- Email: [email protected]
Effect of carbon content on selection of slip system during uniaxial tensile deformation of lath martensite
Materials Science & Engineering A 777 (2020) 139090
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Effect of carbon content on selection of slip system during uniaxial tensile deformation of lath martensite K.H. Ryou *, S. Nambu , T. Koseki Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Electron-backscattering diffraction Martensitic steel Deformation behavior
The deformation behavior of lath martensite in medium-carbon steel was investigated using a scanning electron microscope and electron backscattering diffraction techniques and compared with that of low-carbon steel. There was a significant difference in the occurrence behavior of slip bands between the two grades of steel. In particular, the slip band occurrence is concentrated on specific type in the case of medium-carbon steel. About the origin of this particular phenomenon, it is indicated that the difference in the formation behavior of slip band results from the difference in microstructures due to different carbon content. The noticeable difference in the microstructure was the block width, which affected the selection of the slip system. The block boundary constraint obtained from the block width could be the key factor in the selection of a slip system for martensite that has a relatively higher-carbon content.
1. Introduction Martensite in steels possesses high strength and hardness because of factors such as the super-saturated solid solution of carbon, high dislo cation density, and fine microstructure owing to displacive trans formation. Using the electron-backscattering diffraction (EBSD) techniques, studies have been carried out on the hierarchical micro structure in the prior austenite grain of lath martensite containing 0–0.6% carbon content that includes the lath, sub-block, block, and packet [1–5]. The microstructural complexity of martensite also results in high strength as compared to other phases [6,7]. Due to increasing interest in the potential of lath martensite for industrial applications, the deformation behavior of martensite has been investigated. Khlebnikova et al. [8] and Schastlivtsev et al. [9] reported that a {110} texture similar to that of body-centered cubic (BCC) structure metals is observed in lath martensite and the primary slip planes in lath martensite during cold-rolling deformation are mainly {110} and {112} planes, similar to conventional BCC metals. However, in the case of tensile loading con dition, lath martensite exhibits brittle fracture behavior with extremely small plastic strain range [10]. To overcome the limitation of martensite elongation, some researchers introduced a new approach with specific structured steel to achieve large plastic deformation of lath martensitic steel in the uniaxial loading condition [11–13]. Several studies have been conducted by our research group on the deformation behavior of
lath martensite during tensile loading by employing multi-layered steels that achieved more than 50% strain with even in including martensite [14–20]. Nambu et al. [14] and Ishimoto et al. [15] investigated the deformation behavior of lath martensite in a low-carbon steel during large deformation. They reported that transition behavior and aniso tropic behavior of selection in the slip system of lath martensite that resulted from the anisotropy of morphological feature of lath. The anisotropy of the slip system between the in-lath and out-of-lath plane was also confirmed for crystal rotation behavior [16]. Grain refinement occurred as a result of crystal rotation, and it is clarified that refinement becomes severe along the carbon content of interstitial-free (IF), ultra-low-carbon and low-carbon steels [17]. In addition, the localiza tion of strain is affected by the interaction between adjacent sub-blocks grains during the grain refinement [18]. The occurrence of slip bands and dislocation movements have been investigated to understand the deformation response. The slip bands at the free surface of martensite in low-carbon steel tilt at 45-degree to tensile axis [19]. In perspective of dislocation, a decrease in the annihilation rate of mobile dislocations affects enhancing of work-hardening rate [20]. Ohmura et al. reported that dislocation movement is interrupted by high-angle block boundary in martensitic steel using in-situ nanoindentation [21] and Shibata et al. performed a micro-bending test and demonstrated that the block boundary affects the martensite strengthening behavior [22]. From the result of a micro-tensile test within an extremely restricted condition, Du
* Corresponding author. E-mail address: [email protected] (K.H. Ryou). https://doi.org/10.1016/j.msea.2020.139090 Received 21 November 2019; Received in revised form 10 February 2020; Accepted 10 February 2020 Available online 11 February 2020 0921-5093/© 2020 Elsevier B.V. All rights reserved.
K.H. Ryou et al.
Materials Science & Engineering A 777 (2020) 139090
et al. concluded that block and sub-block boundaries strengthen lath martensite, and concluded that the plastic deformation of martensite depends on the sliding of boundaries when boundary strengthening works [23,24]. Similarly, Mine et al. [25] and Kwak et al. [26] reported the results of micro-tensile tests and concluded that anisotropy resulted from the morphological feature of lath martensite. Anisotropy was also observed even in the bainite, which has similar morphological features with martensite [27]. Numerous studies have been conducted on the morphological features of martensite [28–32]. The effects of block size [28], prior austenite grain size [29], and comprehensive consideration [30] were investigated and it was found that affect global mechanical behavior that decides mechanical properties of martensite. Several factors affect the deformation behavior of martensite. Morsdorf et al. reported that the mechanism of lath martensite plasticity is complex including the effects of size, slip system, and microstructure [31]. It is reported that there are differences between the lath martensite of ultra-low-carbon steel and low-carbon steel in terms of deformed microstructure and deformation behavior [17,32]. The morphological features, orientation relationship between the adjacent blocks, and mechanical properties changed with increasing carbon content, over coming the range of low-carbon steel [20,33,34]. However, previous studies mainly studied martensite in low-carbon and ultra-low-carbon steels for short-range strains and significantly restricted conditions. Therefore, this study aims to investigate the deformation behavior of lath martensite in relatively high-carbon steel and compare the defor mation behavior with that of low-carbon steel. The microstructural features of lath martensite as a polycrystalline during large plastic strain of uniaxial tensile loading are studied.
were investigated with 0.16 μm of step size using EBSD (Tex-SEM Lab oratory (TSL)) technique. 3. Results The results of the tensile test for each carbon content sample are shown as engineering stress-strain curves (Fig. 1). The mechanical properties such as yield strength, ultimate tensile strength (UTS), uni form elongation, and fracture elongation are summarized in Table 2. The uniform elongation of both samples was confirmed that exceeded more than 25% elongation and analysis was conducted in the range of uniform elongation to guarantee the deformation along with the tensile test. 3.1. In-situ observation Fig. 2 shows the microstructure of martensite in low and mediumcarbon steels by Inverse Pole Figure (IPF) map from EBSD analysis, in which the grain boundaries are defined as above 8� of misorientation. Both microstructures of martensite showed similar size of prior austenite grain (60–70 μm). The microstructures of martensite in medium-carbon steel, especially the block widths, became smaller. Thus, the block width was of primary concern. The block widths were measured by a method that considers the angle between the habit plane and the surface [32]. The block width of martensite in low-carbon steel was about 9 μm. On the contrary, the martensite in medium-carbon steel displayed finer microstructure and narrower block width, with an average width of about 3 μm. In-situ observation was carried out during deformation of martensite in low- and medium-carbon steels. The result for medium-carbon steel is shown in Fig. 3, and the result of low-carbon steel exhibited similar behavior to that of previous research [14,15]. Fig. 3(a) illustrates the IPF map before deformation occurred. SEM micrographs were taken at the surface of the samples at every 5% strain until 20%, as shown in Figs. 3 (b)–(f) for martensite in medium-carbon steel. The slip bands were first observed in SEM images at 5% strain already. With increasing strain, the number of slip bands also increased. The formation behavior of slip bands appeared to develop as a unit of packet, as indicated by packet boundaries marked with white dashed lines in Figs. 3(a) and (d). Comparing among the packets, there are differences in deformation even at the same strain. Thus, the occurrence and development of slip bands were mainly investigated among the deformation phenomena of martensite.
2. Experimental Two types of multi-layered steel, including lath martensitic steel with different carbon content, were prepared. The one sample consists of a 0.4 wt% carbon martensitic steel layer and a TWIP steel layer. These four layers of martensitic steel and three layers of TWIP steel were stacked in seven layers that has surface layers with martensitic steel. The initial thicknesses of the constituent steels were 4 mm each, and the thickness of the laminated steel sheets was reduced to 1 mm by hotrolling and cold-rolling processes. For comparison, martensite with relatively low-carbon (0.2 wt%C) steel was prepared following the same procedure of a previous research [17]. The chemical compositions of the steels used in this study are shown in Table 1. To obtain a similar austenite grain size, the fabricated multi-layered steels were austeni tized at 1373 K for 10 min for medium-carbon steel, and 2 min for low-carbon steel. Subsequently, austenitized steels were water-quenched to obtain a full lath martensitic microstructure. The heat-treated samples were cut to tensile test specimen size of 6.25 mm width and 12 mm length of smooth part. Before the tensile test was performed, the surfaces of all samples were polished for observation. In-situ observation during the tensile test was conducted at each 5% elongation until 25% large strain. The strain was measured by an extension meter with an initial length of 10 mm. By using a field-emission scanning electron microscope (FESEM; JSM7001F, JEOL), the formation behavior of the slip bands on the surface was observed. The martensite microstructure and crystallographic features
Table 1 Chemical compositions of the constituent materials (weight percent). Types of steel
C
Medium-carbon MLS TWIP 0.6 Martensite 0.4 Low-carbon MLS [17] SUS 316L 0.02 Martensite 0.2
Mn
Si
Cr
Ni
Mo
Fe
22 1.8
0.25 1.5
– 1.5
– 3.02
– 0.19
Bal. Bal.
0.84 0.25
0.63 0.25
17.76 –
12.09 14
2.12 –
Bal. Bal.
Fig. 1. Engineering stress-strain curves from a tensile test on low-carbon MLS sample (dotted) and a medium-carbon MLS sample (solid). 2
K.H. Ryou et al.
Materials Science & Engineering A 777 (2020) 139090
martensite of both low- and medium-carbon steels. Even though all the four slip band types were observed in both lowand medium-carbon steels, the global developments of the slip bands of low- and medium-carbon steels are quite different. Fig. 5 shows the fraction of slip band types at each strain for both steels. As mentioned in section 3.1, the development of slip bands was different for each packet unit. Thus, hundreds of packets for each steel type were investigated to identify the occurrence tendency of slip band types. For martensite in low-carbon steel, as shown in Fig. 5(a), the large amount of C-type slip bands was observed, and the transition from A-to B-type and from C- to D-type was observed at 10% strain. All the slip band types developed relatively uniformly until 25% strain without concentration on specific slip band type. However, in the case of martensite in medium-carbon steel, as shown in Fig. 5(b), a small fraction of C-type at 5% strain, and smaller transitions from A-to B-type and from C- to D-type at larger strains were observed. The strong dependence phenomenon on the se lection of A-type slip band was found to be consistent.
Table 2 Mechanical properties of multi-layered steels in this present study. Types of steel
0.2% offset yield strength (MPa)
UTS (MPa)
Uniform El. (%)
Fracture El. (%)
Medium-carbon MLS (0.4C) Low-carbon MLS (0.2C)
682
1061
28.5
41.9
337
653
37.9
53.7
3.3. Schmid factor consideration As there are differences in the occurrence of slip bands depending on the region, it is important to clarify the factors affecting the occurrence of slip bands. Thus, considering the microstructure characteristic, the Schmid factor of martensite was investigated. Generally, the BCC martensite has 24 slip systems in total: {110} planes with [111] di rections, and {112} planes with [111] direction. In this study, in-lath plane slip systems among those 24 slip systems were considered based on the previous researches [14–18]. As mentioned in the introduction, the in-lath plane slip system considers the anisotropy that comes from the morphological feature of lath. As shown in the schematic illustra tions of BCC structure and slip systems in Fig. 6, number 1 and 2 are two in-lath plane slip directions on the habit plane indicated by red-colored arrows and red-colored plane. The in-lath plane slip system is catego rized by the existence of in-lath plane slip direction in the slip system. Thus, the slip system that has a green-colored slip plane and number 2 slip direction is also considered as an in-lath plane slip system. However, the other slip system that has a green-colored slip plane and number 3 slip direction does not include in-lath plane slip direction. Therefore, this slip system is the out-of-lath plane slip system. Consequently, with two in-lath plane slip directions accompanied six of slip plane for each direction, there are 12 in-lath plane slip systems. Fig. 7 compares the result between the slip band distribution and Schmid factor map. The SEM image of martensite in low-carbon steel at 5% strain is shown in
Fig. 2. IPF maps from martensite in (a) low-carbon steel and (b) mediumcarbon steel. Grain boundaries for misorientation >8� are drawn.
3.2. Slip band formation behavior The developing patterns of slip bands were categorized into four types as shown in Fig. 4. Fig. 4 also shows the in-situ observation results schematic illustrations of each slip band type. The A-type shown in Fig. 4 (a) indicates the longitudinal-slip-band type where the slip bands form along the block length direction marked with blue-colored arrow, and its tendency is maintained until a large strain region. The B-type initially displayed a similar behavior with the A-type; however, secondary slip bands occurred, crossed to former slip bands and longitudinal direction of the block as shown with yellow-colored arrow in Fig. 4(b). The C-type is fully transverse type, showing transverse slip bands from initial to large strain stages, and slip band is marked with red-colored arrow in Fig. 4(c). The D-type shows a comparable tendency with the C-type, but secondary slip bands occurred randomly crossed at the large strain stage as shown in Fig. 4(d). The four types of slip bands were found in the
Fig. 3. (a) IPF map and SEM images along increasing of 5% strain of martensite in medium-carbon steel from 0% until 20%; (b) 0%, (c) 5%, (d) 10%, (e) 15%, (f) 20% (White dotted line region indicates single packet.). 3
K.H. Ryou et al.
Materials Science & Engineering A 777 (2020) 139090
Fig. 4. IPF map, SEM images along increasing of 5% strain and schematic illustration of categorized slip band types; (a) A-type: longitudinal block length, (b) B-type: Longi. to transverse, (c) C-type: block transverse direction and (d) D-type: multi-crossed.
Fig. 7(a), and the block boundaries above 8-degree of misorientation and slip bands with red line are indicated. Fig. 7(b) indicates the highest value of the Schmid factor among 12 in-lath plane slip system. The value corresponded well with the high-value region of Schmid factor and the slip band distribution, and was in good agreement with our previous research [14]. The slip band formation behavior of martensite in medium-carbon steel was also investigated. The SEM image with slip band distribution marked with red line at 5% strain is shown in Fig. 8(a). Similar to low-
Fig. 5. Development of the slip band type fraction of packets along increasing of strain in (a) low-carbon steel and (b) medium-carbon steel.
Fig. 6. Schematic illustration of slip systems in a crystal structure of BCC lattice.
Fig. 7. (a) SEM image of 5% strain with slip bands (red line) and block boundaries., (b) Schmid factor map of in-lath plane slip system of martensite in low-carbon steel. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 4
K.H. Ryou et al.
Materials Science & Engineering A 777 (2020) 139090
carbon steel, the Schmid factor map of the in-lath plane slip system is shown in Fig. 8(b). Comparing the results of medium-carbon steel case with that of low-carbon steel, a well-matched relationship between the slip band distribution and Schmid factor of the in-lath plane slip system was not noticed. In the region of yellow-dashed circles in Fig. 8(b), several slip bands were seen in the lower Schmid factor regions. Conversely, no slip band was observed in higher Schmid factor region. Consequently, the Schmid factor of in-lath plane slip system cannot explain the slip band formation of martensite in medium-carbon steel.
and the habit plane slip system when martensite deforms. As the habit plane slip system is included among the 12 of in-lath plane slip systems, the cases that indicate high Schmid factors of both slip systems are excluded in the results. Fig. 9(a) shows fraction of selected slip system of martensite in low-carbon steel along the block width. The martensite in low-carbon steel reveals the distribution of relatively wide block widths, and the selection of the slip system is concentrated on the in-lath plane slip system. On the contrary, the martensite in medium-carbon steel distributes of relatively narrower block widths compared with martensite in low-carbon steel (Fig. 9(b)). As shown above, the con centration tendency of selected slip system was also different. In the case of martensite in medium-carbon steel, the selection of the slip system is focused on the habit plane slip system. It seems that the selection of a slip system could be affected by the microstructure resulting from the carbon contents. However, in certain cases such as the narrow-width block of martensite in low-carbon steel and the wide width block of martensite in medium-carbon steel, they do not follow the tendency of selection of slip system along their carbon content. It means that even for martensite in medium-carbon steel, the in-lath plane slip system was activated if blocks had wide width.
4. Discussion The differences in the fraction of slip band types shown in Fig. 5 could be considered as the effect of the microstructure. It is well-known that the development of slip bands is affected by block boundaries because the slip band is the result of the accumulation of dislocation and dislocation cannot propagate the grain boundaries. Thus, among the various kinds of slip bands, the slip band type that has a longitudinal slip direction along the block direction, i.e., the A-type, is easier to form. Considering the finer microstructure of martensite in medium-carbon steel, there is a strong dependence on the A-type slip band because the A-type could be the most favorable type. Narrow block width makes slip bands in the transverse direction of difficult to occur, not only the C-type but also B- and D-types. An additional consideration is needed because the Schmid factor map based on in-lath plane slip system cannot explain the distribution of slip bands for martensite in medium-carbon steel (Fig. 8(b)). From the fraction of slip band types in Fig. 5, the most notable difference in the slip band behavior is the activity of A-type slip bands. As many slip bands of martensite in medium-carbon steel seem to be parallel to the habit plane, it is also necessary to consider the lath plane (habit plane) in addition to the slip directions. Therefore, the two slip systems that have in-lath plane directions on the lath plane (habit plane) were considered in this study. The slip systems are named ‘habit plane slip system’ to distinguish them from the slip systems investigated previously. Fig. 8(c) shows a Schmid factor map of the habit plane slip system. The result indicates that the Schmid factor map can explain the slip band distri bution of martensite in medium-carbon steel much better than the one shown in Fig. 8(b). The most significant difference between the microstructure of martensite in low- and medium-carbon steel is considered as the block width, as shown in Fig. 2. Based on the results of this study, the block width may be the effective factor responsible for the development behavior or selection of slip systems. For example, for martensite in medium-carbon steel, the dependence tendency of the specific slip band types, the low rate of transitions and the delayed strain region of tran sitions to different types of slip bands indicate that the narrower block width makes the other slip band types harder to be active. Fig. 9 shows which slip system is first activated between the in-lath plane slip system
Fig. 9. Frequency of slip system selection along block width of martensite in (a) low-carbon steel and (b) medium-carbon steel.
Fig. 8. (a) SEM image of 5% strain with slip bands (red line) and block boundaries, (b) Schmid factor map of in-lath plane slip system and (c) Schmid factor map of habit plane slip system of martensite in medium-carbon steel. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 5
K.H. Ryou et al.
Materials Science & Engineering A 777 (2020) 139090
Similarly, the habit plane slip system was activated irrespective of the carbon content for narrow-width blocks of martensite in low-carbon steel. Representative examples are illustrated in Fig. 10 and Fig. 11. Fig. 10 shows the case of martensite in medium-carbon steel. A green block whose width is around 15 μm, and its longitudinal direction is parallel with the horizontal is shown in Fig. 10(a). The SEM image of 5% strain for an identical area is also shown in Fig. 10(b). The Schmid factor maps of in-lath plane slip system and habit plane slip system are shown in Figs. 10(c) and (d), respectively. If this block follows the tendency of the medium-carbon content, the slip bands should be propagated within the horizontal direction, just like the longitudinal direction of the block, i.e., the A-type. However, as shown in Fig. 10(b), it is classified as C-type slip bands across the direction of the block, even at 5% strain. Fig. 11 illustrates the case of martensite in low-carbon steel. The block has a width of approximately 2 μm, and the slip band appeared along the longitudinal direction of the block (marked with a white-colored arrow), as shown in Figs. 11(a) and (b). This block has a lower Schmid factor value for the in-lath plane slip system than the surroundings, but a higher Schmid factor value for habit plane slip system, as shown in Figs. 11(c) and (d). As mentioned in a previous example, if this block follows the tendency along the carbon content, slip bands should not occur because the Schmid factor is lower than the surroundings. The direction of the slip band tends to follow the direction of the blue line shown in Fig. 11(e), because the marked {112} plane has the highest Schmid factor. But the slip band is formed according to the result of activation of the habit plane slip system, as shown in Fig. 11(b) and Fig. 11(e). From these results, about the global tendency of slip system selection especially those of Fig. 9, it appears that the carbon content affects the selection of the slip system. The block boundary constraint from the narrow block width affected the selection of the slip system and the development behavior of slip bands. The finer microstructure that resulted in narrower block width was from the increased carbon content. Thus, the influence on the abovementioned differences in deformation behavior between the martensite in low- and medium-carbon steel is
Fig. 11. The block of martensite in low-carbon steel that has narrow width of block. (a) IPF map, (b) SEM image of 5% strain stage, Schmid factor map of (c) in-lath plane slip system, (d) habit plane slip system for an identical area, and (e) the {110} and {112} pole figure of slip system.
exerted by increased carbon content. Accordingly, the deformation behavior of martensite is affected by its microstructure. At the very initial stages of strain, when deformation begins, martensite exhibit isotropic deformation behavior like the con ventional BCC structure in the range of mean free path of dislocation. However, the specific direction of slip direction is anisotropic due to the unique microstructure of martensite and morphological features of lath [15]. Thus, the dislocation movement is interrupted by misorientation between laths or blocks, and in-lath plane slip system demonstrates a more significant effect on the deformation behavior of martensite with a wide block width. For martensite in low-carbon steel, the satisfactory transition of activated slip system occurred because several in-lath plane slip systems can be well-activated even under anisotropic conditions of the slip system [16]. However, if the block width is decreased by increasing the carbon content, the activation of the slip system is also restricted to habit plane slip system with longitudinal direction of block length that results from the constraint effect of the block boundary. Therefore, the transition of slip system activation was not significant compared with the low-carbon case. At the initial strain, Schmid factor with anisotropic conditions could be a significant guideline to explain
Fig. 10. The block of martensite in medium-carbon steel that has wide width of block. (a) IPF map, (b) SEM image of 5% strain stage, Schmid factor map of (c) in-lath plane slip system, and (d) habit plane slip system for an identical area. 6
K.H. Ryou et al.
Materials Science & Engineering A 777 (2020) 139090
the deformation behavior of martensite. In this study, the effect of car bon content was investigated by varying the carbon content, and the results indicate that the direct influence of carbon content is limited. In addition, it is established that the microstructure of martensite was affected by varying carbon content, and the effect of block width that acts as boundary constraint effect was identified by selecting slip system and deformation behavior of martensite.
[8] Y.V. Khlebnikova, D.P. Rodionov, I.L. Yakovleva, V.M. Schastlivtsev, Structural changes in packet martensite of quenched pseudo single crystals of a structural steel upon large plastic deformation, Phys. Met. Metallogr. 86 (1998) 394–399. [9] V.M. Schastlivtsev, D.P. Rodionov, Y.V. Khlebnikova, I.L. Yakovleva, Peculiarity of structure and crystallography of plastic deformation of lath martensite in structural steels, Mater. Sci. Eng. A 275 (1999) 437–442. [10] K. Nakashima, Y. Fujimura, H. Matsubayashi, T. Tsuchiyama, S. Takaki, Yielding behavior and change in dislocation substructure in an UltraLow carbon martensitic steel, Tetsu-To-Hagane 93 (2007) 459–465. [11] J. Inoue, S. Nambu, Y. Ishimoto, T. Koseki, Fracture elongation of brittle/ductile multilayered steel composites with a strong interface, Scripta Mater. 59 (2008) 1055–1058. [12] S. Nambu, M. Michiuchi, J. Inoue, T. Koseki, Effect of interfacial bonding strength on tensile ductility of multilayered steel composites, Compos. Sci. Technol. (2009) 1936–1941. [13] T. Koseki, J. Inoue, S. Nambu, Development of multilayer steels for improved combinations of high strength and high ductility: maters, OR Trans. 55 (2014) 227–237. [14] S. Nambu, M. Michiuchi, Y. Ishimoto, K. Asakura, J. Inoue, T. Koseki, Transition in deformation behavior of martensitic steel during large deformation under uniaxial tensile loading, Scripta Mater. 60 (2009) 221–224. [15] Y. Ishimoto, M. Michiuchi, S. Nambu, K. Asakura, J. Inoue, T. Koseki, Measurement of strain distribution of lath martensite microstructure during tensile deformation, J. Japan Inst. Metals 73 (2009) 720–727. [16] M. Michiuchi, S. Nambu, Y. Ishimoto, J. Inoue, T. Koseki, Relationship between local deformation behavior and crystallographic features of as-quenched lath martensite during uniaxial tensile deformation, Acta Mater. 57 (2009) 5283–5291. [17] H. Na, S. Nambu, M. Ojima, J. Inoue, T. Koseki, Crystallographic and microstructural studies of lath martensitic steel during tensile deformation, Metall. Mat. Trans. 45 (2014) 5029–5043. [18] H. Na, S. Nambu, M. Ojima, J. Inoue, T. Koseki, Strain localization behavior in lowcarbon martensitic steel during tensile deformation, Scripta Mater. 69 (2013) 793–796. [19] J. Inoue, A. Sadeghi, T. Koseki, Slip band formation at free surface of lath martensite in low carbon steel, Acta Mater. 165 (2019) 129–141. [20] T. Niino, J. Inoue, M. Ojima, S. Nambu, T. Koseki, Effects of solute carbon on the work hardening behavior of lath martensite in low-carbon steel, ISIJ Int. 57 (2017) 181–188. [21] T. Ohmura, A.M. Minor, E.A. Stach, J.W. Morris Jr., Dislocation-grain boundary interactions in martensitic steel observed through in situ nanoindentation in a transmission electron microscope, J. Mater. Res. 19 (2004) 3626–3632. [22] A. Shibata, T. Nagoshi, M. Sone, S. Morito, Y. Higo, Evaluation of the block boundary and sub-block boundary strengths of ferrous lath martensite using a micro-bending test, Mater. Sci. Eng. A 527 (2010) 7538–7544. [23] C. Du, J.P.M. Hoefnagels, R. Vaes, M.G.D. Geers, Block and sub-block boundary strengthening in lath martensite, Scripta Mater. 116 (2016) 117–121. [24] C. Du, J.P.M. Hoefnagels, R. Vaes, M.G.D. Geers, Plasticity of lath martensite by sliding of substructure boundaries, Scripta Mater. 120 (2016) 37–40. [25] Y. Mine, K. Hirashita, H. Takashima, M. Matsuda, K. Takashima, Micro-tension behaviour of lath martensite structures of carbon steel, Mater. Sci. Eng. A 560 (2013) 535–544. [26] K. Kwak, T. Mayama, Y. Mine, K. Takashima, Anisotropy of strength and plasticity in lath martensite steel, Mater. Sci. Eng. A 674 (2016) 104–116. [27] K. Kwak, T. Mayama, Y. Mine, K. Takashima, Micro-tensile behaviour of low-alloy steel with bainite/martensite microstructure, ISIJ Int. 56 (2016) 2313–2319. [28] S. Morito, H. Yoshida, T. Maki, X. Huang, Effect of block size on the strength of lath martensite in low carbon steels, Mater. Sci. Eng. A 438–440 (2006) 237–240. [29] Y. Prawoto, N. Jasmawati, K. Sumeru, Effect of prior austenite grain size on the morphology and mechanical properties of martensite in medium carbon steel, J. Mater. Sci. Technol. 28 (2012) 461–466. [30] C. Zhang, Q. Wang, J. Ren, R. Li, M. Wang, F. Zhang, K. Sun, Effect of martensitic morphology on mechanical properties of an as-quenched and tempered 25CrMo48V steel, Mater. Sci. Eng. A 534 (2012) 339–346. [31] L. Morsdorf, O. Jeannin, D. Barbier, M. Mitsuhara, D. Raabe, C.C. Tasan, Multiple mechanisms of lath martensite plasticity, Acta Mater. 121 (2016) 202–214. [32] S. Morito, T. Ohba, T. Maki, Comparison of deformation structure of lath martensite in low carbon and ultra-low carbon steels, Mater. Sci. Forum 558–559 (2007) 933–938. [33] A. Stormvinter, G. Miyamoto, T. Furuhara, P. Hedstrom, A. Borgenstam, Effect of carbon content on variant pairing of martensite in Fe-C alloys, Acta Mater. 60 (2012) 7265–7274. [34] B. Hutchinson, J. Hagstrom, O. Karlsson, D. Lindell, M. Tornberg, F. Lindberg, M. Thuvander, Microstructures and hardness of as-quenched martensites (0.1%0.5%C), Acta Mater. 59 (2011) 5845–5858.
5. Conclusions In this study, in-situ deformation behavior of martensite in mediumcarbon steel was investigated and compared with martensite in lowcarbon steel. Based on the results of the study, the following conclu sions are drawn. 1. Increased carbon content affects the microstructure and deformation behavior. The formation of slip bands is affected by the block width during deformation. 2. The Schmid factor of in-lath plane slip system is not always a key indicator for the selection of the slip system in the case of martensite with narrow block width. 3. Considering dependence on specific slip bands type and slip band distribution with the Schmid factor, the habit plane slip system is preferred in martensite with narrow block width. 4. Block boundary constraint that results from increased carbon content affects the behavior of slip bands and selection of the slip system in martensite. CRediT authorship contribution statement K.H. Ryou: Conceptualization, Methodology, Software, Formal analysis, Investigation, Writing - original draft, Visualization. S. Nambu: Writing - review & editing, Supervision. T. Koseki: Writing review & editing, Supervision, Funding acquisition. Acknowledgement This work has supported by JSPS KAKENHI Grant Number 17H04958. References [1] S. Morito, H. Tanaka, R. Konishi, T. Furuhara, T. Maki, The morphology and crystallography of lath martensite in Fe-C alloys, Acta Mater. 51 (2003) 1789–1799. [2] S. Morito, X. Huang, T. Furuhara, T. Maki, N. Hansen, The morphology and crystallography of lath martensite in alloy steels, Acta Mater. 54 (2006) 5323–5331. [3] H. Kitahara, R. Ueji, N. Tsuji, Y. Minamino, Crystallographic features of lath martensite in low-carbon steel, Acta Mater. 54 (2006) 1279–1288. [4] C.C. Kinney, K.R. Pytlewski, A.G. Khachaturyan, J.W. Morris Jr., The microstructure of lath martensite in quenched 9Ni steel, Acta Mater. 69 (2014) 372–385. [5] P.P. Suikkanen, C. Cayron, A.J. DeArdo, L.P. Karjalainen, Crystallographic analysis of martensite in 0.2C-2.0Mn-1.5Si-0.6Cr steel using EBSD, J. Mater. Sci. Technol. 27 (2011) 920–930. [6] S. Takaki, K. Ngo-huynh, N. Nakada, T. Tsuchiyama, Strengthening mechanism in ultra low carbon martensitic steel, ISIJ Int. 52 (2012) 710–716. [7] G. Badinier, C.W. Sinclair, X. Sauvage, X. Wang, V. Bylik, M. Goune, F. Danoix, Microstructural heterogeneity and its relationship to the strength of martensite, Mater. Sci. Eng. A 638 (2015) 329–339.
7
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: http://www.elsevier.com/locate/msea
Effect of carbon content on selection of slip system during uniaxial tensile deformation of lath martensite K.H. Ryou *, S. Nambu , T. Koseki Department of Materials Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Electron-backscattering diffraction Martensitic steel Deformation behavior
The deformation behavior of lath martensite in medium-carbon steel was investigated using a scanning electron microscope and electron backscattering diffraction techniques and compared with that of low-carbon steel. There was a significant difference in the occurrence behavior of slip bands between the two grades of steel. In particular, the slip band occurrence is concentrated on specific type in the case of medium-carbon steel. About the origin of this particular phenomenon, it is indicated that the difference in the formation behavior of slip band results from the difference in microstructures due to different carbon content. The noticeable difference in the microstructure was the block width, which affected the selection of the slip system. The block boundary constraint obtained from the block width could be the key factor in the selection of a slip system for martensite that has a relatively higher-carbon content.
1. Introduction Martensite in steels possesses high strength and hardness because of factors such as the super-saturated solid solution of carbon, high dislo cation density, and fine microstructure owing to displacive trans formation. Using the electron-backscattering diffraction (EBSD) techniques, studies have been carried out on the hierarchical micro structure in the prior austenite grain of lath martensite containing 0–0.6% carbon content that includes the lath, sub-block, block, and packet [1–5]. The microstructural complexity of martensite also results in high strength as compared to other phases [6,7]. Due to increasing interest in the potential of lath martensite for industrial applications, the deformation behavior of martensite has been investigated. Khlebnikova et al. [8] and Schastlivtsev et al. [9] reported that a {110} texture similar to that of body-centered cubic (BCC) structure metals is observed in lath martensite and the primary slip planes in lath martensite during cold-rolling deformation are mainly {110} and {112} planes, similar to conventional BCC metals. However, in the case of tensile loading con dition, lath martensite exhibits brittle fracture behavior with extremely small plastic strain range [10]. To overcome the limitation of martensite elongation, some researchers introduced a new approach with specific structured steel to achieve large plastic deformation of lath martensitic steel in the uniaxial loading condition [11–13]. Several studies have been conducted by our research group on the deformation behavior of
lath martensite during tensile loading by employing multi-layered steels that achieved more than 50% strain with even in including martensite [14–20]. Nambu et al. [14] and Ishimoto et al. [15] investigated the deformation behavior of lath martensite in a low-carbon steel during large deformation. They reported that transition behavior and aniso tropic behavior of selection in the slip system of lath martensite that resulted from the anisotropy of morphological feature of lath. The anisotropy of the slip system between the in-lath and out-of-lath plane was also confirmed for crystal rotation behavior [16]. Grain refinement occurred as a result of crystal rotation, and it is clarified that refinement becomes severe along the carbon content of interstitial-free (IF), ultra-low-carbon and low-carbon steels [17]. In addition, the localiza tion of strain is affected by the interaction between adjacent sub-blocks grains during the grain refinement [18]. The occurrence of slip bands and dislocation movements have been investigated to understand the deformation response. The slip bands at the free surface of martensite in low-carbon steel tilt at 45-degree to tensile axis [19]. In perspective of dislocation, a decrease in the annihilation rate of mobile dislocations affects enhancing of work-hardening rate [20]. Ohmura et al. reported that dislocation movement is interrupted by high-angle block boundary in martensitic steel using in-situ nanoindentation [21] and Shibata et al. performed a micro-bending test and demonstrated that the block boundary affects the martensite strengthening behavior [22]. From the result of a micro-tensile test within an extremely restricted condition, Du
* Corresponding author. E-mail address: [email protected] (K.H. Ryou). https://doi.org/10.1016/j.msea.2020.139090 Received 21 November 2019; Received in revised form 10 February 2020; Accepted 10 February 2020 Available online 11 February 2020 0921-5093/© 2020 Elsevier B.V. All rights reserved.
K.H. Ryou et al.
Materials Science & Engineering A 777 (2020) 139090
et al. concluded that block and sub-block boundaries strengthen lath martensite, and concluded that the plastic deformation of martensite depends on the sliding of boundaries when boundary strengthening works [23,24]. Similarly, Mine et al. [25] and Kwak et al. [26] reported the results of micro-tensile tests and concluded that anisotropy resulted from the morphological feature of lath martensite. Anisotropy was also observed even in the bainite, which has similar morphological features with martensite [27]. Numerous studies have been conducted on the morphological features of martensite [28–32]. The effects of block size [28], prior austenite grain size [29], and comprehensive consideration [30] were investigated and it was found that affect global mechanical behavior that decides mechanical properties of martensite. Several factors affect the deformation behavior of martensite. Morsdorf et al. reported that the mechanism of lath martensite plasticity is complex including the effects of size, slip system, and microstructure [31]. It is reported that there are differences between the lath martensite of ultra-low-carbon steel and low-carbon steel in terms of deformed microstructure and deformation behavior [17,32]. The morphological features, orientation relationship between the adjacent blocks, and mechanical properties changed with increasing carbon content, over coming the range of low-carbon steel [20,33,34]. However, previous studies mainly studied martensite in low-carbon and ultra-low-carbon steels for short-range strains and significantly restricted conditions. Therefore, this study aims to investigate the deformation behavior of lath martensite in relatively high-carbon steel and compare the defor mation behavior with that of low-carbon steel. The microstructural features of lath martensite as a polycrystalline during large plastic strain of uniaxial tensile loading are studied.
were investigated with 0.16 μm of step size using EBSD (Tex-SEM Lab oratory (TSL)) technique. 3. Results The results of the tensile test for each carbon content sample are shown as engineering stress-strain curves (Fig. 1). The mechanical properties such as yield strength, ultimate tensile strength (UTS), uni form elongation, and fracture elongation are summarized in Table 2. The uniform elongation of both samples was confirmed that exceeded more than 25% elongation and analysis was conducted in the range of uniform elongation to guarantee the deformation along with the tensile test. 3.1. In-situ observation Fig. 2 shows the microstructure of martensite in low and mediumcarbon steels by Inverse Pole Figure (IPF) map from EBSD analysis, in which the grain boundaries are defined as above 8� of misorientation. Both microstructures of martensite showed similar size of prior austenite grain (60–70 μm). The microstructures of martensite in medium-carbon steel, especially the block widths, became smaller. Thus, the block width was of primary concern. The block widths were measured by a method that considers the angle between the habit plane and the surface [32]. The block width of martensite in low-carbon steel was about 9 μm. On the contrary, the martensite in medium-carbon steel displayed finer microstructure and narrower block width, with an average width of about 3 μm. In-situ observation was carried out during deformation of martensite in low- and medium-carbon steels. The result for medium-carbon steel is shown in Fig. 3, and the result of low-carbon steel exhibited similar behavior to that of previous research [14,15]. Fig. 3(a) illustrates the IPF map before deformation occurred. SEM micrographs were taken at the surface of the samples at every 5% strain until 20%, as shown in Figs. 3 (b)–(f) for martensite in medium-carbon steel. The slip bands were first observed in SEM images at 5% strain already. With increasing strain, the number of slip bands also increased. The formation behavior of slip bands appeared to develop as a unit of packet, as indicated by packet boundaries marked with white dashed lines in Figs. 3(a) and (d). Comparing among the packets, there are differences in deformation even at the same strain. Thus, the occurrence and development of slip bands were mainly investigated among the deformation phenomena of martensite.
2. Experimental Two types of multi-layered steel, including lath martensitic steel with different carbon content, were prepared. The one sample consists of a 0.4 wt% carbon martensitic steel layer and a TWIP steel layer. These four layers of martensitic steel and three layers of TWIP steel were stacked in seven layers that has surface layers with martensitic steel. The initial thicknesses of the constituent steels were 4 mm each, and the thickness of the laminated steel sheets was reduced to 1 mm by hotrolling and cold-rolling processes. For comparison, martensite with relatively low-carbon (0.2 wt%C) steel was prepared following the same procedure of a previous research [17]. The chemical compositions of the steels used in this study are shown in Table 1. To obtain a similar austenite grain size, the fabricated multi-layered steels were austeni tized at 1373 K for 10 min for medium-carbon steel, and 2 min for low-carbon steel. Subsequently, austenitized steels were water-quenched to obtain a full lath martensitic microstructure. The heat-treated samples were cut to tensile test specimen size of 6.25 mm width and 12 mm length of smooth part. Before the tensile test was performed, the surfaces of all samples were polished for observation. In-situ observation during the tensile test was conducted at each 5% elongation until 25% large strain. The strain was measured by an extension meter with an initial length of 10 mm. By using a field-emission scanning electron microscope (FESEM; JSM7001F, JEOL), the formation behavior of the slip bands on the surface was observed. The martensite microstructure and crystallographic features
Table 1 Chemical compositions of the constituent materials (weight percent). Types of steel
C
Medium-carbon MLS TWIP 0.6 Martensite 0.4 Low-carbon MLS [17] SUS 316L 0.02 Martensite 0.2
Mn
Si
Cr
Ni
Mo
Fe
22 1.8
0.25 1.5
– 1.5
– 3.02
– 0.19
Bal. Bal.
0.84 0.25
0.63 0.25
17.76 –
12.09 14
2.12 –
Bal. Bal.
Fig. 1. Engineering stress-strain curves from a tensile test on low-carbon MLS sample (dotted) and a medium-carbon MLS sample (solid). 2
K.H. Ryou et al.
Materials Science & Engineering A 777 (2020) 139090
martensite of both low- and medium-carbon steels. Even though all the four slip band types were observed in both lowand medium-carbon steels, the global developments of the slip bands of low- and medium-carbon steels are quite different. Fig. 5 shows the fraction of slip band types at each strain for both steels. As mentioned in section 3.1, the development of slip bands was different for each packet unit. Thus, hundreds of packets for each steel type were investigated to identify the occurrence tendency of slip band types. For martensite in low-carbon steel, as shown in Fig. 5(a), the large amount of C-type slip bands was observed, and the transition from A-to B-type and from C- to D-type was observed at 10% strain. All the slip band types developed relatively uniformly until 25% strain without concentration on specific slip band type. However, in the case of martensite in medium-carbon steel, as shown in Fig. 5(b), a small fraction of C-type at 5% strain, and smaller transitions from A-to B-type and from C- to D-type at larger strains were observed. The strong dependence phenomenon on the se lection of A-type slip band was found to be consistent.
Table 2 Mechanical properties of multi-layered steels in this present study. Types of steel
0.2% offset yield strength (MPa)
UTS (MPa)
Uniform El. (%)
Fracture El. (%)
Medium-carbon MLS (0.4C) Low-carbon MLS (0.2C)
682
1061
28.5
41.9
337
653
37.9
53.7
3.3. Schmid factor consideration As there are differences in the occurrence of slip bands depending on the region, it is important to clarify the factors affecting the occurrence of slip bands. Thus, considering the microstructure characteristic, the Schmid factor of martensite was investigated. Generally, the BCC martensite has 24 slip systems in total: {110} planes with [111] di rections, and {112} planes with [111] direction. In this study, in-lath plane slip systems among those 24 slip systems were considered based on the previous researches [14–18]. As mentioned in the introduction, the in-lath plane slip system considers the anisotropy that comes from the morphological feature of lath. As shown in the schematic illustra tions of BCC structure and slip systems in Fig. 6, number 1 and 2 are two in-lath plane slip directions on the habit plane indicated by red-colored arrows and red-colored plane. The in-lath plane slip system is catego rized by the existence of in-lath plane slip direction in the slip system. Thus, the slip system that has a green-colored slip plane and number 2 slip direction is also considered as an in-lath plane slip system. However, the other slip system that has a green-colored slip plane and number 3 slip direction does not include in-lath plane slip direction. Therefore, this slip system is the out-of-lath plane slip system. Consequently, with two in-lath plane slip directions accompanied six of slip plane for each direction, there are 12 in-lath plane slip systems. Fig. 7 compares the result between the slip band distribution and Schmid factor map. The SEM image of martensite in low-carbon steel at 5% strain is shown in
Fig. 2. IPF maps from martensite in (a) low-carbon steel and (b) mediumcarbon steel. Grain boundaries for misorientation >8� are drawn.
3.2. Slip band formation behavior The developing patterns of slip bands were categorized into four types as shown in Fig. 4. Fig. 4 also shows the in-situ observation results schematic illustrations of each slip band type. The A-type shown in Fig. 4 (a) indicates the longitudinal-slip-band type where the slip bands form along the block length direction marked with blue-colored arrow, and its tendency is maintained until a large strain region. The B-type initially displayed a similar behavior with the A-type; however, secondary slip bands occurred, crossed to former slip bands and longitudinal direction of the block as shown with yellow-colored arrow in Fig. 4(b). The C-type is fully transverse type, showing transverse slip bands from initial to large strain stages, and slip band is marked with red-colored arrow in Fig. 4(c). The D-type shows a comparable tendency with the C-type, but secondary slip bands occurred randomly crossed at the large strain stage as shown in Fig. 4(d). The four types of slip bands were found in the
Fig. 3. (a) IPF map and SEM images along increasing of 5% strain of martensite in medium-carbon steel from 0% until 20%; (b) 0%, (c) 5%, (d) 10%, (e) 15%, (f) 20% (White dotted line region indicates single packet.). 3
K.H. Ryou et al.
Materials Science & Engineering A 777 (2020) 139090
Fig. 4. IPF map, SEM images along increasing of 5% strain and schematic illustration of categorized slip band types; (a) A-type: longitudinal block length, (b) B-type: Longi. to transverse, (c) C-type: block transverse direction and (d) D-type: multi-crossed.
Fig. 7(a), and the block boundaries above 8-degree of misorientation and slip bands with red line are indicated. Fig. 7(b) indicates the highest value of the Schmid factor among 12 in-lath plane slip system. The value corresponded well with the high-value region of Schmid factor and the slip band distribution, and was in good agreement with our previous research [14]. The slip band formation behavior of martensite in medium-carbon steel was also investigated. The SEM image with slip band distribution marked with red line at 5% strain is shown in Fig. 8(a). Similar to low-
Fig. 5. Development of the slip band type fraction of packets along increasing of strain in (a) low-carbon steel and (b) medium-carbon steel.
Fig. 6. Schematic illustration of slip systems in a crystal structure of BCC lattice.
Fig. 7. (a) SEM image of 5% strain with slip bands (red line) and block boundaries., (b) Schmid factor map of in-lath plane slip system of martensite in low-carbon steel. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 4
K.H. Ryou et al.
Materials Science & Engineering A 777 (2020) 139090
carbon steel, the Schmid factor map of the in-lath plane slip system is shown in Fig. 8(b). Comparing the results of medium-carbon steel case with that of low-carbon steel, a well-matched relationship between the slip band distribution and Schmid factor of the in-lath plane slip system was not noticed. In the region of yellow-dashed circles in Fig. 8(b), several slip bands were seen in the lower Schmid factor regions. Conversely, no slip band was observed in higher Schmid factor region. Consequently, the Schmid factor of in-lath plane slip system cannot explain the slip band formation of martensite in medium-carbon steel.
and the habit plane slip system when martensite deforms. As the habit plane slip system is included among the 12 of in-lath plane slip systems, the cases that indicate high Schmid factors of both slip systems are excluded in the results. Fig. 9(a) shows fraction of selected slip system of martensite in low-carbon steel along the block width. The martensite in low-carbon steel reveals the distribution of relatively wide block widths, and the selection of the slip system is concentrated on the in-lath plane slip system. On the contrary, the martensite in medium-carbon steel distributes of relatively narrower block widths compared with martensite in low-carbon steel (Fig. 9(b)). As shown above, the con centration tendency of selected slip system was also different. In the case of martensite in medium-carbon steel, the selection of the slip system is focused on the habit plane slip system. It seems that the selection of a slip system could be affected by the microstructure resulting from the carbon contents. However, in certain cases such as the narrow-width block of martensite in low-carbon steel and the wide width block of martensite in medium-carbon steel, they do not follow the tendency of selection of slip system along their carbon content. It means that even for martensite in medium-carbon steel, the in-lath plane slip system was activated if blocks had wide width.
4. Discussion The differences in the fraction of slip band types shown in Fig. 5 could be considered as the effect of the microstructure. It is well-known that the development of slip bands is affected by block boundaries because the slip band is the result of the accumulation of dislocation and dislocation cannot propagate the grain boundaries. Thus, among the various kinds of slip bands, the slip band type that has a longitudinal slip direction along the block direction, i.e., the A-type, is easier to form. Considering the finer microstructure of martensite in medium-carbon steel, there is a strong dependence on the A-type slip band because the A-type could be the most favorable type. Narrow block width makes slip bands in the transverse direction of difficult to occur, not only the C-type but also B- and D-types. An additional consideration is needed because the Schmid factor map based on in-lath plane slip system cannot explain the distribution of slip bands for martensite in medium-carbon steel (Fig. 8(b)). From the fraction of slip band types in Fig. 5, the most notable difference in the slip band behavior is the activity of A-type slip bands. As many slip bands of martensite in medium-carbon steel seem to be parallel to the habit plane, it is also necessary to consider the lath plane (habit plane) in addition to the slip directions. Therefore, the two slip systems that have in-lath plane directions on the lath plane (habit plane) were considered in this study. The slip systems are named ‘habit plane slip system’ to distinguish them from the slip systems investigated previously. Fig. 8(c) shows a Schmid factor map of the habit plane slip system. The result indicates that the Schmid factor map can explain the slip band distri bution of martensite in medium-carbon steel much better than the one shown in Fig. 8(b). The most significant difference between the microstructure of martensite in low- and medium-carbon steel is considered as the block width, as shown in Fig. 2. Based on the results of this study, the block width may be the effective factor responsible for the development behavior or selection of slip systems. For example, for martensite in medium-carbon steel, the dependence tendency of the specific slip band types, the low rate of transitions and the delayed strain region of tran sitions to different types of slip bands indicate that the narrower block width makes the other slip band types harder to be active. Fig. 9 shows which slip system is first activated between the in-lath plane slip system
Fig. 9. Frequency of slip system selection along block width of martensite in (a) low-carbon steel and (b) medium-carbon steel.
Fig. 8. (a) SEM image of 5% strain with slip bands (red line) and block boundaries, (b) Schmid factor map of in-lath plane slip system and (c) Schmid factor map of habit plane slip system of martensite in medium-carbon steel. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.) 5
K.H. Ryou et al.
Materials Science & Engineering A 777 (2020) 139090
Similarly, the habit plane slip system was activated irrespective of the carbon content for narrow-width blocks of martensite in low-carbon steel. Representative examples are illustrated in Fig. 10 and Fig. 11. Fig. 10 shows the case of martensite in medium-carbon steel. A green block whose width is around 15 μm, and its longitudinal direction is parallel with the horizontal is shown in Fig. 10(a). The SEM image of 5% strain for an identical area is also shown in Fig. 10(b). The Schmid factor maps of in-lath plane slip system and habit plane slip system are shown in Figs. 10(c) and (d), respectively. If this block follows the tendency of the medium-carbon content, the slip bands should be propagated within the horizontal direction, just like the longitudinal direction of the block, i.e., the A-type. However, as shown in Fig. 10(b), it is classified as C-type slip bands across the direction of the block, even at 5% strain. Fig. 11 illustrates the case of martensite in low-carbon steel. The block has a width of approximately 2 μm, and the slip band appeared along the longitudinal direction of the block (marked with a white-colored arrow), as shown in Figs. 11(a) and (b). This block has a lower Schmid factor value for the in-lath plane slip system than the surroundings, but a higher Schmid factor value for habit plane slip system, as shown in Figs. 11(c) and (d). As mentioned in a previous example, if this block follows the tendency along the carbon content, slip bands should not occur because the Schmid factor is lower than the surroundings. The direction of the slip band tends to follow the direction of the blue line shown in Fig. 11(e), because the marked {112} plane has the highest Schmid factor. But the slip band is formed according to the result of activation of the habit plane slip system, as shown in Fig. 11(b) and Fig. 11(e). From these results, about the global tendency of slip system selection especially those of Fig. 9, it appears that the carbon content affects the selection of the slip system. The block boundary constraint from the narrow block width affected the selection of the slip system and the development behavior of slip bands. The finer microstructure that resulted in narrower block width was from the increased carbon content. Thus, the influence on the abovementioned differences in deformation behavior between the martensite in low- and medium-carbon steel is
Fig. 11. The block of martensite in low-carbon steel that has narrow width of block. (a) IPF map, (b) SEM image of 5% strain stage, Schmid factor map of (c) in-lath plane slip system, (d) habit plane slip system for an identical area, and (e) the {110} and {112} pole figure of slip system.
exerted by increased carbon content. Accordingly, the deformation behavior of martensite is affected by its microstructure. At the very initial stages of strain, when deformation begins, martensite exhibit isotropic deformation behavior like the con ventional BCC structure in the range of mean free path of dislocation. However, the specific direction of slip direction is anisotropic due to the unique microstructure of martensite and morphological features of lath [15]. Thus, the dislocation movement is interrupted by misorientation between laths or blocks, and in-lath plane slip system demonstrates a more significant effect on the deformation behavior of martensite with a wide block width. For martensite in low-carbon steel, the satisfactory transition of activated slip system occurred because several in-lath plane slip systems can be well-activated even under anisotropic conditions of the slip system [16]. However, if the block width is decreased by increasing the carbon content, the activation of the slip system is also restricted to habit plane slip system with longitudinal direction of block length that results from the constraint effect of the block boundary. Therefore, the transition of slip system activation was not significant compared with the low-carbon case. At the initial strain, Schmid factor with anisotropic conditions could be a significant guideline to explain
Fig. 10. The block of martensite in medium-carbon steel that has wide width of block. (a) IPF map, (b) SEM image of 5% strain stage, Schmid factor map of (c) in-lath plane slip system, and (d) habit plane slip system for an identical area. 6
K.H. Ryou et al.
Materials Science & Engineering A 777 (2020) 139090
the deformation behavior of martensite. In this study, the effect of car bon content was investigated by varying the carbon content, and the results indicate that the direct influence of carbon content is limited. In addition, it is established that the microstructure of martensite was affected by varying carbon content, and the effect of block width that acts as boundary constraint effect was identified by selecting slip system and deformation behavior of martensite.
[8] Y.V. Khlebnikova, D.P. Rodionov, I.L. Yakovleva, V.M. Schastlivtsev, Structural changes in packet martensite of quenched pseudo single crystals of a structural steel upon large plastic deformation, Phys. Met. Metallogr. 86 (1998) 394–399. [9] V.M. Schastlivtsev, D.P. Rodionov, Y.V. Khlebnikova, I.L. Yakovleva, Peculiarity of structure and crystallography of plastic deformation of lath martensite in structural steels, Mater. Sci. Eng. A 275 (1999) 437–442. [10] K. Nakashima, Y. Fujimura, H. Matsubayashi, T. Tsuchiyama, S. Takaki, Yielding behavior and change in dislocation substructure in an UltraLow carbon martensitic steel, Tetsu-To-Hagane 93 (2007) 459–465. [11] J. Inoue, S. Nambu, Y. Ishimoto, T. Koseki, Fracture elongation of brittle/ductile multilayered steel composites with a strong interface, Scripta Mater. 59 (2008) 1055–1058. [12] S. Nambu, M. Michiuchi, J. Inoue, T. Koseki, Effect of interfacial bonding strength on tensile ductility of multilayered steel composites, Compos. Sci. Technol. (2009) 1936–1941. [13] T. Koseki, J. Inoue, S. Nambu, Development of multilayer steels for improved combinations of high strength and high ductility: maters, OR Trans. 55 (2014) 227–237. [14] S. Nambu, M. Michiuchi, Y. Ishimoto, K. Asakura, J. Inoue, T. Koseki, Transition in deformation behavior of martensitic steel during large deformation under uniaxial tensile loading, Scripta Mater. 60 (2009) 221–224. [15] Y. Ishimoto, M. Michiuchi, S. Nambu, K. Asakura, J. Inoue, T. Koseki, Measurement of strain distribution of lath martensite microstructure during tensile deformation, J. Japan Inst. Metals 73 (2009) 720–727. [16] M. Michiuchi, S. Nambu, Y. Ishimoto, J. Inoue, T. Koseki, Relationship between local deformation behavior and crystallographic features of as-quenched lath martensite during uniaxial tensile deformation, Acta Mater. 57 (2009) 5283–5291. [17] H. Na, S. Nambu, M. Ojima, J. Inoue, T. Koseki, Crystallographic and microstructural studies of lath martensitic steel during tensile deformation, Metall. Mat. Trans. 45 (2014) 5029–5043. [18] H. Na, S. Nambu, M. Ojima, J. Inoue, T. Koseki, Strain localization behavior in lowcarbon martensitic steel during tensile deformation, Scripta Mater. 69 (2013) 793–796. [19] J. Inoue, A. Sadeghi, T. Koseki, Slip band formation at free surface of lath martensite in low carbon steel, Acta Mater. 165 (2019) 129–141. [20] T. Niino, J. Inoue, M. Ojima, S. Nambu, T. Koseki, Effects of solute carbon on the work hardening behavior of lath martensite in low-carbon steel, ISIJ Int. 57 (2017) 181–188. [21] T. Ohmura, A.M. Minor, E.A. Stach, J.W. Morris Jr., Dislocation-grain boundary interactions in martensitic steel observed through in situ nanoindentation in a transmission electron microscope, J. Mater. Res. 19 (2004) 3626–3632. [22] A. Shibata, T. Nagoshi, M. Sone, S. Morito, Y. Higo, Evaluation of the block boundary and sub-block boundary strengths of ferrous lath martensite using a micro-bending test, Mater. Sci. Eng. A 527 (2010) 7538–7544. [23] C. Du, J.P.M. Hoefnagels, R. Vaes, M.G.D. Geers, Block and sub-block boundary strengthening in lath martensite, Scripta Mater. 116 (2016) 117–121. [24] C. Du, J.P.M. Hoefnagels, R. Vaes, M.G.D. Geers, Plasticity of lath martensite by sliding of substructure boundaries, Scripta Mater. 120 (2016) 37–40. [25] Y. Mine, K. Hirashita, H. Takashima, M. Matsuda, K. Takashima, Micro-tension behaviour of lath martensite structures of carbon steel, Mater. Sci. Eng. A 560 (2013) 535–544. [26] K. Kwak, T. Mayama, Y. Mine, K. Takashima, Anisotropy of strength and plasticity in lath martensite steel, Mater. Sci. Eng. A 674 (2016) 104–116. [27] K. Kwak, T. Mayama, Y. Mine, K. Takashima, Micro-tensile behaviour of low-alloy steel with bainite/martensite microstructure, ISIJ Int. 56 (2016) 2313–2319. [28] S. Morito, H. Yoshida, T. Maki, X. Huang, Effect of block size on the strength of lath martensite in low carbon steels, Mater. Sci. Eng. A 438–440 (2006) 237–240. [29] Y. Prawoto, N. Jasmawati, K. Sumeru, Effect of prior austenite grain size on the morphology and mechanical properties of martensite in medium carbon steel, J. Mater. Sci. Technol. 28 (2012) 461–466. [30] C. Zhang, Q. Wang, J. Ren, R. Li, M. Wang, F. Zhang, K. Sun, Effect of martensitic morphology on mechanical properties of an as-quenched and tempered 25CrMo48V steel, Mater. Sci. Eng. A 534 (2012) 339–346. [31] L. Morsdorf, O. Jeannin, D. Barbier, M. Mitsuhara, D. Raabe, C.C. Tasan, Multiple mechanisms of lath martensite plasticity, Acta Mater. 121 (2016) 202–214. [32] S. Morito, T. Ohba, T. Maki, Comparison of deformation structure of lath martensite in low carbon and ultra-low carbon steels, Mater. Sci. Forum 558–559 (2007) 933–938. [33] A. Stormvinter, G. Miyamoto, T. Furuhara, P. Hedstrom, A. Borgenstam, Effect of carbon content on variant pairing of martensite in Fe-C alloys, Acta Mater. 60 (2012) 7265–7274. [34] B. Hutchinson, J. Hagstrom, O. Karlsson, D. Lindell, M. Tornberg, F. Lindberg, M. Thuvander, Microstructures and hardness of as-quenched martensites (0.1%0.5%C), Acta Mater. 59 (2011) 5845–5858.
5. Conclusions In this study, in-situ deformation behavior of martensite in mediumcarbon steel was investigated and compared with martensite in lowcarbon steel. Based on the results of the study, the following conclu sions are drawn. 1. Increased carbon content affects the microstructure and deformation behavior. The formation of slip bands is affected by the block width during deformation. 2. The Schmid factor of in-lath plane slip system is not always a key indicator for the selection of the slip system in the case of martensite with narrow block width. 3. Considering dependence on specific slip bands type and slip band distribution with the Schmid factor, the habit plane slip system is preferred in martensite with narrow block width. 4. Block boundary constraint that results from increased carbon content affects the behavior of slip bands and selection of the slip system in martensite. CRediT authorship contribution statement K.H. Ryou: Conceptualization, Methodology, Software, Formal analysis, Investigation, Writing - original draft, Visualization. S. Nambu: Writing - review & editing, Supervision. T. Koseki: Writing review & editing, Supervision, Funding acquisition. Acknowledgement This work has supported by JSPS KAKENHI Grant Number 17H04958. References [1] S. Morito, H. Tanaka, R. Konishi, T. Furuhara, T. Maki, The morphology and crystallography of lath martensite in Fe-C alloys, Acta Mater. 51 (2003) 1789–1799. [2] S. Morito, X. Huang, T. Furuhara, T. Maki, N. Hansen, The morphology and crystallography of lath martensite in alloy steels, Acta Mater. 54 (2006) 5323–5331. [3] H. Kitahara, R. Ueji, N. Tsuji, Y. Minamino, Crystallographic features of lath martensite in low-carbon steel, Acta Mater. 54 (2006) 1279–1288. [4] C.C. Kinney, K.R. Pytlewski, A.G. Khachaturyan, J.W. Morris Jr., The microstructure of lath martensite in quenched 9Ni steel, Acta Mater. 69 (2014) 372–385. [5] P.P. Suikkanen, C. Cayron, A.J. DeArdo, L.P. Karjalainen, Crystallographic analysis of martensite in 0.2C-2.0Mn-1.5Si-0.6Cr steel using EBSD, J. Mater. Sci. Technol. 27 (2011) 920–930. [6] S. Takaki, K. Ngo-huynh, N. Nakada, T. Tsuchiyama, Strengthening mechanism in ultra low carbon martensitic steel, ISIJ Int. 52 (2012) 710–716. [7] G. Badinier, C.W. Sinclair, X. Sauvage, X. Wang, V. Bylik, M. Goune, F. Danoix, Microstructural heterogeneity and its relationship to the strength of martensite, Mater. Sci. Eng. A 638 (2015) 329–339.
7