m . T h e wall-channel structure is forming and occupies about 15% in volume: it appears as an intermediate structure between the cells and the wall-channel structure which occurs only for a greater n u m b e r of cycles. T h e final width of the channels is difficult to m e a s u r e but is
9O TABLE 3 Proportions and dimensions of different structures of dislocations observed in the 17-12 S P H stainless steel according to the number of cycles for the plastic strain amplitude Aep/2 = 5 × 10-3
Structures
Number of cycles 25
300
4200
81O0
10 540
Cells
Tendency 0.5-0.7/~m
Walls-channels
15% 0.5-0.6/~m
50%-60% 0.4-0.7/~m L a = 0.6-3/~m I b = 0.3-0.5/~m 20%-25% 0.3-0.6/~m 5% 0.2-0.4/~m 2% Very rare Very rare Very rare 15%
50%-60% 0.5-0.7/~m L a = 0.6-2.5 ktm / b = 0.3-0.5/~m 20% 0.4-0.6/~m 4% 0.2-0.4/~m 2% Very rare Very rare Very rare -<10%
60% 0.3-0.7/zm L a = 0.4-2.5/~m l b = 0.2-0.4 ktm 15% 0.3-0.5/~m 2% 0.2-0.3/~m 2% 5%-10%
70% 0.5-0.7/~m L a = 0.2-2.5/~m / b = 0.2-0.5/~m 10%- 15% 0.3-0.4/~m 5% 0.2-0.4/~m
Labyrinths PSBs with ladder Microtwins Planar slip bands Stacking faults Tangles
3%
20%-30%
-<10%
Very rare Very rare Very rare Very rare
aMajor dimension of cell. bMinor dimension of cell.
Fig. 18. Wall structure already drawn after 25 cycles at A%/2 = 5 x 10 -3. Foil normal close to [011].
Fig. 19. Cell structure already drawn after 25 cycles at Aep/2 = 5 x 10 3. Foil normal close to [112].
estimated between 0.5 and 0.6/am. Besides these two structures, tangles (about 30%) (Fig. 20) and some microtwins are noted. During the softening stage (25-300 cycles), the most significant fact is the increase of the wall-channel structure (20%-25%) and its improvement although walls are still generally badly drawn and numerous dislocations are seen inside the channels (Fig. 21). Labyrinth structure can also be noted (5%) with spacing between walls from 0.2 to 0.4/am, and some PSBs with a ladder structure (about 2%)(Fig. 22). The description of the structural evolution during the saturation stage has been described in previous sections, when comparing the structures at failure in air and i n v a c u o . This evolution is basically characterized by a better definition of the structures and a transformation of the walls
and channels into cells. Actually, it can be seen in Table 3 that the proportion of walls and channels tends to decrease with cycling while the proportion of cells increases at the same time. For the sample cycled up to 10540 cycles it should be noted above all that the whole structures are geometrically well defined and that dislocations are well condensed in the walls and in the cell boundaries. About 70% of the observed areas are covered by cells with their interior well free of dislocations. However large cell misorientations can be noted (Fig. 23). Part of the cell structure consists of elongated cells aligned in channels and the majority of these cells seem to form from the wall-channel structure (Fig. 24) which is present to a lesser degree than in the sample cycled up to failure in v a c u o . The walls are well defined and straight and there are very few dislocations inside
91
Fig. 20. Tangles formed after 25 cycles at A%/2 =5 x 10 3. Foil normal close to [011].
the channels whose width varies from 0.3 to 0.4 /zm. Labyrinth structures also show very fine and straight walls, but sometimes seem in the process of transformation into cells. In this sample, some microtwins, stacking faults and planar slip bands have also been seen but we have not observed PSBs with a ladder structure.
Fig. 21. Wall and channel structure after 300 cycles at A%/2 = 5 x 10 -3. Foil normal close to [112].
4. D i s c u s s i o n
4.1. Formation and evolution of microstructures T h e present quantitative study of microstructures in austenitic 17-12 SPH stainless steel (cycled at 20 °C in vacuo in the plastic fatigue range) has allowed us to specify the proportions occupied in volume by the different structures observed and the dimensions of these structures. This quantification permits to establish the influence of the cumulative plastic strain at a constant strain amplitude. In the 17-12 SPH stainless steel with a relatively low stacking fault energy (SFE) (about 28 mJ m 2) [35, 36] we have observed structures such as wall and channel, ladder or labyrinth similar to those observed in copper, a material with a higher SFE (about 50 mJ m-2). However, these structures are obtained for higher amplitudes than in copper. For instance, for the lowest strain amplitude Atp/2-~ 6 × 10 -4, the wall structure is missing in the 17-12 SPH stainless steel (see Table 1) while it is predominant in c o p p e r [37]. Moreover, when these structures form they are heterogeneous and in steel at 20 °C, never reach such large proportions as in c o p p e r at 20 °C or even as in steel at 600 °C [26].
Fig. 22. Ladder-type PSB structure after 3(10 cycles at Aq,/2 = 5 x 10 3. Foil normal close to [013]. Thus, in this stainless steel we have observed very different structures such as planar arrangements and cells and intermediate configurations like walls, labyrinths and ladder-like structures. Planar arrangements are basically seen at the low strain amplitudes and for a small n u m b e r o f
92
a0C
50
s.l~~ Fig. 23. Cell structure formed after 10540 Aep/2 = 5 x 10 -3. Foil normal close to [011]..
cycles at
Fig. 24. Elongated cells formed after 10540 cycles at Aep/2 = 5 x 10 -3. Foil normal close to [001].
cycles. In contrast, cells are in the majority for high amplitudes and at failure. Figure 25 shows that a critical value of the amplitude (8 x 10 -4) exists above which planar arrangements are in the minority and cells are in the majority. PSBs, and wall and labyrinth structures were observed for medium amplitude and during saturation. We have shown that in the 17-12 SPH steel these structures were transitory; they are not formed in the early cycles but they are almost all
103
102
Fig. 25. Variation of cell and wall-channel proportions vs. plastic strain amplitude: open symbols, failure in air; full symbols, failure in v a c u o ; squares, wall-channel structure; circles, cell structure.
transformed to cells at failure in vacuo. However, it should be noted that cells are not exclusively obtained by the gradual transformation of unstable structures. In fact, for high amplitudes, the majority of cells are already shaped from the early cycles (see Table 3), even ff they close up completely much later. However, it is certain that the increase of the proportion of cells, which appears during cycling, is essentially because of the transformation of these unstable structures. Figure 25 shows that from the number of cycles at failure in air to the number of cycles at failure in vacuo, the increase of the cell proportion is compensated by an equivalent reduction of walls and channels. The elongated cells are the results of this transformation and, as a consequence, they are not formed from the early cycles contrary to the case for the equiaxed cells (see Table 3). Thus, in this material various microstructures can form according to the applied strain amplitude and the cumulated plastic strain, with some of them are characteristic of high SFE materials and others of low SFE materials. Taking a value of 28 mJ m -2 for the SFE of the 17-12 SPH stainless steel, this is in a good agreement with relations given by Feltner and Laird [38] between microstructure, SFE, plastic strain and temperature. In this material we have even observed, in the same grain, the coexistence of two structures, one typical of a material with a high SFE and the other typical of a material with a low SFE (see for example, Fig. 26 (Channels and microtwins) and Fig. 27 (cells and microtwins)).
93 d(}Jml I
t 2 0
i
I
t
L
I
i
i
,
-3 0 10 Fig. 28. Evolutions of cell size (
-2 10
) and of channel width
( - - - - --) vs. plastic strain amplitude at failure in vacuo.
Fig. 26. Coexistence of wall-channel structure and microtwins for Aep/2 = 10 -' at Nfv = 950 cycles. Foil normal close to [011 ].
loaded with dislocations which in the great majority exhibit a screw character. After a more significant amount of cycling, the walls condense progressively and the channels are emptied. Then a misorientation between adjacent channels can occur, just preceeding the transformation of this structure into cells. In the same way, cells which are formed from the first cycles have thick, uncondensed and badly closed boundaries and the interior contains numerous dislocations. With cycling, the boundaries become thinner, the interior contains less dislocations and cells finally close up completely. After this a misorientation occurs which is still very slight at failure in air but which becomes more pronounced when cycling is prolonged in v a c u o . This misorientation appears particularly marked in the sample cycled at Aep/2 = 5 x 10 .3 up to 10 540 cycles beyond the number of cycles corresponding to failure in v a c u o . These evolutions are in a good agreement with those already described in the literature [39,
401
Fig. 27. Coexistence of cells and microtwins after 10 540 cycles at A e o / 2 = 5 x l 0 3. Foil normal close to [011]: (a) bright field; (b) dark field.
In the wall-channel and labyrinth structures a marked evolution of the dislocation arrangements can be seen with cycling. Firstly, the walls are thick, uncondensed, and the channels are highly
Concerning the evolution in size of the different structures, we have noted on samples cycled up to failure in v a c u o a decrease of their dimension with an increase of plastic strain amplitude. The decrease is very weak in the case of cells and slightly more marked for the width of the channels. In this way mean values pass from 0.6 to 0.5 ktm for cells and from 0.6 to 0.3 ktm for channels when Aep/2 increases from 2 x 10 3 t o 10 .2 (Fig. 28). However, for a given value of strain amplitude, the sizes of the structures vary very little with cycling (see Table 3) which is in a good agreement with results of Plumtree [40] and Plumtree and Pawlus [39] on aluminium at least for the cellular structure. The measured values for the 17-12 SPH stainless steel are similar to those obtained by Bernard e t al. [23] but lower, concerning the channels, than those obtained by
94 l'Esperance et al. [29] in a 316L-type stainless steel. 4.2. Microstructure evolution a n d m e c h a n i c a l behaviour
Data given in Table 3 concerning the different structures during fatigue life for the 5 x 10 -3 amplitude, show a relative stability concerning their nature as well as their proportion. It appears effectively that from the early cycles, cell formation represents 50% of the observed surfaces although cells are badly formed (Fig. 18) and that the proportion stays practically the same until failure in vacuo, i.e. during 8000 cycles. During the same time the proportion of wall structure increases until the beginning of the saturation stage then slowly decreases, while the proportion of tangles decreases constantly. However the rate of decrease of the tangles is higher between 25 and 300 cycles than afterwards. These evolutions perfectly explain the mechanical behaviour given by the cyclic hardening curves A o / 2 = f ( N ) ( F i g . 1). The primary hardening, very short and all the more limited in intensity and in duration since the plastic strain amplitude is low, is due to the interaction of the dislocations between themselves and the alloy elements. These interactions are of course more important when the density of dislocations is higher, i.e. at elevated amplitudes. The slight cyclic softening which occurs after the primary hardening stage can be associated with the rearrangement of the dislocations which formed tangles into configurations that accommodate well for plastic strain, namely walls-channels with one or two directions (dipolar walls or labyrinths). Indeed, for the 5 x 10 -3 amplitude, after 25 cycles (the end of tlae primary hardening), tangles represent 20%-30% of the surface while walls-channels occupy 15% of it. At the end of the softening stage (300 cycles), tangles only represent 15% of the surface while wallschannels (and labyrinths) occupy 25%-30% of it. As the other structures have practically not changed in proportion in the same time, one can thus consider that the rearrangement of the dislocations forming tangles into walls separating channels is responsible for the observed softening. The percentages of tangles and wall-channel structures which are more important for the lower strain amplutides (see Tables 1 and 2) explain the variations of the intensity of the softening with the strain amplitude.
At higher amplitudes, the greater stability of the A a = f ( N ) curves reflect the stability in the proportion of the structures. In fact, the most pronounced evolutionary feature concerns the constant improvement of the different structures during fatigue life: at the beginning of the saturation the different structures are badly organized, even if they are defined from the first cycles (Figs. 18 and 19). After further cycling, these structures improve slowly, the cells close up, the walls line up and boundaries condense progressively, but the sizes do not change significantly. However, the decrease of both the cell size and the channel width when the plastic strain amplitude increases (Fig. 28) expresses the increase of the saturation stress with the strain amplitude. When the microstructure is in form of walls, mobile dislocations can move easily in the channels; however, their transformation into cells by mechanisms proposed in a previous paper for the same material cycled at 600 °C [26] can locally reduce cyclic strain accommodation. However, compared to more stable ones, these structures are in a minority for all the strain amplitudes; thus, their evolution alone cannot modify the macroscopic cyclic behaviour. However, when cells reach an advanced degree of organization (condensed and completely closed up boundaries), the accommodation of plastic strain must become more difficult (dislocations cannot pass from one cell to another). This is another factor that would induce cyclic hardening but which may be compensated by an easier mobility of dislocations related to the fact that the interior of the cells are at the same time free of dislocations. However, all these transformations arrive at the end of the fatigue life in vacuo when microcracks are already forming [34] and thus the presence of these microcracks would reduce the stress amplitude and balance cyclic hardening. 5. Conclusions
The present work on a 316L-type austenitic stainless steel cycled in vacuo at room temperature in the low cycle fatigue range has characterized the different dislocation structures formed by cyclic deformation and the evolution of their proportions in the bulk with cyclic plastic amplitude or cumulative plastic strain. In particular it has been shown that "unstable" wall and channel, labyrinth and ladder structures can be formed at the same time as cells or micro-
95
twins. It has been demonstrated that the proportions occupied in the bulk by the former structures increase in the first part of the test, then decrease all along the fatigue life in v a c u o , a part of them being transformed into cells. The proportion of those transformable structures is however limited compared to the stable ones; this explains why cyclic behaviour remains unchanged (no secondary hardening is observed) when cycling is prolonged by performing tests in vacuo.
References 1 P.J. Woods, Philos. Mag., 28 (1973) 155. 2 H. Mughrabi, Proc. 5th Int. Conf. on the Strength of Metals and Alloys, Aachen, 1979, Vol. 3, Pergamon, Oxford, 1979, p. 1615. 3 K. V. Rasmussen and O. B. Pedersen, Acta Metall., 28 (1980) 1467. 4 J. C. Figueroa, S. E Bhat, R. Delaveaux, S. Murzenski and C. Laird, A cta Metall., 29 ( 1981 ) 1667. 5 A.T. Winter, O. B. Pedersen and K. V. Rasmussen, Acta MetalL, 29(1981) 735. 6 T. Lepisto and E Kettunen, Scr. Metall., I6 ( 1982) 1145. 7 E Ackermann, L. P. Kubin, J. Lepinoux and H. Maghrabi, Acta Metall., 32 (1984) 715. 8 N.Y. JinandA. T. Winter, ActaMetalL, 32(1984) 1173. 9 N.Y. Jin and A. T. Winter, Acta Metall., 32 (1984) 989. 10 T. Lepisto, V. T. Kuokkala and P. Kettunen, Scr. Metall., 18(1984) 245. 11 J. Polak and M. Klesnil, Mater. Sci. Eng., 63 (1984) 189. 12 R. Wang, H. Mughrabi, S. McGovern, M. Rapp, Mater. Sci. 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 L.L. Lisiecki and J. R. Weertman, Scr. Metall., 20 (1986) 249. 16 T. K. Lepisto, V. T. Kuokkala and P. O. Kettunen, Mater. Sci. Eng., 81 (1986)457. 17 Z. Wang and C. Laird, Mater. Sci. Eng., 100 (1988) 57.
18 P. Charsley and D. Kuhlmann-Wilsdorf, Philos. Mag. A, 44(1981) 1351. 19 E Charsley, Mater. Sci. Eng., 47(1981) 181. 20 C. Laird, S. Stanzl, R. de la Veaux and L. Buchinger, Mater. Sci. Eng., 80(1986) 143. 21 L. Buchinger, A. S. Cheng, S. Stanzl and C. Laird, Mater. Sci. Eng., 80(1986) 155. 22 J. O. Nilsson, Scr. Metall., 17(1983) 593. 23 M. Bernard, J. B. Vogt, T. Bui-Quoc and J. I. Dickson, in C. J. Beevers (ed.), Proc. of 2nd Int. Conf on Fatigue and Fatigue Thresholds, 1984, Vol. II, 1029. 24 C. Gorlier, Thesis, D. I. Saint-Etienne 1984. 25 L. Boulanger, A. Bisson and A. A. Tavassoli, Philos. Mag. A, 51 2 (1985) L5. 26 M. Gerland and P. Violan, Mater. Sci. Eng., 84 (1986) 23. 27 B. Ait Saadi, Thesis, Poitiers, 1988. 28 M. Gerland, B. Ait Saadi and P. Violan, Mater. Sci. Eng., 96 (1987) L1. 29 G. L'Esperance, J. B. Vogt and J. i. Dickson, Mater. Sci. Eng., 79(1986) 141. 30 G. Brun, J. P. Gauthier and P. Petrequin, Mem. Sci. Rev. MetalL, 7-8 ( 1976 ) 461. 31 J. Mendez and P. Violan, A S T M Spec. Tech. Publ., 924 (1988) 196-210. 32 J. Mendez, P. Violan and C. Gasc, in K. J. Miller and E. R. de los Rios (eds.), The Behaviour of Short Fatigue Cracks, EGF Pub. 1, 1986, Mechanical Engineering Publications, London, pp. 145-161. 33 J. H. Driver, G. Gorlier, C. Belamri, E Violan and C. Amzallag, Low Cycle Fatigue, ASTM Spec. Tech. Publ., 942 (1987) 438-455. 34 J. Mendez, P. Violan, to be published. 35 R.E. Schramm and R. E Reed, Metall. Trans. A, 6(1975) 1345-1351. 36 T. Magnin, C. Ramade, J. Lepinoux and L. P. Kubin, Mater. Sci. Eng., A l l 8 (1989) 41. 37 P. Villechaise, J. Mendez and M. Gerland, Unpublished results. 38 C.E. Feltner and C. Laird, Trans. Metall. Soc. A1ME, 242 (1968) 1253-1257. 39 A. Plumtree and L. D. Pawlus, A S T M Spec. Tech. Publ., 924 (1988) 81-97. 40 A. Plumtree, in K. T. Rie (ed.), Low Cycle Fatigue and Elasto-Plastic Behaviour o]" Materials, Elsevier, Amsterdam, 1987, pp. 19-30. Amsterdam, 1987, pp. 19-30.
83
Evolution of Dislocation Structures and Cyclic Behaviour of a 316L-type Austenitic Stainless Steel Cycled in vacuo at Room Temperature M. GERLAND, J. MENDEZ, P. VIOLAN and B. AIT SAADI Laboratoire de Mkcanique et de Physique des Mat~riaux, URA CNRS 863, 86034 Poitiers Cedex (France)
(Received January 23, 1989; in revised form April 17, 1989)
Abstract
Dislocation structures formed during cycfic deformation at room temperature in vacuo of a 316L-type austenitic stainless steel are presented. It has been shown that for this material having a rather low stacking fault energy of about 28 mJ m - 2, dislocation structures exhibit a planar slip or a wavy slip character depending on the cyclic plastic strain amplitude. In particular the existence of wall and channel, labyrinth or ladder structures has been shown; these structures have been observed to evolve progressively into cells during extensive cycling in vacuo. The volume fraction of each type of structure in the specimen has been evaluated quantitatively as a function of the cyclic plastic strain level and the number of cycles throughout the fatigue life in vacuo.
1. Introduction
During the last 10 years the numerous studies which have been made on the cyclic behaviour of metals and alloys have considerably improved our understanding of the internal dislocation arrangements in relation with the cyclic strain-stress behaviour and the crack initiation processes. Most of these investigations have concerned f.c.c, materials and in particular monocrystalline and polycrystalline pure copper [1-17] or copper alloys [1, 18-21] cycled at room temperature, i.e. in the low homologous temperature range ( T< 0.25 Tr). One of the most interesting findings from these studies on cyclic deformation of copper, obtained in particular on single crystals, concerns the existence of well defined "soft" structures such as the wall and channel structure or the ladder structure which clearly differ from a surrounding much less 0921-5093/89/$3.50
deformable matrix. The analysis of the dislocation motion into these zones of high localized slip activity and the characterization of the associated surface deformation features as extrusion-intrusion pairs, led to a better understanding of the surface crack initiation phenomena in copper. Such detailed work has not been conducted so far for other f.c.c, materials, in particular for industrial materials, although some studies conducted on anstenitic stainless steels have revealed structures very similar to those observed in copper [22-30]. An important aspect of cyclic deformation processes which has received little attention, with the exception of a few works [26, 27], concerns the stability of the different dislocation structures with cycling. This question was often ignored because most of the fatigue tests were conducted in air; now, it has been clearly shown that in this environment the fatigue lives can be considerably reduced compared with those observed in vacuo or in inert atmospheres (even in the low cycle fatigue range and for low temperatures) [26, 28, 31-33]. As a consequence, dislocation structures appear without appreciable change during cycling at the scale of the fatigue life in air; prolonging considerably the fatigue life and therefore the accumulated plastic strain by performing the fatigue tests in vacuo, in some cases reveals that some types of dislocation structures which would appear stable in air evolve in fact progressively in a more stable structure. Examples of such a transformation are given in the work of Wang and Mughrabi [13] on monocrystalline copper cycled at room temperatm'e in vacuo or by the work of Gerland and Violan [26] on an austenitic stainless steel cycled in vacuo at 600 °C. In both studies these authors have observed that the wall and channel structure corresponding to persistent slip bands progress© Elsevier Sequoia/Printed in The Netherlands
84 ively transforms into a cell structure; such an evolution is accompanied in some cases by a modification of cyclic behaviour (secondary cyclic hardening). In this paper a study is presented of the internal dislocation structure induced by cyclic deformation in vacuo at room temperature in a 316L-type austenitic stainless steel. The respective roles of the cyclic plastic strain amplitude and the number of cycles have been considered. In particular, we have compared, at a fixed plastic strain amplitude, the microstructural states associated with the cumulative plastic strains reached at failure in air and in vacuo; the differences in fatigue damage (crack initiation processes or crack propagation kinetics) in both environments are not considered in this paper but will be published separately [34].
2. Experimental conditions The austenitic stainless steel studied in this work is a 316L-type steel denoted by 17-12 SPH whose composition has been indicated elsewhere [26, 28]. Cylindrical specimens with a diameter of 6 mm and a gauge length of 13 mm were used. They were annealed before fatigue by heating for 1 h at 1050 °C in vacuo and water quenched. The specimens were carefully electropolished in a solution of 10% perchloric acid and 90% acetic acid. Cyclic deformation tests were performed in a symmetrical uniaxial push-pull mode on an electromechanical machine at 25 °C in laboratory air or in vacuo (pressure lower than 10 -3 Pa). Tests were performed in plastic strain controlled mode with constant plastic strain rate (2 x 1 0 - 3 s-l). The cyclic plastic strain amplitudes Aep/2 investigated were in the range 6 x 10 - 4 10 - 2 The fatigue tests were performed to failure in each environment or stopped at a fixed number of cycles. The specimens were cut by spark machining, then mechanically polished to 0.1 mm; the final thin foils were then obtained using the electrolytic double-jet technique with a solution of acetic acid and perchloric acid solution. Observations were performed on a JEOL-100 B microscope at 100 kV with a double-tilt stage. For each sample, 6-8 thin foils were studied with at least two of them perpendicular to the stress axis. Each foil generally has 15-20 observable grains, which corresponds to a minimum of 100 grains observed for each specimen.
The volume fractions of the different structures described in Section 3 are obtained by averaging the whole observations realized on all the observed grains in a given sample.
3. Experimental results 3.1. Cyclichardening
Figure 1 shows the curves giving the evolution of the stress amplitude A a/2 as a function of the number of cycles in a semilogarithmic diagram, for the different strain amplitudes investigated. These curves correspond to the vacuum-tested specimens; several of them are marked by an arrow which indicates the number of cycles leading to failure in air; details of the air and vacuum test data will be given in a separate paper [34]. The cyclic behaviour of the 17-12 SPH stainless steel is characterized by a primary hardening over a number of cycles which increases with the cyclic strain amplitude but which always remains limited; it varies from 5 to 500 cycles when Aep/2 varies from 6 x 10 - 4 to 10- 2. This short period of hardening is followed by a period of softening, except for the highest strain amplitude of 10 -2 . This softening period is longer at low Aep/2 values and is much more extended, in all cases, than the previous hardening stage; however, it remains small compared to the total fatigue life. For all the strain amplitudes the stress level is stabilized during most of the test and no trace of secondary hardening is observed even when the test in vacuo is significantly prolonged beyond the fatigue life in air.
'
.......
[
[
i
......
I
........
]
........
I
i
~
. . . . .
t
........
'Ill
........
~
. . . . . . .
t
.....
_
~0[
30[ 20C
10C
.......
J
10~
. . . . .
Jill
10 z
i
.......
103
.......
10 ~
I
10s
........
106
N
Fig. 1. Cyclic hardening curves of 17-12 SPH stainless steel cycled at 20 °C in vacuo. NfA indicates the number of cycles at failure in air.
85
Figure 2 gives the cyclic stress-strain curve (CSSC) and the corresponding monotonic curve established from the first quarter-cycle of each fatigue test. Up to an amplitude of about 2 x 10 -3 the softening period balances the initial hardening and no significant cyclic hardening is observed on the CSSC; however, beyond 2 x 10 -3 the stabilized cyclic stress amplitude is up to 50% higher than the corresponding monotonic stress. 3.2. Types and proportions of the different dislocation structures at failure in vacuo: influence of the cyclic plastic strain amplitude Depending on the cyclic plastic strain amplitude, different types of dislocation structures have been observed in the specimens cycled up to failure in vacuo. These different types can be
classified as: cells; labyrinths; walls and channels; ladder structures; microtwins; stacking faults; planar slip bands; dipoles and multipoles; tangles. The existence of each structure and its volumic fraction depend on the plastic strain amplitude: the higher Aep/2 the higher the proportion of cells; the lower Aep/2 the greater the tendency of tangles to prevail. Table 1 gives all the details concerning the dimensions of each structure and its proportion in the bulk of the specimens cycled to failure in vacuo, for the different cyclic plastic strain amplitudes. 3.2.1. Cells Although the cellular structure is very rarely observed at Aep/2=6 X 10 -4, it occupies 20%-30% in volume at 8 x 10-4; in this case, the
A(~2MPa 5OO
400 300
200
10 -4
/ Cgctic //~~j
l -3
10
Monotonic
I -2
a~P/2
~_
10
Fig. 2. C y c l i c s t r e s s - s t r a i n curve and corresponding m o n o t o n i c c u r v e of t h e 1 7 - 1 2 S P H steel at 2 0 °C.
Fig. 3. C e l l u l a r s t r u c t u r e for Ae_/2 = 8 x 10 -4 at Ntv = 380 0 0 0 cycles. Foil n o r m a l c l o s e to [12 [].
Proportions and dimensions of different structures of dislocations observed in 1 7 - 1 2 S P H stainless steel cycled up to failure in v a c u o for several plastic strain amplitudes
TABLE 1
Structures
Cells
Walls-channels Labyrinths PSBs with ladder Microtwins P l a n a r slip b a n d s Stacking faults Dipoles and multipoles Tangles a M a j o r d i m e n s i o n o f cell. b M i n o r d i m e n s i o n of cell.
Aep/2 10 2
5 x 1 0 -3
2xlO 3
8.3×10-4
6x10-4
70% L a= 0.6-2.5/~m l b = 0.2-0.6/~m 10% 0.2-0.3/,m 5%- 10% 0.2-0.4/~m 2% 10%- 15% -----
60% L a = 0.4-2.5/~m l b = 0.2-0.4 pm 15% 0.3-0.5/,m 2% 0.2-0.3/~m 2% 5%- 10% ---10%
50% L a = 0.8-3.5/~m Ib = 0.3-0.6/~m 20% 0.5-0.8 pm 4%
20%-30%
Very r a r e
2% 2% 5% --15%
20%-30%
-Very r a r e 10% Very r a r e 5% 20%
-Very r a r e -Very r a r e 5% 90%
86
Fig. 4. Cellular structure for Ae_/2 = 2 × cycles. Foil normal close to [011~.
1 0 -3
at Nfv = 50 000
Fig. 6. Cellular structure for Aep/2 = 5 × 10 -3 at Nfv= 8100 cycles. Foil normal close to [111].
Fig. 5. UncondensedceUsforAep/2=5x10-3atNfv=8100 cycles. Foil normal close to [011].
Fig. 7. Elongated cells for A e J 2 = 5 x l 0 cycles. Foil normal close to [112].
cells are badly formed with poor uncondensed walls (Fig. 3). At 2 x 10-3 the proportion of cells is twice as high (about 50%), but the cells still remain badly formed (Fig. 4); for this amplitude two types of cells are observed, either equiaxed cells with a mean diameter of about 0.4-0.8/~m, or elongated cells of length up to 3/~m. For the amplitude of 5 x 10 -3 uncondensed cells (Fig. 5) as well as cells in a more advanced stage with fine condensed walls (Fig. 6) can be observed. The majority of the cells is equiaxed with a mean diameter which varies between 0.3 and 0.7/~m; however, one can also observe elongated cells (Fig. 7) up to 2.5/~m in length located in channels 0.2-0.4/~m in width. At the highest amplitude of 1 0 - 2 the cells are the predominant structure (about 70%) and have, in general, fine condensed walls (Fig. 8); the
geometrical features remain very similar to those observed at 5 x 10-3.
-3 at Nfv=8100
3.2.2. Walls and channels, ladder structures and labyrinths Walls and channels are observed from the amplitude of 8 x 10- 4, but even for amplitudes of 2 x 10- 3 they remain badly formed although they cover about 20% in volume; the walls are thick and wavy and their width varies between 0.5 and 0.8/~m. Beyond this amplitude a decrease of the volume covered by this type of structure is observed when Aep/2 increases (about 15%); the walls become regular but remain thick and badly formed, and the interior of the channels 0.3-0.5 ~m in width, contains numerous dislocations (Fig. 9). At 1 0 - 2 the wall and channel structure covers only 10% of the bulk and the wails are badly aligned, wavy and thick; the width of the
87
Fig. 8. Condensed cells for Aep/2 = 10 -2 at Nrv= 950 cycles. Foil normal close to [011].
Fig. 9. Wall and channel structure for A%/2=5 ×10 -3 at Nfv= 8100 cycles.Foil normal close to [011]. channels varies between 0.2 and 0.3 ktm, the dislocation density is rather important inside and a high tendency to transformation of this structure into cells is observed (Fig. 10). A ladder-type persistent slip band (PSB) structure (regular arranged walls in dislocation free channels) was observed for the amplitudes 2 x 10 -3, 5 × 10 -3 and 10 .2 (Fig. 11). This structure, which only covers about 2% of the total area of the observed foils, presents some differences for the classical ladder structure in copper; in the case of this stainless steel the surrounding structure is c o m p o s e d of badly formed cells or other channels instead of the classical structure of veins in copper. T h e labyrinth structure was observed at the same plastic strain amplitudes as the wall and channel or the ladder structures; it has been estimated to cover about 4 % in volume for A~p/2 =
Fig. 10. Beginning of transformation from wall structure into cells for Aep/2= 10 2 at Nfv=950 cycles. Foil normal close to [012].
Fig. 11. Ladder-type PSB structure for Aep/2= 10 2 at Nw= 950 cycles.Foil normal close to [123]. 2 x 10 -3 and near to 7% for the highest amplitude of 10 -2 (Fig. 12). T h e channel width varies little with Aep/2 and is about 0.3 #m. For high amplitudes numerous traces of local transformation of this structure into cells were found. 3.2.3. Microtwins T h e existence of mechanical microtwinning was noted from the lowest amplitudes but with very low p r o p o r t i o n at 6 x 10 -4 or 8 × 10 -4. Microtwins cover about 2% of the foil area at A%/2 = 2 x 10 -3, 7% at 5 × 10 -3 and about 12% at 10 -2. At 10 -2 microtwins cover up to 50% of certain grains (Fig. 13). 3.2.4. Other structures Less well-organized structures, such as tangles, are rarely observed in specimens tested at high
88
Fig. 12. Labyrinth structure for Aep/2= 10 -2 at cycles. Foil normal close to [011].
Nfv =
Fig. 13. Extended microtwinning for Aep/2= 1 0 - 2 Nfv = 950 cycles. Foil normal close to [110]; dark field.
950
at
amplitudes, but represent the main part of the dislocation structure for low plastic amplitudes (about 90% at Aep/2 = 6 x 10-4). In the specimens cycled at 6 × 1 0 -4 or 8 × 10-4 some planar slip bands, stacking faults or dipoles and multipoles have also been observed.
3.3. Microstructural evolution during cycling 3.3.1. Dislocation structures after failure in air Observations were made o n samples fatigued in air up to failure for three plastic strain amplitudes Aep/2 = 7 x 10 -4, 2 x 10 -3 and 5 x 10 -3. T h e same structures as those observed on samples cycled up to failure in vacuo were seen but with slightly different proportions (Figs. 14-17). Table 2 shows that m o r e walls and channels are observed at failure in air but less cells, microtwins or labyrinth structures.
Fig. 14. Ladder-type PSB structure for Aep/2=5 x 10 -3 at NfA=4200 cycles.Foil normal close to [112].
Fig. 15. Labyrinth structure for AeJ2=5xl0-3 NfA=4200 cycles.Foil normal close to [112].
at
Although the dimensions of these structures are quite similar to those in samples cycled in vacuo, we have noted that the structures are less well developed such that the cell boundaries are rather thick and loose and the majority of the cells are not completely closed with numerous dislocations inside. Also, the walls are thick and often appear to be in the process of being transformed into cell structure especially for the amplitudes 2 x 1 0 - 3 and 5 x 10-3.
3.3.2. Microstructural evolution during saturation stage for Aep/2 = 5 x 10 -3 Comparison between structures after failure in air (generally corresponding to half of the saturation stage in vacuo) and after failure in vacuo, shows an evolution of some of these structures though the stress remains stable. A detailed study of the evolution of structures was carried out for
89
K
Fig. 16. Badly formed cells for A%/2=7× 10 -4 at NeA= 145 100 cycles. Foil normal close to [111].
Fig. 17. Wall and channel structure for Aep/2 = 5 × NfA = 4200
10 -3
at
cycles. Foil normal close to [114].
TABLE 2 Proportions and dimensions of different structures of dislocations observed in 17-12 SPH stainless steel cycled up to failure in air for several plastic strain amplitudes Structures
Cells
Walls-channels Labyrinths PSBs with ladder Microtwins Planar slip bands Stacking faults Dipoles and multipoles Tangles
A ep/2 5 × 1 0 -.1
2 x l O -,~
7×10 ~
50%-60% 0.5-0.7/~m La = 0.6-2.5/am l b = 0.3-0.5/~m 20% 0.4-0.6 pm 4% 0.2-0.4 ktm 2% Very rare Very rare Very rare
50% 0.6-0.8 pm La= 1-4.5/~m l b= 0.3-0.8 pm 25% 0.4-0.7 #m
5%- 10%
10%
25%
20%
Very rare 10% Very rare 50/0- 10% 5O%
"Major dimension of cell. bMinor dimension of cell.
the amplitude 5 × 1 0 - 3. Samples were thus cycled up to different n u m b e r s of cycles: (i) 25 cycles corresponding to the end of the primary hardening stage; (ii) 300 cycles corresponding to the end of the softening stage and to the beginning of the saturation stage; (iii) 4 2 0 0 cycles corresponding to failure in air; (iv) 8100 cycles corresponding to failure in vacuo;
(v) 1 0 5 4 0 cycles; this sample was cycled b e y o n d the fatigue life in v a c u o by performing several polishings during the test. Table 3 gives the characteristics of each structure and its p r o p o r t i o n inside the sample accord-
ing to the n u m b e r of cycles (or the cumulative plastic strain). After 25 cycles cellular and wall structures are already developing (Figs. 18 and 19) although the distribution of the dislocations is very heterogeneous f r o m grain to grain and inside the same grain. T h e interiors of these structures contain n u m e r o u s dislocations, and the walls or boundaries are thick and badly formed. T h e proportion occupied by cells is about 5 0 % and an evaluation of the m e a n cell diameter give a value of 0.5-0.7 p m . T h e wall-channel structure is forming and occupies about 15% in volume: it appears as an intermediate structure between the cells and the wall-channel structure which occurs only for a greater n u m b e r of cycles. T h e final width of the channels is difficult to m e a s u r e but is
9O TABLE 3 Proportions and dimensions of different structures of dislocations observed in the 17-12 S P H stainless steel according to the number of cycles for the plastic strain amplitude Aep/2 = 5 × 10-3
Structures
Number of cycles 25
300
4200
81O0
10 540
Cells
Tendency 0.5-0.7/~m
Walls-channels
15% 0.5-0.6/~m
50%-60% 0.4-0.7/~m L a = 0.6-3/~m I b = 0.3-0.5/~m 20%-25% 0.3-0.6/~m 5% 0.2-0.4/~m 2% Very rare Very rare Very rare 15%
50%-60% 0.5-0.7/~m L a = 0.6-2.5 ktm / b = 0.3-0.5/~m 20% 0.4-0.6/~m 4% 0.2-0.4/~m 2% Very rare Very rare Very rare -<10%
60% 0.3-0.7/zm L a = 0.4-2.5/~m l b = 0.2-0.4 ktm 15% 0.3-0.5/~m 2% 0.2-0.3/~m 2% 5%-10%
70% 0.5-0.7/~m L a = 0.2-2.5/~m / b = 0.2-0.5/~m 10%- 15% 0.3-0.4/~m 5% 0.2-0.4/~m
Labyrinths PSBs with ladder Microtwins Planar slip bands Stacking faults Tangles
3%
20%-30%
-<10%
Very rare Very rare Very rare Very rare
aMajor dimension of cell. bMinor dimension of cell.
Fig. 18. Wall structure already drawn after 25 cycles at A%/2 = 5 x 10 -3. Foil normal close to [011].
Fig. 19. Cell structure already drawn after 25 cycles at Aep/2 = 5 x 10 3. Foil normal close to [112].
estimated between 0.5 and 0.6/am. Besides these two structures, tangles (about 30%) (Fig. 20) and some microtwins are noted. During the softening stage (25-300 cycles), the most significant fact is the increase of the wall-channel structure (20%-25%) and its improvement although walls are still generally badly drawn and numerous dislocations are seen inside the channels (Fig. 21). Labyrinth structure can also be noted (5%) with spacing between walls from 0.2 to 0.4/am, and some PSBs with a ladder structure (about 2%)(Fig. 22). The description of the structural evolution during the saturation stage has been described in previous sections, when comparing the structures at failure in air and i n v a c u o . This evolution is basically characterized by a better definition of the structures and a transformation of the walls
and channels into cells. Actually, it can be seen in Table 3 that the proportion of walls and channels tends to decrease with cycling while the proportion of cells increases at the same time. For the sample cycled up to 10540 cycles it should be noted above all that the whole structures are geometrically well defined and that dislocations are well condensed in the walls and in the cell boundaries. About 70% of the observed areas are covered by cells with their interior well free of dislocations. However large cell misorientations can be noted (Fig. 23). Part of the cell structure consists of elongated cells aligned in channels and the majority of these cells seem to form from the wall-channel structure (Fig. 24) which is present to a lesser degree than in the sample cycled up to failure in v a c u o . The walls are well defined and straight and there are very few dislocations inside
91
Fig. 20. Tangles formed after 25 cycles at A%/2 =5 x 10 3. Foil normal close to [011].
the channels whose width varies from 0.3 to 0.4 /zm. Labyrinth structures also show very fine and straight walls, but sometimes seem in the process of transformation into cells. In this sample, some microtwins, stacking faults and planar slip bands have also been seen but we have not observed PSBs with a ladder structure.
Fig. 21. Wall and channel structure after 300 cycles at A%/2 = 5 x 10 -3. Foil normal close to [112].
4. D i s c u s s i o n
4.1. Formation and evolution of microstructures T h e present quantitative study of microstructures in austenitic 17-12 SPH stainless steel (cycled at 20 °C in vacuo in the plastic fatigue range) has allowed us to specify the proportions occupied in volume by the different structures observed and the dimensions of these structures. This quantification permits to establish the influence of the cumulative plastic strain at a constant strain amplitude. In the 17-12 SPH stainless steel with a relatively low stacking fault energy (SFE) (about 28 mJ m 2) [35, 36] we have observed structures such as wall and channel, ladder or labyrinth similar to those observed in copper, a material with a higher SFE (about 50 mJ m-2). However, these structures are obtained for higher amplitudes than in copper. For instance, for the lowest strain amplitude Atp/2-~ 6 × 10 -4, the wall structure is missing in the 17-12 SPH stainless steel (see Table 1) while it is predominant in c o p p e r [37]. Moreover, when these structures form they are heterogeneous and in steel at 20 °C, never reach such large proportions as in c o p p e r at 20 °C or even as in steel at 600 °C [26].
Fig. 22. Ladder-type PSB structure after 3(10 cycles at Aq,/2 = 5 x 10 3. Foil normal close to [013]. Thus, in this stainless steel we have observed very different structures such as planar arrangements and cells and intermediate configurations like walls, labyrinths and ladder-like structures. Planar arrangements are basically seen at the low strain amplitudes and for a small n u m b e r o f
92
a0C
50
s.l~~ Fig. 23. Cell structure formed after 10540 Aep/2 = 5 x 10 -3. Foil normal close to [011]..
cycles at
Fig. 24. Elongated cells formed after 10540 cycles at Aep/2 = 5 x 10 -3. Foil normal close to [001].
cycles. In contrast, cells are in the majority for high amplitudes and at failure. Figure 25 shows that a critical value of the amplitude (8 x 10 -4) exists above which planar arrangements are in the minority and cells are in the majority. PSBs, and wall and labyrinth structures were observed for medium amplitude and during saturation. We have shown that in the 17-12 SPH steel these structures were transitory; they are not formed in the early cycles but they are almost all
103
102
Fig. 25. Variation of cell and wall-channel proportions vs. plastic strain amplitude: open symbols, failure in air; full symbols, failure in v a c u o ; squares, wall-channel structure; circles, cell structure.
transformed to cells at failure in vacuo. However, it should be noted that cells are not exclusively obtained by the gradual transformation of unstable structures. In fact, for high amplitudes, the majority of cells are already shaped from the early cycles (see Table 3), even ff they close up completely much later. However, it is certain that the increase of the proportion of cells, which appears during cycling, is essentially because of the transformation of these unstable structures. Figure 25 shows that from the number of cycles at failure in air to the number of cycles at failure in vacuo, the increase of the cell proportion is compensated by an equivalent reduction of walls and channels. The elongated cells are the results of this transformation and, as a consequence, they are not formed from the early cycles contrary to the case for the equiaxed cells (see Table 3). Thus, in this material various microstructures can form according to the applied strain amplitude and the cumulated plastic strain, with some of them are characteristic of high SFE materials and others of low SFE materials. Taking a value of 28 mJ m -2 for the SFE of the 17-12 SPH stainless steel, this is in a good agreement with relations given by Feltner and Laird [38] between microstructure, SFE, plastic strain and temperature. In this material we have even observed, in the same grain, the coexistence of two structures, one typical of a material with a high SFE and the other typical of a material with a low SFE (see for example, Fig. 26 (Channels and microtwins) and Fig. 27 (cells and microtwins)).
93 d(}Jml I
t 2 0
i
I
t
L
I
i
i
,
-3 0 10 Fig. 28. Evolutions of cell size (
-2 10
) and of channel width
( - - - - --) vs. plastic strain amplitude at failure in vacuo.
Fig. 26. Coexistence of wall-channel structure and microtwins for Aep/2 = 10 -' at Nfv = 950 cycles. Foil normal close to [011 ].
loaded with dislocations which in the great majority exhibit a screw character. After a more significant amount of cycling, the walls condense progressively and the channels are emptied. Then a misorientation between adjacent channels can occur, just preceeding the transformation of this structure into cells. In the same way, cells which are formed from the first cycles have thick, uncondensed and badly closed boundaries and the interior contains numerous dislocations. With cycling, the boundaries become thinner, the interior contains less dislocations and cells finally close up completely. After this a misorientation occurs which is still very slight at failure in air but which becomes more pronounced when cycling is prolonged in v a c u o . This misorientation appears particularly marked in the sample cycled at Aep/2 = 5 x 10 .3 up to 10 540 cycles beyond the number of cycles corresponding to failure in v a c u o . These evolutions are in a good agreement with those already described in the literature [39,
401
Fig. 27. Coexistence of cells and microtwins after 10 540 cycles at A e o / 2 = 5 x l 0 3. Foil normal close to [011]: (a) bright field; (b) dark field.
In the wall-channel and labyrinth structures a marked evolution of the dislocation arrangements can be seen with cycling. Firstly, the walls are thick, uncondensed, and the channels are highly
Concerning the evolution in size of the different structures, we have noted on samples cycled up to failure in v a c u o a decrease of their dimension with an increase of plastic strain amplitude. The decrease is very weak in the case of cells and slightly more marked for the width of the channels. In this way mean values pass from 0.6 to 0.5 ktm for cells and from 0.6 to 0.3 ktm for channels when Aep/2 increases from 2 x 10 3 t o 10 .2 (Fig. 28). However, for a given value of strain amplitude, the sizes of the structures vary very little with cycling (see Table 3) which is in a good agreement with results of Plumtree [40] and Plumtree and Pawlus [39] on aluminium at least for the cellular structure. The measured values for the 17-12 SPH stainless steel are similar to those obtained by Bernard e t al. [23] but lower, concerning the channels, than those obtained by
94 l'Esperance et al. [29] in a 316L-type stainless steel. 4.2. Microstructure evolution a n d m e c h a n i c a l behaviour
Data given in Table 3 concerning the different structures during fatigue life for the 5 x 10 -3 amplitude, show a relative stability concerning their nature as well as their proportion. It appears effectively that from the early cycles, cell formation represents 50% of the observed surfaces although cells are badly formed (Fig. 18) and that the proportion stays practically the same until failure in vacuo, i.e. during 8000 cycles. During the same time the proportion of wall structure increases until the beginning of the saturation stage then slowly decreases, while the proportion of tangles decreases constantly. However the rate of decrease of the tangles is higher between 25 and 300 cycles than afterwards. These evolutions perfectly explain the mechanical behaviour given by the cyclic hardening curves A o / 2 = f ( N ) ( F i g . 1). The primary hardening, very short and all the more limited in intensity and in duration since the plastic strain amplitude is low, is due to the interaction of the dislocations between themselves and the alloy elements. These interactions are of course more important when the density of dislocations is higher, i.e. at elevated amplitudes. The slight cyclic softening which occurs after the primary hardening stage can be associated with the rearrangement of the dislocations which formed tangles into configurations that accommodate well for plastic strain, namely walls-channels with one or two directions (dipolar walls or labyrinths). Indeed, for the 5 x 10 -3 amplitude, after 25 cycles (the end of tlae primary hardening), tangles represent 20%-30% of the surface while walls-channels occupy 15% of it. At the end of the softening stage (300 cycles), tangles only represent 15% of the surface while wallschannels (and labyrinths) occupy 25%-30% of it. As the other structures have practically not changed in proportion in the same time, one can thus consider that the rearrangement of the dislocations forming tangles into walls separating channels is responsible for the observed softening. The percentages of tangles and wall-channel structures which are more important for the lower strain amplutides (see Tables 1 and 2) explain the variations of the intensity of the softening with the strain amplitude.
At higher amplitudes, the greater stability of the A a = f ( N ) curves reflect the stability in the proportion of the structures. In fact, the most pronounced evolutionary feature concerns the constant improvement of the different structures during fatigue life: at the beginning of the saturation the different structures are badly organized, even if they are defined from the first cycles (Figs. 18 and 19). After further cycling, these structures improve slowly, the cells close up, the walls line up and boundaries condense progressively, but the sizes do not change significantly. However, the decrease of both the cell size and the channel width when the plastic strain amplitude increases (Fig. 28) expresses the increase of the saturation stress with the strain amplitude. When the microstructure is in form of walls, mobile dislocations can move easily in the channels; however, their transformation into cells by mechanisms proposed in a previous paper for the same material cycled at 600 °C [26] can locally reduce cyclic strain accommodation. However, compared to more stable ones, these structures are in a minority for all the strain amplitudes; thus, their evolution alone cannot modify the macroscopic cyclic behaviour. However, when cells reach an advanced degree of organization (condensed and completely closed up boundaries), the accommodation of plastic strain must become more difficult (dislocations cannot pass from one cell to another). This is another factor that would induce cyclic hardening but which may be compensated by an easier mobility of dislocations related to the fact that the interior of the cells are at the same time free of dislocations. However, all these transformations arrive at the end of the fatigue life in vacuo when microcracks are already forming [34] and thus the presence of these microcracks would reduce the stress amplitude and balance cyclic hardening. 5. Conclusions
The present work on a 316L-type austenitic stainless steel cycled in vacuo at room temperature in the low cycle fatigue range has characterized the different dislocation structures formed by cyclic deformation and the evolution of their proportions in the bulk with cyclic plastic amplitude or cumulative plastic strain. In particular it has been shown that "unstable" wall and channel, labyrinth and ladder structures can be formed at the same time as cells or micro-
95
twins. It has been demonstrated that the proportions occupied in the bulk by the former structures increase in the first part of the test, then decrease all along the fatigue life in v a c u o , a part of them being transformed into cells. The proportion of those transformable structures is however limited compared to the stable ones; this explains why cyclic behaviour remains unchanged (no secondary hardening is observed) when cycling is prolonged by performing tests in vacuo.
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