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Kinetics of fatigue-induced phase transformation in a metastable austenitic 304 L-type steel at low temperatures
Scripta METALLURGICA et MATERIALIA
Vol. 29, pp. 521-526, 1993 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
KINETICS OF F A T I G U E - I N D U C E D P H A S E T R A N S F O R M A T I O N IN A METASTABLE AUSTENITIC 304 L - T Y P E STEEL AT L O W T E M P E R A T U R E S H.J. Maier 1, O. Schneeweiss 2 and B. Donth 1 llnstitut fiir Werkstoffwissenschaften, Lehrstuhl I, Universit~it Erlangen-Niirnberg Martensstr. 5, D-W-8520 Erlangen, Fed. Rep. Germany 2Czech Academy of Sciences, Institute of Physical Metallurgy Zi~kova 22, CS-616 62 Brno, Czech Republic (Received May 21, 1993)
Introduction Austenitic stainless steels are widely used mainly because of their excellent corrosion resistance. For applications where high strength is required, metastable variants of these materials may be used as the strength of these metastable steels can be increased via the formation of martensite [1,2]. The martensitic transformation can occur spontaneously by quenching the steel below the martensite start temperature (M~). Above Me the phase transformation can be assisted either by elastic stresses or plastic deformation. Cyclic deformation at low temperatures can strongly enhance the strength of metastable steel with acceptable decrease in ductility and toughness [3]. Investigations based on fatigue tests have shown that the fatigue life depends strongly on the kinetics of the martensite formation [4]. The temperature dependence of the kinetics of the martensitic transformation with and without superimposed external stresses below the yield point and during plastic deformation have been intensely investigated. For a recent review see [5]. In this paper, the kinetics of the martensitic transformation are investigated during low-temperature cyclic deformation. Experimental The material used in this investigation was an AISI 304 L-type metastable austenitic stainless steel (German designation X2 CrNi 19 11, 1.4306) whose nominal composition is given in table i. TABLE 1 - Alloy Composition (weight percent) I C ] !3 0.01 0 5
I V
Mn I P I S I Cr I M;0 1.25 0.024 0.023 18.38 Al 0.042 0.001
o.o o oo l
0.08 I W
I
I
Ni] 10.05 0.0012
The fatigue specimens having a 4.8 mm gauge length and a gauge diameter of 2.7 mm were machined from 30 mm diameter bars by electrical discharge machining. Prior to testing the entire gauge section was electropolished using a solution of 125g CrO3, 675ml CHaCOOH and 35ml H20 and a current density of 2000Am -2 at a temperature of 18 °C. Fatigue testing involved closed-loop plastic strain control in symmetrical push-pull using a sinusoidal waveform. All specimens were fatigue tested at a plastic strain range (A%l) of 2.5 × 10-2. The tests were run at a constant frequency of 0.2 Hz resulting in a mean plastic strain rate of 1 × 10-2 s -1. Test temperatures were 103K and 203K. As the test material has low thermal conductivity the tests were run in vacuum ( P t o t ___ 1 × 10-3 Pa ) to improve the cooling efficiency. Details of specimen heat treatment and the equipment used for low temperature testing have been described elsewhere [3]. The average grain size was 38 #m. After a given number of cycles the tests were stopped during the increasing half cycle at the peak stress and the resulting microstructure was studied b~ transmission electron microscopy (TEM). The TEM specimens were 521 0956-716X/93 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.
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prepared from thin (approximately 0.5 mm) slices cut perpendicular to the stress axis and carefully ground to 40/~m thickness. Comparisons with specimens prepared completely by electropolishing indicated that no phase transformation had occurred during mechanical specimen preparation. Final preparation for TEM observation involved double-jet electropolishing using a 10:1 solution of acetic acid and perchloric acid with a potential of 42 V at a temperature of 10 °C. All TEM work was performed at a nominal accelerating voltage of 120 kV. MSssbauer spectroscopy was used to quantify the volume content of a'-martensite transformed during cyclic deformation. Thin slices of ~ 40/~m were prepared in the same way as those used for TEM observation and 57Fe MSssbauer spectra were obtained in transmission geometry using 57Co in Cr as a source. Calibration was performed against an a - i r o n foil and the spectra were deconvoluted using the CONFIT program package [6]. These results were used for calculation of the atomic fractions of iron atoms in the phases present in the samples. The volume concentration of these phases is equal to the atomic fractions, since a homogeneous iron distribution in all phases can be expected. Results and Discussion Prior to fatigue testing the as-received material showed planar dislocation arrangements with a low overall dislocation density, a few stacking faults and twins characteristic of this type of material. Figures 1 a) and b) show a bright and a dark field TEM micrograph of the microstrJacture observed in a specimen fatigued at the lower temperature of 103 K. Only part of the a'-marteusite shows up in the dark field micrograph obtained using the (202) a'-reflection due to the misorientation between individual martensite laths. Austenite or emartensite could not be detected in this sample. The microstructures at both test temperatures showed only slight differences. At the lower temperature the a'-martensite appears more lath-like while at the higher test temperature it was more needle-like. The similarity of the microstructures formed at both test temperatures was also confirmed by tensile tests of low-temperature pre-fatigued specimens reported elsewhere [4, 7]. These tests indicated that the ultimate tensile strength at room temperature is governed primarily by the amount of fatigueinduced a'-martensite. Lath-shaped and needle-like a'-martensite were found to give identical mechanical properties when tested at room temperature. In specimens from tests that were interrupted before the austenite to martensite transformation was completed, TEM investigations revealed fine needle-like a'-martensite and austenite with intense shear bands and micro-twins. Shear band intersections have been identified as preferred nucleation sites for the formation of a'-martensite, e.g. [8-10]. Preliminary tests indicated that even at temperatures down to 4.2 K spontaneous phase transformation does not occur in this material. In tests with purely elastic loading run at 7 K the material also did not show any phase transformation. Therefore, under the test conditions used, the martensite transformation requires plastic deformation to occur. As seen in fig. 2 a), only a paramagnetic phase is detected by MSssbauer spectroscopy after fatigue deformation to 1.25 cycles at 103 K. For the case of room temperature fatigue deformation several authors have already found that a certain cumulative plastic incubation strain exists that must be exceeded before the martensitic transformation sets in [11-14]. Fig. 2 b) shows the MSssbauer spectrum obtained from a sample fatigued to 20 cycles at a temperature of 103 K. In the spectrum the following phases were identified: (i) a ferromagnetic phase, represented by three Zeeman sextets with a mean hyperfine induction Bhf = 24.4 T and (ii) a paramagnetic phase, represented by the singlet with an isomer shift (IS) of-0.1 mm s -1, the doublet with IS ~ = -0.1 mm s -1 and a quadrupole splitting (QS) ~ 0.14 mm s -x. Paramagnetic phases present in the samples are austenite and ~-martensite. Ferromagnetic phases are a'-martensite and 6-ferrite. Measurements of the magnetic saturation indicated that the 8-ferrite content in the as-received condition is below 0.5vo1.%. Therefore, in the following the 6-ferrite content will be neglected. The paramagnetic phase ~-martensite, which is often found as an intermediate phase in the transformation from austenite to a'-martensite [8-10], has been reported to have a volume fraction below 10% in tensile tests [15]. Furthermore, the e-martensite content is known to peak already at low strains in such tests [9]. Similar results were obtained in the present work. TEM inspection has shown that e-martensite could be observed only in the early stages of fatigue and even then the volume fractions were below the detection limit of MSssbauer spectroscopy. Therefore, the paramagnetic phase content measured by MSssbauer spectroscopy was
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assumed to be equal to the austenite content and the ferromagnetic phase equal to the a'-content. Carbides were neglected in this case as the carbon content is very low. For intermediate a ' - m a r t e n s i t e contents MSssbauer spectroscopy gave results that were equal within the experimental scatter to those measured by conventional X-ray analysis [16] and in measurements of the magnetic saturation. Regarding the detection of phases with a volume content below approximately 10%, however, MSssbauer spectroscopy was found to be superior to the X-ray technique. The cyclic stress amplitudes vs. number of cycles are shown in fig. 3 for both of the low test temperatures. For comparison, the cyclic response curve at a test temperture above the threshold temperature for deformationinduced martensite deformation (Md) is also included. After a small initial hardening stage followed by weak cyclic softenting, cyclic saturation is observed for the test run at a temperature above Md. This type of deformation behaviour has already been reported for stable austenitic stainless steels [11, 17]. In contrast, at both low temperatures pronounced cyclic hardening is apparent. At temperatures below Md the magnitude of the cyclic stress amplitude is governed by: (i) the thermal component of the yield strength, (ii) the amount of a ' - m a r t e n s i t e formed during deformation and (iii) the dislocation density and arrangement. Consequently, the phase transformation behaviour cannot be inferred directly from the shape of the cyclic stress response curve. In fig. 4 the increase in cyclic stress amplitude normalized with respect to the first quarter cycle is plotted together with the martensite volume content as a function of the number of cycles for a specimen fatigued at 203 K. It is seen that the rapid increase in the stress amplitude correlates with the martensite volume content. This direct correlation between martensite volume content and the cyclic stress amplitude also holds for the tests run at 103 K. A direct correlation between the a ' - m a r t e n s i t e volume content and the yield stress is also reported from fatigue tests run at room temperature [17]. Fig. 5 indicates that decreasing temperature accelerates the transformation kinetics. Such a behaviour is expected as (i) the difference in Gibbs free enthalphy between austenite and a'-martensite increases at lower temperatures [18] and (ii) the cyclic stress amplitude is increased as the thermal component of the yield strength increases thus providing additional driving force for the phase transformation. However, the maximum martensite content for both temperatures is identical within the experimental scatter. As the microstructures formed in the fatigued samples and the maximum a ' - m a r t e n s i t e volume contents are quite similar at both of the low test temperatures the difference in the magnitude of the cyclic stress amplitude is mainly due to the difference in the thermal component of the yield strength. In a related study, it was shown that at temperatures below about 220 K a martensite volume content of about 83 % is obtained in specimens cycled until the maximum stress amplitude is reached [7]. Similarly, Hecker et al. [19] have shown for 304 stainless steel that for roomtemperature tensile deformation the maximum a ' - m a r t e n s i t e content is about 85 %. However, the ferromagnetic phase content in samples deformed cyclically until failure was found to be 96 % for both test temperatures, cf. fig. 5. This difference will be addressed below. From fig. 4 it is obvious that the cyclic stress amplitude peaks around 100 cycles and drops slightly until macrocrack growth sets in. In contrast, the MSssbauer data indicate that the ferromagnetic phase content increases continuously until failure. It should be noted that the samples prepared for MSssbauer spectroscopy were prepared from areas of the samples not traversed by the crack. The slight decrease in stress amplitude despite the increase in ferromagnetic phase content can be due to either of the following. First, the decrease of the stress amplitude can be a consequence of a change in dislocation arrangement and/or a reduction of dislocation density during cyclic deformation. Such microstructural changes may well compensate the effect of an increase in martensite volume content. Second, the amount of ferromagnetic phase measured by MSssbauer spectroscopy might not necessarily be identical with the true a ' - m a r t e n s i t e volume content. The measurement error observed for ferromagnetic phases is below 1%. However, one might speculate that austenite with an extremely high dislocation density might appear as a ferromagnetic phase in a MSssbauer spectra as this technique is sensitive to the local atomic arrangement. Such local transformation to ferromagetic state was already observed in other alloys [20]. An increase in the dislocation density could also affect the Lamb-MSssbauer factor f which gives the probability of nuclear resonance. As f depends on the local stress amplitude such an effect seems feasible. However, it has been observed that residual internal stresses associated with dislocation structures formed during cyclic
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deformation are lower than those measured in monotonically deformed materials [21,22]. The maximum amount of martensite formed remains below 100% as elastic stresses which build up in the austenite hinders further transformation [23]. IIence, if cyclic deformation results in dislocation structures with lower internal stresses than monotonic deformation, a higher volume fraction of transformed a'-martensite can be obtained. Thus we propose that the dislocation arrangement and density do not have a significant effect on the MSssbauer spectra and cyclic deformation can give higher volume fraction of c~'-martensite than monotonic deformation. Conclusions Metastable austenitic stainless steel was cyclically deformed at temperatures of 103 K and 203 K. The results can be summarized as follows. 1. The rate of fatigue-induced martensite formation increases with decreasing temperatures while the maximum volume fraction of a'-martensite obtainable remains constant. 2. The increase of the cyclic stress amplitude during low-temperature fatigue deformation is determined mainly by the amount of fatigue-induced a'-martensite. 3. It appears that cyclic deformation can give higher maximum martensite volume contents than monotonic deformation. Acknowledgments The authors would like to thank Dr. H. Nyilas for the measurements done under elastic loading at 7 K. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
R.W.K. IIoneycombe, Steels Microstructure and Properties, p. 230, Edward Arnold Ltd., London (1981). U. Reichel, B. Gabriel, M. Kesten, B. Meier and W. Dahl, steel research 10, 464 (1989). M. Bayerlein, It. Mughrabi, M. Kesten and B. Meier, Mater. Sci. Eng. A159; 35 (1992). H.J. Maier, B. Donth, M. Bayerlein, H. Mughrabi, B. Meier and M. Kesten, submitted to Z. Metallkunde. V. Raghavan, in: Martensite, Eds. G.B. Oison and W.S. Owen, Chap. 11, ASM International (1992). V. VeselS', Nucl. Instr. Meth. Phys. Res. B18, 88 (1986). II.J. Maier, B. Donth, M. Bayerlein and H. Mughrabi, in: Proc. 5th Int. Conf. on Fatigue and Fatigue Thresholds, FATIGUE '93, Vol. I, Eds. J.-P. Bailon and J.I. Dickson, p. 85, EMAS Ltd., UK (1993). L.E. Murr, K.P. Staudhammer and S.S. ttecker, Metall. Trans. 13A, 627 (1982). P.L. Mangonon, Jr. and G. Thomas, Metall. Trans. 1, 1577 (1970). D. IIennessy, G. Steckel and C. Altstetter, Metall. Trans. 7A, 415 (1976). K. Tsuzaki, T. Maki and I. Tamura, J. Physique Coll. C4, sfipplement au no 43,423 (1982). M. Bayerlein, II.-J. Christ and It. Mughrabi, Mater. Sci. Eng. A l l 4 , L l l (1989). G.R. Chanani and S.D. Antolovich, Metall. Trans. 5, 217 (1974). G. Baudry and A. Pineau, Mater. Sci. Eng. 28, 229 (1977). Y. Katz, A. Bussiba and I-I. Mathias, in: Proc. 4th European Conf. on Fracture: "Fracture and the Role of Microstructure", Vol. II, Eds. K.L. Maurer and F.E. Matzer, p. 503, Leoben (1982). C.F. Jatczak, J.A. Larson and S.W. Shin, Retained Austenite and its Measurements by X-Ray Diffraction, p. 1, Soc. of Automotive Eng., Warrendale (1980). K. Tsuzaki, E. Nakanishim, T. Maki and I. Tamura, Trans. ISIJ 23, 834 (1983). R.P. Reed, in: Austenitic Steels at Low Temperature, Eds. R.P. Reed and T. Horiuchi, p. 41, Plenum, New York (1983). S.S. ttecker, M.G. Stout, K.P. Staudhammer and J.L. Smith, Metall. Trans. 13A, 619 (1982). S. Takahashi and A.Y. Takahashi, J. Phys.: Condens. Matter 4, L339 (1992). M. Wilkens, K. IIerz, It. Mughrabi, Z. Metallkunde. 71,376 (1980). T. Unggr, I-I. Biermann and I-I. Mughrabi, Mater. Sci. Eng., in press. D.A. Porter and K.E. Easterling, Phase Transformations in Metals and Alloys, p. 382, Van Nostrand Reinhold Co. Ltd., Wokingham (1981).
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Figure 1: Microstructure (TEM) of a specimen fatigued to fracture (N = 271 cycles) at Aepl = 2.5 × 10 -2 and 103 K; (a) Bright field and (b) corresponding dark field micrograph. ~tl~_'Y77._;;-Z..--£---~-: . . . . .
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Figure 2: M6ssbauer spectra of specimens fatigued to (a) N = 1.25, (b) N---20 and (c) N = 271 cycles at Aepl of 2.5 x 10-2 and 103 K. The dotted lines denote the paramagnetic component (austenite).
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1600 1400
~ 103 K /
~1200 1000
203 K
423 K
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.
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1;o0'2o'00'2,oo Figure 3: Cyclic stress response curves for dif200 300 i number of cycles ferent test temperatures at ~X%l of 2.5 × 10 -2.
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Figure 4: Increase in cyclic stress amplitude normalized with respect to the stress amplitude in the first cycle and (I) normalized evolution of ferromagnetic phase volume content vs. number of cycles at a test temperature of 203 K.
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Figure 5: Volume fraction of ferromagnetic phase as a function of number of cycles for specimens fatigued at Aepl = 2.5 x 10 -2 at temperatures of 103 K and 203 K.
4
Vol. 29, pp. 521-526, 1993 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
KINETICS OF F A T I G U E - I N D U C E D P H A S E T R A N S F O R M A T I O N IN A METASTABLE AUSTENITIC 304 L - T Y P E STEEL AT L O W T E M P E R A T U R E S H.J. Maier 1, O. Schneeweiss 2 and B. Donth 1 llnstitut fiir Werkstoffwissenschaften, Lehrstuhl I, Universit~it Erlangen-Niirnberg Martensstr. 5, D-W-8520 Erlangen, Fed. Rep. Germany 2Czech Academy of Sciences, Institute of Physical Metallurgy Zi~kova 22, CS-616 62 Brno, Czech Republic (Received May 21, 1993)
Introduction Austenitic stainless steels are widely used mainly because of their excellent corrosion resistance. For applications where high strength is required, metastable variants of these materials may be used as the strength of these metastable steels can be increased via the formation of martensite [1,2]. The martensitic transformation can occur spontaneously by quenching the steel below the martensite start temperature (M~). Above Me the phase transformation can be assisted either by elastic stresses or plastic deformation. Cyclic deformation at low temperatures can strongly enhance the strength of metastable steel with acceptable decrease in ductility and toughness [3]. Investigations based on fatigue tests have shown that the fatigue life depends strongly on the kinetics of the martensite formation [4]. The temperature dependence of the kinetics of the martensitic transformation with and without superimposed external stresses below the yield point and during plastic deformation have been intensely investigated. For a recent review see [5]. In this paper, the kinetics of the martensitic transformation are investigated during low-temperature cyclic deformation. Experimental The material used in this investigation was an AISI 304 L-type metastable austenitic stainless steel (German designation X2 CrNi 19 11, 1.4306) whose nominal composition is given in table i. TABLE 1 - Alloy Composition (weight percent) I C ] !3 0.01 0 5
I V
Mn I P I S I Cr I M;0 1.25 0.024 0.023 18.38 Al 0.042 0.001
o.o o oo l
0.08 I W
I
I
Ni] 10.05 0.0012
The fatigue specimens having a 4.8 mm gauge length and a gauge diameter of 2.7 mm were machined from 30 mm diameter bars by electrical discharge machining. Prior to testing the entire gauge section was electropolished using a solution of 125g CrO3, 675ml CHaCOOH and 35ml H20 and a current density of 2000Am -2 at a temperature of 18 °C. Fatigue testing involved closed-loop plastic strain control in symmetrical push-pull using a sinusoidal waveform. All specimens were fatigue tested at a plastic strain range (A%l) of 2.5 × 10-2. The tests were run at a constant frequency of 0.2 Hz resulting in a mean plastic strain rate of 1 × 10-2 s -1. Test temperatures were 103K and 203K. As the test material has low thermal conductivity the tests were run in vacuum ( P t o t ___ 1 × 10-3 Pa ) to improve the cooling efficiency. Details of specimen heat treatment and the equipment used for low temperature testing have been described elsewhere [3]. The average grain size was 38 #m. After a given number of cycles the tests were stopped during the increasing half cycle at the peak stress and the resulting microstructure was studied b~ transmission electron microscopy (TEM). The TEM specimens were 521 0956-716X/93 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.
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prepared from thin (approximately 0.5 mm) slices cut perpendicular to the stress axis and carefully ground to 40/~m thickness. Comparisons with specimens prepared completely by electropolishing indicated that no phase transformation had occurred during mechanical specimen preparation. Final preparation for TEM observation involved double-jet electropolishing using a 10:1 solution of acetic acid and perchloric acid with a potential of 42 V at a temperature of 10 °C. All TEM work was performed at a nominal accelerating voltage of 120 kV. MSssbauer spectroscopy was used to quantify the volume content of a'-martensite transformed during cyclic deformation. Thin slices of ~ 40/~m were prepared in the same way as those used for TEM observation and 57Fe MSssbauer spectra were obtained in transmission geometry using 57Co in Cr as a source. Calibration was performed against an a - i r o n foil and the spectra were deconvoluted using the CONFIT program package [6]. These results were used for calculation of the atomic fractions of iron atoms in the phases present in the samples. The volume concentration of these phases is equal to the atomic fractions, since a homogeneous iron distribution in all phases can be expected. Results and Discussion Prior to fatigue testing the as-received material showed planar dislocation arrangements with a low overall dislocation density, a few stacking faults and twins characteristic of this type of material. Figures 1 a) and b) show a bright and a dark field TEM micrograph of the microstrJacture observed in a specimen fatigued at the lower temperature of 103 K. Only part of the a'-marteusite shows up in the dark field micrograph obtained using the (202) a'-reflection due to the misorientation between individual martensite laths. Austenite or emartensite could not be detected in this sample. The microstructures at both test temperatures showed only slight differences. At the lower temperature the a'-martensite appears more lath-like while at the higher test temperature it was more needle-like. The similarity of the microstructures formed at both test temperatures was also confirmed by tensile tests of low-temperature pre-fatigued specimens reported elsewhere [4, 7]. These tests indicated that the ultimate tensile strength at room temperature is governed primarily by the amount of fatigueinduced a'-martensite. Lath-shaped and needle-like a'-martensite were found to give identical mechanical properties when tested at room temperature. In specimens from tests that were interrupted before the austenite to martensite transformation was completed, TEM investigations revealed fine needle-like a'-martensite and austenite with intense shear bands and micro-twins. Shear band intersections have been identified as preferred nucleation sites for the formation of a'-martensite, e.g. [8-10]. Preliminary tests indicated that even at temperatures down to 4.2 K spontaneous phase transformation does not occur in this material. In tests with purely elastic loading run at 7 K the material also did not show any phase transformation. Therefore, under the test conditions used, the martensite transformation requires plastic deformation to occur. As seen in fig. 2 a), only a paramagnetic phase is detected by MSssbauer spectroscopy after fatigue deformation to 1.25 cycles at 103 K. For the case of room temperature fatigue deformation several authors have already found that a certain cumulative plastic incubation strain exists that must be exceeded before the martensitic transformation sets in [11-14]. Fig. 2 b) shows the MSssbauer spectrum obtained from a sample fatigued to 20 cycles at a temperature of 103 K. In the spectrum the following phases were identified: (i) a ferromagnetic phase, represented by three Zeeman sextets with a mean hyperfine induction Bhf = 24.4 T and (ii) a paramagnetic phase, represented by the singlet with an isomer shift (IS) of-0.1 mm s -1, the doublet with IS ~ = -0.1 mm s -1 and a quadrupole splitting (QS) ~ 0.14 mm s -x. Paramagnetic phases present in the samples are austenite and ~-martensite. Ferromagnetic phases are a'-martensite and 6-ferrite. Measurements of the magnetic saturation indicated that the 8-ferrite content in the as-received condition is below 0.5vo1.%. Therefore, in the following the 6-ferrite content will be neglected. The paramagnetic phase ~-martensite, which is often found as an intermediate phase in the transformation from austenite to a'-martensite [8-10], has been reported to have a volume fraction below 10% in tensile tests [15]. Furthermore, the e-martensite content is known to peak already at low strains in such tests [9]. Similar results were obtained in the present work. TEM inspection has shown that e-martensite could be observed only in the early stages of fatigue and even then the volume fractions were below the detection limit of MSssbauer spectroscopy. Therefore, the paramagnetic phase content measured by MSssbauer spectroscopy was
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assumed to be equal to the austenite content and the ferromagnetic phase equal to the a'-content. Carbides were neglected in this case as the carbon content is very low. For intermediate a ' - m a r t e n s i t e contents MSssbauer spectroscopy gave results that were equal within the experimental scatter to those measured by conventional X-ray analysis [16] and in measurements of the magnetic saturation. Regarding the detection of phases with a volume content below approximately 10%, however, MSssbauer spectroscopy was found to be superior to the X-ray technique. The cyclic stress amplitudes vs. number of cycles are shown in fig. 3 for both of the low test temperatures. For comparison, the cyclic response curve at a test temperture above the threshold temperature for deformationinduced martensite deformation (Md) is also included. After a small initial hardening stage followed by weak cyclic softenting, cyclic saturation is observed for the test run at a temperature above Md. This type of deformation behaviour has already been reported for stable austenitic stainless steels [11, 17]. In contrast, at both low temperatures pronounced cyclic hardening is apparent. At temperatures below Md the magnitude of the cyclic stress amplitude is governed by: (i) the thermal component of the yield strength, (ii) the amount of a ' - m a r t e n s i t e formed during deformation and (iii) the dislocation density and arrangement. Consequently, the phase transformation behaviour cannot be inferred directly from the shape of the cyclic stress response curve. In fig. 4 the increase in cyclic stress amplitude normalized with respect to the first quarter cycle is plotted together with the martensite volume content as a function of the number of cycles for a specimen fatigued at 203 K. It is seen that the rapid increase in the stress amplitude correlates with the martensite volume content. This direct correlation between martensite volume content and the cyclic stress amplitude also holds for the tests run at 103 K. A direct correlation between the a ' - m a r t e n s i t e volume content and the yield stress is also reported from fatigue tests run at room temperature [17]. Fig. 5 indicates that decreasing temperature accelerates the transformation kinetics. Such a behaviour is expected as (i) the difference in Gibbs free enthalphy between austenite and a'-martensite increases at lower temperatures [18] and (ii) the cyclic stress amplitude is increased as the thermal component of the yield strength increases thus providing additional driving force for the phase transformation. However, the maximum martensite content for both temperatures is identical within the experimental scatter. As the microstructures formed in the fatigued samples and the maximum a ' - m a r t e n s i t e volume contents are quite similar at both of the low test temperatures the difference in the magnitude of the cyclic stress amplitude is mainly due to the difference in the thermal component of the yield strength. In a related study, it was shown that at temperatures below about 220 K a martensite volume content of about 83 % is obtained in specimens cycled until the maximum stress amplitude is reached [7]. Similarly, Hecker et al. [19] have shown for 304 stainless steel that for roomtemperature tensile deformation the maximum a ' - m a r t e n s i t e content is about 85 %. However, the ferromagnetic phase content in samples deformed cyclically until failure was found to be 96 % for both test temperatures, cf. fig. 5. This difference will be addressed below. From fig. 4 it is obvious that the cyclic stress amplitude peaks around 100 cycles and drops slightly until macrocrack growth sets in. In contrast, the MSssbauer data indicate that the ferromagnetic phase content increases continuously until failure. It should be noted that the samples prepared for MSssbauer spectroscopy were prepared from areas of the samples not traversed by the crack. The slight decrease in stress amplitude despite the increase in ferromagnetic phase content can be due to either of the following. First, the decrease of the stress amplitude can be a consequence of a change in dislocation arrangement and/or a reduction of dislocation density during cyclic deformation. Such microstructural changes may well compensate the effect of an increase in martensite volume content. Second, the amount of ferromagnetic phase measured by MSssbauer spectroscopy might not necessarily be identical with the true a ' - m a r t e n s i t e volume content. The measurement error observed for ferromagnetic phases is below 1%. However, one might speculate that austenite with an extremely high dislocation density might appear as a ferromagnetic phase in a MSssbauer spectra as this technique is sensitive to the local atomic arrangement. Such local transformation to ferromagetic state was already observed in other alloys [20]. An increase in the dislocation density could also affect the Lamb-MSssbauer factor f which gives the probability of nuclear resonance. As f depends on the local stress amplitude such an effect seems feasible. However, it has been observed that residual internal stresses associated with dislocation structures formed during cyclic
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deformation are lower than those measured in monotonically deformed materials [21,22]. The maximum amount of martensite formed remains below 100% as elastic stresses which build up in the austenite hinders further transformation [23]. IIence, if cyclic deformation results in dislocation structures with lower internal stresses than monotonic deformation, a higher volume fraction of transformed a'-martensite can be obtained. Thus we propose that the dislocation arrangement and density do not have a significant effect on the MSssbauer spectra and cyclic deformation can give higher volume fraction of c~'-martensite than monotonic deformation. Conclusions Metastable austenitic stainless steel was cyclically deformed at temperatures of 103 K and 203 K. The results can be summarized as follows. 1. The rate of fatigue-induced martensite formation increases with decreasing temperatures while the maximum volume fraction of a'-martensite obtainable remains constant. 2. The increase of the cyclic stress amplitude during low-temperature fatigue deformation is determined mainly by the amount of fatigue-induced a'-martensite. 3. It appears that cyclic deformation can give higher maximum martensite volume contents than monotonic deformation. Acknowledgments The authors would like to thank Dr. H. Nyilas for the measurements done under elastic loading at 7 K. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.
R.W.K. IIoneycombe, Steels Microstructure and Properties, p. 230, Edward Arnold Ltd., London (1981). U. Reichel, B. Gabriel, M. Kesten, B. Meier and W. Dahl, steel research 10, 464 (1989). M. Bayerlein, It. Mughrabi, M. Kesten and B. Meier, Mater. Sci. Eng. A159; 35 (1992). H.J. Maier, B. Donth, M. Bayerlein, H. Mughrabi, B. Meier and M. Kesten, submitted to Z. Metallkunde. V. Raghavan, in: Martensite, Eds. G.B. Oison and W.S. Owen, Chap. 11, ASM International (1992). V. VeselS', Nucl. Instr. Meth. Phys. Res. B18, 88 (1986). II.J. Maier, B. Donth, M. Bayerlein and H. Mughrabi, in: Proc. 5th Int. Conf. on Fatigue and Fatigue Thresholds, FATIGUE '93, Vol. I, Eds. J.-P. Bailon and J.I. Dickson, p. 85, EMAS Ltd., UK (1993). L.E. Murr, K.P. Staudhammer and S.S. ttecker, Metall. Trans. 13A, 627 (1982). P.L. Mangonon, Jr. and G. Thomas, Metall. Trans. 1, 1577 (1970). D. IIennessy, G. Steckel and C. Altstetter, Metall. Trans. 7A, 415 (1976). K. Tsuzaki, T. Maki and I. Tamura, J. Physique Coll. C4, sfipplement au no 43,423 (1982). M. Bayerlein, II.-J. Christ and It. Mughrabi, Mater. Sci. Eng. A l l 4 , L l l (1989). G.R. Chanani and S.D. Antolovich, Metall. Trans. 5, 217 (1974). G. Baudry and A. Pineau, Mater. Sci. Eng. 28, 229 (1977). Y. Katz, A. Bussiba and I-I. Mathias, in: Proc. 4th European Conf. on Fracture: "Fracture and the Role of Microstructure", Vol. II, Eds. K.L. Maurer and F.E. Matzer, p. 503, Leoben (1982). C.F. Jatczak, J.A. Larson and S.W. Shin, Retained Austenite and its Measurements by X-Ray Diffraction, p. 1, Soc. of Automotive Eng., Warrendale (1980). K. Tsuzaki, E. Nakanishim, T. Maki and I. Tamura, Trans. ISIJ 23, 834 (1983). R.P. Reed, in: Austenitic Steels at Low Temperature, Eds. R.P. Reed and T. Horiuchi, p. 41, Plenum, New York (1983). S.S. ttecker, M.G. Stout, K.P. Staudhammer and J.L. Smith, Metall. Trans. 13A, 619 (1982). S. Takahashi and A.Y. Takahashi, J. Phys.: Condens. Matter 4, L339 (1992). M. Wilkens, K. IIerz, It. Mughrabi, Z. Metallkunde. 71,376 (1980). T. Unggr, I-I. Biermann and I-I. Mughrabi, Mater. Sci. Eng., in press. D.A. Porter and K.E. Easterling, Phase Transformations in Metals and Alloys, p. 382, Van Nostrand Reinhold Co. Ltd., Wokingham (1981).
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4
FATIGUE-INDUCED
TRANSFORMATION
525
Figure 1: Microstructure (TEM) of a specimen fatigued to fracture (N = 271 cycles) at Aepl = 2.5 × 10 -2 and 103 K; (a) Bright field and (b) corresponding dark field micrograph. ~tl~_'Y77._;;-Z..--£---~-: . . . . .
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Figure 2: M6ssbauer spectra of specimens fatigued to (a) N = 1.25, (b) N---20 and (c) N = 271 cycles at Aepl of 2.5 x 10-2 and 103 K. The dotted lines denote the paramagnetic component (austenite).
526
FATIGUE-INDUCED TRANSFORMATION
Vol.
29, No.
1600 1400
~ 103 K /
~1200 1000
203 K
423 K
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Figure 4: Increase in cyclic stress amplitude normalized with respect to the stress amplitude in the first cycle and (I) normalized evolution of ferromagnetic phase volume content vs. number of cycles at a test temperature of 203 K.
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Figure 5: Volume fraction of ferromagnetic phase as a function of number of cycles for specimens fatigued at Aepl = 2.5 x 10 -2 at temperatures of 103 K and 203 K.
4