- Email: [email protected]
Effect of dislocation substructure of crack tip on near fatigue threshold in dual-phase steels
Materials Science and Engineering, A 176 (1994) 393-396
393
Effect of dislocation substructure of crack tip on near fatigue threshold in dual-phase steels Y. S. Zheng*, Z. G. Wang and S. H. Ai The State Key Laboratory for Fatigue and Fracture of Materials, Institute of Metal Research, Academia Sinica, Sehnyang 110015 (China) Abstract The effects of stabilized subgrain cell and wall structures on the near-threshold of physical short cracks in dual-phase steels have been found for the first time in the work reported in this paper. These effects result from both the larger number of cycles in the original position and the higher stress-strain field of the crack tip. For the near-threshold of long cracks, there was a tendency for dislocation cells to form. These behaviours illustrate that the dislocation morphologies of crack tips are the products of strain history. It is proposed for the first time in this paper that threshold values computed by the subgrain model should be related to effective fatigue threshold values: the dislocation cell is one of the intrinsic toughening factors and also has an effect on the microstructure.
It is well known that dual-phase (DP) ferritemartensite (FM) microstructures offer attractive combinations of mechanical properties, namely high strength, good ductility and higher fatigue resistance of crack growth. Until now the dislocation configurations induced by strain control in this composite-like structure have been reported on in a few publications [1-4], but there are still no such reports on stress control in the nearthreshold range, especially on the near-threshold for physical short cracks (PSCs). This is very difficult experimental work because it is necessary not only to retain details of fracture surfaces but also to protecct the nickel plating layer from falling off the fracture surface in the preparation of thin foil samples for transmission electron microscopy (TEM). T h e aim of the present work is to attempt to explain the intrinsic relationship between the dislocation structure and the long-short fatigue crack growth threshold in DP F M microstructure.
T h e foil samples for T E M were prepared to identify dislocation configurations before fatigue and after the fatigue crack growth threshold. In order to obtain information on the dislocation configuration of the fatigue crack tip, special care needs to be taken in the preparation of foil samples. Discs with a thickness of about 0.5 mm were spark cut from the centre of the specimen. First, the fracture surfaces were thoroughly washed and were then nickel plated chemically to a thickness of about 23 /~m (the details of the nickel plated fracture surfaces are omitted here). T h e samples with 500 # m thickness were ground to 150 /~m thickness with emery paper and then hand ground from both sides down to about 30 #m. T h e nickel plating layer on the fracture surfaces was still retained well in this state. Finally, twin-jet electropolishing was used to prepare the foil samples. Details of the dislocations of fracture surfaces and subsurfaces near the nickel plating layer could be observed under TEM. T h e foils were examined using a Philips E M 4 2 0 electron microscope operating at 100 or 120 kV.
2. Materials and experimental details
3. Results and discussion
T h e material used in this study contains 0.12-0.18 C, 0.12-1.6 Mn, 0.2-0.55 Si, less than 0.045 S, less than 0. P and balance Fe.
3.1. Characterization of the microstructure
1. Introduction
*Present address: The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi r.d., Shanghai 200050, China. 0921-5093/94/$7.00 SSDI tl921-5093(93)02529-C
T h e microstructure after intercritical annealing is shown in Fig. 1. Clearly, it has a nearly continuous network (M) surrounding ferrite grains. T h e volume fraction V,~ of martensite was determined to be 23.6% by quantitative metallography. T h e dislocation configuration before fatigue is shown in Fig. 2. T h e dislocation density decreased © 1994 ElsevierSequoia. All rights reserved
394
Y. S. Zheng et al.
/
Fatigue threshold in dual-phase steels
Fig. l. The F matrix surrounded by M in the DP FM microstructure.
Fig. 2. TEM image showing the dislocation distribution in F close to the phase interface before fatigue.
rapidly from the M - F interface to the interior of E The interior regions of F far from the phase interface are completely dislocation free. 3.2. Dislocation structures on the near-threshold of physical short cracks
It was found first that the dislocation cell and wall structures had been formed on near-threshold of PSC (Fig. 3). These kinds of dislocation cell and wall structures were also reported to appear at the saturation stage of a medium plastic strain amplitude of 2.15 x 10 -3 in DP steel (DPS)[1]. It is clear that the walls are built up of high density edge dislocations. In contrast, the. dislocation density between walls is much lower. The wall spacing is approximately 0.5-1 /~m; the direction parallel to the wall was vertical with respect to the direction of cyclic deformation. Such a parallel wall structure appears similar to that of persistent slip bands observed in fatigued copper single crystals [5], in stainless steel [6] and in other DPSs [ 1, 7].
Fig. 3. (a) The dislocation cell and (b) the wall structure in the F matrix near the phase interface on the near-threshold of PSCs.
The equiaxed cells preferred to form in the area adjacent to the phase interface (see Fig. 3(a)), with diameters of about 0.4-0.7 /~m, whereas the parallel walls were the striking feature of dislocation structures in the interior of F. This arrangement might imply an inhomogeneous strain distribution from the interior of F to the phase interface, as suggested previously [1-4,
7]. The dislocation morphologies of the fatigue crack tip are the products of strain history. The plastic zone of the crack tip on the near-threshold of PSCs had experienced long-term cyclic load stepping up in the original position and a high stress-strain field so that stable dislocation cell and wall structures can be formed in the F grains of the crack tip vicinity. 3.3. Dislocation structure on the near-threshold of long cracks
Because the fatigue threshold of long cracks is obtained by load shedding, it neither experienced the longer numbers of cycles in the original position, as for
E S. Zheng et al.
/
kiltigue threshoM in dual-phase steels
the short crack threshold, nor experienced the smaller numbers of cycles and the higher crack tip stress as for the crack growth stage. The dislocation configuration of the crack tip was denser on near-threshold of long cracks, but the dislocation cell and wall structures had still not formed, only a tendency for the latter being observed (not shown here). 3.4. 7he relationship between fatigue crack growth threshold and dislocation substructure The subgrain cell structure develops within the grains in the vicinity of the crack tip as a result of the applied cyclic load. Iron and low alloy steels in general exhibit a high propensity for subgrain cell formation duing cyclic plastic deformation. At the threshold stress intensity level after numerous fatigue cycles, the subgrain cells achieved a critical (saturation) size with impenetrable cell walls to free dislocations. At the threshold, dislocations being emitted from the crack tip or dislocations at a distance from the crack tip are simply shuttled back and forth within the boundary of the cell wall in compliance with the frequency of the applied load. In other words, the cell wall acts as formidable barriers to dislocation slip bands approaching the-dell wall as well as to dislocations attempting to move within the walls. It should be noted that, as the saturated cell size becomes larger, the slip dislocations can accommodate additional dislocations. Consequently, the back stress would be higher (hence higher AK~h) owing to an increase in the number of dislocations in the slip band pile-up. The threshold condition will persist until the applied stress intensity is increased to a level sufficient to break down the stable subgrain cell structure. Furthermore, this consitutes a fatigue threshold condition. The subgrain (dislocation) cells form a characteristic threshold size which will depend on the saturation stress and the initial grain size of the material. Lucas and Gerberich [8] proposed a model for fatigue threshold based on the dislocation subgrain cell structure, as follows: AKth=2(~pmbXE/3oy) ....
(1)
(szX) ,'-
where fl~, is the cyclic strain hardening exponent, oy is the yield stress, X is the subgrain cell size, ~ is an
395
appropriate constant for plastic flow in a polycrystalline material, Pm is the average mobile dislocation density, b is the magnitude of the Burgers vector and E is the elastic modulus. This model proposes that the threshold is influenced by microstructure, particularly the dislocation substructure. Several mechanical properties and structure parameters, as found in eqn. (1), as well as observed (apparent), effective (subtract crack closure) and calculated thresholds for PSCs in DPS are shown in Table 1. It is found from Table 1 and ref. 8 that the calculated threshold values are lower generally than the threshold values for a variety of steels, but the effective threshold values coincide well with those predicted. It is seen that subgrains, i.e. dislocation cells, are one of the intrinsic toughness factors that influence the fatigue crack growth threshold, and obviously also one of the microstructural parameters, but they do not contribute to the phenomena of macro-extrinsic toughness, such as crack closure.
4. Conclusion ( 1) It was found initially that the dislocation cell and wall structures were formed in the F grains on the nearthreshold for PSCs at the crack tip of PSCs having experienced large numbers of cycles in which the load was stepped up at the original position and a high crack tip stress-strain field. In contrast, on the near-threshold for long cracks, although the dislocation density was larger, the dislocation cells were still not formed. (2) It is raised for the first time in this paper that the dislocation cell is a micro-intrinsic toughness factor, but it should not be interrelated with macro-crack closure. Thus it makes sense that the calculated threshold values should coincide with effective threshold values but not with apparent threshold values.
Acknowledgment This work was supported by the National Natural Science of China under Grants 853263 and 95787005. We are grateful for this support.
TABLE 1. Mechanical properties, structure parameters, and apparent and effective as well as predicted threshold values of physical short cracks in dual-phase steel O'y E (MPa) (MPa) 5(15 2.(17x10 -s
/3c
P,1 (m -z)
qb
0.1Na
4×1014b
0.5 b
b (m) 2.5x10 H,
"See ref. 9: cyclic strain hardening exponent of DPS. bRef. 8.
X (m) 0.6xl0-~,
AKth(Observed ) (MPam ~'2) 4.46
AKth(effective) AKth(Calculated) (MPam ~2) (MPam ~/z) 3.12 3.19
396
Y. S. Zheng et al.
/
Fatigue threshold in dual-phase steels
References 1 Z. G Wang, Z. M. Sun and S. H. Ai, Mater. Sci. Eng. A, 113 (1989) 259-265. 2 A. M. Sherman and R. G. Davies, Metall. Trans. A, 10 (1979) 927. 3 Z.G. Wang, G. N. Wang, W. Ke and H. C. He, Mater. Sci. Eng., 91 (1987)39. 4 R. M. Ramage, K. V. Jata, G. J. Shifter and E. A. Starke, Jr., Metall. Trans. A, 18 (1987) 1291.
5 S. Kocanda, Fatigue Failure of Metals, Vol. A16, Sijthoff and Noorhoff, 1985, p. 543. 6 Y. B. Xia and Z. G. Wang, Phys. Status Solidi A, 103 (1987) 389. 7 J. H. Beatty, G. J. Shiflet and K. V. Jata, Metall. Trans. A, 19 (1988) 973. 8 J. P. Lucas and W. W. Gerberich, Fatigue Eng. Mater. Struct., 6 (3)(1983)271-280. 9 J. L. Horng and M. E. Fine, Scr. Metall., 17 (1983) 1427-1430.
393
Effect of dislocation substructure of crack tip on near fatigue threshold in dual-phase steels Y. S. Zheng*, Z. G. Wang and S. H. Ai The State Key Laboratory for Fatigue and Fracture of Materials, Institute of Metal Research, Academia Sinica, Sehnyang 110015 (China) Abstract The effects of stabilized subgrain cell and wall structures on the near-threshold of physical short cracks in dual-phase steels have been found for the first time in the work reported in this paper. These effects result from both the larger number of cycles in the original position and the higher stress-strain field of the crack tip. For the near-threshold of long cracks, there was a tendency for dislocation cells to form. These behaviours illustrate that the dislocation morphologies of crack tips are the products of strain history. It is proposed for the first time in this paper that threshold values computed by the subgrain model should be related to effective fatigue threshold values: the dislocation cell is one of the intrinsic toughening factors and also has an effect on the microstructure.
It is well known that dual-phase (DP) ferritemartensite (FM) microstructures offer attractive combinations of mechanical properties, namely high strength, good ductility and higher fatigue resistance of crack growth. Until now the dislocation configurations induced by strain control in this composite-like structure have been reported on in a few publications [1-4], but there are still no such reports on stress control in the nearthreshold range, especially on the near-threshold for physical short cracks (PSCs). This is very difficult experimental work because it is necessary not only to retain details of fracture surfaces but also to protecct the nickel plating layer from falling off the fracture surface in the preparation of thin foil samples for transmission electron microscopy (TEM). T h e aim of the present work is to attempt to explain the intrinsic relationship between the dislocation structure and the long-short fatigue crack growth threshold in DP F M microstructure.
T h e foil samples for T E M were prepared to identify dislocation configurations before fatigue and after the fatigue crack growth threshold. In order to obtain information on the dislocation configuration of the fatigue crack tip, special care needs to be taken in the preparation of foil samples. Discs with a thickness of about 0.5 mm were spark cut from the centre of the specimen. First, the fracture surfaces were thoroughly washed and were then nickel plated chemically to a thickness of about 23 /~m (the details of the nickel plated fracture surfaces are omitted here). T h e samples with 500 # m thickness were ground to 150 /~m thickness with emery paper and then hand ground from both sides down to about 30 #m. T h e nickel plating layer on the fracture surfaces was still retained well in this state. Finally, twin-jet electropolishing was used to prepare the foil samples. Details of the dislocations of fracture surfaces and subsurfaces near the nickel plating layer could be observed under TEM. T h e foils were examined using a Philips E M 4 2 0 electron microscope operating at 100 or 120 kV.
2. Materials and experimental details
3. Results and discussion
T h e material used in this study contains 0.12-0.18 C, 0.12-1.6 Mn, 0.2-0.55 Si, less than 0.045 S, less than 0. P and balance Fe.
3.1. Characterization of the microstructure
1. Introduction
*Present address: The State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Ding Xi r.d., Shanghai 200050, China. 0921-5093/94/$7.00 SSDI tl921-5093(93)02529-C
T h e microstructure after intercritical annealing is shown in Fig. 1. Clearly, it has a nearly continuous network (M) surrounding ferrite grains. T h e volume fraction V,~ of martensite was determined to be 23.6% by quantitative metallography. T h e dislocation configuration before fatigue is shown in Fig. 2. T h e dislocation density decreased © 1994 ElsevierSequoia. All rights reserved
394
Y. S. Zheng et al.
/
Fatigue threshold in dual-phase steels
Fig. l. The F matrix surrounded by M in the DP FM microstructure.
Fig. 2. TEM image showing the dislocation distribution in F close to the phase interface before fatigue.
rapidly from the M - F interface to the interior of E The interior regions of F far from the phase interface are completely dislocation free. 3.2. Dislocation structures on the near-threshold of physical short cracks
It was found first that the dislocation cell and wall structures had been formed on near-threshold of PSC (Fig. 3). These kinds of dislocation cell and wall structures were also reported to appear at the saturation stage of a medium plastic strain amplitude of 2.15 x 10 -3 in DP steel (DPS)[1]. It is clear that the walls are built up of high density edge dislocations. In contrast, the. dislocation density between walls is much lower. The wall spacing is approximately 0.5-1 /~m; the direction parallel to the wall was vertical with respect to the direction of cyclic deformation. Such a parallel wall structure appears similar to that of persistent slip bands observed in fatigued copper single crystals [5], in stainless steel [6] and in other DPSs [ 1, 7].
Fig. 3. (a) The dislocation cell and (b) the wall structure in the F matrix near the phase interface on the near-threshold of PSCs.
The equiaxed cells preferred to form in the area adjacent to the phase interface (see Fig. 3(a)), with diameters of about 0.4-0.7 /~m, whereas the parallel walls were the striking feature of dislocation structures in the interior of F. This arrangement might imply an inhomogeneous strain distribution from the interior of F to the phase interface, as suggested previously [1-4,
7]. The dislocation morphologies of the fatigue crack tip are the products of strain history. The plastic zone of the crack tip on the near-threshold of PSCs had experienced long-term cyclic load stepping up in the original position and a high stress-strain field so that stable dislocation cell and wall structures can be formed in the F grains of the crack tip vicinity. 3.3. Dislocation structure on the near-threshold of long cracks
Because the fatigue threshold of long cracks is obtained by load shedding, it neither experienced the longer numbers of cycles in the original position, as for
E S. Zheng et al.
/
kiltigue threshoM in dual-phase steels
the short crack threshold, nor experienced the smaller numbers of cycles and the higher crack tip stress as for the crack growth stage. The dislocation configuration of the crack tip was denser on near-threshold of long cracks, but the dislocation cell and wall structures had still not formed, only a tendency for the latter being observed (not shown here). 3.4. 7he relationship between fatigue crack growth threshold and dislocation substructure The subgrain cell structure develops within the grains in the vicinity of the crack tip as a result of the applied cyclic load. Iron and low alloy steels in general exhibit a high propensity for subgrain cell formation duing cyclic plastic deformation. At the threshold stress intensity level after numerous fatigue cycles, the subgrain cells achieved a critical (saturation) size with impenetrable cell walls to free dislocations. At the threshold, dislocations being emitted from the crack tip or dislocations at a distance from the crack tip are simply shuttled back and forth within the boundary of the cell wall in compliance with the frequency of the applied load. In other words, the cell wall acts as formidable barriers to dislocation slip bands approaching the-dell wall as well as to dislocations attempting to move within the walls. It should be noted that, as the saturated cell size becomes larger, the slip dislocations can accommodate additional dislocations. Consequently, the back stress would be higher (hence higher AK~h) owing to an increase in the number of dislocations in the slip band pile-up. The threshold condition will persist until the applied stress intensity is increased to a level sufficient to break down the stable subgrain cell structure. Furthermore, this consitutes a fatigue threshold condition. The subgrain (dislocation) cells form a characteristic threshold size which will depend on the saturation stress and the initial grain size of the material. Lucas and Gerberich [8] proposed a model for fatigue threshold based on the dislocation subgrain cell structure, as follows: AKth=2(~pmbXE/3oy) ....
(1)
(szX) ,'-
where fl~, is the cyclic strain hardening exponent, oy is the yield stress, X is the subgrain cell size, ~ is an
395
appropriate constant for plastic flow in a polycrystalline material, Pm is the average mobile dislocation density, b is the magnitude of the Burgers vector and E is the elastic modulus. This model proposes that the threshold is influenced by microstructure, particularly the dislocation substructure. Several mechanical properties and structure parameters, as found in eqn. (1), as well as observed (apparent), effective (subtract crack closure) and calculated thresholds for PSCs in DPS are shown in Table 1. It is found from Table 1 and ref. 8 that the calculated threshold values are lower generally than the threshold values for a variety of steels, but the effective threshold values coincide well with those predicted. It is seen that subgrains, i.e. dislocation cells, are one of the intrinsic toughness factors that influence the fatigue crack growth threshold, and obviously also one of the microstructural parameters, but they do not contribute to the phenomena of macro-extrinsic toughness, such as crack closure.
4. Conclusion ( 1) It was found initially that the dislocation cell and wall structures were formed in the F grains on the nearthreshold for PSCs at the crack tip of PSCs having experienced large numbers of cycles in which the load was stepped up at the original position and a high crack tip stress-strain field. In contrast, on the near-threshold for long cracks, although the dislocation density was larger, the dislocation cells were still not formed. (2) It is raised for the first time in this paper that the dislocation cell is a micro-intrinsic toughness factor, but it should not be interrelated with macro-crack closure. Thus it makes sense that the calculated threshold values should coincide with effective threshold values but not with apparent threshold values.
Acknowledgment This work was supported by the National Natural Science of China under Grants 853263 and 95787005. We are grateful for this support.
TABLE 1. Mechanical properties, structure parameters, and apparent and effective as well as predicted threshold values of physical short cracks in dual-phase steel O'y E (MPa) (MPa) 5(15 2.(17x10 -s
/3c
P,1 (m -z)
qb
0.1Na
4×1014b
0.5 b
b (m) 2.5x10 H,
"See ref. 9: cyclic strain hardening exponent of DPS. bRef. 8.
X (m) 0.6xl0-~,
AKth(Observed ) (MPam ~'2) 4.46
AKth(effective) AKth(Calculated) (MPam ~2) (MPam ~/z) 3.12 3.19
396
Y. S. Zheng et al.
/
Fatigue threshold in dual-phase steels
References 1 Z. G Wang, Z. M. Sun and S. H. Ai, Mater. Sci. Eng. A, 113 (1989) 259-265. 2 A. M. Sherman and R. G. Davies, Metall. Trans. A, 10 (1979) 927. 3 Z.G. Wang, G. N. Wang, W. Ke and H. C. He, Mater. Sci. Eng., 91 (1987)39. 4 R. M. Ramage, K. V. Jata, G. J. Shifter and E. A. Starke, Jr., Metall. Trans. A, 18 (1987) 1291.
5 S. Kocanda, Fatigue Failure of Metals, Vol. A16, Sijthoff and Noorhoff, 1985, p. 543. 6 Y. B. Xia and Z. G. Wang, Phys. Status Solidi A, 103 (1987) 389. 7 J. H. Beatty, G. J. Shiflet and K. V. Jata, Metall. Trans. A, 19 (1988) 973. 8 J. P. Lucas and W. W. Gerberich, Fatigue Eng. Mater. Struct., 6 (3)(1983)271-280. 9 J. L. Horng and M. E. Fine, Scr. Metall., 17 (1983) 1427-1430.