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Effect of MnS inclusions on deformation behavior of matrix based on in-situ experiment
Materials Science & Engineering A 746 (2019) 239–247
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of MnS inclusions on deformation behavior of matrix based on in-situ experiment
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Xin-gang Liu, Can Wang , Jiang-tao Gui, Qi-qi Xiao, Bao-feng Guo College of Mechanical Engineering (Yanshan University), Qinhuangdao 066004, Hebei, China
A R T I C LE I N FO
A B S T R A C T
Keywords: In-situ Inclusions Deformation Fracture
In order to study the influence of inclusions on the deformation behavior of matrix, SEM data and EBSD data were collected under various loads by means of an in-situ tensile test. Specifically, the effect of MnS inclusions on the deformation behavior of matrix was studied at various stages. The experimental results show that the influence of different forms of inclusions on the deformation of the substrate differs at normal temperatures. The long-axis direction of inclusions perpendicular to the load direction influences the deformation behavior more than that of the long-axis direction parallel to the load direction and near-spherical inclusions. An analysis of the EBSD data reveals that the morphology and polymorphism of in-situ MnS inclusions coexist, and the deformation behavior of the two during the deformation process is significantly different, as is the influence on the deformation behavior of the matrix. SEM fracture scanning analysis shows that the presence of MnS particles causes the material to exhibit brittle fracture characteristics and promote material failure. In this study, the microscopic topography and the final load-displacement curve of each stage of the experiment are analyzed in detail, and the influence of inclusions on the deformation behavior of the matrix is explained in detail.
1. Introduction MnS is a common inclusion in steel, which seriously affects the mechanical properties (strength, toughness, impact properties, etc.) of the metal matrix. In order to clarify how inclusions affect the matrix, many scholars have conducted a lot of scientific research on this. Fernandes et al. [1] used Scanning Electron Microscope (SEM) data to obtain the basic chemical composition of non-metallic inclusions, and they then used a computer program to draw the chemical composition of the inclusions and explain the location of non-metallic inclusions in the appropriate ternary- or quaternary-phase diagram. The presence of non-metallic inclusions has a relatively large impact on the matrix properties. In-depth research and analysis has been conducted on the effect of MnS on matrix anisotropy. Joo et al. [2] and Victor et al. [3] showed that the uneven distribution of non-metallic inclusions of various shapes and sizes has an important influence on the anisotropy of matrix toughness. Kaddouri et al. [4] studied the influence of inclusions on the thermal conductivity of materials, and proposed an analytical expression to estimate the effective thermal conductivity of heterogeneous materials by considering the morphology of inclusions. Angeles-Herrera et al. [5] pointed out that the number and length of non-metallic inclusions have a strong negative impact on the performance of welded joints. ⁎
Further, many scholars have studied the effects of inclusions on the impact fatigue life of materials. For instance, Hashimoto et al. [6] studied the influence of inclusions of different compositions on the impact fatigue life of the matrix by controlling the chemical composition of inclusions in the matrix. They concluded that MnS and TiN have less influence on the impact fatigue life of the material than SiO2 and Al2O3. They further concluded that oxides play a crucial role in the impact fatigue life of materials. Fujimatsu et al. [7] closed the interface cavity between the inclusions and the matrix by using HIP treatment to delay both the initiation of pores and the significant growth of existing pores. Doing so reduced the stress concentration between the inclusions and the matrix, thereby reducing the impact of the inclusions on the matrix. In order to clarify the quantitative influence of material parameters on crack initiation and propagation, many scholars have developed finite element models to show the influence of inclusions on the matrix during deformation [8–13]. Makino et al. [14] used the finite element method to accurately explain the increase of stress caused by inclusions and interface separation, revealing the influence of inclusions on rolling control fatigue (RCF) intensity. Moghaddam et al. [15] proposed a model to simulate the formation of bearing steel from the initiation of microcracks around non-metallic inclusions to the final destruction of underground cracks. They showed that the stiffness and position of inclusions considerably influences the life of RCF. The
Corresponding author. E-mail address: [email protected] (C. Wang).
https://doi.org/10.1016/j.msea.2018.12.121 Received 3 November 2018; Received in revised form 26 December 2018; Accepted 29 December 2018 Available online 31 December 2018 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
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2.2. In-situ tensile test
problem is severe at critical depths, where it can eventually lead to material failure when the size of the inclusions is large. Neishi et al. [16] demonstrated that vertical cracks caused by MnS inclusions are closely related to the length of the MnS. Guan et al. [17] studied the influence of inclusions on crack initiation and propagation by means of the finite element method. They concluded that softer inclusions are smaller in size and that these inclusions more easily initiate and propagate cracks. Furthermore, they showed that the closer the inclusions are to the surface, the more the cracks will propagate. Siruguet et al. [18] and Jean-Baptiste et al. [19] studied the influence of non-metallic inclusions on the growth of porous ductile materials during the deformation process using the finite element method to reflect the influence of non-metallic inclusions in the deformation of the matrix. Ghosh et al. [20] investigated the tensile and impact properties of samples under different conditions by controlling the amount and morphology of MnS inclusions. Finally, research has shown that with higher-content MnS, the shape of the strip appears to be more destructive to the material properties. Du et al. [21] observed the position of hydrogen-induced cracks, and concluded that cracks tend to grow at the interface between the matrix and two-phase and non-metallic inclusions, causing premature material failure. This paper focuses on the influence of MnS inclusions on the deformation behavior of the matrix. It further investigates the causes of pore initiation, along with those of crack propagation and premature material failure. To do so, SEM data and Electron Backscatter Diffraction (EBSD) data were collected under various loads by means of in-situ tensile experiments to analyze the influence of inclusions on the deformation behavior of 304 stainless steel matrix.
Generally, the problem of the influence of inclusions on the properties of the matrix under tensile conditions can only be studied by the near-in situ test method. It is only possible to stretch with several specimens whose tissue state is basically the same, and then stop loading and unload the specimen when the predetermined load is reached. Then, the surface of each sample was polished to observe the deformation of the surface of the sample. This method has a lot of drawbacks. It can not guarantee the uniformity of each sample, can not accurately obtain the load value when the typical phenomenon occurs, and can not obtain the deformation under different stress states in the same area. These drawbacks make it impossible to ensure the uniqueness of the factors. In-situ tensile test has the following three advantages compared with it The observation area can be accurately located, and the area can be found by coordinates under any load. Accurately collect SEM and EBSD information under different stress states in the same region. It can accurately find the moment when microcrack initiation, expansion and macroscopic fracture occur. Because of the three advantages of the in-situ tensile test, this research problem can be solved. The results of the in-situ tensile test will be analyzed in detail in the following results and discussion. An in-situ tensile test requires an in-situ tensile test bench. Because the test bench has high processing precision for the sample, the machining method was used to meet the corresponding requirements. The experimental schematic is shown in Fig. 2. The thickness of the sample is 1 mm. The In order to clarify the influence of MnS on the deformation behavior of the substrate, part of casting A was separately processed into sample A, and part of casting B into sample B. The two samples were then in-situ tested. SEM and EBSD information was collected on the surface of the sample during the experiment. Before the experiment, a small observation area was collected in the sample gauge section and coordinate data was recorded. A small area is necessary because the whole area in the gauge length cannot be observed in-situ during the experiment. Doing so is time consuming and unnecessary. It is only necessary to find a typical small area in each area of the gauge length to represent the entire deformation situation of the gauge length. Since the sample calibration area (2 × 1 mm2) was observed in-situ, the sample was pre-stretched in advance to understand the load during each deformation stage of the sample such that the load pause point could be found to better achieve the purpose of the experiment and to understand the influence of inclusions on the deformation behavior of the substrate in each deformation stage. During the experiment, the EBSD information was collected regarding the specimen gauge length, so the specimen was placed at a 70° inclination from the in-situ stretching table.
2. Experimental procedures 2.1. Materials Forged 304 stainless steel was used as the raw material. This was heated to 1600 °C for 10 min and then cooled to 1400 °C by vacuum smelting. Then, 182 g of 85% pure FeS powder and a 368 g electrolytic manganese sheet were added successively. Finally, electromagnetic stirring was carried out for 5 min and then cooled to obtain casting A. Casting B underwent the same smelting process, except that no ingredients were added during the smelting process. In order to clarify the chemical composition, microstructure, and inclusion composition of the two materials, ICP-OES data, SEM data, and energy spectrum information were collected for the two materials. See Table 1 and Fig. 1 for details. Fig. 1(a) is a photomicrograph of casting A, in which we can clearly see the inclusions in the uniform dispersion of casting A. Fig. 1(b) shows the microstructure of casting B, where it can be seen that there are few impurities. To more accurately understand the composition of the inclusions in castings A and B, the energy spectrum information was collected for the two materials. Fig. 1(c) is a photograph of the energy spectrum information of casting A. It can be seen from the photograph that the inclusions mainly contain Mn and S elements, which can be basically determined to be MnS inclusions. Fig. 1(d) is a photograph of the energy spectrum information of casting B. It can be seen from the figure that a small amount of impurities mainly contain Mn and Si elements. These may be considered MnS and SiO2 inclusions from the viewpoint of the chemical elements and smelting processes.
3. Results and discussions 3.1. Deformation behavior and fracture analysis It can be seen from the experimental load-displacement curves of sample A and sample B that the plasticity and strength of sample A are lower than those of sample B, as shown in Fig. 3. In order to have a complete and accurate explanation of the reasons for the differences between the two, a detailed analysis was carried out. When the displacement is less than 0.15 mm, both are in the elastic deformation stage, and the curves of the two are basically consistent. That is, the MnS particles have no influence on the matrix at this time. Comparing the two graphs (a) and (b) in Fig. 3, it can be seen that no significant deformation of the matrix and MnS under such load and displacement is observed, and the uncoordinated deformation behavior of the MnS and the matrix is not prominent. Thus, the load-displacement curves of both samples are basically the same.
Table 1 The chemical compositions of Casting A and Casting B wt (%). Element
C
Mn
Si
P
S
Cr
Ni
Fe
Casting A Casting B
0.061 0.061
2.70 0.86
0.55 0.55
0.029 0.029
0.30 0.018
18.06 18.06
8.05 8.05
Balance Balance
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Fig. 1. Microstructure and energy spectrum. (a) Casting A; (b) Casting B; (c) Spectrum of Casting A; (d) Spectrum of Casting B.
load, it is clear that the former is considerably larger. The reason for this is that the displacement of sample B is exclusively the plastic deformation displacement of the matrix. The displacement of sample A, by contrast, includes not only the plastic deformation displacement of the matrix but also the width of the pores after interface debonding of the MnS particles and internal fracture. There are three reasons why there is a more of a difference in the displacement than previously. First, the number of debonded and internally fractured MnS particles increases, resulting in an increase in pores due to overall debonding and internal fracture and an increase in the rate of pore growth. Second, the convergence of microcracks occurs at this time, making for larger cracks with an increase in the opening degree. This causes the total displacement to increase. Third, the microcrack confluence and the generation of debonding and internal fracture of the MnS particles cause the material strength to be greatly reduced. Thus, the loadbearing capacity of the substrate environment is reduced. Compared to sample B, displacement can be produced at a lower load value. In comparing the highest load value and displacement amount of samples A and B at the final fracture, our results show that the highest load value of sample B is 352.59 N higher than that of the sample A, and the displacement amount is 1.02 mm more than sample A. In order to explain this phenomenon, SEM image analysis of the fracture was carried out, as shown in Fig. 4. Comparing Figs. 4(a) and 4(c), it can be seen that the fracture depth of sample A is shallower than that of sample B, indicating that sample A did not undergo large plastic deformation and that the fracture time was short. Sample A has more dimples, a smaller dimple diameter, and
When the displacement is between 0.15 and 0.5 mm, it is obvious that sample A has a lower load value than sample B under the same displacement, and that the larger the displacement, the greater the difference in the load value. This is due to the uncoordinated deformation between MnS and the matrix, and internal fracture and debonding of the MnS particles. This results in discontinuous strain of the matrix material, a decrease in the ability to transfer loads, and a decrease in material strength. As shown in Fig. 3(c), as the displacement increases, the load value increases, resulting in an increase in the number of MnS particles in which debonding and internal fracture occur. This, in turn, leads to a further decrease in the ability of the matrix to transfer loads, further decreasing material strength. When the displacement is greater than 0.5 mm, the load values of samples A and B differ increasingly, as shown in Fig. 3(d–e). At this time, a large amount of MnS particles on the surface of the sample A are debonded and internally cracked. Furthermore, microcracks that are initiated between the MnS particles start to merge and become larger cracks. This is because, in the case of an increase in load, the matrix produces greater plastic deformation. More MnS causes the interface to debond or internally crack due to the value of its interface strength, and the MnS particles will be removed as the load increases. Sticking and internal cracking are more serious, eventually leading to more stress concentrated at the interface between the MnS and the substrate. This causes the microcracks to expand into the matrix and merge with other microcracks produced by the nearest neighboring MnS particles. Ultimately, the matrix environment is seriously damaged. Comparing the displacement of the samples A and B with the same 241
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Fig. 2. Experimental scheme diagram.
Fig. 3. The micrograph of In situ test result. (a) F = 0 N(δ = 0 mm); (b) F = 300 N(δ = 0.061 mm); (c) F = 600 N(δ = 0.417 mm); (d) F = 800 N(δ = 1.102 mm); (e) F= 800 N(δ = 1.102 mm); (f) F= 1130 N(δ = 2.233 mm); (g) F < 1130 N(δ = 2.25 mm); (h) F < 1130 N(δ = 2.261 mm). 242
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Fig. 4. Fracture scan. (a)(b) are the fracture diagram of sample A; (c)(d) are the fracture diagram of sample B.
most of it is the fracture caused by the growth of the microscopic pores in the metal matrix under external stress. However, the remarkable feature is the fracture morphology of the dimple.
a shallower depth. It is known from the fracture mechanism that the microscopic pores inside the metal grow under the action of external stress, and the cross-sectional area of the matrix between adjacent pores is continuously reduced until the adjacent pores are polymerized with each other. The shape of the dimple port is formed after the final fracture, because in the metal matrix, the holes will undergo nucleation at the second-phase particles, and the MnS particles provide nucleation points for the holes, inducing nucleation. During the increase in the load value, the cross-section of the matrix between several adjacent holes is reduced until they are connected to each other to cause breakage. That is, the presence of MnS particles accelerates fracture failure in the material. A closer look at Fig. 4(b) reveals that there are many secondary cracks in the fracture and a large number of MnS particles in the secondary crack. This is because when the stress intensity at the interface between the MnS particles and the matrix is greater than the interface strength, debonding occurs and pores are generated. During the increase in load, pores merge, grow, and eventually tear, forming secondary cracks. It can also be observed that there is a cleavage step around the secondary crack. The local cleavage step is a remarkable feature of the quasi-cleavage fracture. The crack source of the cleavage step mostly appears in the pores and inclusions inside the grain, and the discontinuous state seriously affects the plasticity and strength of the matrix. The occurrence of secondary cracks and cleavage steps indicates that MnS particles are the main cause of the quasicleavage fracture form of sample A [22]. The fracture of sample B had a nearly 45° slope, indicating that it had large plastic deformation at the time of fracture with dimples due to shear stress. Although there were not many dimples, they were considerably deep. The above judgment can be made by combining the above phenomena. The fracture form of sample A has ductile fracture characteristics and quasi-cleavage fracture characteristics. MnS induces pore nucleation throughout the fracture process, promoting secondary crack formation and a quasi-solution. The formation of cracks accelerates material failure. The fracture mode of sample B is a ductile fracture, and
3.2. Influence of MnS positional relationship on deformation behavior In order to observe the influence of the orientation relationship between inclusions and the tensile load on the deformation behavior of the matrix, the field of view of the inclusions with different orientation relationships was selected during the experiment, as shown in Fig. 5. In the figure, the long axis of the No. 1 MnS inclusion is perpendicular to the tensile load direction, whereas the long axis of the No. 2 MnS inclusion is parallel to the direction of the tensile load. It can be seen from the figure that when the tensile load is 600 N, the material has been plastically deformed. Some joints at the interface between the inclusions and the substrate have debonded, and some inclusions have cracks inside. It can be found that the direction of cracks appearing inside the inclusions is perpendicular to the direction of the tensile load, and as the tensile load increases, the opening degree of the crack gradually increases, aggravating the fragmentation inside the inclusions. By observing inclusions No. 1 and No. 2, it was found that the internal crack length and width of the two particles increased to different degrees as the tensile load increased. When the tensile load was 600 N, the internal crack of No. 1 was 11 µm and the width was 1.07 µm; the internal crack of No. 2 was 4.64 µm and the width was 0.71 µm. When the tensile load was 1100 N, the length of the internal crack of No. 1 inclusion was 11.87 µm and the width was 2.5 µm; the length of the internal crack of No. 2 was 4.65 µm and the width was 1.01 µm. Comparing the above data, it can be seen that with the increase of tensile load, the crack length of No. 1 did not significantly increase. The crack opening degree approximately doubled, and the crack length and opening degree of No. 2 did not change significantly. It can be seen that when the long-axis direction of the inclusion was perpendicular to the direction of the tensile load, the expansion tendency of the pores and 243
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Fig. 5. Morphology of inclusions in different stretching stages. (a) F= 600 N; (b) F = 700 N; (c) F= 800 N; (d) F = 900 N; (e) F = 1000 N; (f) F= 1100 N.
particles, the MnS particles in the austenite region were selected for observation and analysis. In order to clarify the deformation behavior of single crystal and polycrystalline MnS particles during the stretching process and the influence of both on the deformation of the matrix, the two MnS particles were observed during the experiment. Fig. 6 shows the change in the single crystal MnS particles with increasing load, whereas Fig. 7 shows the change in the polycrystalline MnS particles with increasing load. It can be observed from Fig. 6 that there is no significant change in the MnS particles and the matrix of the single crystal when the load value is lower than 500 N. When the load reaches 500 N, an obvious slip line appears in the matrix and the interface debonds between the single crystal MnS particles and the matrix. This indicates that the matrix has undergone plastic deformation and that there is deformation inconsistency between the MnS particles and the matrix. The debonding point occurs at the pole of the long-axis direction of the single crystal MnS particles and has a tendency to extend toward both ends. At 600 N, the interfacial debonding opening degree of the single crystal MnS particles is enlarged.
microcracks was most severe, and the occurrence of the fracture behavior was promoted. When the long-axis direction of the inclusions was parallel to the tensile load, the expansion of the pores and microcracks was more gradual, and the damage to the fracture behavior was relatively small [20]. 3.3. Influence of MnS orientation difference on deformation behavior The study of MnS particles by means of EBSD means that in-situgenerated MnS particles (i.e., individual particles) do not always have the same orientation. When processing with Channel 5 software, we found that there were always multiple orientations inside the MnS particles. In order to avoid the influence of load on the orientation of MnS particles, we performed EBSD information acquisition on MnS particles when no load was applied. In order to avoid the influence of the orientation on the subsequent deformation behavior of single crystals and polycrystalline MnS particles, MnS particles with an approximately circular shape were selected for observation and analysis. In order to avoid the influence of the matrix properties of the MnS 244
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Fig. 6. Change process of MnS particles with the same orientation under different loads. (a) F= 0 N; (b) F = 300 N; (c) F = 500 N; (d) F = 600 N.
Fig. 7. Variation process of MnS particles with different orientations under different loads. (a) F= 0 N; (b) F= 300 N; (c) F= 500 N; (d) F= 600 N. 245
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3.4. Analysis of deformation inconsistency
It can be seen from Fig. 7 that the deformation of the polycrystalline MnS particles and the matrix is approximately the same as that reflected in Fig. 6 when the load is less than 500 N. When the load value is greater than 500 N, the polycrystalline MnS particles are internally broken without debonding. As the load increases, so too does the internal crack opening degree. The degree of this increase is equivalent to the degree of debonding. It can be seen from these two figures that the single crystal MnS particles are debonded without internal fracture, whereas the polycrystalline MnS particles are prone to internal fracture. The occurrence of these two cases is not accidental. The deformation process of these two types of MnS in the statistical matrix is consistent with this law. The reason why MnS particles are only somewhat debonded after internal fracture is that some debonding does not occur. The reason for this is clear from the point of view of energy. When stress is concentrated around the internally cracked MnS particles, the MnS particles can absorb part of the energy to increase the internal crack opening degree. This reduces the stress on the interface between the MnS particles and the substrate. When the stress after weakening does not exceed the critical stress at the time of debonding, the MnS does not debond. However, when the weakened stress exceeds the critical stress at the time of debonding, the MnS debonds. Not all MnS produces debonding or internal fracture at the same time, because the specimens are not subjected to the same stress during the loading process [23].
In order to clarify the influence of MnS on the matrix, EBSD data acquisition was performed on the deformation of MnS under each load. After this analysis, the so-called local misorientation was obtained to reflect the influence of MnS particles on the matrix, as shown in Fig. 8. The local misorientation function takes the difference in the orientation of a point and its five points as a statistical value, and all the points in the graph are counted according to an algorithm. In Fig. 8, regions with a larger difference in orientation are more red. During the deformation process, plastic deformation occurs at each point in the sample, but the amount of deformation differs. This difference leads to the existence of a difference in orientation. In this case, local misorientation can be used to characterize the large strains of those regions more accurately. Indeed, the strain and stress are closely related. The stress in regions with considerable strain is correspondingly high, as can be seen from Fig. 8. The orientation difference at 0 N is not large, and regions with slightly larger orientations are dispersed in the matrix. Therefore, this is the internal stress generated at the time of casting. A partially red area appears at the grain boundary of the matrix and the grain boundary of the MnS inclusion. When loaded to 300 N, the matrix has a slight plastic flow and there is a certain coordinated deformation behavior between the grains in the matrix. The residual stress in the matrix is offset by the loading method. However, the MnS inclusions have almost no plasticity at
Fig. 8. Local Misorientation Diagram. (a) F= 0 N; (b) F= 300 N; (c) F= 500 N; (d) F= 600 N. 246
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normal temperature, and the crystal grains are not deformed. Moreover, the difference in orientation between the crystal grains cannot be reduced by the deformation of the crystal grains, so the stress at the inner grain boundary of the MnS inclusion is not offset. When the load is 500 N, the single crystal MnS particles are debonded, and the stress concentration in the region after debonding is obvious. The polycrystalline MnS particles have not yet generated internal fracture at this time, and the stress concentration around them is not obvious, indicating that more energy is still absorbed by the internal grain boundaries at this time. When the load reaches 600 N, the MnS particles and the matrix are severely debonded and fractures occur inside the MnS. At this time, the stress is more concentrated at the debonding site, seriously affecting the performance of the matrix. After analyzing the experimental phenomena, it is clear that the MnS particles and the matrix are subjected to the same load and the interface between them and the inside of the MnS particles is more likely to fail. It is at this interface where pore nucleation begins. The pores generated after debonding and breaking render the substrate unable to produce a continuous load and more likely to cause the dislocations to entangle there, since the stress concentration is abnormal. This amount of stress promotes the generation of microcracks, which, in turn, provide conditions for the subsequent generation of macroscopic cracks. Thus, serious damage can result to the subsequent matrix.
[2] [3]
[4]
[5]
[6]
[7]
[8] [9] [10]
[11]
[12]
4. Conclusions In this paper, SEM images and EBSD data were analyzed in an effort to understand the influence of MnS inclusions on the deformation behavior of 304 stainless steel. The conclusions obtained are as follows:
[13]
1. The MnS particles generated in situ in the metal matrix are coherent with the single crystal and the polycrystalline state. 2. The single crystal MnS particles will only debond from the matrix interface during the deformation process; the polycrystalline MnS particles will internally fracture and occasionally debond from the matrix interface. 3. During the deformation process, the MnS particles in the long-axis direction perpendicular to the stretching direction are more significant to the matrix than MnS particles in the long-axis direction parallel to the stretching direction; the former result in more damage to the matrix. 4. The presence of MnS particles promotes pore nucleation during the deformation process, providing an opportunity for the pores to aggregate. This promotes quasi-cleavage fractures in the material, causing the material to weaken or even fail early.
[15]
[14]
[16]
[17] [18]
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[22]
Acknowledgements [23]
This research was supported by the National Natural Science Foundation of China (Grant no. 51575475 and Grant no. 51675465). References [1] M. Fernandes, J.C. Pires, N. Cheung, A. Garcia, Investigation of the chemical
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Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Effect of MnS inclusions on deformation behavior of matrix based on in-situ experiment
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Xin-gang Liu, Can Wang , Jiang-tao Gui, Qi-qi Xiao, Bao-feng Guo College of Mechanical Engineering (Yanshan University), Qinhuangdao 066004, Hebei, China
A R T I C LE I N FO
A B S T R A C T
Keywords: In-situ Inclusions Deformation Fracture
In order to study the influence of inclusions on the deformation behavior of matrix, SEM data and EBSD data were collected under various loads by means of an in-situ tensile test. Specifically, the effect of MnS inclusions on the deformation behavior of matrix was studied at various stages. The experimental results show that the influence of different forms of inclusions on the deformation of the substrate differs at normal temperatures. The long-axis direction of inclusions perpendicular to the load direction influences the deformation behavior more than that of the long-axis direction parallel to the load direction and near-spherical inclusions. An analysis of the EBSD data reveals that the morphology and polymorphism of in-situ MnS inclusions coexist, and the deformation behavior of the two during the deformation process is significantly different, as is the influence on the deformation behavior of the matrix. SEM fracture scanning analysis shows that the presence of MnS particles causes the material to exhibit brittle fracture characteristics and promote material failure. In this study, the microscopic topography and the final load-displacement curve of each stage of the experiment are analyzed in detail, and the influence of inclusions on the deformation behavior of the matrix is explained in detail.
1. Introduction MnS is a common inclusion in steel, which seriously affects the mechanical properties (strength, toughness, impact properties, etc.) of the metal matrix. In order to clarify how inclusions affect the matrix, many scholars have conducted a lot of scientific research on this. Fernandes et al. [1] used Scanning Electron Microscope (SEM) data to obtain the basic chemical composition of non-metallic inclusions, and they then used a computer program to draw the chemical composition of the inclusions and explain the location of non-metallic inclusions in the appropriate ternary- or quaternary-phase diagram. The presence of non-metallic inclusions has a relatively large impact on the matrix properties. In-depth research and analysis has been conducted on the effect of MnS on matrix anisotropy. Joo et al. [2] and Victor et al. [3] showed that the uneven distribution of non-metallic inclusions of various shapes and sizes has an important influence on the anisotropy of matrix toughness. Kaddouri et al. [4] studied the influence of inclusions on the thermal conductivity of materials, and proposed an analytical expression to estimate the effective thermal conductivity of heterogeneous materials by considering the morphology of inclusions. Angeles-Herrera et al. [5] pointed out that the number and length of non-metallic inclusions have a strong negative impact on the performance of welded joints. ⁎
Further, many scholars have studied the effects of inclusions on the impact fatigue life of materials. For instance, Hashimoto et al. [6] studied the influence of inclusions of different compositions on the impact fatigue life of the matrix by controlling the chemical composition of inclusions in the matrix. They concluded that MnS and TiN have less influence on the impact fatigue life of the material than SiO2 and Al2O3. They further concluded that oxides play a crucial role in the impact fatigue life of materials. Fujimatsu et al. [7] closed the interface cavity between the inclusions and the matrix by using HIP treatment to delay both the initiation of pores and the significant growth of existing pores. Doing so reduced the stress concentration between the inclusions and the matrix, thereby reducing the impact of the inclusions on the matrix. In order to clarify the quantitative influence of material parameters on crack initiation and propagation, many scholars have developed finite element models to show the influence of inclusions on the matrix during deformation [8–13]. Makino et al. [14] used the finite element method to accurately explain the increase of stress caused by inclusions and interface separation, revealing the influence of inclusions on rolling control fatigue (RCF) intensity. Moghaddam et al. [15] proposed a model to simulate the formation of bearing steel from the initiation of microcracks around non-metallic inclusions to the final destruction of underground cracks. They showed that the stiffness and position of inclusions considerably influences the life of RCF. The
Corresponding author. E-mail address: [email protected] (C. Wang).
https://doi.org/10.1016/j.msea.2018.12.121 Received 3 November 2018; Received in revised form 26 December 2018; Accepted 29 December 2018 Available online 31 December 2018 0921-5093/ © 2019 Elsevier B.V. All rights reserved.
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2.2. In-situ tensile test
problem is severe at critical depths, where it can eventually lead to material failure when the size of the inclusions is large. Neishi et al. [16] demonstrated that vertical cracks caused by MnS inclusions are closely related to the length of the MnS. Guan et al. [17] studied the influence of inclusions on crack initiation and propagation by means of the finite element method. They concluded that softer inclusions are smaller in size and that these inclusions more easily initiate and propagate cracks. Furthermore, they showed that the closer the inclusions are to the surface, the more the cracks will propagate. Siruguet et al. [18] and Jean-Baptiste et al. [19] studied the influence of non-metallic inclusions on the growth of porous ductile materials during the deformation process using the finite element method to reflect the influence of non-metallic inclusions in the deformation of the matrix. Ghosh et al. [20] investigated the tensile and impact properties of samples under different conditions by controlling the amount and morphology of MnS inclusions. Finally, research has shown that with higher-content MnS, the shape of the strip appears to be more destructive to the material properties. Du et al. [21] observed the position of hydrogen-induced cracks, and concluded that cracks tend to grow at the interface between the matrix and two-phase and non-metallic inclusions, causing premature material failure. This paper focuses on the influence of MnS inclusions on the deformation behavior of the matrix. It further investigates the causes of pore initiation, along with those of crack propagation and premature material failure. To do so, SEM data and Electron Backscatter Diffraction (EBSD) data were collected under various loads by means of in-situ tensile experiments to analyze the influence of inclusions on the deformation behavior of 304 stainless steel matrix.
Generally, the problem of the influence of inclusions on the properties of the matrix under tensile conditions can only be studied by the near-in situ test method. It is only possible to stretch with several specimens whose tissue state is basically the same, and then stop loading and unload the specimen when the predetermined load is reached. Then, the surface of each sample was polished to observe the deformation of the surface of the sample. This method has a lot of drawbacks. It can not guarantee the uniformity of each sample, can not accurately obtain the load value when the typical phenomenon occurs, and can not obtain the deformation under different stress states in the same area. These drawbacks make it impossible to ensure the uniqueness of the factors. In-situ tensile test has the following three advantages compared with it The observation area can be accurately located, and the area can be found by coordinates under any load. Accurately collect SEM and EBSD information under different stress states in the same region. It can accurately find the moment when microcrack initiation, expansion and macroscopic fracture occur. Because of the three advantages of the in-situ tensile test, this research problem can be solved. The results of the in-situ tensile test will be analyzed in detail in the following results and discussion. An in-situ tensile test requires an in-situ tensile test bench. Because the test bench has high processing precision for the sample, the machining method was used to meet the corresponding requirements. The experimental schematic is shown in Fig. 2. The thickness of the sample is 1 mm. The In order to clarify the influence of MnS on the deformation behavior of the substrate, part of casting A was separately processed into sample A, and part of casting B into sample B. The two samples were then in-situ tested. SEM and EBSD information was collected on the surface of the sample during the experiment. Before the experiment, a small observation area was collected in the sample gauge section and coordinate data was recorded. A small area is necessary because the whole area in the gauge length cannot be observed in-situ during the experiment. Doing so is time consuming and unnecessary. It is only necessary to find a typical small area in each area of the gauge length to represent the entire deformation situation of the gauge length. Since the sample calibration area (2 × 1 mm2) was observed in-situ, the sample was pre-stretched in advance to understand the load during each deformation stage of the sample such that the load pause point could be found to better achieve the purpose of the experiment and to understand the influence of inclusions on the deformation behavior of the substrate in each deformation stage. During the experiment, the EBSD information was collected regarding the specimen gauge length, so the specimen was placed at a 70° inclination from the in-situ stretching table.
2. Experimental procedures 2.1. Materials Forged 304 stainless steel was used as the raw material. This was heated to 1600 °C for 10 min and then cooled to 1400 °C by vacuum smelting. Then, 182 g of 85% pure FeS powder and a 368 g electrolytic manganese sheet were added successively. Finally, electromagnetic stirring was carried out for 5 min and then cooled to obtain casting A. Casting B underwent the same smelting process, except that no ingredients were added during the smelting process. In order to clarify the chemical composition, microstructure, and inclusion composition of the two materials, ICP-OES data, SEM data, and energy spectrum information were collected for the two materials. See Table 1 and Fig. 1 for details. Fig. 1(a) is a photomicrograph of casting A, in which we can clearly see the inclusions in the uniform dispersion of casting A. Fig. 1(b) shows the microstructure of casting B, where it can be seen that there are few impurities. To more accurately understand the composition of the inclusions in castings A and B, the energy spectrum information was collected for the two materials. Fig. 1(c) is a photograph of the energy spectrum information of casting A. It can be seen from the photograph that the inclusions mainly contain Mn and S elements, which can be basically determined to be MnS inclusions. Fig. 1(d) is a photograph of the energy spectrum information of casting B. It can be seen from the figure that a small amount of impurities mainly contain Mn and Si elements. These may be considered MnS and SiO2 inclusions from the viewpoint of the chemical elements and smelting processes.
3. Results and discussions 3.1. Deformation behavior and fracture analysis It can be seen from the experimental load-displacement curves of sample A and sample B that the plasticity and strength of sample A are lower than those of sample B, as shown in Fig. 3. In order to have a complete and accurate explanation of the reasons for the differences between the two, a detailed analysis was carried out. When the displacement is less than 0.15 mm, both are in the elastic deformation stage, and the curves of the two are basically consistent. That is, the MnS particles have no influence on the matrix at this time. Comparing the two graphs (a) and (b) in Fig. 3, it can be seen that no significant deformation of the matrix and MnS under such load and displacement is observed, and the uncoordinated deformation behavior of the MnS and the matrix is not prominent. Thus, the load-displacement curves of both samples are basically the same.
Table 1 The chemical compositions of Casting A and Casting B wt (%). Element
C
Mn
Si
P
S
Cr
Ni
Fe
Casting A Casting B
0.061 0.061
2.70 0.86
0.55 0.55
0.029 0.029
0.30 0.018
18.06 18.06
8.05 8.05
Balance Balance
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Fig. 1. Microstructure and energy spectrum. (a) Casting A; (b) Casting B; (c) Spectrum of Casting A; (d) Spectrum of Casting B.
load, it is clear that the former is considerably larger. The reason for this is that the displacement of sample B is exclusively the plastic deformation displacement of the matrix. The displacement of sample A, by contrast, includes not only the plastic deformation displacement of the matrix but also the width of the pores after interface debonding of the MnS particles and internal fracture. There are three reasons why there is a more of a difference in the displacement than previously. First, the number of debonded and internally fractured MnS particles increases, resulting in an increase in pores due to overall debonding and internal fracture and an increase in the rate of pore growth. Second, the convergence of microcracks occurs at this time, making for larger cracks with an increase in the opening degree. This causes the total displacement to increase. Third, the microcrack confluence and the generation of debonding and internal fracture of the MnS particles cause the material strength to be greatly reduced. Thus, the loadbearing capacity of the substrate environment is reduced. Compared to sample B, displacement can be produced at a lower load value. In comparing the highest load value and displacement amount of samples A and B at the final fracture, our results show that the highest load value of sample B is 352.59 N higher than that of the sample A, and the displacement amount is 1.02 mm more than sample A. In order to explain this phenomenon, SEM image analysis of the fracture was carried out, as shown in Fig. 4. Comparing Figs. 4(a) and 4(c), it can be seen that the fracture depth of sample A is shallower than that of sample B, indicating that sample A did not undergo large plastic deformation and that the fracture time was short. Sample A has more dimples, a smaller dimple diameter, and
When the displacement is between 0.15 and 0.5 mm, it is obvious that sample A has a lower load value than sample B under the same displacement, and that the larger the displacement, the greater the difference in the load value. This is due to the uncoordinated deformation between MnS and the matrix, and internal fracture and debonding of the MnS particles. This results in discontinuous strain of the matrix material, a decrease in the ability to transfer loads, and a decrease in material strength. As shown in Fig. 3(c), as the displacement increases, the load value increases, resulting in an increase in the number of MnS particles in which debonding and internal fracture occur. This, in turn, leads to a further decrease in the ability of the matrix to transfer loads, further decreasing material strength. When the displacement is greater than 0.5 mm, the load values of samples A and B differ increasingly, as shown in Fig. 3(d–e). At this time, a large amount of MnS particles on the surface of the sample A are debonded and internally cracked. Furthermore, microcracks that are initiated between the MnS particles start to merge and become larger cracks. This is because, in the case of an increase in load, the matrix produces greater plastic deformation. More MnS causes the interface to debond or internally crack due to the value of its interface strength, and the MnS particles will be removed as the load increases. Sticking and internal cracking are more serious, eventually leading to more stress concentrated at the interface between the MnS and the substrate. This causes the microcracks to expand into the matrix and merge with other microcracks produced by the nearest neighboring MnS particles. Ultimately, the matrix environment is seriously damaged. Comparing the displacement of the samples A and B with the same 241
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Fig. 2. Experimental scheme diagram.
Fig. 3. The micrograph of In situ test result. (a) F = 0 N(δ = 0 mm); (b) F = 300 N(δ = 0.061 mm); (c) F = 600 N(δ = 0.417 mm); (d) F = 800 N(δ = 1.102 mm); (e) F= 800 N(δ = 1.102 mm); (f) F= 1130 N(δ = 2.233 mm); (g) F < 1130 N(δ = 2.25 mm); (h) F < 1130 N(δ = 2.261 mm). 242
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Fig. 4. Fracture scan. (a)(b) are the fracture diagram of sample A; (c)(d) are the fracture diagram of sample B.
most of it is the fracture caused by the growth of the microscopic pores in the metal matrix under external stress. However, the remarkable feature is the fracture morphology of the dimple.
a shallower depth. It is known from the fracture mechanism that the microscopic pores inside the metal grow under the action of external stress, and the cross-sectional area of the matrix between adjacent pores is continuously reduced until the adjacent pores are polymerized with each other. The shape of the dimple port is formed after the final fracture, because in the metal matrix, the holes will undergo nucleation at the second-phase particles, and the MnS particles provide nucleation points for the holes, inducing nucleation. During the increase in the load value, the cross-section of the matrix between several adjacent holes is reduced until they are connected to each other to cause breakage. That is, the presence of MnS particles accelerates fracture failure in the material. A closer look at Fig. 4(b) reveals that there are many secondary cracks in the fracture and a large number of MnS particles in the secondary crack. This is because when the stress intensity at the interface between the MnS particles and the matrix is greater than the interface strength, debonding occurs and pores are generated. During the increase in load, pores merge, grow, and eventually tear, forming secondary cracks. It can also be observed that there is a cleavage step around the secondary crack. The local cleavage step is a remarkable feature of the quasi-cleavage fracture. The crack source of the cleavage step mostly appears in the pores and inclusions inside the grain, and the discontinuous state seriously affects the plasticity and strength of the matrix. The occurrence of secondary cracks and cleavage steps indicates that MnS particles are the main cause of the quasicleavage fracture form of sample A [22]. The fracture of sample B had a nearly 45° slope, indicating that it had large plastic deformation at the time of fracture with dimples due to shear stress. Although there were not many dimples, they were considerably deep. The above judgment can be made by combining the above phenomena. The fracture form of sample A has ductile fracture characteristics and quasi-cleavage fracture characteristics. MnS induces pore nucleation throughout the fracture process, promoting secondary crack formation and a quasi-solution. The formation of cracks accelerates material failure. The fracture mode of sample B is a ductile fracture, and
3.2. Influence of MnS positional relationship on deformation behavior In order to observe the influence of the orientation relationship between inclusions and the tensile load on the deformation behavior of the matrix, the field of view of the inclusions with different orientation relationships was selected during the experiment, as shown in Fig. 5. In the figure, the long axis of the No. 1 MnS inclusion is perpendicular to the tensile load direction, whereas the long axis of the No. 2 MnS inclusion is parallel to the direction of the tensile load. It can be seen from the figure that when the tensile load is 600 N, the material has been plastically deformed. Some joints at the interface between the inclusions and the substrate have debonded, and some inclusions have cracks inside. It can be found that the direction of cracks appearing inside the inclusions is perpendicular to the direction of the tensile load, and as the tensile load increases, the opening degree of the crack gradually increases, aggravating the fragmentation inside the inclusions. By observing inclusions No. 1 and No. 2, it was found that the internal crack length and width of the two particles increased to different degrees as the tensile load increased. When the tensile load was 600 N, the internal crack of No. 1 was 11 µm and the width was 1.07 µm; the internal crack of No. 2 was 4.64 µm and the width was 0.71 µm. When the tensile load was 1100 N, the length of the internal crack of No. 1 inclusion was 11.87 µm and the width was 2.5 µm; the length of the internal crack of No. 2 was 4.65 µm and the width was 1.01 µm. Comparing the above data, it can be seen that with the increase of tensile load, the crack length of No. 1 did not significantly increase. The crack opening degree approximately doubled, and the crack length and opening degree of No. 2 did not change significantly. It can be seen that when the long-axis direction of the inclusion was perpendicular to the direction of the tensile load, the expansion tendency of the pores and 243
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Fig. 5. Morphology of inclusions in different stretching stages. (a) F= 600 N; (b) F = 700 N; (c) F= 800 N; (d) F = 900 N; (e) F = 1000 N; (f) F= 1100 N.
particles, the MnS particles in the austenite region were selected for observation and analysis. In order to clarify the deformation behavior of single crystal and polycrystalline MnS particles during the stretching process and the influence of both on the deformation of the matrix, the two MnS particles were observed during the experiment. Fig. 6 shows the change in the single crystal MnS particles with increasing load, whereas Fig. 7 shows the change in the polycrystalline MnS particles with increasing load. It can be observed from Fig. 6 that there is no significant change in the MnS particles and the matrix of the single crystal when the load value is lower than 500 N. When the load reaches 500 N, an obvious slip line appears in the matrix and the interface debonds between the single crystal MnS particles and the matrix. This indicates that the matrix has undergone plastic deformation and that there is deformation inconsistency between the MnS particles and the matrix. The debonding point occurs at the pole of the long-axis direction of the single crystal MnS particles and has a tendency to extend toward both ends. At 600 N, the interfacial debonding opening degree of the single crystal MnS particles is enlarged.
microcracks was most severe, and the occurrence of the fracture behavior was promoted. When the long-axis direction of the inclusions was parallel to the tensile load, the expansion of the pores and microcracks was more gradual, and the damage to the fracture behavior was relatively small [20]. 3.3. Influence of MnS orientation difference on deformation behavior The study of MnS particles by means of EBSD means that in-situgenerated MnS particles (i.e., individual particles) do not always have the same orientation. When processing with Channel 5 software, we found that there were always multiple orientations inside the MnS particles. In order to avoid the influence of load on the orientation of MnS particles, we performed EBSD information acquisition on MnS particles when no load was applied. In order to avoid the influence of the orientation on the subsequent deformation behavior of single crystals and polycrystalline MnS particles, MnS particles with an approximately circular shape were selected for observation and analysis. In order to avoid the influence of the matrix properties of the MnS 244
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Fig. 6. Change process of MnS particles with the same orientation under different loads. (a) F= 0 N; (b) F = 300 N; (c) F = 500 N; (d) F = 600 N.
Fig. 7. Variation process of MnS particles with different orientations under different loads. (a) F= 0 N; (b) F= 300 N; (c) F= 500 N; (d) F= 600 N. 245
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3.4. Analysis of deformation inconsistency
It can be seen from Fig. 7 that the deformation of the polycrystalline MnS particles and the matrix is approximately the same as that reflected in Fig. 6 when the load is less than 500 N. When the load value is greater than 500 N, the polycrystalline MnS particles are internally broken without debonding. As the load increases, so too does the internal crack opening degree. The degree of this increase is equivalent to the degree of debonding. It can be seen from these two figures that the single crystal MnS particles are debonded without internal fracture, whereas the polycrystalline MnS particles are prone to internal fracture. The occurrence of these two cases is not accidental. The deformation process of these two types of MnS in the statistical matrix is consistent with this law. The reason why MnS particles are only somewhat debonded after internal fracture is that some debonding does not occur. The reason for this is clear from the point of view of energy. When stress is concentrated around the internally cracked MnS particles, the MnS particles can absorb part of the energy to increase the internal crack opening degree. This reduces the stress on the interface between the MnS particles and the substrate. When the stress after weakening does not exceed the critical stress at the time of debonding, the MnS does not debond. However, when the weakened stress exceeds the critical stress at the time of debonding, the MnS debonds. Not all MnS produces debonding or internal fracture at the same time, because the specimens are not subjected to the same stress during the loading process [23].
In order to clarify the influence of MnS on the matrix, EBSD data acquisition was performed on the deformation of MnS under each load. After this analysis, the so-called local misorientation was obtained to reflect the influence of MnS particles on the matrix, as shown in Fig. 8. The local misorientation function takes the difference in the orientation of a point and its five points as a statistical value, and all the points in the graph are counted according to an algorithm. In Fig. 8, regions with a larger difference in orientation are more red. During the deformation process, plastic deformation occurs at each point in the sample, but the amount of deformation differs. This difference leads to the existence of a difference in orientation. In this case, local misorientation can be used to characterize the large strains of those regions more accurately. Indeed, the strain and stress are closely related. The stress in regions with considerable strain is correspondingly high, as can be seen from Fig. 8. The orientation difference at 0 N is not large, and regions with slightly larger orientations are dispersed in the matrix. Therefore, this is the internal stress generated at the time of casting. A partially red area appears at the grain boundary of the matrix and the grain boundary of the MnS inclusion. When loaded to 300 N, the matrix has a slight plastic flow and there is a certain coordinated deformation behavior between the grains in the matrix. The residual stress in the matrix is offset by the loading method. However, the MnS inclusions have almost no plasticity at
Fig. 8. Local Misorientation Diagram. (a) F= 0 N; (b) F= 300 N; (c) F= 500 N; (d) F= 600 N. 246
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normal temperature, and the crystal grains are not deformed. Moreover, the difference in orientation between the crystal grains cannot be reduced by the deformation of the crystal grains, so the stress at the inner grain boundary of the MnS inclusion is not offset. When the load is 500 N, the single crystal MnS particles are debonded, and the stress concentration in the region after debonding is obvious. The polycrystalline MnS particles have not yet generated internal fracture at this time, and the stress concentration around them is not obvious, indicating that more energy is still absorbed by the internal grain boundaries at this time. When the load reaches 600 N, the MnS particles and the matrix are severely debonded and fractures occur inside the MnS. At this time, the stress is more concentrated at the debonding site, seriously affecting the performance of the matrix. After analyzing the experimental phenomena, it is clear that the MnS particles and the matrix are subjected to the same load and the interface between them and the inside of the MnS particles is more likely to fail. It is at this interface where pore nucleation begins. The pores generated after debonding and breaking render the substrate unable to produce a continuous load and more likely to cause the dislocations to entangle there, since the stress concentration is abnormal. This amount of stress promotes the generation of microcracks, which, in turn, provide conditions for the subsequent generation of macroscopic cracks. Thus, serious damage can result to the subsequent matrix.
[2] [3]
[4]
[5]
[6]
[7]
[8] [9] [10]
[11]
[12]
4. Conclusions In this paper, SEM images and EBSD data were analyzed in an effort to understand the influence of MnS inclusions on the deformation behavior of 304 stainless steel. The conclusions obtained are as follows:
[13]
1. The MnS particles generated in situ in the metal matrix are coherent with the single crystal and the polycrystalline state. 2. The single crystal MnS particles will only debond from the matrix interface during the deformation process; the polycrystalline MnS particles will internally fracture and occasionally debond from the matrix interface. 3. During the deformation process, the MnS particles in the long-axis direction perpendicular to the stretching direction are more significant to the matrix than MnS particles in the long-axis direction parallel to the stretching direction; the former result in more damage to the matrix. 4. The presence of MnS particles promotes pore nucleation during the deformation process, providing an opportunity for the pores to aggregate. This promotes quasi-cleavage fractures in the material, causing the material to weaken or even fail early.
[15]
[14]
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