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Asynchronous effect of N+Cr alloying on the monotonic and cyclic deformation behaviors of Hadfield steel
Materials Science & Engineering A 761 (2019) 138015
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Asynchronous effect of N+Cr alloying on the monotonic and cyclic deformation behaviors of Hadfield steel
T
Chen Chena, Fucheng Zhanga,b,*, Bo Lvc, Hua Maa, Lin Wanga, Hongwang Zhangb, Wei Shend a
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, 066004, China National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao, 066004, China c College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao, 066004, China d China Railway Shanhaiguan Bridge Group Co. LTD, Qinhuangdao, 066205, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Monotonic deformation Cyclic deformation Planar slip Deformation twins N+Cr alloying Hadfield steel
Alloying treatment has always been an effective way to improve the mechanical performance of Hadfield steel. In the present study, Hadfield steel was alloyed with N + Cr, named Mn12CrN steel. Its monotonic and cyclic deformation behaviors were comparatively studied with traditional Hadfield steel, named Mn12 steel. Results showed that the stress responses during monotonic and cyclic deformations were different for these two steels, presenting an asynchronous effect of N + Cr alloying. During monotonic deformation, high-density deformation twins were formed in the Mn12CrN steel at large strain. Together with the solid solution strengthening effect, the Mn12CrN steel obtained higher yield and tensile strengths than the Mn12 steel. However, the maximum cyclic peak stress of the Mn12CrN steel was lower than that of the Mn12 steel during cyclic deformation, though a higher initial cyclic peak stress was obtained in the Mn12CrN steel. The cyclic deformation strain was much smaller than the monotonic deformation. In the strain range of cyclic deformation, planar dislocation configuration was mainly formed in the Mn12CrN steel at the total strain amplitude of 0.4 × 10−2–0.6 × 10−2. While deformation twins and mixed dislocation configurations were observed in the Mn12CrN steel when the total strain amplitude was above 0.8 × 10−2. With these microstructure evolutions in the Mn12CrN steel, low maximum cyclic peak stress but prolonged fatigue life was obtained compared with the Mn12 steel.
1. Introduction Stable austenitic microstructure can be retained after water toughening treatment in traditional Hadfield steel because of the high carbon and manganese contents. These specific chemical compositions determine medium-low stacking fault energy (SFE) in the Hadfield steel. As a result, both dislocation slipping and twinning are plastic deformation mechanisms, and excellent work hardening characteristic can be obtained [1,2]. However, dislocation slipping and twinning behaviors would be changed in the metal materials with the alterations in chemical compositions (e.g., alloying treatment) and test conditions (e.g., strain rates, temperatures, and pre-hardening treatment), which finally influence its deformation behavior [2–5]. Reddy et al. [6,7] studied the effect of N content on cyclic deformation behaviors in 316 austenitic stainless steel. They affirmed that N changed the dislocation slip mode in the stainless steel during cyclic deformation. The increased N content promoted the planar slip tendency. However, the effect of N on fatigue
*
lives varied depending on the fracture modes (trans-granular or intergranular fractures) of the test samples. In the report of He et al. [8], Cr was found to enhance the ordering tendency of alloying elements and intensify the gathering of C–Mn atoms in high-manganese steels by Xray diffraction analysis. Therefore, the dislocation slip behavior was changed, and the work hardening capacity was improved after Cr alloying treatment. In our previous work [3], the tensile property of the N + Cr alloyed Hadfield steel was demonstrated to be insensitive to strain rate. The active twining behavior and improved dynamic strain aging (DSA) effect were responsible for the changed performance. At this point, extraordinary results could be expected during cyclic deformation in the N + Cr alloyed Hadfield steel compared with the traditional Hadfield steel. Commonly, cyclic and monotonic deformation performances are highly related to each other [9,10]. However, some exceptional cases always attract the researchers’ attention. Shao et al. [11] reported a decreased low-cycle fatigue resistance in high-manganese steels with synchronously improved strength and ductility. In another paper of
Corresponding author. State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, 066004, China. E-mail address: [email protected] (F. Zhang).
https://doi.org/10.1016/j.msea.2019.06.025 Received 9 April 2019; Received in revised form 6 June 2019; Accepted 8 June 2019 Available online 09 June 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 Chemical compositions of the test steels (wt.%). Test steels
C
Mn
Cr
N
Si
Al
Cu
P
S
Mn12CrN Mn12
1.10 1.14
11.6 11.7
2.14 0.07
0.052 0.016
0.42 0.42
0.024 0.027
0.014 0.009
≤0.001 ≤0.001
0.005 0.005
their group, wide variations in cyclic stress response and fatigue life were observed in high-manganese steels with a slight variation in monotonic flow stress [12]. They explained it as the slight difference in the initial work hardening of those steels. Meanwhile, materials with similar tensile properties could present remarkable variations in lowcycle fatigue resistance, depending on the dislocation structure evolution during cyclic deformation [13]. These results all claimed a noninvariable relationship between monotonic and cyclic deformation behaviors of steels. In the present study, an asynchronous effect of N + Cr alloying on the monotonic and cyclic deformation behaviors of the Hadfield steel was illustrated. This effect was discussed by comparing the microstructure evolutions in the N + Cr alloyed and traditional Hadfield steels under different deformation conditions.
Fig. 1. Engineering stress-strain curves of Mn12CrN and Mn12 steels.
3. Results 3.1. Monotonic deformation characteristics The engineering stress-strain curves in Fig. 1 show simultaneously improved strength and plasticity of the Mn12CrN steel compared with the Mn12 steel. The yield and tensile strengths of the Mn12CrN steel were 478 MPa and 1043 MPa, respectively, 16.9% and 21.1% higher than those of the Mn12 steel. While the elongation of the Mn12CrN steel was 39.6%, slightly higher than that of the Mn12 steel. This improving effect on the strength and plasticity of the N + Cr alloyed Hadfield steel was explained by the promoted twinning behavior and solid solution strengthening effect in our previous work [3].
2. Materials and experimental methods The test steels in the present study were N + Cr alloyed Hadfield steel, named Mn12CrN steel, and traditional Hadfield steel, named Mn12 steel. Their chemical compositions are listed in Table 1. The steels were smelted in a 50 kg vacuum induction furnace, and then forged into square blocks with a size of 60 mm × 60 mm. The blocks were cut into pieces with a length of 110 mm for solid solution treatment. The austenitizing temperature of the Mn12 steel was 1050 °C, while that of the Mn12CrN steel was increased to 1100 °C to ensure the dissolution of Cr-rich carbides. After holding at austenitizing temperature for 1 h, the test steels were quenched with water. Subsequently, the test steels were machined into cylindrical specimens for tensile and lowcycle fatigue tests. The gauge length and diameter of the deformation part of the cylindrical specimens were 10 and 5 mm, respectively. To reduce the effect of surface condition on fatigue property, the specimens were grounded into smooth and mirror-like surfaces. The tensile and low-cycle fatigue tests were conducted on an MTS hydraulic servo fatigue testing machine with a same strain rate of 5 × 10−3 s−1. A strain-controlled symmetric tension-compression loading method was used in the low-cycle fatigue test. Total strain amplitudes of 0.4 × 10−2, 0.6 × 10−2, 0.8 × 10−2 and 1.0 × 10−2 were selected. Five specimens were tested at each strain amplitude. The failure of specimens was determined by a reduction of 25% in peak tensile load as compared with the maximum peak value. The tensile and fatigue curves were plotted in accordance with the test results. The internal and effective stresses of the two test steels during cyclic deformation were calculated using the Handfield-Dickson method [4]. Samples used for microstructure observation were cut using an electric spark wire cutting machine, and all of the observed surfaces were parallel to the loading direction. Optical microstructures of the Mn12CrN and Mn12 steels after low-cycle fatigue test were observed under an Axiover 200MAT optical microscope. After mechanical grinding and polishing, the samples were etched by a 4% nitric acid alcohol solution. Fine microstructures of the Mn12CrN steel were examined using a JEM-2010 transmission electron microscope (TEM). The operating voltage was 200 kV. Thin foils were prepared through a precision ion polishing system (Gatan).
3.2. Cyclic deformation characteristics 3.2.1. Cyclic deformation behavior Fig. 2 illustrates the changing curves of tensile peak stress as the number of cycles and fraction of the lifetime of the two test steels at different total strain amplitudes. With the increase in total strain amplitude, the cyclic peak stresses of the two test steels increased gradually, but the fatigue lives decreased. At the total strain amplitudes of 0.4 × 10−2 ≤ εt/2 ≤ 0.6 × 10−2, the cyclic deformation behaviors of the two test steels both displayed cyclic hardening in the first tens of cycles. When the peak stress reached a certain value, cyclic softening followed by cyclic stability happened until fatigue failure. At the total strain amplitudes of 0.8 × 10−2 ≤ εt/2 ≤ 1.0 × 10−2, only cyclic hardening and softening behaviors were observed before fatigue failure (Fig. 2a and c). The two test steels presented similar changing trend in the cyclic deformation behavior. However, some interesting points were also observed. The initial cyclic peak stress of the Mn12CrN steel was higher than that of the Mn12 steel at each total strain amplitude, while a contrary result was obtained in the maximum cyclic peak stress (Fig. 3a and Table 2). Number of cycles to reach the maximum cyclic peak stress of the Mn12CrN steel was less than that of the Mn12 steel at different strain amplitudes (Fig. 3b). However, the fatigue life of the Mn12CrN steel was much higher than that of the Mn12 steel especially when the total strain amplitude was above 0.6 × 10−2 (Fig. 3b and Table 2). Meanwhile, the proportion of hardening cycles in the whole fatigue life of the Mn12CrN steel was smaller than that of the Mn12 steel (Fig. 2b and d). Apparently, the Mn12CrN steel possessed higher strength and plasticity than the Mn12 steel during monotonic deformation (Fig. 1), but the cyclic deformation behaviors did not respond asynchronously (Figs. 2 and 3). Fig. 4 shows the relationships among total strain amplitudes, elastic strain amplitudes, plastic strain amplitudes and number of reversals to failure of the test steels. The elastic and plastic strain amplitudes of the 2
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Fig. 2. Tensile peak stress as a function of number of cycles and fraction of the lifetime of Mn12CrN (a, b) and Mn12 (c, d) steels at different total strain amplitudes.
two test steels both declined gradually with the decrease in total strain amplitude. However, the plastic strain amplitude reduced more rapidly than the elastic strain amplitude. Specifically, the plastic strain amplitude of the Mn12CrN steel played a dominant role at the total strain amplitude of εt/2 ≥ 0.6 × 10−2. When the total strain amplitude was 0.4 × 10−2, the plastic and elastic strain amplitudes were almost equivalent (Fig. 4a). For the Mn12 steel, the plastic strain amplitude dominated at the total strain amplitude of 0.8 × 10−2 ≤ εt/ 2 ≤ 1.0 × 10−2. While at the total strain amplitude of 0.6 × 10−2, the plastic strain amplitude was nearly equal to the elastic strain amplitude. At the total strain amplitude of 0.4 × 10−2, the elastic strain amplitude dominated (Fig. 4b). It was obvious that, compared with the Mn12 steel, the Mn12CrN steel possessed low cyclic peak stress at half lifetime with the same total strain amplitude but large plastic strain amplitude (Figs. 2 and 4 and Table 2). This result was unexpected when the responded stress of the Mn12CrN steel during monotonic deformation was higher than that of the Mn12 steel at the same strain (Fig. 1). Cyclic hardening ratio (CHR) and cyclic softening ratio (CSR) are important parameters to characterize the hardening and softening degrees of materials during cyclic deformation. The values of CHR and CSR can be calculated by CHR=(σmax-σ1)/σ1 and CSR= (σmax-σhalf)/ σmax, where σ1, σmax, and σhalf are stress amplitude at the first cycle, the
Table 2 Low cycle fatigue results of Mn12CrN and Mn12 steels. Test steel
Total strain amplitude/×10−2
Maximum peak stress amplitude/ MPa
Plastic strain amplitude at half-life cycle/×10−2
Fatigue life
Mn12CrN
0.4 0.6 0.8 1.0 0.4 0.6 0.8 1.0
485 558 598 709 526 627 678 798
0.19 0.35 0.52 0.65 0.15 0.32 0.44 0.57
12864 4978 1120 926 11388 1666 654 311
Mn12
maximum stress amplitude, and the stress amplitude at the half lifetime, respectively. Fig. 5 shows the CHRs and CSRs of the Mn12CrN and Mn12 steels at different strain amplitudes. The CHRs of the two test steels increased gradually with the increase in total strain amplitudes, whereas the CSRs decreased. The CHR of the Mn12CrN steel was much smaller than that of the Mn12 steel at the same total strain amplitude, and the CSR was slightly greater than that of the Mn12 steel. When the
Fig. 3. Initial and maximum cyclic tensile stress (a), and cycles to reach maximum cyclic stress and fatigue life (b) of Mn12CrN and Mn12 steels as functions of total strain amplitude. 3
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Fig. 4. Total, elastic and plastic strain amplitudes of Mn12CrN (a) and Mn12 (b) steels as a function of number of reversals to failure.
amplitude selected in the present study, which was in accordance with the difference in maximum cyclic peak stresses of the Mn12CrN and Mn12 steels (Figs. 2 and 3). Different deformation band densities in the two test steels were macroscopic expressions of diversity in fine microstructures. Therefore, TEM micrographs of the two test steels at different total strain amplitudes were studied. Fine microstructures of traditional Hadfield steel after the fatigue test have been observed in detail in our previous study [14]. At a relatively low total strain amplitude of 0.4 × 10−2, entangled dislocations were the main characteristic, and dislocation cell structures were hardly formed. With the increase in total strain amplitude, obvious dislocation cell structures were formed, but no evidence of deformation twins was found. Wavy-slip structured dislocations coordinated the plastic deformation process [14,15]. Fine microstructures of the Mn12CrN steel after the fatigue test were observed in the present study. TEM micrographs are shown in Figs. 7–9. At a relatively low total strain amplitude of 0.4 × 10−2, the microstructure was dominated by planar-slip structured dislocation arrangements (Fig. 7). Dislocation-rich and dislocation-poor bands were formed in the austenite matrix, and they extended to the grain boundaries (Fig. 7a). Moreover, typical planar-slip structured dislocation configurations, such as dislocation bundles (Fig. 7b), dislocation arrays (Fig. 7c), and intersected slip bands (Fig. 7d) were also observed. At this total strain amplitude, typical planar slip was the main slip mode of dislocations in the Mn12CrN steel, which was different from that in the Mn12 steel. When the total strain amplitude was increased to 0.6 × 10−2 (Fig. 8), except for the unidirectional planar slip bands in Fig. 8a, the intersected slip bands were frequently formed (Fig. 8b). Entangled dislocations (Fig. 8c) and numerous stacking faults (Fig. 8d) were also observed in some grains. The dislocation slip mode transited into the mixed slip. At the total strain amplitude of 0.8 × 10−2, though some planar-slip structured dislocation configurations still existed in a few grains (Fig. 9b), wavy-slip structured dislocations became the main configurations (Fig. 9a, c, and 9d). The dislocations interacted with each other to form cell structures, and wavy slip mode became the main slip mode in the Mn12CrN steel. Meanwhile, unidirectional and intersected deformation twins were observed in the Mn12CrN steel (Fig. 9c and d). Incomplete cell structures were also formed in the matrix between the adjacent deformation twins (Fig. 9c). During cyclic deformation, many factors may influence the dislocation slip modes and the formation of twins. These microstructural configurations will directly determine the fatigue behaviors and lives. Therefore, the differences in cyclic deformation behaviors of the Mn12CrN and Mn12 steels were analyzed in combination with the chemical compositions and microstructure evolutions in the next section.
Fig. 5. Cyclic hardening and softening ratios as a function of total strain amplitudes in Mn12CrN and Mn12 steels.
relationships between CHRs, CSRs and the total strain amplitudes were approximately fitted into straight lines, the CHRs and CSRs curves of the Mn12CrN steel presented an approaching tendency to those of the Mn12 steel. This quick glance maybe caused by a gradually approaching microstructure in the two test steels. However, The CHR and CSR differences of the Mn12CrN and Mn12 steels indicated that the microstructural evolutions during cyclic deformation should be distinct. 3.3. Microstructural features Uniform austenitic microstructures were obtained in the two test steels after solid solution treatment. The grain sizes of the Mn12CrN and Mn12 steels were 79 and 89 μm, respectively, in accordance with our previous work [3]. The microstructures of the two test steels after monotonic deformation had been studied in detail in our previous work [3]. High-density dislocations and deformation twins were generated in the two test steels. The Mn12CrN steel obtained more deformation twins than the Mn12 steel at the same tensile strain. In the present study, microstructures after cyclic deformation were studied on focus. Optical images of the two test steels after cyclic deformation are listed in Fig. 6. Numerous deformation bands occurred in the grains of Hadfield steel after the fatigue test. At a relatively low total strain amplitude (εt/2 = 0.4 × 10−2), unidirectional deformation bands were the main characteristics (Fig. 6a and b). Moreover, no apparent deformation bands were formed in some grains because of the grain orientation and observation direction. With the increase in the total strain amplitude, the deformation bands began to intersect with each other, and the deformation band density increased gradually (Fig. 6c, d, 6e, and 6f). Specifically, the deformation band density of the Mn12CrN steel was smaller than that of the Mn12 steel at the same total strain 4
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Fig. 6. Optical images of the two test steels after fatigue failure at different total strain amplitudes: a, Mn12CrN steel-0.4 × 10−2; b, Mn12 steel-0.4 × 10−2; c, Mn12CrN steel-0.6 × 10−2; d, Mn12 steel-0.6 × 10−2; e, Mn12CrN steel-0.8 × 10−2; f, Mn12 steel-0.8 × 10−2.
0.8 × 10−2 (Fig. 9), which were not found in traditional Hadfield steel. In comparison with the traditional Hadfield steel, i.e., the Mn12 steel, N and Cr were added in the Mn12CrN steel. The difference in grain sizes of the two test steels could be neglected. Therefore, different microstructural characteristics in the Mn12CrN steel were caused by alloying treatment. In other words, alloying treatment in the Mn12CrN steel depressed the cross slip of dislocations and promoted the formation of deformation twins. Factors that influence the slip modes of dislocations include SFE [11,17], short range order (SRO) [18], and DSA [6]. In general, a large stacking fault width in metal materials with low SFE was unfavorable to the cross slip of dislocations, thereby the planar slip mode dominated [19]. The SFE of traditional Hadfield steel was ~50 mJm−2 [1]. Wavy slip was the main dislocation slip mode in traditional Hadfield steel with this SFE. However, N was regarded as an effective element that could decrease the SFE of austenitic manganese steels [20,21], and Cr was also an SFE-decreasing element [22]. Therefore, the SFE of the Mn12CrN steel was reduced because of N + Cr alloying, which became a favorable factor that promoted slip planarity. SRO is another factor that can promote dislocation slip planarity. Gerold and Karnthaler [18] proposed that SROs softened the slip planes, and was the main factor to promote slip planarity in face-centered cubic (fcc) metals. When a leading dislocation passed through the slip plane, high stress was required to overcome the increased energy associated with the disordering of SROs. However, subsequent passage of dislocations hardly further disordered the SROs, and the required activation stress was reduced [23]. The dislocation slip was therefore
4. Discussion 4.1. Effect of N + Cr alloying on the monotonic deformation behavior of the hadfield steel The N + Cr alloying treatment in the Hadfield steel has been proved to reduce the SFE in our previous works [3,16]. The monotonic deformation microstructures in the Mn12CrN steel were high-density deformation twins and dislocations. The number of deformation twins in the Mn12CrN steel was much greater than that in the Mn12 steel at the same strain [3]. Together with the solid solution strengthening and DSA effects of N + Cr alloying, the Mn12CrN steel obtained higher tensile and yield strengths than the Mn12 steel during monotonic deformation (Fig. 1). 4.2. Effect of N + Cr alloying on the cyclic deformation behavior of the hadfield steel 4.2.1. Planar slip characteristic in the N + Cr alloyed hadfield steel During cyclic deformation of traditional Hadfield steel, wavy slip is the main slip mode. In the strain-controlled low cycle fatigue test, wavy-slip structured dislocations dominated even though the total strain amplitude was as low as 0.4 × 10−2 [14,15]. However, the slip mode transited from planar to mixed and finally wavy slip when the total strain amplitude was increased from 0.4 × 10−2 to 0.8 × 10−2 in the Mn12CrN steel (Figs. 7–9). Moreover, numerous deformation twins were observed in the Mn12CrN steel at the total strain amplitude of 5
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Fig. 7. TEM micrographs of the Mn12CrN steel after fatigue failure at total strain amplitude of 0.4 × 10−2, showing (a) planar arrangement of dislocations, (b) dislocation bundles, (c) dislocation arrays and (d) two sets of planar dislocations.
The screw dislocation annihilation distance (ys) determines the difficulty level of cross-slip in steels. A lower value of ys makes cross slip more difficult, thereby facilitating planar slip [18]. On the contrary, it facilitates wavy slip. ys can be estimated through the following formula:where G is the shear modulus, b is the Burgers vector, θ is the included angle of primary slip plane and cross slip plane (70.5° in fcc crystals), τ0, τa, and τi are critical shear stress, applied stress, and internal stress, respectively, and S is the ratio of Schmid factors of (11 ‾ 1)[1 ‾ 01] and (11 ‾ 1)[1 ‾ 01] dislocations. During cyclic deformation in the present study, changes in G, b and τ0 with the total strain amplitudes could be neglected. Therefore, ys is only determined by τa and τi. The stress amplitudes and internal stresses at different total strain amplitudes of steels can be calculated using the Handfield-Dickson method [4]. It indicated that the stress amplitude and internal stress at the half life cycle improved with increasing total strain amplitudes (Fig. 11), which meant increasing values of τa and τi. As a result, the screw dislocation annihilation distance increased, which facilitated the wavy slip of dislocations. Thus, the slip mode in the Mn12CrN steel transited to wavy slip at relatively high total strain amplitudes (Fig. 9). In polycrystalline materials, a critical stress σT is needed for the formation of deformation twins [38]. σT is expressed as follows:where γSF is the SFE, and b is the Burgers vector. The critical stress is determined by the SFE and Burgers vector. The critical stress during cyclic deformation almost remained unchanged at different total strain amplitudes. When the applied stress at a certain total strain amplitude exceeded the critical stress, deformation twins formed. Deformation twins in Fig. 9c and d proved that the applied stress at the total strain amplitude of 0.8 × 10−2 exceeded the critical stress, whereas that at the total strain amplitude of εt/2 ≤ 0.6 × 10−2 did not, and no evidence of deformation twins was found (Figs. 7 and 8). Compared with the Mn12 steel, N + Cr alloying in the Mn12CrN steel increased the
confined to the slip plane, and slip planarity was promoted. In Fe–Mn–Cr–N or Fe–Ni–Cr–N alloys, Cr–N SROs had been detected through various test methods [24,25]. In the present study, Cr–N SROs caused by N + Cr alloying that might influence the microstructures and properties of Hadfield steel were considered during the composition design. Therefore, the promoted effect of SROs on slip planarity in the Mn12CrN steel during cyclic deformation could be expected. DSA effect is caused by the interactions between the mobile dislocations and solute atoms. The flow stress rises sufficiently to break away from the pinning atoms or to generate new dislocations to maintain the imposed strain rate. Serrations in the hysteresis loop were macro evidence of the DSA effect during plastic deformation. Fig. 10 portrays the hysteresis loops and corresponding amplified curves of the local areas of the two test steels. Apparent serrations were observed, which indicated the occurrence of the DSA effect. In DSA, the solute atoms played pinning and dragging roles to mobile dislocations. The dislocations could not easily transit to other slip planes through cross slip, thereby promoting the slip planarity of dislocations [6]. However, apparent DSA effect both occurred in the two test steels, and no major difference was observed. Thus, the low SFE and Cr–N SROs in the Mn12CrN steel were the main reasons that caused the slip planarity of dislocations during cyclic deformation. 4.2.2. The change of slip mode and formation of deformation twins in the N + Cr alloyed hadfield steel In traditional Hadfield steel, only wavy-slip structured dislocations were observed during cyclic deformation because of the high SFE [14,15]. However, the slip mode might change with increasing total strain or stress amplitude in metal materials with relatively low SFE [20]. The Mn12CrN steel displayed different slip modes with varying total strain amplitudes (Figs. 7–9). 6
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Fig. 8. TEM micrographs of the Mn12CrN steel after fatigue failure at total strain amplitude of 0.6 × 10−2, showing (a) planar arrangement of dislocations, (b) two sets of planar dislocations, (c) entangled dislocations and (d) stacking faults.
we found that the DSA effect in the Mn12CrN steel was depressed at the primary stage of cyclic deformation. Although N + Cr alloying increased the number of solute atoms in Hadfield steel, Cr and N atoms existed in the form of Cr–N SROs at the initial stage of cyclic deformation, which made the aggregation of solute atoms more difficult. Therefore, the interactions between dislocations and solute atoms were weakened. Second, after leading dislocations passed through the Cr–N SRO areas, the integrity of SRO structures was destroyed. The stress needed for the subsequent dislocations to pass through these areas decreased. Third, the planar slip characteristic of the Mn12CrN steel resulted in planar band dislocation configurations during cyclic deformation (Figs. 7 and 8). This type of dislocation configuration weakened the strengthening effect from dislocation interactions. Therefore, the effective stresses induced in the short-range motion of dislocations and the internal stresses induced in the long-range motion of dislocations of the Mn12CrN steel were both lower than those of the Mn12 steel (Fig. 11). A relatively short dislocation annihilation distance in planar slip also accelerated the dislocation annihilation rate. Therefore, Cr–N SROs, suppressed DSA effect and weakened interactions among dislocations resulted in relatively low maximum cyclic stresses and CHRs but high CSRs in the Mn12CrN steel in comparison with those in the Mn12 steel (Figs. 3 and 5). With the increase in total strain amplitude, the integrity of Cr–N SROs was further destroyed. The weakening effect on DSA eased. The serration shapes in the hysteresis loops of the Mn12CrN steel were similar to those of the Mn12 steel at total strain amplitudes of 0.8 × 10−2 and 1.0 × 10−2 in Fig. 10. In addition, the dislocation configurations gradually transited from planar-slip structured dislocations to wavy-slip structured dislocations. Therefore, cycles to reach the maximum cyclic peak stress of the Mn12CrN steel approached those of the Mn12 steel (Fig. 3).
lattice constant, resulting in an increased Burgers vector, but decreased the SFE. The critical stress to form deformation twins in the Mn12CrN steel was smaller than that in the Mn12 steel at the same test conditions. Therefore, deformation twins were observed in the Mn12CrN steel, and no evidence was found in the Mn12 steel at each total strain amplitude selected in the present study. 4.2.3. Cyclic stress responses and fatigue lives in the N + Cr alloyed hadfield steel At the total strain amplitudes selected in the present study, the initial cyclic peak stress of the Mn12CrN steel was greater than that of the Mn12 steel, whereas the maximum cyclic peak stress of the Mn12CrN steel was lower (Figs. 2 and 3). After N + Cr alloying, the solid solution strengthening effect and increased stress which was needed for leading dislocations to disorder the Cr–N SROs explained the enhanced initial cyclic peak stress in the Mn12CrN steel. However, in the subsequent cyclic deformation process, the Mn12CrN steel reached the maximum cyclic peak stress after very short cycles (Fig. 2). Subsequently, cyclic softening until fracture failure (εt/2 ≥ 0.8 × 10−2) or cyclic softening to cyclic stability followed by fracture failure (εt/2 ≤ 0.6 × 10−2) occurred. Compared with the Mn12 steel, the shortened hardening behavior and decreased maximum cyclic peak stress in the Mn12CrN steel were not expected. Cyclic hardening behavior is determined by dislocation activities in metal materials during cyclic deformation. The low maximum cyclic peak stress in the Mn12CrN steel could be explained as follows. First, the DSA effect is one of the reasons that cause cyclic hardening in metal materials [16,21]. Except for the serrations in the hysteresis loops (Fig. 10), the DSA effect can also prolong cyclic hardening cycles. By comparing the serration shapes in hysteresis loops of the second cycle in Fig. 10 and the cyclic hardening cycles of the two test steels in Fig. 3, 7
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Fig. 9. TEM micrographs of the Mn12CrN steel after fatigue failure at total strain amplitude of 0.8 × 10−2, showing (a) dislocations cells, (b) planar arrangement of dislocations, (c) deformation twins and (d) intersected deformation twins.
Fig. 10. Hysteresis loops of Mn12CrN (a, b) and Mn12 (c, d) steels at different total strain amplitudes during cyclic deformation, Fig. 10b and d are amplified pictures in the blue boxes in Fig. 10a and b, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
8
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Fig. 11. Stress amplitudes, internal stresses and effective stresses of Mn12CrN (a) and Mn12 (b) steels at the half life cycle as functions of strain amplitude.
dislocations and released the local plastic strain [26]. On the other hand, the deformation twins were formed only after tens of cycles at the total strain amplitude of 0.8 × 10−2 (Fig. 12). The twin boundaries (TBs) interacted with movable dislocations during the subsequent deformation. Their interaction destroyed the coherency of TBs (Fig. 12b, c, 12d and 12e). With the increase in the cyclic deformation cycles, the interaction frequency of TBs and dislocations increased (Fig. 13). The TB-dislocation interaction was helpful for releasing the local stress concentration at the grain boundaries, therefore alleviating the accumulation of fatigue damage [27]. The two effects of deformation twins finally prolonged the fatigue lives of the Mn12CrN steel at high total strain amplitudes.
Based on the discussion above, the microstructural characteristics in the Mn12CrN steel were not favorable to rapid hardening at low strain. However, with the increase in the deformation strain, the Cr–N SROs were seriously destroyed, and additional Cr and N atoms would gather to the center of dislocations to intensify the DSA effect. Moreover, the low SFE in the Mn12CrN steel promoted the formation of deformation twins. Fine-size and high-density deformation twins were obtained in the Mn12CrN steel at the same deformation strain [3]. Therefore, an intensified DSA effect and high-density deformation twins resulted in the increase in strength and plasticity of the Mn12CrN steel at large strain during monotonic deformation. In terms of the fatigue life, the Mn12CrN steel possessed longer lives than the Mn12 steel at each total strain amplitude. Fig. 4 and Table 2 show that the plastic strain amplitudes of the Mn12CrN steel dominated the total strain amplitudes, except for the total strain amplitude of 0.4 × 10−2. Generally, high plastic strain amplitude could facilitate the generation and slip of dislocations, resulting in high cyclic peak stress. However, dislocation pile-up was also intensified during this process, easily leading to local stress concentration and fracture failure. In that way, the fatigue life would be eventually shortened. The plastic strain amplitude of the Mn12CrN steel was larger than that of the Mn12 steel, but the maximum cyclic peak stress was smaller and the fatigue life was longer at the same total strain amplitude. These results were determined by the microstructure evolution during cyclic deformation. At the total strain amplitude of εt/2 ≤ 0.6 × 10−2, the dislocation slip mode in the Mn12CrN steel was planar slip (Figs. 7 and 8). Compared with wavy-slip structured dislocations, the resistance to generation and motion of planar-slip structured dislocations were small, which was propitious to plastic deformation. Meanwhile, planar slip of dislocations increased the slip reversibility [24]. When the total strain amplitude exceeded 0.8 × 10−2, wavy-slip structured dislocations dominated (Fig. 9a, c, and 9d). Part of planar-slip structured dislocations were also involved (Fig. 9b). This part of planar-slip structured dislocations could bear some cyclic plastic strain. In addition, deformation twins were formed in some grains in the Mn12CrN steel (Fig. 9c and d). On the one hand, twinning changed the orientation of the matrix, which was favorable for the generation and slip of
5. Conclusions The monotonic and cyclic deformation behaviors of the N + Cr alloyed Hadfield steel, Mn12CrN steel, and traditional Hadfield steel, Mn12 steel, were comparatively studied in the present study. The asynchronous effect of N + Cr alloying on the monotonic and cyclic deformation behaviors of the Hadfield steel were analyzed. The following conclusions could be drawn. 1. The yield and tensile strengths of the Mn12CrN steel were much higher than those of the Mn12 steel during monotonic deformation. Solid solution strengthening, DSA effects and high-density deformation twins formed at large strain were responsible for the improved strengths in the Mn12CrN steel. 2. The maximum cyclic peak stress and cycles to reach this value of the Mn12CrN steel at each of the total strain amplitudes were smaller than those of the Mn12 steel, while the initial cyclic peak stress was greater than that of the Mn12 steel during cyclic deformation. 3. Different microstructure evolutions in the Mn12CrN steel during cyclic deformation were observed compared with those in the Mn12 steel. At the total strain amplitude of 0.4 × 10−2, typical planar-slip structured dislocations were the main dislocation configurations. With the increase in the total strain amplitude, the dislocation slip mode started transiting to wavy slip. When the total strain Fig. 12. Deformation twin structures of the Mn12CrN steel at the total strain amplitude of 0.8 × 10−2 after 20 cycles: a, low-magnification image showing deformation twins and the diffraction pattern; b, HRTEM image of the red box in Fig. 12a and the corresponding fast Fourier-transformed image; c, d, e, inverse fast Fourier-transformed images of area c, d and e in Fig. 12b. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 13. Deformation twin structures of the Mn12CrN steel at the total strain amplitude of 0.8 × 10−2 after 100 cycles: a, lowmagnification image showing deformation twins and the diffraction pattern; b, HRTEM image of the red box in Fig. 13a and the corresponding fast Fourier-transformed image; c, d, inverse fast Fourier-transformed images of area c and d in Fig. 13b. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
amplitude was increased to 0.8 × 10−2, wavy-slip structured dislocations dominated, and deformation twins were formed in the Mn12CrN steel. 4. The solid solution strengthening effect by N + Cr alloying and high stress which was needed for leading dislocations to pass through the Cr–N SRO areas finally resulted in higher initial cyclic peak stresses in the Mn12CrN steel than those in the Mn12 steel. As the cyclic deformation continued, the SROs were partially destroyed. The stress needed for dislocations to pass through these areas decreased. The SROs also had a negative influence on the DSA effect. Moreover, interactions among the planar-slip structured dislocations were weakened. The factors above eventually resulted in lower maximum cyclic peak stresses in the Mn12CrN steel than in the Mn12 steel. 5. In comparison with the wavy-slip structured dislocations, the resistances to generation and motion of the planar-slip structured dislocations were small, which was favorable for slip reversibility and plastic deformation. It was the reason that caused longer fatigue lives in the Mn12CrN steel than those in the Mn12 steel at low total strain amplitudes. At high total strain amplitudes, partial planar-slip structured dislocations and deformation twins in the Mn12CrN steel helped in prolonging the fatigue lives.
642 (2015) 71–83. [6] G.V.P. Reddy, R. Kannan, K. Mariappan, R. Sandhya, S. Sankaran, K. Bhanu Sankara Rao, Effect of strain rate on low cycle fatigue of 316LN stainless steel with varying nitrogen content: Part-I cyclic deformation behavior, Int. J. Fatigue 81 (2015) 299–308. [7] G.V.P. Reddy, K. Mariappan, R. Kannan, R. Sandhya, S. Sankaran, K. Bhanu Sankara Rao, Effect of strain rate on low cycle fatigue of 316LN stainless steel with varying nitrogen content: Part-II fatigue life and fracture, Int. J. Fatigue 81 (2015) 309–317. [8] L. He, Z.H. Jin, J.D. Lu, X.L. Chen, J. Fu, C.W. Sun, Effect of chromium on the microstructure of Hadfield manganese steel, Iron Steel 35 (2000) 48–50. [9] H. Mughrabi, H.W. Höppel, M. Kautz, Fatigue and microstructure of ultrafinegrained metals produced by severe plastic deformation, Scripta Mater. 51 (2004) 807–812. [10] S.G.S. Raman, K.A. Padmanabhan, Effect of prior cold work on the room-temperature low-cycle fatigue behaviour of AISI 304LN stainless steel, Int. J. Fatigue 18 (1996) 71–79. [11] C.W. Shao, P. Zhang, R. Liu, Z.J. Zhang, J.C. Pang, Z.F. Zhang, Low-cycle and extremely-low-cycle fatigue behaviors of high-Mn austenitic TRIP/TWIP alloys: property evaluation, damage mechanisms and life prediction, Acta Mater. 103 (2016) 781–795. [12] C.W. Shao, P. Zhang, Z.J. Zhang, Z.F. Zhang, Butterfly effect in low-cycle fatigue: importance of microscopic damage mechanism, Scripta Mater. 140 (2017) 76–81. [13] C.W. Shao, P. Zhang, R. Liu, Z.J. Zhang, J.C. Pang, Q.Q. Duan, Z.F. Zhang, A remarkable improvement of low-cycle fatigue resistance of high-Mn austenitic TWIP alloys with similar tensile properties: importance of slip mode, Acta Mater. 118 (2016) 196–212. [14] J. Kang, F.C. Zhang, X.Y. Long, B. Lv, Cyclic deformation and fatigue behaviors of Hadfield manganese steel, Mater. Sci. Eng. A 591 (2014) 59–68. [15] C. Chen, B. Lv, F. Wang, F.C. Zhang, Low-cycle fatigue behaviors of pre-hardening Hadfield steel, Mater. Sci. Eng. A 695 (2017) 144–153. [16] C. Chen, B. Lv, H. Ma, D.Y. Sun, F.C. Zhang, Wear behavior and the corresponding work hardening characteristics of Hadfield steel, Tribol. Int. 121 (2018) 389–399. [17] X.W. Li, Y.M. Hu, Z.G. Wang, Investigation of dislocation structure in a cyclically deformed copper single crystal using electron channeling contrast technique in SEM, Mater. Sci. Eng. A 248 (1998) 299–303. [18] V. Gerold, H.P. Karnthaler, On the origin of planar slip in fcc alloys, Acta Metall. 37 (1989) 2177–2183. [19] L. Chen, H.S. Kim, S.K. Kim, Localized deformation due to Portevin–LeChatelier effect in 18Mn–0.6 C TWIP austenitic steel, ISIJ Int. 47 (2007) 1804–1812. [20] C.W. Shao, F. Shi, X.W. Li, Influence of cyclic stress amplitude on mechanisms of deformation of a high nitrogen austenitic stainless steel, Mater. Sci. Eng. A 667 (2016) 208–216. [21] R.E. Schramm, R.P. Reed, Stacking fault energies of seven commercial austenitic stainless steels, Metall. Trans. A 6 (1975) 1345–1351. [22] Y.J. Dai, Z.L. Mi, D. Tang, J.C. Lv, Effect of aluminium, copper and chromium on the stacking fault energy and mechanical properties of Fe-21Mn-0.4C TWIP/TRIP steel, J. Iron Steel Res. 23 (2011) 32–34. [23] J. Friedel, Dislocations: International Series of Monographs on Solid State Physics, Elsevier, Amsterdam, 2013. [24] M. Grujicic, X.W. Zhou, W.S. Owen, Monte Carlo analysis of short-range order in nitrogen-strengthened Fe–Ni–Cr–N austenitic Alloys, Mater. Sci. Eng. A 169 (1993) 103–110. [25] M. Murayama, K. Hono, H. Hirukawa, T. Ohmura, S. Matsuoka, The combined effect of molybdenum and nitrogen on the fatigued microstructure of 316 type austenitic stainless steel, Scripta Mater. 41 (1999) 467–473. [26] C. Chen, B. Lv, F. Wang, F.C. Zhang, Low-cycle fatigue behaviors of pre-hardening Hadfield steel, Mater. Sci. Eng. A 695 (2017) 144–153. [27] Q.S. Pan, H.F. Zhou, Q.H. Lu, H.J. Gao, L. Lu, History-independent cyclic response of nanotwinned metals, Nature (2017) 1–4.
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Acknowledgements The authors gratefully acknowledge the PhD Foundation Project of Yanshan University (NO. BL19007) and the Youth Fund of National Natural Science Fund (No. 51501161). References [1] I. Karaman, H. Sehitoglu, K. Gall, Y.I. Chumlyakov, H.J. Maier, Deformation of single crystal Hadfield steel by twinning and slip, Acta Mater. 48 (2000) 1345–1359. [2] F.C. Liu, Z.N. Yang, C.L. Zheng, F.C. Zhang, Simultaneously improving the strength and ductility of coarse-grained Hadfield steel with increasing strain rate, Scripta Mater. 66 (2012) 431–434. [3] C. Chen, F.C. Zhang, F. Wang, H. Liu, B.D. Yu, Effect of N+Cr alloying on the microstructures and tensile properties of Hadfield steel, Mater. Sci. Eng. A 679 (2017) 95–103. [4] J. Kang, F.C. Zhang, X.Y. Long, Z.N. Yang, Synergistic enhancing effect of N+C alloying on cyclic deformation behaviors in austenitic steel, Mater. Sci. Eng. A 610 (2014) 427–435. [5] L. Mosecker, D.T. Pierce, A. Schwedt, M. Beighmohamadi, J. Mayer, W. Bleck, J.E. Wittig, Temperature effect on deformation mechanisms and mechanical properties of a high manganese C+N alloyed austenitic stainless steel, Mater. Sci. Eng. A
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Asynchronous effect of N+Cr alloying on the monotonic and cyclic deformation behaviors of Hadfield steel
T
Chen Chena, Fucheng Zhanga,b,*, Bo Lvc, Hua Maa, Lin Wanga, Hongwang Zhangb, Wei Shend a
State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, 066004, China National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao, 066004, China c College of Environmental and Chemical Engineering, Yanshan University, Qinhuangdao, 066004, China d China Railway Shanhaiguan Bridge Group Co. LTD, Qinhuangdao, 066205, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Monotonic deformation Cyclic deformation Planar slip Deformation twins N+Cr alloying Hadfield steel
Alloying treatment has always been an effective way to improve the mechanical performance of Hadfield steel. In the present study, Hadfield steel was alloyed with N + Cr, named Mn12CrN steel. Its monotonic and cyclic deformation behaviors were comparatively studied with traditional Hadfield steel, named Mn12 steel. Results showed that the stress responses during monotonic and cyclic deformations were different for these two steels, presenting an asynchronous effect of N + Cr alloying. During monotonic deformation, high-density deformation twins were formed in the Mn12CrN steel at large strain. Together with the solid solution strengthening effect, the Mn12CrN steel obtained higher yield and tensile strengths than the Mn12 steel. However, the maximum cyclic peak stress of the Mn12CrN steel was lower than that of the Mn12 steel during cyclic deformation, though a higher initial cyclic peak stress was obtained in the Mn12CrN steel. The cyclic deformation strain was much smaller than the monotonic deformation. In the strain range of cyclic deformation, planar dislocation configuration was mainly formed in the Mn12CrN steel at the total strain amplitude of 0.4 × 10−2–0.6 × 10−2. While deformation twins and mixed dislocation configurations were observed in the Mn12CrN steel when the total strain amplitude was above 0.8 × 10−2. With these microstructure evolutions in the Mn12CrN steel, low maximum cyclic peak stress but prolonged fatigue life was obtained compared with the Mn12 steel.
1. Introduction Stable austenitic microstructure can be retained after water toughening treatment in traditional Hadfield steel because of the high carbon and manganese contents. These specific chemical compositions determine medium-low stacking fault energy (SFE) in the Hadfield steel. As a result, both dislocation slipping and twinning are plastic deformation mechanisms, and excellent work hardening characteristic can be obtained [1,2]. However, dislocation slipping and twinning behaviors would be changed in the metal materials with the alterations in chemical compositions (e.g., alloying treatment) and test conditions (e.g., strain rates, temperatures, and pre-hardening treatment), which finally influence its deformation behavior [2–5]. Reddy et al. [6,7] studied the effect of N content on cyclic deformation behaviors in 316 austenitic stainless steel. They affirmed that N changed the dislocation slip mode in the stainless steel during cyclic deformation. The increased N content promoted the planar slip tendency. However, the effect of N on fatigue
*
lives varied depending on the fracture modes (trans-granular or intergranular fractures) of the test samples. In the report of He et al. [8], Cr was found to enhance the ordering tendency of alloying elements and intensify the gathering of C–Mn atoms in high-manganese steels by Xray diffraction analysis. Therefore, the dislocation slip behavior was changed, and the work hardening capacity was improved after Cr alloying treatment. In our previous work [3], the tensile property of the N + Cr alloyed Hadfield steel was demonstrated to be insensitive to strain rate. The active twining behavior and improved dynamic strain aging (DSA) effect were responsible for the changed performance. At this point, extraordinary results could be expected during cyclic deformation in the N + Cr alloyed Hadfield steel compared with the traditional Hadfield steel. Commonly, cyclic and monotonic deformation performances are highly related to each other [9,10]. However, some exceptional cases always attract the researchers’ attention. Shao et al. [11] reported a decreased low-cycle fatigue resistance in high-manganese steels with synchronously improved strength and ductility. In another paper of
Corresponding author. State Key Laboratory of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao, 066004, China. E-mail address: [email protected] (F. Zhang).
https://doi.org/10.1016/j.msea.2019.06.025 Received 9 April 2019; Received in revised form 6 June 2019; Accepted 8 June 2019 Available online 09 June 2019 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
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Table 1 Chemical compositions of the test steels (wt.%). Test steels
C
Mn
Cr
N
Si
Al
Cu
P
S
Mn12CrN Mn12
1.10 1.14
11.6 11.7
2.14 0.07
0.052 0.016
0.42 0.42
0.024 0.027
0.014 0.009
≤0.001 ≤0.001
0.005 0.005
their group, wide variations in cyclic stress response and fatigue life were observed in high-manganese steels with a slight variation in monotonic flow stress [12]. They explained it as the slight difference in the initial work hardening of those steels. Meanwhile, materials with similar tensile properties could present remarkable variations in lowcycle fatigue resistance, depending on the dislocation structure evolution during cyclic deformation [13]. These results all claimed a noninvariable relationship between monotonic and cyclic deformation behaviors of steels. In the present study, an asynchronous effect of N + Cr alloying on the monotonic and cyclic deformation behaviors of the Hadfield steel was illustrated. This effect was discussed by comparing the microstructure evolutions in the N + Cr alloyed and traditional Hadfield steels under different deformation conditions.
Fig. 1. Engineering stress-strain curves of Mn12CrN and Mn12 steels.
3. Results 3.1. Monotonic deformation characteristics The engineering stress-strain curves in Fig. 1 show simultaneously improved strength and plasticity of the Mn12CrN steel compared with the Mn12 steel. The yield and tensile strengths of the Mn12CrN steel were 478 MPa and 1043 MPa, respectively, 16.9% and 21.1% higher than those of the Mn12 steel. While the elongation of the Mn12CrN steel was 39.6%, slightly higher than that of the Mn12 steel. This improving effect on the strength and plasticity of the N + Cr alloyed Hadfield steel was explained by the promoted twinning behavior and solid solution strengthening effect in our previous work [3].
2. Materials and experimental methods The test steels in the present study were N + Cr alloyed Hadfield steel, named Mn12CrN steel, and traditional Hadfield steel, named Mn12 steel. Their chemical compositions are listed in Table 1. The steels were smelted in a 50 kg vacuum induction furnace, and then forged into square blocks with a size of 60 mm × 60 mm. The blocks were cut into pieces with a length of 110 mm for solid solution treatment. The austenitizing temperature of the Mn12 steel was 1050 °C, while that of the Mn12CrN steel was increased to 1100 °C to ensure the dissolution of Cr-rich carbides. After holding at austenitizing temperature for 1 h, the test steels were quenched with water. Subsequently, the test steels were machined into cylindrical specimens for tensile and lowcycle fatigue tests. The gauge length and diameter of the deformation part of the cylindrical specimens were 10 and 5 mm, respectively. To reduce the effect of surface condition on fatigue property, the specimens were grounded into smooth and mirror-like surfaces. The tensile and low-cycle fatigue tests were conducted on an MTS hydraulic servo fatigue testing machine with a same strain rate of 5 × 10−3 s−1. A strain-controlled symmetric tension-compression loading method was used in the low-cycle fatigue test. Total strain amplitudes of 0.4 × 10−2, 0.6 × 10−2, 0.8 × 10−2 and 1.0 × 10−2 were selected. Five specimens were tested at each strain amplitude. The failure of specimens was determined by a reduction of 25% in peak tensile load as compared with the maximum peak value. The tensile and fatigue curves were plotted in accordance with the test results. The internal and effective stresses of the two test steels during cyclic deformation were calculated using the Handfield-Dickson method [4]. Samples used for microstructure observation were cut using an electric spark wire cutting machine, and all of the observed surfaces were parallel to the loading direction. Optical microstructures of the Mn12CrN and Mn12 steels after low-cycle fatigue test were observed under an Axiover 200MAT optical microscope. After mechanical grinding and polishing, the samples were etched by a 4% nitric acid alcohol solution. Fine microstructures of the Mn12CrN steel were examined using a JEM-2010 transmission electron microscope (TEM). The operating voltage was 200 kV. Thin foils were prepared through a precision ion polishing system (Gatan).
3.2. Cyclic deformation characteristics 3.2.1. Cyclic deformation behavior Fig. 2 illustrates the changing curves of tensile peak stress as the number of cycles and fraction of the lifetime of the two test steels at different total strain amplitudes. With the increase in total strain amplitude, the cyclic peak stresses of the two test steels increased gradually, but the fatigue lives decreased. At the total strain amplitudes of 0.4 × 10−2 ≤ εt/2 ≤ 0.6 × 10−2, the cyclic deformation behaviors of the two test steels both displayed cyclic hardening in the first tens of cycles. When the peak stress reached a certain value, cyclic softening followed by cyclic stability happened until fatigue failure. At the total strain amplitudes of 0.8 × 10−2 ≤ εt/2 ≤ 1.0 × 10−2, only cyclic hardening and softening behaviors were observed before fatigue failure (Fig. 2a and c). The two test steels presented similar changing trend in the cyclic deformation behavior. However, some interesting points were also observed. The initial cyclic peak stress of the Mn12CrN steel was higher than that of the Mn12 steel at each total strain amplitude, while a contrary result was obtained in the maximum cyclic peak stress (Fig. 3a and Table 2). Number of cycles to reach the maximum cyclic peak stress of the Mn12CrN steel was less than that of the Mn12 steel at different strain amplitudes (Fig. 3b). However, the fatigue life of the Mn12CrN steel was much higher than that of the Mn12 steel especially when the total strain amplitude was above 0.6 × 10−2 (Fig. 3b and Table 2). Meanwhile, the proportion of hardening cycles in the whole fatigue life of the Mn12CrN steel was smaller than that of the Mn12 steel (Fig. 2b and d). Apparently, the Mn12CrN steel possessed higher strength and plasticity than the Mn12 steel during monotonic deformation (Fig. 1), but the cyclic deformation behaviors did not respond asynchronously (Figs. 2 and 3). Fig. 4 shows the relationships among total strain amplitudes, elastic strain amplitudes, plastic strain amplitudes and number of reversals to failure of the test steels. The elastic and plastic strain amplitudes of the 2
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Fig. 2. Tensile peak stress as a function of number of cycles and fraction of the lifetime of Mn12CrN (a, b) and Mn12 (c, d) steels at different total strain amplitudes.
two test steels both declined gradually with the decrease in total strain amplitude. However, the plastic strain amplitude reduced more rapidly than the elastic strain amplitude. Specifically, the plastic strain amplitude of the Mn12CrN steel played a dominant role at the total strain amplitude of εt/2 ≥ 0.6 × 10−2. When the total strain amplitude was 0.4 × 10−2, the plastic and elastic strain amplitudes were almost equivalent (Fig. 4a). For the Mn12 steel, the plastic strain amplitude dominated at the total strain amplitude of 0.8 × 10−2 ≤ εt/ 2 ≤ 1.0 × 10−2. While at the total strain amplitude of 0.6 × 10−2, the plastic strain amplitude was nearly equal to the elastic strain amplitude. At the total strain amplitude of 0.4 × 10−2, the elastic strain amplitude dominated (Fig. 4b). It was obvious that, compared with the Mn12 steel, the Mn12CrN steel possessed low cyclic peak stress at half lifetime with the same total strain amplitude but large plastic strain amplitude (Figs. 2 and 4 and Table 2). This result was unexpected when the responded stress of the Mn12CrN steel during monotonic deformation was higher than that of the Mn12 steel at the same strain (Fig. 1). Cyclic hardening ratio (CHR) and cyclic softening ratio (CSR) are important parameters to characterize the hardening and softening degrees of materials during cyclic deformation. The values of CHR and CSR can be calculated by CHR=(σmax-σ1)/σ1 and CSR= (σmax-σhalf)/ σmax, where σ1, σmax, and σhalf are stress amplitude at the first cycle, the
Table 2 Low cycle fatigue results of Mn12CrN and Mn12 steels. Test steel
Total strain amplitude/×10−2
Maximum peak stress amplitude/ MPa
Plastic strain amplitude at half-life cycle/×10−2
Fatigue life
Mn12CrN
0.4 0.6 0.8 1.0 0.4 0.6 0.8 1.0
485 558 598 709 526 627 678 798
0.19 0.35 0.52 0.65 0.15 0.32 0.44 0.57
12864 4978 1120 926 11388 1666 654 311
Mn12
maximum stress amplitude, and the stress amplitude at the half lifetime, respectively. Fig. 5 shows the CHRs and CSRs of the Mn12CrN and Mn12 steels at different strain amplitudes. The CHRs of the two test steels increased gradually with the increase in total strain amplitudes, whereas the CSRs decreased. The CHR of the Mn12CrN steel was much smaller than that of the Mn12 steel at the same total strain amplitude, and the CSR was slightly greater than that of the Mn12 steel. When the
Fig. 3. Initial and maximum cyclic tensile stress (a), and cycles to reach maximum cyclic stress and fatigue life (b) of Mn12CrN and Mn12 steels as functions of total strain amplitude. 3
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Fig. 4. Total, elastic and plastic strain amplitudes of Mn12CrN (a) and Mn12 (b) steels as a function of number of reversals to failure.
amplitude selected in the present study, which was in accordance with the difference in maximum cyclic peak stresses of the Mn12CrN and Mn12 steels (Figs. 2 and 3). Different deformation band densities in the two test steels were macroscopic expressions of diversity in fine microstructures. Therefore, TEM micrographs of the two test steels at different total strain amplitudes were studied. Fine microstructures of traditional Hadfield steel after the fatigue test have been observed in detail in our previous study [14]. At a relatively low total strain amplitude of 0.4 × 10−2, entangled dislocations were the main characteristic, and dislocation cell structures were hardly formed. With the increase in total strain amplitude, obvious dislocation cell structures were formed, but no evidence of deformation twins was found. Wavy-slip structured dislocations coordinated the plastic deformation process [14,15]. Fine microstructures of the Mn12CrN steel after the fatigue test were observed in the present study. TEM micrographs are shown in Figs. 7–9. At a relatively low total strain amplitude of 0.4 × 10−2, the microstructure was dominated by planar-slip structured dislocation arrangements (Fig. 7). Dislocation-rich and dislocation-poor bands were formed in the austenite matrix, and they extended to the grain boundaries (Fig. 7a). Moreover, typical planar-slip structured dislocation configurations, such as dislocation bundles (Fig. 7b), dislocation arrays (Fig. 7c), and intersected slip bands (Fig. 7d) were also observed. At this total strain amplitude, typical planar slip was the main slip mode of dislocations in the Mn12CrN steel, which was different from that in the Mn12 steel. When the total strain amplitude was increased to 0.6 × 10−2 (Fig. 8), except for the unidirectional planar slip bands in Fig. 8a, the intersected slip bands were frequently formed (Fig. 8b). Entangled dislocations (Fig. 8c) and numerous stacking faults (Fig. 8d) were also observed in some grains. The dislocation slip mode transited into the mixed slip. At the total strain amplitude of 0.8 × 10−2, though some planar-slip structured dislocation configurations still existed in a few grains (Fig. 9b), wavy-slip structured dislocations became the main configurations (Fig. 9a, c, and 9d). The dislocations interacted with each other to form cell structures, and wavy slip mode became the main slip mode in the Mn12CrN steel. Meanwhile, unidirectional and intersected deformation twins were observed in the Mn12CrN steel (Fig. 9c and d). Incomplete cell structures were also formed in the matrix between the adjacent deformation twins (Fig. 9c). During cyclic deformation, many factors may influence the dislocation slip modes and the formation of twins. These microstructural configurations will directly determine the fatigue behaviors and lives. Therefore, the differences in cyclic deformation behaviors of the Mn12CrN and Mn12 steels were analyzed in combination with the chemical compositions and microstructure evolutions in the next section.
Fig. 5. Cyclic hardening and softening ratios as a function of total strain amplitudes in Mn12CrN and Mn12 steels.
relationships between CHRs, CSRs and the total strain amplitudes were approximately fitted into straight lines, the CHRs and CSRs curves of the Mn12CrN steel presented an approaching tendency to those of the Mn12 steel. This quick glance maybe caused by a gradually approaching microstructure in the two test steels. However, The CHR and CSR differences of the Mn12CrN and Mn12 steels indicated that the microstructural evolutions during cyclic deformation should be distinct. 3.3. Microstructural features Uniform austenitic microstructures were obtained in the two test steels after solid solution treatment. The grain sizes of the Mn12CrN and Mn12 steels were 79 and 89 μm, respectively, in accordance with our previous work [3]. The microstructures of the two test steels after monotonic deformation had been studied in detail in our previous work [3]. High-density dislocations and deformation twins were generated in the two test steels. The Mn12CrN steel obtained more deformation twins than the Mn12 steel at the same tensile strain. In the present study, microstructures after cyclic deformation were studied on focus. Optical images of the two test steels after cyclic deformation are listed in Fig. 6. Numerous deformation bands occurred in the grains of Hadfield steel after the fatigue test. At a relatively low total strain amplitude (εt/2 = 0.4 × 10−2), unidirectional deformation bands were the main characteristics (Fig. 6a and b). Moreover, no apparent deformation bands were formed in some grains because of the grain orientation and observation direction. With the increase in the total strain amplitude, the deformation bands began to intersect with each other, and the deformation band density increased gradually (Fig. 6c, d, 6e, and 6f). Specifically, the deformation band density of the Mn12CrN steel was smaller than that of the Mn12 steel at the same total strain 4
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Fig. 6. Optical images of the two test steels after fatigue failure at different total strain amplitudes: a, Mn12CrN steel-0.4 × 10−2; b, Mn12 steel-0.4 × 10−2; c, Mn12CrN steel-0.6 × 10−2; d, Mn12 steel-0.6 × 10−2; e, Mn12CrN steel-0.8 × 10−2; f, Mn12 steel-0.8 × 10−2.
0.8 × 10−2 (Fig. 9), which were not found in traditional Hadfield steel. In comparison with the traditional Hadfield steel, i.e., the Mn12 steel, N and Cr were added in the Mn12CrN steel. The difference in grain sizes of the two test steels could be neglected. Therefore, different microstructural characteristics in the Mn12CrN steel were caused by alloying treatment. In other words, alloying treatment in the Mn12CrN steel depressed the cross slip of dislocations and promoted the formation of deformation twins. Factors that influence the slip modes of dislocations include SFE [11,17], short range order (SRO) [18], and DSA [6]. In general, a large stacking fault width in metal materials with low SFE was unfavorable to the cross slip of dislocations, thereby the planar slip mode dominated [19]. The SFE of traditional Hadfield steel was ~50 mJm−2 [1]. Wavy slip was the main dislocation slip mode in traditional Hadfield steel with this SFE. However, N was regarded as an effective element that could decrease the SFE of austenitic manganese steels [20,21], and Cr was also an SFE-decreasing element [22]. Therefore, the SFE of the Mn12CrN steel was reduced because of N + Cr alloying, which became a favorable factor that promoted slip planarity. SRO is another factor that can promote dislocation slip planarity. Gerold and Karnthaler [18] proposed that SROs softened the slip planes, and was the main factor to promote slip planarity in face-centered cubic (fcc) metals. When a leading dislocation passed through the slip plane, high stress was required to overcome the increased energy associated with the disordering of SROs. However, subsequent passage of dislocations hardly further disordered the SROs, and the required activation stress was reduced [23]. The dislocation slip was therefore
4. Discussion 4.1. Effect of N + Cr alloying on the monotonic deformation behavior of the hadfield steel The N + Cr alloying treatment in the Hadfield steel has been proved to reduce the SFE in our previous works [3,16]. The monotonic deformation microstructures in the Mn12CrN steel were high-density deformation twins and dislocations. The number of deformation twins in the Mn12CrN steel was much greater than that in the Mn12 steel at the same strain [3]. Together with the solid solution strengthening and DSA effects of N + Cr alloying, the Mn12CrN steel obtained higher tensile and yield strengths than the Mn12 steel during monotonic deformation (Fig. 1). 4.2. Effect of N + Cr alloying on the cyclic deformation behavior of the hadfield steel 4.2.1. Planar slip characteristic in the N + Cr alloyed hadfield steel During cyclic deformation of traditional Hadfield steel, wavy slip is the main slip mode. In the strain-controlled low cycle fatigue test, wavy-slip structured dislocations dominated even though the total strain amplitude was as low as 0.4 × 10−2 [14,15]. However, the slip mode transited from planar to mixed and finally wavy slip when the total strain amplitude was increased from 0.4 × 10−2 to 0.8 × 10−2 in the Mn12CrN steel (Figs. 7–9). Moreover, numerous deformation twins were observed in the Mn12CrN steel at the total strain amplitude of 5
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Fig. 7. TEM micrographs of the Mn12CrN steel after fatigue failure at total strain amplitude of 0.4 × 10−2, showing (a) planar arrangement of dislocations, (b) dislocation bundles, (c) dislocation arrays and (d) two sets of planar dislocations.
The screw dislocation annihilation distance (ys) determines the difficulty level of cross-slip in steels. A lower value of ys makes cross slip more difficult, thereby facilitating planar slip [18]. On the contrary, it facilitates wavy slip. ys can be estimated through the following formula:where G is the shear modulus, b is the Burgers vector, θ is the included angle of primary slip plane and cross slip plane (70.5° in fcc crystals), τ0, τa, and τi are critical shear stress, applied stress, and internal stress, respectively, and S is the ratio of Schmid factors of (11 ‾ 1)[1 ‾ 01] and (11 ‾ 1)[1 ‾ 01] dislocations. During cyclic deformation in the present study, changes in G, b and τ0 with the total strain amplitudes could be neglected. Therefore, ys is only determined by τa and τi. The stress amplitudes and internal stresses at different total strain amplitudes of steels can be calculated using the Handfield-Dickson method [4]. It indicated that the stress amplitude and internal stress at the half life cycle improved with increasing total strain amplitudes (Fig. 11), which meant increasing values of τa and τi. As a result, the screw dislocation annihilation distance increased, which facilitated the wavy slip of dislocations. Thus, the slip mode in the Mn12CrN steel transited to wavy slip at relatively high total strain amplitudes (Fig. 9). In polycrystalline materials, a critical stress σT is needed for the formation of deformation twins [38]. σT is expressed as follows:where γSF is the SFE, and b is the Burgers vector. The critical stress is determined by the SFE and Burgers vector. The critical stress during cyclic deformation almost remained unchanged at different total strain amplitudes. When the applied stress at a certain total strain amplitude exceeded the critical stress, deformation twins formed. Deformation twins in Fig. 9c and d proved that the applied stress at the total strain amplitude of 0.8 × 10−2 exceeded the critical stress, whereas that at the total strain amplitude of εt/2 ≤ 0.6 × 10−2 did not, and no evidence of deformation twins was found (Figs. 7 and 8). Compared with the Mn12 steel, N + Cr alloying in the Mn12CrN steel increased the
confined to the slip plane, and slip planarity was promoted. In Fe–Mn–Cr–N or Fe–Ni–Cr–N alloys, Cr–N SROs had been detected through various test methods [24,25]. In the present study, Cr–N SROs caused by N + Cr alloying that might influence the microstructures and properties of Hadfield steel were considered during the composition design. Therefore, the promoted effect of SROs on slip planarity in the Mn12CrN steel during cyclic deformation could be expected. DSA effect is caused by the interactions between the mobile dislocations and solute atoms. The flow stress rises sufficiently to break away from the pinning atoms or to generate new dislocations to maintain the imposed strain rate. Serrations in the hysteresis loop were macro evidence of the DSA effect during plastic deformation. Fig. 10 portrays the hysteresis loops and corresponding amplified curves of the local areas of the two test steels. Apparent serrations were observed, which indicated the occurrence of the DSA effect. In DSA, the solute atoms played pinning and dragging roles to mobile dislocations. The dislocations could not easily transit to other slip planes through cross slip, thereby promoting the slip planarity of dislocations [6]. However, apparent DSA effect both occurred in the two test steels, and no major difference was observed. Thus, the low SFE and Cr–N SROs in the Mn12CrN steel were the main reasons that caused the slip planarity of dislocations during cyclic deformation. 4.2.2. The change of slip mode and formation of deformation twins in the N + Cr alloyed hadfield steel In traditional Hadfield steel, only wavy-slip structured dislocations were observed during cyclic deformation because of the high SFE [14,15]. However, the slip mode might change with increasing total strain or stress amplitude in metal materials with relatively low SFE [20]. The Mn12CrN steel displayed different slip modes with varying total strain amplitudes (Figs. 7–9). 6
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Fig. 8. TEM micrographs of the Mn12CrN steel after fatigue failure at total strain amplitude of 0.6 × 10−2, showing (a) planar arrangement of dislocations, (b) two sets of planar dislocations, (c) entangled dislocations and (d) stacking faults.
we found that the DSA effect in the Mn12CrN steel was depressed at the primary stage of cyclic deformation. Although N + Cr alloying increased the number of solute atoms in Hadfield steel, Cr and N atoms existed in the form of Cr–N SROs at the initial stage of cyclic deformation, which made the aggregation of solute atoms more difficult. Therefore, the interactions between dislocations and solute atoms were weakened. Second, after leading dislocations passed through the Cr–N SRO areas, the integrity of SRO structures was destroyed. The stress needed for the subsequent dislocations to pass through these areas decreased. Third, the planar slip characteristic of the Mn12CrN steel resulted in planar band dislocation configurations during cyclic deformation (Figs. 7 and 8). This type of dislocation configuration weakened the strengthening effect from dislocation interactions. Therefore, the effective stresses induced in the short-range motion of dislocations and the internal stresses induced in the long-range motion of dislocations of the Mn12CrN steel were both lower than those of the Mn12 steel (Fig. 11). A relatively short dislocation annihilation distance in planar slip also accelerated the dislocation annihilation rate. Therefore, Cr–N SROs, suppressed DSA effect and weakened interactions among dislocations resulted in relatively low maximum cyclic stresses and CHRs but high CSRs in the Mn12CrN steel in comparison with those in the Mn12 steel (Figs. 3 and 5). With the increase in total strain amplitude, the integrity of Cr–N SROs was further destroyed. The weakening effect on DSA eased. The serration shapes in the hysteresis loops of the Mn12CrN steel were similar to those of the Mn12 steel at total strain amplitudes of 0.8 × 10−2 and 1.0 × 10−2 in Fig. 10. In addition, the dislocation configurations gradually transited from planar-slip structured dislocations to wavy-slip structured dislocations. Therefore, cycles to reach the maximum cyclic peak stress of the Mn12CrN steel approached those of the Mn12 steel (Fig. 3).
lattice constant, resulting in an increased Burgers vector, but decreased the SFE. The critical stress to form deformation twins in the Mn12CrN steel was smaller than that in the Mn12 steel at the same test conditions. Therefore, deformation twins were observed in the Mn12CrN steel, and no evidence was found in the Mn12 steel at each total strain amplitude selected in the present study. 4.2.3. Cyclic stress responses and fatigue lives in the N + Cr alloyed hadfield steel At the total strain amplitudes selected in the present study, the initial cyclic peak stress of the Mn12CrN steel was greater than that of the Mn12 steel, whereas the maximum cyclic peak stress of the Mn12CrN steel was lower (Figs. 2 and 3). After N + Cr alloying, the solid solution strengthening effect and increased stress which was needed for leading dislocations to disorder the Cr–N SROs explained the enhanced initial cyclic peak stress in the Mn12CrN steel. However, in the subsequent cyclic deformation process, the Mn12CrN steel reached the maximum cyclic peak stress after very short cycles (Fig. 2). Subsequently, cyclic softening until fracture failure (εt/2 ≥ 0.8 × 10−2) or cyclic softening to cyclic stability followed by fracture failure (εt/2 ≤ 0.6 × 10−2) occurred. Compared with the Mn12 steel, the shortened hardening behavior and decreased maximum cyclic peak stress in the Mn12CrN steel were not expected. Cyclic hardening behavior is determined by dislocation activities in metal materials during cyclic deformation. The low maximum cyclic peak stress in the Mn12CrN steel could be explained as follows. First, the DSA effect is one of the reasons that cause cyclic hardening in metal materials [16,21]. Except for the serrations in the hysteresis loops (Fig. 10), the DSA effect can also prolong cyclic hardening cycles. By comparing the serration shapes in hysteresis loops of the second cycle in Fig. 10 and the cyclic hardening cycles of the two test steels in Fig. 3, 7
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Fig. 9. TEM micrographs of the Mn12CrN steel after fatigue failure at total strain amplitude of 0.8 × 10−2, showing (a) dislocations cells, (b) planar arrangement of dislocations, (c) deformation twins and (d) intersected deformation twins.
Fig. 10. Hysteresis loops of Mn12CrN (a, b) and Mn12 (c, d) steels at different total strain amplitudes during cyclic deformation, Fig. 10b and d are amplified pictures in the blue boxes in Fig. 10a and b, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 11. Stress amplitudes, internal stresses and effective stresses of Mn12CrN (a) and Mn12 (b) steels at the half life cycle as functions of strain amplitude.
dislocations and released the local plastic strain [26]. On the other hand, the deformation twins were formed only after tens of cycles at the total strain amplitude of 0.8 × 10−2 (Fig. 12). The twin boundaries (TBs) interacted with movable dislocations during the subsequent deformation. Their interaction destroyed the coherency of TBs (Fig. 12b, c, 12d and 12e). With the increase in the cyclic deformation cycles, the interaction frequency of TBs and dislocations increased (Fig. 13). The TB-dislocation interaction was helpful for releasing the local stress concentration at the grain boundaries, therefore alleviating the accumulation of fatigue damage [27]. The two effects of deformation twins finally prolonged the fatigue lives of the Mn12CrN steel at high total strain amplitudes.
Based on the discussion above, the microstructural characteristics in the Mn12CrN steel were not favorable to rapid hardening at low strain. However, with the increase in the deformation strain, the Cr–N SROs were seriously destroyed, and additional Cr and N atoms would gather to the center of dislocations to intensify the DSA effect. Moreover, the low SFE in the Mn12CrN steel promoted the formation of deformation twins. Fine-size and high-density deformation twins were obtained in the Mn12CrN steel at the same deformation strain [3]. Therefore, an intensified DSA effect and high-density deformation twins resulted in the increase in strength and plasticity of the Mn12CrN steel at large strain during monotonic deformation. In terms of the fatigue life, the Mn12CrN steel possessed longer lives than the Mn12 steel at each total strain amplitude. Fig. 4 and Table 2 show that the plastic strain amplitudes of the Mn12CrN steel dominated the total strain amplitudes, except for the total strain amplitude of 0.4 × 10−2. Generally, high plastic strain amplitude could facilitate the generation and slip of dislocations, resulting in high cyclic peak stress. However, dislocation pile-up was also intensified during this process, easily leading to local stress concentration and fracture failure. In that way, the fatigue life would be eventually shortened. The plastic strain amplitude of the Mn12CrN steel was larger than that of the Mn12 steel, but the maximum cyclic peak stress was smaller and the fatigue life was longer at the same total strain amplitude. These results were determined by the microstructure evolution during cyclic deformation. At the total strain amplitude of εt/2 ≤ 0.6 × 10−2, the dislocation slip mode in the Mn12CrN steel was planar slip (Figs. 7 and 8). Compared with wavy-slip structured dislocations, the resistance to generation and motion of planar-slip structured dislocations were small, which was propitious to plastic deformation. Meanwhile, planar slip of dislocations increased the slip reversibility [24]. When the total strain amplitude exceeded 0.8 × 10−2, wavy-slip structured dislocations dominated (Fig. 9a, c, and 9d). Part of planar-slip structured dislocations were also involved (Fig. 9b). This part of planar-slip structured dislocations could bear some cyclic plastic strain. In addition, deformation twins were formed in some grains in the Mn12CrN steel (Fig. 9c and d). On the one hand, twinning changed the orientation of the matrix, which was favorable for the generation and slip of
5. Conclusions The monotonic and cyclic deformation behaviors of the N + Cr alloyed Hadfield steel, Mn12CrN steel, and traditional Hadfield steel, Mn12 steel, were comparatively studied in the present study. The asynchronous effect of N + Cr alloying on the monotonic and cyclic deformation behaviors of the Hadfield steel were analyzed. The following conclusions could be drawn. 1. The yield and tensile strengths of the Mn12CrN steel were much higher than those of the Mn12 steel during monotonic deformation. Solid solution strengthening, DSA effects and high-density deformation twins formed at large strain were responsible for the improved strengths in the Mn12CrN steel. 2. The maximum cyclic peak stress and cycles to reach this value of the Mn12CrN steel at each of the total strain amplitudes were smaller than those of the Mn12 steel, while the initial cyclic peak stress was greater than that of the Mn12 steel during cyclic deformation. 3. Different microstructure evolutions in the Mn12CrN steel during cyclic deformation were observed compared with those in the Mn12 steel. At the total strain amplitude of 0.4 × 10−2, typical planar-slip structured dislocations were the main dislocation configurations. With the increase in the total strain amplitude, the dislocation slip mode started transiting to wavy slip. When the total strain Fig. 12. Deformation twin structures of the Mn12CrN steel at the total strain amplitude of 0.8 × 10−2 after 20 cycles: a, low-magnification image showing deformation twins and the diffraction pattern; b, HRTEM image of the red box in Fig. 12a and the corresponding fast Fourier-transformed image; c, d, e, inverse fast Fourier-transformed images of area c, d and e in Fig. 12b. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 13. Deformation twin structures of the Mn12CrN steel at the total strain amplitude of 0.8 × 10−2 after 100 cycles: a, lowmagnification image showing deformation twins and the diffraction pattern; b, HRTEM image of the red box in Fig. 13a and the corresponding fast Fourier-transformed image; c, d, inverse fast Fourier-transformed images of area c and d in Fig. 13b. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
amplitude was increased to 0.8 × 10−2, wavy-slip structured dislocations dominated, and deformation twins were formed in the Mn12CrN steel. 4. The solid solution strengthening effect by N + Cr alloying and high stress which was needed for leading dislocations to pass through the Cr–N SRO areas finally resulted in higher initial cyclic peak stresses in the Mn12CrN steel than those in the Mn12 steel. As the cyclic deformation continued, the SROs were partially destroyed. The stress needed for dislocations to pass through these areas decreased. The SROs also had a negative influence on the DSA effect. Moreover, interactions among the planar-slip structured dislocations were weakened. The factors above eventually resulted in lower maximum cyclic peak stresses in the Mn12CrN steel than in the Mn12 steel. 5. In comparison with the wavy-slip structured dislocations, the resistances to generation and motion of the planar-slip structured dislocations were small, which was favorable for slip reversibility and plastic deformation. It was the reason that caused longer fatigue lives in the Mn12CrN steel than those in the Mn12 steel at low total strain amplitudes. At high total strain amplitudes, partial planar-slip structured dislocations and deformation twins in the Mn12CrN steel helped in prolonging the fatigue lives.
642 (2015) 71–83. [6] G.V.P. Reddy, R. Kannan, K. Mariappan, R. Sandhya, S. Sankaran, K. Bhanu Sankara Rao, Effect of strain rate on low cycle fatigue of 316LN stainless steel with varying nitrogen content: Part-I cyclic deformation behavior, Int. J. Fatigue 81 (2015) 299–308. [7] G.V.P. Reddy, K. Mariappan, R. Kannan, R. Sandhya, S. Sankaran, K. Bhanu Sankara Rao, Effect of strain rate on low cycle fatigue of 316LN stainless steel with varying nitrogen content: Part-II fatigue life and fracture, Int. J. Fatigue 81 (2015) 309–317. [8] L. He, Z.H. Jin, J.D. Lu, X.L. Chen, J. Fu, C.W. Sun, Effect of chromium on the microstructure of Hadfield manganese steel, Iron Steel 35 (2000) 48–50. [9] H. Mughrabi, H.W. Höppel, M. Kautz, Fatigue and microstructure of ultrafinegrained metals produced by severe plastic deformation, Scripta Mater. 51 (2004) 807–812. [10] S.G.S. Raman, K.A. Padmanabhan, Effect of prior cold work on the room-temperature low-cycle fatigue behaviour of AISI 304LN stainless steel, Int. J. Fatigue 18 (1996) 71–79. [11] C.W. Shao, P. Zhang, R. Liu, Z.J. Zhang, J.C. Pang, Z.F. Zhang, Low-cycle and extremely-low-cycle fatigue behaviors of high-Mn austenitic TRIP/TWIP alloys: property evaluation, damage mechanisms and life prediction, Acta Mater. 103 (2016) 781–795. [12] C.W. Shao, P. Zhang, Z.J. Zhang, Z.F. Zhang, Butterfly effect in low-cycle fatigue: importance of microscopic damage mechanism, Scripta Mater. 140 (2017) 76–81. [13] C.W. Shao, P. Zhang, R. Liu, Z.J. Zhang, J.C. Pang, Q.Q. Duan, Z.F. Zhang, A remarkable improvement of low-cycle fatigue resistance of high-Mn austenitic TWIP alloys with similar tensile properties: importance of slip mode, Acta Mater. 118 (2016) 196–212. [14] J. Kang, F.C. Zhang, X.Y. Long, B. Lv, Cyclic deformation and fatigue behaviors of Hadfield manganese steel, Mater. Sci. Eng. A 591 (2014) 59–68. [15] C. Chen, B. Lv, F. Wang, F.C. Zhang, Low-cycle fatigue behaviors of pre-hardening Hadfield steel, Mater. Sci. Eng. A 695 (2017) 144–153. [16] C. Chen, B. Lv, H. Ma, D.Y. Sun, F.C. Zhang, Wear behavior and the corresponding work hardening characteristics of Hadfield steel, Tribol. Int. 121 (2018) 389–399. [17] X.W. Li, Y.M. Hu, Z.G. Wang, Investigation of dislocation structure in a cyclically deformed copper single crystal using electron channeling contrast technique in SEM, Mater. Sci. Eng. A 248 (1998) 299–303. [18] V. Gerold, H.P. Karnthaler, On the origin of planar slip in fcc alloys, Acta Metall. 37 (1989) 2177–2183. [19] L. Chen, H.S. Kim, S.K. Kim, Localized deformation due to Portevin–LeChatelier effect in 18Mn–0.6 C TWIP austenitic steel, ISIJ Int. 47 (2007) 1804–1812. [20] C.W. Shao, F. Shi, X.W. Li, Influence of cyclic stress amplitude on mechanisms of deformation of a high nitrogen austenitic stainless steel, Mater. Sci. Eng. A 667 (2016) 208–216. [21] R.E. Schramm, R.P. Reed, Stacking fault energies of seven commercial austenitic stainless steels, Metall. Trans. A 6 (1975) 1345–1351. [22] Y.J. Dai, Z.L. Mi, D. Tang, J.C. Lv, Effect of aluminium, copper and chromium on the stacking fault energy and mechanical properties of Fe-21Mn-0.4C TWIP/TRIP steel, J. Iron Steel Res. 23 (2011) 32–34. [23] J. Friedel, Dislocations: International Series of Monographs on Solid State Physics, Elsevier, Amsterdam, 2013. [24] M. Grujicic, X.W. Zhou, W.S. Owen, Monte Carlo analysis of short-range order in nitrogen-strengthened Fe–Ni–Cr–N austenitic Alloys, Mater. Sci. Eng. A 169 (1993) 103–110. [25] M. Murayama, K. Hono, H. Hirukawa, T. Ohmura, S. Matsuoka, The combined effect of molybdenum and nitrogen on the fatigued microstructure of 316 type austenitic stainless steel, Scripta Mater. 41 (1999) 467–473. [26] C. Chen, B. Lv, F. Wang, F.C. Zhang, Low-cycle fatigue behaviors of pre-hardening Hadfield steel, Mater. Sci. Eng. A 695 (2017) 144–153. [27] Q.S. Pan, H.F. Zhou, Q.H. Lu, H.J. Gao, L. Lu, History-independent cyclic response of nanotwinned metals, Nature (2017) 1–4.
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study. Acknowledgements The authors gratefully acknowledge the PhD Foundation Project of Yanshan University (NO. BL19007) and the Youth Fund of National Natural Science Fund (No. 51501161). References [1] I. Karaman, H. Sehitoglu, K. Gall, Y.I. Chumlyakov, H.J. Maier, Deformation of single crystal Hadfield steel by twinning and slip, Acta Mater. 48 (2000) 1345–1359. [2] F.C. Liu, Z.N. Yang, C.L. Zheng, F.C. Zhang, Simultaneously improving the strength and ductility of coarse-grained Hadfield steel with increasing strain rate, Scripta Mater. 66 (2012) 431–434. [3] C. Chen, F.C. Zhang, F. Wang, H. Liu, B.D. Yu, Effect of N+Cr alloying on the microstructures and tensile properties of Hadfield steel, Mater. Sci. Eng. A 679 (2017) 95–103. [4] J. Kang, F.C. Zhang, X.Y. Long, Z.N. Yang, Synergistic enhancing effect of N+C alloying on cyclic deformation behaviors in austenitic steel, Mater. Sci. Eng. A 610 (2014) 427–435. [5] L. Mosecker, D.T. Pierce, A. Schwedt, M. Beighmohamadi, J. Mayer, W. Bleck, J.E. Wittig, Temperature effect on deformation mechanisms and mechanical properties of a high manganese C+N alloyed austenitic stainless steel, Mater. Sci. Eng. A
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