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Deformation dislocation structure and fracture in a low carbon steel
Materials Science and Engineering, 84 (1986) 137-145
137
Deformation Dislocation Structure and Fracture in a Low Carbon Steel LIU YUMEN and ZHOU JINGEN
Department of Materials Engineering, Xi'an diaotong University, Xi'an (China) (Received January 15, 1986; in revised form April 14, 1986)
ABSTRACT
The dislocation structures produced in 12Cr2Ni4A steel during plastic deformation under tensile loading were studied. A sharp yield phenomenon was observed in the steel investigated. The multiplication o f fresh dislocations and the formation o f slip bands are the fundamental mechanisms for plastic deformation. Dislocation tangles and cells with a high dislocation density are gradually developed within ferrite laths with increasing strain, leading to strain hardening. The formation o f dislocation structures is strongly affected by the carbides in the ferrite laths. It is found that microcracks nucleate at ferrite lath boundaries and the fracture unit is a single ferrite lath or the substructure o f the recovered ferrite.
1. INTRODUCTION The mechanical behaviour of a quenchedand-tempered steel strongly depends on its microstructure. Thus, the study of effects of the microstructure and dislocation structure of a steel on strength, ductility and fracture characteristics is of importance from the viewpoint of both t h e o r y and practice. For instance, a better understanding of the flow process and accompanying changes in dislocation structures in quenched-and-tempered steel is essential for improving the yield strength and flow property of the steel.
The workability and the application of steels directly depend on the flow properties such as strain hardening, which can be improved by the study of the dislocation structures developed during tensile deformation loading. In addition, the study of the fracture mechanism of steels under tensile stress is also important for developing high strength steel. In previous papers the dislocation structures produced during plastic deformation in a ferrite-pearlite steel [1] and quenched-andtempered alloy steel [2] have been reported by one of the present authors and coworkers. This paper is an extension of the previous work.
2. EXPERIMENTALPROCEDURE The composition of the 12Cr2Ni4A steel used in this study is shown in Table 1. The steel was austenitized at 900 °C, oil quenched and tempered at 600 °C for 1 h. The tension tests were carried out on an Instron machine using specimens with a diameter of 10 mm. The specimens were divided into a number of groups. Each group of specimens were deformed to a pre-fixed strain; then the tests were interrupted and the dislocation structures of the deformed specimens were examined using JEM 200CX and JEM 1000 transmission electron microscopes. A Neophot 21 optical microscope and a JSM35C scanning electron microscope were employed
TABLE 1 Chemical composition of 12Cr2Ni4A steel
Element Amount (wt.%) 0025-5416/86/$3.50
C
Cr
Ni
P
S
Fe
0.14
1.96
3.6
0.027
0.018
Balance
© Etsevier SeQuoia/Printed in The Netherlands
138
to study the microstructure and fracture surface respectively.
3. EXPERIMENTAL RESULTS
3.1. Microstructure o f the steel investigated The resultant microstructure of lath ferrite and cementite is obtained for the steel quenched-and-tempered at 600 °C (Fig. 1). Although the recovery of martensite has occurred, the lath feature of the low carbon martensite still remains in the steel investigated. The optical observation shows that some laths have grown to larger laths (Fig. 1). Two operating mechanisms should be responsible for the lath growth. The larger laths may be produced from the m o v e m e n t of lath boundaries. Alternatively, the elimination of lath boundaries due to m o v e m e n t and annihilation of dislocations at the boundaries m a y also lead to the growth of laths (Figs. 1 and 2). Two types of carbide with different shapes and sizes have been f o u n d in the quenchedand-tempered steel. The small rod-shaped carbides are distributed within lath ferrites in specific directions, and the larger carbides are distributed along lath ferrite boundaries, as shown in Fig. 2. The analysis of electron diffraction patterns shows that both carbides are cementite (F%C). The dislocation structure in the steel investigated is shown in Fig. 2(a). Although disloca-
tions are rarely present in some ferrite laths, some dislocations on which carbides are precipitated are still observed in other ferrite laths. The carbides precipitated on dislocations during tempering immobilize the dislocations and cannot form dislocation arrays with a low energy such as those at small-angle grain boundaries.
3.2. Mechanical properties o f the steel investigated The tensile properties of the steel are shown in Table 2. The true stress-strain curve is shown in Fig. 3. The pre-fixed true strains used to study the effect of deformation on dislocation structures are given in Table 3. 3.3. Dislocation structures in the deformed steel 3.3.1. Specimen with a 3.84% true strain The dislocation structures in the deformed ferrite are shown in Figs. 4-8. The ferrite lath boundaries are indicated by the arrows in Fig. 4. Some carbides can be found at the boundaries, and dislocations released from the boundaries are also observed. The characteristics of the movement and the distribution of dislocations on the slip planes of ferrite are shown in Fig. 5. Figure 6 is the dark field image of the dislocations at small-angle subgrain boundaries. The interaction between dislocations and carbides given in Fig. 7 shows
Fig. 1. Optical microscopy image showing tempered martensite in quenched-and-tempered 12Cr2Ni4A steel.
139
Fig. 2. (a) Recovery microstructure in quenched-and-tempered steel, g l l 0 a ; (b) electron microscopy image showing ferrite and carbides in quenched-and-tempered steel. TABLE 2 Tensile properties of the steel investigated Yield strength oys Ultimate tensile strength auts Fracture strength af Uniform reduction ~B in area Reduction ~f in area for fracture Elongation 5 B at maximum force
746 MPa 867 MPa 1442 MPa 6.88% 68.08% 7.38%
that the dislocation tangles occur around the carbides during plastic deformation. In this stage of deformation the dislocation tangles a n d c e l l s w e r e f o r m e d in a f e w r e g i o n s o f t h e d e f o r m e d s p e c i m e n s as s h o w n in F i g . 8.
3.3.2. Specimens with higher true strains In the specimen with a 7.54% true strain, although more dislocation tangles and cells
140 9O0 8O0 7OO 1600 I
2 co Lo
%
60C
J
1400
50C
1200 I000
40C
800
50C
eoo
20C
400
IO0
2oo o
' 6
' 2~
' 3~
(o)
'4b
,
L
50
,
i
,
60
:
70
True strain ~
,
i
80
,
i
90
,
,
tO0
Fig. 3. T r u e s t r e s s S vs. t r u e s t r a i n 41 f o r q u e n c h e d - a n d - t e m p e r e d
The pre-fixed true strains Test
T r u e strain
1 2 3 4
3.84 7.54 20.85 41.22
~ (%)
are formed, dislocation movement on the slip planes of ferrite and dislocation pileups at ferrite lath boundaries can still be observed. The dislocation structure in the specimen with a 20.85% true strain is given in Fig. 9. For the specimen with a 41.22% true strain the dislocation tangles and cells are the d o m i n a n t structures. The high dislocation density and smaller dislocation cells are the two characteristics in dislocation structures, as shown in Fig. 10.
L
,
o
I10
(b)
(%)
TABLE 3
,
i
2
3
~s (%)
l o w c a r b o n a l l o y steel.
the loading direction has taken place. This is ascribed to ferrite lath rotation during plastic deformation. It is obvious that the rotation of the ferrite laths is incompatible and thus the microcracks inevitably form at the ferrite boundaries with increasing deformation (Fig. 11).
3.5. Examination of fracture surface Observation of the fracture surface shows that heavy necking has taken place during tensile loading. The typical ductile "cup-andc o n e " fracture surface includes three parts: the fibrous region in the centre, the outside smaller shear lip and the larger radial region between the two. The appearance of the fibrous region is illustrated in Fig. 12 and consists of two types of dimple. The larger dimples result from inclusions and the smaller dimples are ascribed to characteristics of the microstructure.
4. D I S C U S S I O N
3.4. Microstructures of deformed specimens The examination on the longitudinally sectioned surface of the fractured specimen shows that there is no significant difference between the microstructure of the uniformly deformed region and the microstructure of the virgin specimen. However, in the former case, microcracks are nucleated from inclusions or occur at ferrite boundaries. Figure 11 is a scanning electron micrograph taken from the longitudinally sectioned surface of the necked region, showing the characteristics of microstructure. It can be seen that the alignment of the ferrite laths in
4.1. Yielding and early strain hardening in the steel investigated Since the t h e o r y of the interstitial atom atmosphere was proposed by Cottrell and Bilby [3] to explain the yield point phenomenon, the dislocation theory has been widely used to study the flow mechanisms [4]. In the quenched-and-tempered 12Cr2Ni4A steel, most of the carbon atoms have precipitated from the ferrite lattice to form carbides, but a small a m o u n t of carbon still remains in solution. These carbon atoms segregate at the dislocation lines and thus pin the dislocations.
141
Fig. 4. Dislocations from ferrite lath boundaries (~1 = 3.84%). The orientation of the ferrite lath is close to (011)a.
Fig. 5. Dark field image obtained using the ferrite (110)a reflection, showing dislocation glide band in a ferrite lath (~1 = 3.84%).
H o w e v e r , the p r e c i p i t a t e d carbides o n t h e d i s l o c a t i o n c a n p i n t h e d i s l o c a t i o n s as well. This is a r e a s o n a b l e a r g u m e n t f o r t h e yield point phenomenon. I t is f o u n d t h a t a large a m o u n t o f fine carbide p r e c i p i t a t e s o c c u r s at d i s l o c a t i o n s (see
Fig. 2) and, in c o n s e q u e n c e , t h e m o v e m e n t and multiplication of the pinned dislocations c a n n o t take place during yield deformation. In o r d e r to e x p l a i n t h e n u c l e a t i o n m e c h anism o f t h e Lfiders b a n d , t h e m u l t i p l i c a t i o n o f d i s l o c a t i o n s o c c u r r i n g in t h e yield
142
Fig. 6. Dark field image formed using the ferrite ( l l 0 ) a reflection, showing subgrain boundaries between ferrite laths (~I = 3.84%).
Fig. 7. Carbides and a dislocation in a deformed ferrite lath (~1 = 3.84%).
p r o c e s s has t o be c o n s i d e r e d . I t is f o u n d t h a t d i f f e r e n t d i s l o c a t i o n s t r u c t u r e s m a y be dev e l o p e d at ferrite b o u n d a r i e s in t h e q u e n c h e d a n d - t e m p e r e d 1 2 C r 2 N i 4 A steel, especially
a f t e r tensile plastic d e f o r m a t i o n (see Figs. 4 6). I t is a p p a r e n t t h a t an e x t r a strain m u s t be introduced because of the precipitation of c a r b i d e s on t h e lath b o u n d a r i e s . T h u s , w h e n
143
Fig. 8. Dislocation cells in deformed ferrite laths (41 = 3.84%).
Fig. 9. Dislocation structure in deformed ferrite laths (41 = 20.85%).
an e x t e r n a l stress is a p p l i e d t o t h e s p e c i m e n , t h e d i s l o c a t i o n s are readily released f r o m t h e lath b o u n d a r i e s , as s h o w n in Fig. 4. Figure 7 s h o w s t h a t d i s l o c a t i o n s m a y also be released f r o m t h e d e f o r m e d ferrite a r o u n d carbides, as s h o w n b y t h e a r r o w s in this figure. This is a n o t h e r possible m e c h a n i s m f o r d i s l o c a t i o n m u l t i p l i c a t i o n . I t s e e m s t h a t , even if t h e disl o c a t i o n s p r e s e n t in t h e ferrite laths b e f o r e l o a d i n g c a n n o t be u n p i n n e d b y loading, fresh d i s l o c a t i o n s m a y be g e n e r a t e d w h e n an ex-
t e r n a l l o a d is applied, leading t o t h e nucleat i o n o f L/]ders bands. O n c e Lfiders b a n d s n u c l e a t e in a ferrite lath, t h e plastic d e f o r m a t i o n will p r o p a g a t e t o o t h e r laths. W h e n a large n u m b e r o f e x t r a d i s l o c a t i o n s pile u p at t h e ferrite lath b o u n d ary, t h e stress c o n c e n t r a t i o n causes t h e release o f d i s l o c a t i o n s in t h e a d j a c e n t ferrite lath. In this w a y t h e slip d e f o r m a t i o n a n d Lfiders b a n d s p r o p a g a t e in the ferrite laths o n e b y one. T h e p r o p o s e d m e c h a n i s m is
144
Fig. 10. Dislocation structure in deformed ferrite laths (~1 = 41.22%).
Fig. 11. Scanning electron micrograph in the necking region of deformed steel.
Fig. 12. Electron fractograph showing the dimples on the fracture surface of the quenched-and-tempered steel.
s u p p o r ted by the presence of slip d e f o r m a t i o n bands in Fig. 5. Figure 3 shows t hat the strain hardening occurs with increasing plastic de f or m at i on. The dislocation structures observed in Figs. 5-7 in the early stage of plastic deformation are ascribed to interactions between dislocations and carbides, between dislocations and lath boundaries and between dislocations and dislocations. These dislocation structures
result in the strain hardening. The dislocation tangles are inevitably f o r m e d by the interactions m e n t i o n e d above. In addition, a few dislocation cells also develop in this stage which cont ri but e to strain hardening as well.
4.2. Changes in dislocation structures with increasing plastic deformation Although the dislocation tangles and cells have f o r m e d already in the early stage of
145 plastic deformation (41 = 4%), the regions of dislocation cells only take up a small fraction of the specimen and the sizes of the cells are relatively large. As the plastic deformation continues, the regions with dislocation tangle and cell structures increase. For the specimens with a 7.54% true strain, some longer dislocations m a y still be f o u n d at the ferrite boundaries. At a true strain of 20.85%, only dislocation tangle and cell structures are observed. Apparently, the dislocation structures are directly affected by the distribution of carbides. The carbides are uniformly distributed within the ferrite lath and thus a uniform distribution of dislocations is formed. Precisely because of the uniform distribution of dislocations, heavy stress concentrations cannot develop within a ferrite lath and thereby the steel exhibits a higher ~l (greater than 110%). A higher dislocation density, however, m a y develop at ferrite boundaries because the boundaries act as barriers to dislocation motion, and the incompatible flow of adjacent ferrite laths occurs. Moreover, the carbides precipitated at the boundaries may further impede the dislocation movement. The dislocation density at ferrite boundaries increases with increasing plastic deformation. Figures 9 and 10 illustrate the characteristics of dislocation structures for the specimen with larger true strains, showing a further increase in dislocation density within dislocation tangles and a decrease in the sizes of dislocation cells. 4.3. F r a c t u r e o f t h e s t e e l i n v e s t i g a t e d
In the early stage of plastic deformation, the dislocation cells develop in a few ferrite grains. A higher dislocation density occurs at the boundaries of ferrite laths. The stress concentration at the boundaries increases with increasing deformation, eventually leading to the nucleation of microcracks. At a small a m o u n t of deformation (41 = 7.54%), only a few microcracks have nucleated. The microcracks cannot propagate to fatal cracks because of the restraint of the surrounding soft ferrite matrix and its strain hardening. Since the realignment of ferrite laths along the axis of external stress develops with in-
creasing deformation and this process cannot occur simultaneously between adjacent ferrite laths, a heavier stress concentration is produced and more microcracks will inevitably nucleate at ferrite boundaries. Because the regions with a higher dislocation density have been formed within the ferrite lath for the heavily deformed specimen, the propagation of microcracks can occur easily, leading to fracture of the specimen. The examination of the microstructure and fracture surface shows t h a t the average dimple size is of the same order as the width of the ferrite lath. This finding suggests that the fracture unit is the ferrite lath or the substructures (dislocation cell etc.) of the recovered ferrite.
5. CONCLUSIONS The following conclusions can be drawn from this investigation. (1) For the quenched-and-tempered 12Cr2Ni4A steel the L6ders strain occurs in the process of tensile loading. (2) The dislocation tangles and cells with a high dislocation density are gradually formed within ferrite laths with increasing deformation. These dislocation structures should be responsible for strain hardening. (3) The distribution of carbides directly affects the formation of dislocation structures. The higher 41 (greater than 110%) of the steel investigated is ascribed to its fine ferrite laths in which carbides are uniformly distributed. (4) The ferrite lath and the substructure (dislocation cell etc.) of the recovered ferrite are the fracture unit in the steel investigated.
REFERENCES 1 L. Yumen, Trans. Met. Heat Treat., 3 (1) (1982) 1 (in Chinese). 2 L. Yumen and Yang Gui-Yang, Acta Metall. Sin., 20 (8) (1984) A242 (in Chinese). 3 A. H. Cottrell and B. A. Bilby, Proc. Phys. Soc. London, Sect. A, 62 (1949) 49. 4 E. O. Hall, Yield Point Phenomena in Metals and Alloys, Plenum, New York, 1970, 16.
137
Deformation Dislocation Structure and Fracture in a Low Carbon Steel LIU YUMEN and ZHOU JINGEN
Department of Materials Engineering, Xi'an diaotong University, Xi'an (China) (Received January 15, 1986; in revised form April 14, 1986)
ABSTRACT
The dislocation structures produced in 12Cr2Ni4A steel during plastic deformation under tensile loading were studied. A sharp yield phenomenon was observed in the steel investigated. The multiplication o f fresh dislocations and the formation o f slip bands are the fundamental mechanisms for plastic deformation. Dislocation tangles and cells with a high dislocation density are gradually developed within ferrite laths with increasing strain, leading to strain hardening. The formation o f dislocation structures is strongly affected by the carbides in the ferrite laths. It is found that microcracks nucleate at ferrite lath boundaries and the fracture unit is a single ferrite lath or the substructure o f the recovered ferrite.
1. INTRODUCTION The mechanical behaviour of a quenchedand-tempered steel strongly depends on its microstructure. Thus, the study of effects of the microstructure and dislocation structure of a steel on strength, ductility and fracture characteristics is of importance from the viewpoint of both t h e o r y and practice. For instance, a better understanding of the flow process and accompanying changes in dislocation structures in quenched-and-tempered steel is essential for improving the yield strength and flow property of the steel.
The workability and the application of steels directly depend on the flow properties such as strain hardening, which can be improved by the study of the dislocation structures developed during tensile deformation loading. In addition, the study of the fracture mechanism of steels under tensile stress is also important for developing high strength steel. In previous papers the dislocation structures produced during plastic deformation in a ferrite-pearlite steel [1] and quenched-andtempered alloy steel [2] have been reported by one of the present authors and coworkers. This paper is an extension of the previous work.
2. EXPERIMENTALPROCEDURE The composition of the 12Cr2Ni4A steel used in this study is shown in Table 1. The steel was austenitized at 900 °C, oil quenched and tempered at 600 °C for 1 h. The tension tests were carried out on an Instron machine using specimens with a diameter of 10 mm. The specimens were divided into a number of groups. Each group of specimens were deformed to a pre-fixed strain; then the tests were interrupted and the dislocation structures of the deformed specimens were examined using JEM 200CX and JEM 1000 transmission electron microscopes. A Neophot 21 optical microscope and a JSM35C scanning electron microscope were employed
TABLE 1 Chemical composition of 12Cr2Ni4A steel
Element Amount (wt.%) 0025-5416/86/$3.50
C
Cr
Ni
P
S
Fe
0.14
1.96
3.6
0.027
0.018
Balance
© Etsevier SeQuoia/Printed in The Netherlands
138
to study the microstructure and fracture surface respectively.
3. EXPERIMENTAL RESULTS
3.1. Microstructure o f the steel investigated The resultant microstructure of lath ferrite and cementite is obtained for the steel quenched-and-tempered at 600 °C (Fig. 1). Although the recovery of martensite has occurred, the lath feature of the low carbon martensite still remains in the steel investigated. The optical observation shows that some laths have grown to larger laths (Fig. 1). Two operating mechanisms should be responsible for the lath growth. The larger laths may be produced from the m o v e m e n t of lath boundaries. Alternatively, the elimination of lath boundaries due to m o v e m e n t and annihilation of dislocations at the boundaries m a y also lead to the growth of laths (Figs. 1 and 2). Two types of carbide with different shapes and sizes have been f o u n d in the quenchedand-tempered steel. The small rod-shaped carbides are distributed within lath ferrites in specific directions, and the larger carbides are distributed along lath ferrite boundaries, as shown in Fig. 2. The analysis of electron diffraction patterns shows that both carbides are cementite (F%C). The dislocation structure in the steel investigated is shown in Fig. 2(a). Although disloca-
tions are rarely present in some ferrite laths, some dislocations on which carbides are precipitated are still observed in other ferrite laths. The carbides precipitated on dislocations during tempering immobilize the dislocations and cannot form dislocation arrays with a low energy such as those at small-angle grain boundaries.
3.2. Mechanical properties o f the steel investigated The tensile properties of the steel are shown in Table 2. The true stress-strain curve is shown in Fig. 3. The pre-fixed true strains used to study the effect of deformation on dislocation structures are given in Table 3. 3.3. Dislocation structures in the deformed steel 3.3.1. Specimen with a 3.84% true strain The dislocation structures in the deformed ferrite are shown in Figs. 4-8. The ferrite lath boundaries are indicated by the arrows in Fig. 4. Some carbides can be found at the boundaries, and dislocations released from the boundaries are also observed. The characteristics of the movement and the distribution of dislocations on the slip planes of ferrite are shown in Fig. 5. Figure 6 is the dark field image of the dislocations at small-angle subgrain boundaries. The interaction between dislocations and carbides given in Fig. 7 shows
Fig. 1. Optical microscopy image showing tempered martensite in quenched-and-tempered 12Cr2Ni4A steel.
139
Fig. 2. (a) Recovery microstructure in quenched-and-tempered steel, g l l 0 a ; (b) electron microscopy image showing ferrite and carbides in quenched-and-tempered steel. TABLE 2 Tensile properties of the steel investigated Yield strength oys Ultimate tensile strength auts Fracture strength af Uniform reduction ~B in area Reduction ~f in area for fracture Elongation 5 B at maximum force
746 MPa 867 MPa 1442 MPa 6.88% 68.08% 7.38%
that the dislocation tangles occur around the carbides during plastic deformation. In this stage of deformation the dislocation tangles a n d c e l l s w e r e f o r m e d in a f e w r e g i o n s o f t h e d e f o r m e d s p e c i m e n s as s h o w n in F i g . 8.
3.3.2. Specimens with higher true strains In the specimen with a 7.54% true strain, although more dislocation tangles and cells
140 9O0 8O0 7OO 1600 I
2 co Lo
%
60C
J
1400
50C
1200 I000
40C
800
50C
eoo
20C
400
IO0
2oo o
' 6
' 2~
' 3~
(o)
'4b
,
L
50
,
i
,
60
:
70
True strain ~
,
i
80
,
i
90
,
,
tO0
Fig. 3. T r u e s t r e s s S vs. t r u e s t r a i n 41 f o r q u e n c h e d - a n d - t e m p e r e d
The pre-fixed true strains Test
T r u e strain
1 2 3 4
3.84 7.54 20.85 41.22
~ (%)
are formed, dislocation movement on the slip planes of ferrite and dislocation pileups at ferrite lath boundaries can still be observed. The dislocation structure in the specimen with a 20.85% true strain is given in Fig. 9. For the specimen with a 41.22% true strain the dislocation tangles and cells are the d o m i n a n t structures. The high dislocation density and smaller dislocation cells are the two characteristics in dislocation structures, as shown in Fig. 10.
L
,
o
I10
(b)
(%)
TABLE 3
,
i
2
3
~s (%)
l o w c a r b o n a l l o y steel.
the loading direction has taken place. This is ascribed to ferrite lath rotation during plastic deformation. It is obvious that the rotation of the ferrite laths is incompatible and thus the microcracks inevitably form at the ferrite boundaries with increasing deformation (Fig. 11).
3.5. Examination of fracture surface Observation of the fracture surface shows that heavy necking has taken place during tensile loading. The typical ductile "cup-andc o n e " fracture surface includes three parts: the fibrous region in the centre, the outside smaller shear lip and the larger radial region between the two. The appearance of the fibrous region is illustrated in Fig. 12 and consists of two types of dimple. The larger dimples result from inclusions and the smaller dimples are ascribed to characteristics of the microstructure.
4. D I S C U S S I O N
3.4. Microstructures of deformed specimens The examination on the longitudinally sectioned surface of the fractured specimen shows that there is no significant difference between the microstructure of the uniformly deformed region and the microstructure of the virgin specimen. However, in the former case, microcracks are nucleated from inclusions or occur at ferrite boundaries. Figure 11 is a scanning electron micrograph taken from the longitudinally sectioned surface of the necked region, showing the characteristics of microstructure. It can be seen that the alignment of the ferrite laths in
4.1. Yielding and early strain hardening in the steel investigated Since the t h e o r y of the interstitial atom atmosphere was proposed by Cottrell and Bilby [3] to explain the yield point phenomenon, the dislocation theory has been widely used to study the flow mechanisms [4]. In the quenched-and-tempered 12Cr2Ni4A steel, most of the carbon atoms have precipitated from the ferrite lattice to form carbides, but a small a m o u n t of carbon still remains in solution. These carbon atoms segregate at the dislocation lines and thus pin the dislocations.
141
Fig. 4. Dislocations from ferrite lath boundaries (~1 = 3.84%). The orientation of the ferrite lath is close to (011)a.
Fig. 5. Dark field image obtained using the ferrite (110)a reflection, showing dislocation glide band in a ferrite lath (~1 = 3.84%).
H o w e v e r , the p r e c i p i t a t e d carbides o n t h e d i s l o c a t i o n c a n p i n t h e d i s l o c a t i o n s as well. This is a r e a s o n a b l e a r g u m e n t f o r t h e yield point phenomenon. I t is f o u n d t h a t a large a m o u n t o f fine carbide p r e c i p i t a t e s o c c u r s at d i s l o c a t i o n s (see
Fig. 2) and, in c o n s e q u e n c e , t h e m o v e m e n t and multiplication of the pinned dislocations c a n n o t take place during yield deformation. In o r d e r to e x p l a i n t h e n u c l e a t i o n m e c h anism o f t h e Lfiders b a n d , t h e m u l t i p l i c a t i o n o f d i s l o c a t i o n s o c c u r r i n g in t h e yield
142
Fig. 6. Dark field image formed using the ferrite ( l l 0 ) a reflection, showing subgrain boundaries between ferrite laths (~I = 3.84%).
Fig. 7. Carbides and a dislocation in a deformed ferrite lath (~1 = 3.84%).
p r o c e s s has t o be c o n s i d e r e d . I t is f o u n d t h a t d i f f e r e n t d i s l o c a t i o n s t r u c t u r e s m a y be dev e l o p e d at ferrite b o u n d a r i e s in t h e q u e n c h e d a n d - t e m p e r e d 1 2 C r 2 N i 4 A steel, especially
a f t e r tensile plastic d e f o r m a t i o n (see Figs. 4 6). I t is a p p a r e n t t h a t an e x t r a strain m u s t be introduced because of the precipitation of c a r b i d e s on t h e lath b o u n d a r i e s . T h u s , w h e n
143
Fig. 8. Dislocation cells in deformed ferrite laths (41 = 3.84%).
Fig. 9. Dislocation structure in deformed ferrite laths (41 = 20.85%).
an e x t e r n a l stress is a p p l i e d t o t h e s p e c i m e n , t h e d i s l o c a t i o n s are readily released f r o m t h e lath b o u n d a r i e s , as s h o w n in Fig. 4. Figure 7 s h o w s t h a t d i s l o c a t i o n s m a y also be released f r o m t h e d e f o r m e d ferrite a r o u n d carbides, as s h o w n b y t h e a r r o w s in this figure. This is a n o t h e r possible m e c h a n i s m f o r d i s l o c a t i o n m u l t i p l i c a t i o n . I t s e e m s t h a t , even if t h e disl o c a t i o n s p r e s e n t in t h e ferrite laths b e f o r e l o a d i n g c a n n o t be u n p i n n e d b y loading, fresh d i s l o c a t i o n s m a y be g e n e r a t e d w h e n an ex-
t e r n a l l o a d is applied, leading t o t h e nucleat i o n o f L/]ders bands. O n c e Lfiders b a n d s n u c l e a t e in a ferrite lath, t h e plastic d e f o r m a t i o n will p r o p a g a t e t o o t h e r laths. W h e n a large n u m b e r o f e x t r a d i s l o c a t i o n s pile u p at t h e ferrite lath b o u n d ary, t h e stress c o n c e n t r a t i o n causes t h e release o f d i s l o c a t i o n s in t h e a d j a c e n t ferrite lath. In this w a y t h e slip d e f o r m a t i o n a n d Lfiders b a n d s p r o p a g a t e in the ferrite laths o n e b y one. T h e p r o p o s e d m e c h a n i s m is
144
Fig. 10. Dislocation structure in deformed ferrite laths (~1 = 41.22%).
Fig. 11. Scanning electron micrograph in the necking region of deformed steel.
Fig. 12. Electron fractograph showing the dimples on the fracture surface of the quenched-and-tempered steel.
s u p p o r ted by the presence of slip d e f o r m a t i o n bands in Fig. 5. Figure 3 shows t hat the strain hardening occurs with increasing plastic de f or m at i on. The dislocation structures observed in Figs. 5-7 in the early stage of plastic deformation are ascribed to interactions between dislocations and carbides, between dislocations and lath boundaries and between dislocations and dislocations. These dislocation structures
result in the strain hardening. The dislocation tangles are inevitably f o r m e d by the interactions m e n t i o n e d above. In addition, a few dislocation cells also develop in this stage which cont ri but e to strain hardening as well.
4.2. Changes in dislocation structures with increasing plastic deformation Although the dislocation tangles and cells have f o r m e d already in the early stage of
145 plastic deformation (41 = 4%), the regions of dislocation cells only take up a small fraction of the specimen and the sizes of the cells are relatively large. As the plastic deformation continues, the regions with dislocation tangle and cell structures increase. For the specimens with a 7.54% true strain, some longer dislocations m a y still be f o u n d at the ferrite boundaries. At a true strain of 20.85%, only dislocation tangle and cell structures are observed. Apparently, the dislocation structures are directly affected by the distribution of carbides. The carbides are uniformly distributed within the ferrite lath and thus a uniform distribution of dislocations is formed. Precisely because of the uniform distribution of dislocations, heavy stress concentrations cannot develop within a ferrite lath and thereby the steel exhibits a higher ~l (greater than 110%). A higher dislocation density, however, m a y develop at ferrite boundaries because the boundaries act as barriers to dislocation motion, and the incompatible flow of adjacent ferrite laths occurs. Moreover, the carbides precipitated at the boundaries may further impede the dislocation movement. The dislocation density at ferrite boundaries increases with increasing plastic deformation. Figures 9 and 10 illustrate the characteristics of dislocation structures for the specimen with larger true strains, showing a further increase in dislocation density within dislocation tangles and a decrease in the sizes of dislocation cells. 4.3. F r a c t u r e o f t h e s t e e l i n v e s t i g a t e d
In the early stage of plastic deformation, the dislocation cells develop in a few ferrite grains. A higher dislocation density occurs at the boundaries of ferrite laths. The stress concentration at the boundaries increases with increasing deformation, eventually leading to the nucleation of microcracks. At a small a m o u n t of deformation (41 = 7.54%), only a few microcracks have nucleated. The microcracks cannot propagate to fatal cracks because of the restraint of the surrounding soft ferrite matrix and its strain hardening. Since the realignment of ferrite laths along the axis of external stress develops with in-
creasing deformation and this process cannot occur simultaneously between adjacent ferrite laths, a heavier stress concentration is produced and more microcracks will inevitably nucleate at ferrite boundaries. Because the regions with a higher dislocation density have been formed within the ferrite lath for the heavily deformed specimen, the propagation of microcracks can occur easily, leading to fracture of the specimen. The examination of the microstructure and fracture surface shows t h a t the average dimple size is of the same order as the width of the ferrite lath. This finding suggests that the fracture unit is the ferrite lath or the substructures (dislocation cell etc.) of the recovered ferrite.
5. CONCLUSIONS The following conclusions can be drawn from this investigation. (1) For the quenched-and-tempered 12Cr2Ni4A steel the L6ders strain occurs in the process of tensile loading. (2) The dislocation tangles and cells with a high dislocation density are gradually formed within ferrite laths with increasing deformation. These dislocation structures should be responsible for strain hardening. (3) The distribution of carbides directly affects the formation of dislocation structures. The higher 41 (greater than 110%) of the steel investigated is ascribed to its fine ferrite laths in which carbides are uniformly distributed. (4) The ferrite lath and the substructure (dislocation cell etc.) of the recovered ferrite are the fracture unit in the steel investigated.
REFERENCES 1 L. Yumen, Trans. Met. Heat Treat., 3 (1) (1982) 1 (in Chinese). 2 L. Yumen and Yang Gui-Yang, Acta Metall. Sin., 20 (8) (1984) A242 (in Chinese). 3 A. H. Cottrell and B. A. Bilby, Proc. Phys. Soc. London, Sect. A, 62 (1949) 49. 4 E. O. Hall, Yield Point Phenomena in Metals and Alloys, Plenum, New York, 1970, 16.