Scholarly article on topic 'Characteristic microstructure of polycrystalline Fe-Mn-C alloys deformed by tensile test'

Characteristic microstructure of polycrystalline Fe-Mn-C alloys deformed by tensile test Academic research paper on "Materials engineering"

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{"Fe-Mn-C alloy" / "X-ray diffraction" / "electron backscattering diffraction"}

Abstract of research paper on Materials engineering, author of scientific article — S. Suzuki, T. Yoshimura, E.P. Kwon, S. Fujieda, K. Shinoda, et al.

Abstract Some Fe-based alloys containing Mn exhibit characteristic plastic deformation modes such as twin induced plasticity (TWIP) and transformation induced plasticity (TRIP). In this study, microstructural changes induced by a tensile test in polycrystalline Fe-25%Mn-0.6%C alloys exhibiting TWIP were characterized. Electron backscatter diffraction (EBSD) and two-dimensional (2D) X-ray diffraction (XRD) measurements using synchrotron radiation were carried out to characterize the microstructure and crystallographic orientation of the polycrystalline alloy samples. The samples were deformed by 10%, 30%, and 60% in a tensile test. The EBSD results showed that deformation twins appeared to form preferentially in grains with large Schmid factors for twinning, and these had an orientation of nearly <011> - <111> parallel to the tensile direction. In addition to twinning, plastic deformation by dislocation slip was also observed in the interior of grains and near grain boundaries. In the polycrystalline sample, the heterogeneous strain by twinning and dislocation slip evolved overall with tensile strain. The 2D-XRD results also indicated that dislocation slip as well as twinning occurs in tensile deformed samples, and the contribution of twinning and dislocation slip in grains depends on tensile strain. Therefore, multiple plastic deformation modes consisting of different deformation twins and dislocation slips are attributed to large elongation in Fe-Mn-C alloys.

Academic research paper on topic "Characteristic microstructure of polycrystalline Fe-Mn-C alloys deformed by tensile test"

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ProcediaEngineeringlO (2011) 88-93

Engineering

Procedia

Characteristic microstructure of polycrystalline Fe-Mn-C alloys

deformed by tensile test

S. Suzukia' *, T. Yoshimuraa, E. P. Kwona, S. Fujiedaa, K. Shinodaa, S. Satob

aInstitute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan bInstitute for Materials Research, Tohoku University, Katahira, Aoba-ku, Sendai 980-8577, Japan

Abstract

Some Fe-based alloys containing Mn exhibit characteristic plastic deformation modes such as twin induced plasticity (TWIP) and transformation induced plasticity (TRIP). In this study, microstructural changes induced by a tensile test in polycrystalline Fe-25%Mn-0.6%C alloys exhibiting TWIP were characterized. Electron backscatter diffraction (EBSD) and two-dimensional (2D) X-ray diffraction (XRD) measurements using synchrotron radiation were carried out to characterize the microstructure and crystallographic orientation of the polycrystalline alloy samples. The samples were deformed by 10%, 30%, and 60% in a tensile test. The EBSD results showed that deformation twins appeared to form preferentially in grains with large Schmid factors for twinning, and these had an orientation of nearly <011> - <111> parallel to the tensile direction. In addition to twinning, plastic deformation by dislocation slip was also observed in the interior of grains and near grain boundaries. In the polycrystalline sample, the heterogeneous strain by twinning and dislocation slip evolved overall with tensile strain. The 2D-XRD results also indicated that dislocation slip as well as twinning occurs in tensile deformed samples, and the contribution of twinning and dislocation slip in grains depends on tensile strain. Therefore, multiple plastic deformation modes consisting of different deformation twins and dislocation slips are attributed to large elongation in Fe-Mn-C alloys.

© 2011 Published by Elsevier Ltd. Selection and peer-review under responsibility of ICM11 Keywords: Fe-Mn-C alloy; X-ray diffraction; electron backscattering diffraction

1. Introduction

Many studies have recently focused on twin formation and martensitic transformation in Fe-Mn based alloys because they are known to occasionally exhibit characteristic deformation modes such as twin induced plasticity (TWIP) and transformation induced plasticity (TRIP). In previous studies on TWIP, the microstructure and texture of Fe-Mn-Si-Al alloys [1-3] and Fe-Mn-C alloys [3-12] subjected to cold rolling or tensile deformation were characterized by techniques such as transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). For example, it has been shown that the characteristic texture evolved during tensile deformation of austenitic Fe-Mn-C steels with TWIP, in which two fiber textures belonging to <111> and <100> parallel to the tensile direction were formed during tensile deformation [10]. Slip deformation as well as deformation twinning may be the underlying deformation mechanisms observed in Fe-Mn-C steels with TWIP, because the deformation of Fe-Mn-Si shape memory alloys evolved not only by martensitic transformation but also by slip deformation [13, 14].

* Corresponding author. Tel.: +81-(0)22-217-5168. E-mail address: ssuzuki@tagen.thoku.ac.jp.

1877-7058 © 2011 Published by Elsevier Ltd. doi:10.1016/j.proeng.2011.04.017

Thus, although studies have focused on the microstructure and texture evolution in Fe-Mn-C alloys, the relationship between the microstructural evolution and the wide-range structural change in TWIP samples by tensile deformation remains unclear. The objective of this study is to investigate the evolution of the microstructure and texture of a polycrystalline Fe-Mn-C alloy exhibiting TWIP due to tensile deformation using EBSD and two-dimensional (2D) X-ray diffraction (XRD) measurements. This study focused on the relationship between twin formation and accompanied deformation in many grains in TWIP alloys. 2D-XRD measurements using monochromatized synchrotron radiation were carried out to characterize changes in the overall fiber orientation in samples by tensile deformation.

2. Experimental

Fe-25%Mn-0.6%C (mass%) alloy sheets were cut into tensile samples with a gauge size of 10 x 0.5 x 1 mm3. They were recrystallized at 1173 K in vacuum for 1800 s and tensile deformed up to different strains using a SHIMADZU autograph AG-X series tensile machine at a strain rate of 6.9 x 10-4 s-1. The tensile strength was approximately 750 MPa and the elongation to rupture was 70% for the Fe-Mn-C alloy.

The EBSD method was used to characterize the microstructure and texture evolution of the samples before and after deformation. An optical microscope was also used to observe microstructural changes with increasing strain. The recrystallized samples were tensile deformed to different strain levels at a strain rate of 5.3 x 10-4 s-1 at room temperature to observe twin deformed microstructures. EBSD patterns were obtained using a Nordlys II EBSD detector mounted on a Hitachi SU-6600 scanning electron microscope coupled with an HKL CHANNEL 5 flamenco software. During the measurement, the high indexing rate of Kikuchi patterns was maintained by adjusting the step size and accelerating voltage, and non-indexed points were removed.

2D-XRD rings were measured by synchrotron radiation using a method described in a previous work [13]. The XRD measurements were carried out at beamline BL19B2 of SPring-8. Figure 1 shows the geometry of the 2D-XRD measurements of a tensile sample and the procedure using herein to analyze the orientation [13]. In simple tensile deformation, the apparent orientation [uvw] of the tensile axis may be estimated from changes in diffraction intensities by tensile deformation. The orientation of the tensile axis depends on deformation, because the diffraction intensities are composed of components of different deformation modes. The tensile sample with the [uvw] is oriented toward NS, which is perpendicular to IC. CP is the normal to a set of (hkl) planes, which are inclined to the incident beam at an angle of 0. While the (hkl) pole is on the circle PUQV, the (hkl) pole confined to the circle PAQB is on a circle around the [uvw] axis. Thus, diffraction of the (hkl) pole occurs at P and Q, which are intersections of PUQV and PAQB. Thereby, the corresponding diffraction spots appear at R and T on the imaging plate. For the spherical triangle, IPN, the relationship between the angles p, 8, and a is given by cosp = cos6> cosa . By measuring a in the diffraction rings on the imaging plate and calculating 0, p can be estimated. The index [uvw] can be estimated from a set of values of p measured for different (hkl) poles. Twinning is shown in Fig. 2, which illustrates the relationship between the orientation of deformation twinning and the fcc matrix by tensile test.

Twin formation in fee crystal

Reference sphere

view from fee ( III ) view troll) tee (011)

Fig. 1. Geometry of (hkl) diffraction from a crystal with [uvw] axis oriented toward tensile axis (T.A.).

Fig. 2. Atomistic movements in deformation twin formed in fcc structure by tensile test.

3. Results and Discussion

3.1 EBSD maps of deformation twins

Figures 3 (a) and (b) show the inverse pole figure (IPF) maps obtained by EBSD measurements for samples deformed by 10% and 30%, respectively. Annealing twins are occasionally visible in the interior of the grains because this alloy consists of recrystallized fcc grains with low stacking fault energy. The tensile direction of this sample was horizontal in these maps. With increasing tensile strain, characteristic changes in the micro structure were found in each grain. In addition to heterogeneous misorientation formation by tensile test in the polycrystalline sample, thin plate-like deformation twins formed in the 10% strained sample and they evolved further in the 30% strained sample. Such deformation twins were primarily observed in grains with <011> (green) and <111> (blue) nearly parallel to the tensile direction.

To focus on the details of the evolution of the deformation twin, EBSD IPF maps for a grain with <011> parallel to the tensile direction were observed with high magnification, as shown in Figs. 4 (a) and (b). As a reference, a contour map of Schmid factors for deformation twinning ([-211](111)) and that for dislocation slip ([-1-12](111)) are shown in Fig. 4 (c). The contours of Schmid factors for the deformation twin and dislocation slip are indicated by red and green in the stereographic triangle, respectively. Because the crystallographically resolved shear direction in deformation twinning (Fig. 3 (c)) differs from that in dislocation slip, the two regions are separated in the contour map of Schmid factors in Fig. 4 (c). The contour map suggests that deformation twins may frequently form in grains where <011> and <111> are nearly parallel to the tensile direction. Actually, such deformation twins were found in the green matrix grain shown in Figs. 4 (a) and (b), while twins rarely formed in grains with <001> orientation. The thin red bands in the grain have twin relations with the matrix. These results indicate that deformation twins preferentially formed in specific grains when shear stress by tensile test reached a critical resolved shear stress for twinning. Thus, the crystallographic orientation with respect to external stress direction is an important factor in the appearance of deformation twins. It should also be noted that deformation twin bands heterogeneously formed even within a grain, because they may have been initiated by local characteristic stress concentration fields, such as a stress field constrained by grain boundaries.

In the sample deformed by 30%, deformation twin bands were grown but they were bent due to plastic deformation induced by dislocation slip, as shown in Fig. 4 (b). The critical resolved shear stresses for dislocation slip are considered to be slightly higher than those for twinning in this grain. Although such dislocation slips contributed to the plastic behavior depending on the grain orientation, twin bands seemed to easily form in grains with close to <011> and <111> orientation in the sample deformed by 30%, as shown in Fig. 3 (b). Here, it is interesting to note that the fcc matrix microstructure changed to different refined microstructures by dividing by twins during deformation. In addition, Kikuchi patterns in heavily strained samples could not be measured; such parts were assigned to dark areas in the EBSD-IPF maps. These microstructural changes by deformation contribute, more or less, to the hardening and high elongation of this alloy, although microscopic components should be characterized further in future experiments.

Figure 5 summarizes the IPFs of tensile direction in 0%, 10%, and 30% strained samples. The results indicate that with increasing strain, the tensile direction of the samples oriented close to <011>-<111>. This is consistent with 2D-XRD results shown later.

Fig. 3. EBSD IPF maps of a sample deformed up to 10% (a) and 30% (b). Grains with <001>, <011 >, and <111>, which are oriented toward the tensile axis, are indicated by red, green, and blue, respectively, as shown in (c).

Fig. 4. EBSD IPF maps of a grain with <011> in a sample deformed up to 10% (a) and 30% (b). The Schmid factor for deformation twinning in this grain is relatively large as indicated by the red contours in (c), where the Schmid factor contour is also indicated by the green contour.

Fig.5. IPF of tensile direction in samples deformed by 0% (a), 10% (b), and 30% (c), in which the densities are indicated by RGB colors. With increasing strain, the tensile direction of the sample oriented toward <011>-<111>.

3.2 X-ray diffraction rings

XRD measurements are used to analyze the structure of crystalline materials irrespective of strain degree, whereas EBSD patterns of heavily strained areas are rarely used to identify the crystallographic orientation, as shown in Fig. 4 (b). Thereby, XRD measurements of 0% and 60% strained samples were performed to study the influence of heavy strain on the microstructure. Figure 6 shows 2D-XRD rings for the recrystallized Fe-Mn-C alloy. These rings are composed of discontinuous diffraction spots, implying that fcc grains in the polycrystalline Fe-Mn-C alloy recrystallized well or contain a lower dislocation density. On the other hand, 2D-XRD rings for Fe-Mn-C alloys deformed by 60% tensile deformation are shown in Figs. 7. The basic shape of the rings is similar to that of the recrystallized sample; however, the diffraction rings appear to be continuous. This suggests that many dislocations were introduced in the interior of grains by deformation, while the basic structure remained. It should be noted that the diffraction intensities of diffraction rings depend on the circular angle, indicating that the fiber texture formed during deformation. For example, the {220} diffraction intensities from the fcc matrix in the tensile direction decreased considerably with a 60% strain. This may correspond to twin formation accompanied by plastic deformation in grains of polycrystalline Fe-Mn-C alloy by tensile deformation, from which the texture formed in polycrystalline grains following external stress.

Information about the fibrous texture of the fcc structure, including twin deformation, can be obtained from angles denoted in the diffraction rings shown in Fig. 8. Tables 1 and 2 summarize the angles a1, a2, p1, and p2, at which the diffraction intensities decrease in the 10% and 60% deformed Fe-Mn-C alloys, respectively. The fiber axis was estimated from these values as shown in Fig. 9, which is a stereographic projection with a shear direction of [-211] and a shear plane of (111). The fiber orientation of the 10% strained sample (gray circle) and 60% strained sample (black circle) was plotted from decreases in the diffraction intensities. The grain with <144> parallel to the tensile axis has the largest Schmid factor for twinning, which is close to the fiber axis estimated above. These results imply that the fiber axis or the contribution of twinning and dislocation slip to deformation changes with strain. Figure 10

shows the fiber orientation of fcc grains estimated from increases in the diffraction intensities in the 60% strained sample (black circle). It is displayed in a stereographic projection with a shear direction of [2-1-1] and a shear plane of (111) in twin, whose atomic arrangements are shown in Fig. 2. Although these results indicate that the diffraction intensities close to <001> increased by deformation twinning, it is noted that the diffraction intensities may be influenced by other fiber components. Thus, the present XRD results are consistent with the EBSD results that the fiber orientation of fcc grains is formed by tensile deformation, as shown in Fig. 5, although further detailed texture analysis is required.

/ wn//,' A ;

fee] 2001' Veejay;/

Fig. 7. 2D-XRD ring for Fe-Mn-C alloy deformed by 60% by tensile test at room temperature.

Fig. 8. A quarter of 2D-XRD ring for 60% strained Fe-Mn-C alloy. The angles a\, 02, pi, and p2, for 10% and 60% deformed aloys are summarized in Tables 1 and 2.

ÎV12I /

1 \ 121

/M21 J

111 120 131

111 102 113

Fig. 9. Stereographic projection showing fiber orientation of overall fcc grains estimated from decrease in diffraction intensities for 10% strained sample (gray circle) and 60% strained sample (black circle). Shear direction and plane are shown in [-211] and (111) in the fcc matrix, respectively. The angles listed in Table 1 were used to plot the projection. <144> orientation has the largest Schmid factor for fcc twinning, as shown in Fig. 2.

Fig. 10. Stereographic projection showing fiber orientation of fcc grains estimated from increase in diffraction intensities in 60% strained sample (black circle). Shear direction and plane for twinning are [2-1-1] and (111), respectively. The projection is displayed in the twin side, as shown in Fig. 2.

Table 1. Values of 6, ai, 0.2, pi, and p2 obtained from 2D-XRD rings for 10% deformed Fe-Mn-C alloy.

Reflection 0 (°) «1 (°) P1 (°) «2 (°) P2 (°)

111 5.7 84-90 84-90 10-11 11-12

200 6.5 42-46 42-46 0-4 6-7

220 9.3 0-12 6-14 25-35 25-35

Table 2. Values of 6, a1, a2, p1, and p2 obtained from 2D-XRD rings for 60% deformed Fe-Mn-C alloy.

Reflection e (°) «1 (°) P1 (°) «2 (°) P2 (°)

111 5.7 24-45 25-46 0-7 6-9

200 6.5 27-45 28-45 0-8 6-10

220 9.3 0-21 6-22 32-45 33-45

4. Concluding remarks

EBSD and X-ray diffraction rings were measured to characterize the evolution of twin deformation and accompanied deformation in fcc Fe-Mn-C alloys by tensile test. The EBSD results showed that deformation twins formed in grains with large Schmid factors for twins parallel to the tensile direction. In addition, plastic strain by dislocation slip was observed in the microstructure. These results indicate that the alloys are deformed in different deformation modes, depending on the grain orientation and the resolved shear stress for each deformation mode. In addition, the formation of deformation twins seems to cause refinements in the microstructure during deformation. Thus, a combination of twinning and dislocation slip in the alloys may be attributed to large overall elongation. The X-ray diffraction rings also showed that twinning and dislocation slip occur during tensile deformation, and a strong texture also formed during the tensile test. These results indicate that both twinning and dislocation slip, which depend on crystallographic orientation and tensile strain, play important roles in large plastic deformation.

Acknowledgements

The authors would like to express their gratitude to Dr. M. Sato for valuable discussions. They would also like to thank the Japan Synchrotron Radiation Research Institute (JASRI) for their assistance with the XRD measurements at SPring-8. This work was supported by Global COE Program "Materials Integration (International Center of Education and Research), Tohoku University" of MEXT, Japan.

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