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Microstructure evolution and deformation behaviors of E-form and AZ31 Mg alloys during ex-situ mini-V-bending tests
Journal of Alloys and Compounds 778 (2019) 124e133
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure evolution and deformation behaviors of E-form and AZ31 Mg alloys during ex-situ mini-V-bending tests Jaiveer Singh a, Min-Seong Kim a, Ji-Hyun Lee b, Hwanuk Guim b, Shi-Hoon Choi a, * a b
Department of Printed Electronics Engineering, Sunchon National University, 255 Jungang-ro, Suncheon, Jeonnam, 57922, Republic of Korea Electron Microscopy Research Center, Korea Basic Science Institute, Daejeon 34133, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2018 Received in revised form 1 November 2018 Accepted 11 November 2018 Available online 13 November 2018
Microstructure evolution in E-form and AZ31 magnesium (Mg) alloys was studied via ex-situ mini-Vbending tests. Initially, the E-form and AZ31 Mg alloys had different crystallographic textures and average grain-size distributions. Direct observation of microstructural evolution during the ex-situ miniV-bending tests was experimentally observed via electron back-scatter diffraction (EBSD) technique. The EBSD results revealed how twin bands (TBs) developed at different punch strokes (PSs) in the deformed grains made a significant contribution to the localized deformation zones in both Mg alloys under the mini-V-bending process. Eventually, the TBs and grain boundaries (GBs) in the localized deformation zones were responsible for crack initiation sites in the tension region. At lower PSs, compression (CTW) and double (DTW) twins were more prominent in E-form than in AZ31 under the mini-V-bending process. High-resolution cross-sectional t-EBSD analysis showed that surface relief allowed the grains residing on the free surface to be less affected by stress concentration while the sub-surface grains were more affected by stress concentration, which promoted the development of twinning. © 2018 Elsevier B.V. All rights reserved.
Keywords: Mg alloys V-bending Twinning Plastic deformation EBSD
1. Introduction
deformations [8,9]. The activation of basal slip and f1012g tension twin (TTW) systems is insufficient to accommodate the large plastic
Magnesium (Mg) alloys are the lightest structural alloys available, and this property is used by the automobile/aircraft industry to replace denser materials such as steels, cast irons, copper alloys, and even aluminum alloys [1,2]. Mg has a hexagonal close-packed (hcp) crystal structure that affects the basic properties and deformation behavior [3,4]. The low symmetry crystal structure of hcp Mg has a limited number of active slip systems at room temperature (RT), which results in the poor ductility. Besides slip systems, active twin systems also play an important role in the plastic deformation behavior in Mg alloys. However, Mg alloys have several advantages that include excellent castability and high specific strength. Also, the addition of the appropriate alloying elements can also significantly improve the corrosion behaviors of Mg alloys [5e7]. The bending test is a typical forming method that is used to evaluate the formability of sheet metals [2,8]. However, Mg alloys exhibit poor bendability due to an insufficient number of active slip and twin systems at RT which are required to accommodate plastic
strains at RT. Therefore, the activation of non-basal slips and f1011g compression twin (CTW) systems are mandatory to accommodate the large deformations in order to improve the RT formability [3]. Methods have been developed to further improve formability at RT, and these include texture modifications via thermomechanical processing [10], pre-twinning [11e13], the addition of rare earth (RE) elements [14e17], and equal channel angular extrusion (ECAE) [18]. The plastic deformation behavior of Mg alloys during the bending process is significantly more complicated than the uniaxial loading. Recent studies have focused mainly on explaining the roles of deformation twinning mechanisms [5,19,20] and second-phase particles [21e23] affecting the fracture behavior of Mg alloys. A few experimental studies have reported the roles that slip/twin activities play in localized deformation and in crack regions [24e26]. Moreover, much effort has been focused on understand-
* Corresponding author. E-mail address: [email protected] (S.-H. Choi). https://doi.org/10.1016/j.jallcom.2018.11.138 0925-8388/© 2018 Elsevier B.V. All rights reserved.
ing the activation of CTW and f1011g f1012g double twins (DTW) in Mg alloys due to c-axis compression under bending [2,8,9,27e30]. The results of previous studies [19,20] have documented clear evidence of twin-sized microcracks parallel to the DTW. The activation of DTW plays a significant role in the crack
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initiation sites and in the propagation of cracks that occur along the TTW's boundaries [20]. The formation of micro-voids at the crack initiation sites could be attributed to microcracks in localized deformation regions. Kondori and Benzerga [21,31] described how plastic anisotropy in AZ31 alloys affects microcrack formation due to micro-void coalescence under triaxial loading. Furthermore, the second-phase particles at the grain boundaries, deformation twinning, and texture are also known to play important roles in the fracture behavior of RE-containing Mg alloys [31]. Three-point bending tests with in-situ EBSD have been performed for Mg alloys [8,27]. Jin et al. [8] focused on the deformation and fracture behaviors in the thickness direction of a specimen and showed that macro-scale strain gradients from tension to compression regions induce the heterogeneous evolution of both microstructure and texture under the bending process. Baird et al. [27] emphasized the generation of localized deformation bands in the thickness direction under three-point bending and discussed the possible mechanisms for such localized twin bands (TBs). The probable sites for crack initiation under the V-bending test are either in the normal direction (ND) plane or on the bent surface of a deformed specimen rather than in the sub-surface regions. Therefore, observation of the deformation and fracture behaviors on the surface of a deformed specimen is more effective than observation throughout the thickness direction. It is difficult to measure the direct observations of microstructure evolution, however, due to the occurrence of deformation twins and crack initiation sites on the surface of deformed specimens that is caused by conventional V-bending processes. Hence, in this work, an ex-situ mini-V-bending test using EBSD was focused on studying the effects of the initial microstructure and texture in order to explain the deformation and fracture behaviors on the surface where the cracks actually begin during mini-Vbending. Most of the previous in-situ/ex-situ research works have involved the study of microstructural changes under uniaxial tension or compression [32e35]. However, Yang et al. [34] showed that the strain path along with strong initial basal texture and grain size are factors that can significantly affect the amount of deformation twinning. In the present study, the microstructural evolution in E-form and AZ31 Mg alloys was directly observed for different PSs during ex-situ mini-V-bending. In addition, the objective of this work was to understand the effects that initial texture and deformation twinning can exert on failure behaviors. The deformation mechanisms due to microcracks and surface relief during mini-V-bending tests were also identified. The EBSD technique was effectively used to investigate the twin morphologies and localized deformation zones. High-resolution cross-section t-EBSD analysis was performed to investigate the role of stress concentration on grains residing at the free and sub-surface levels. Throughout this study, ex-situ mini-V-bending was an effective testing method for not only evaluating RT formability, but also for developing new Mg alloys with a desirable combination of mechanical properties and RT formability.
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TD) 1.2 mm (ND). The cut specimens were bent perpendicular to the RD at a speed of 20 mm/s in a mini-V-bending jig [28,30]. 2.2. Microstructure and crystallographic texture measurements The microstructural measurements of specimens deformed by ex-situ mini-V-bending were observed using the EBSD system. The specimens were polished via a standard metallography polishing procedure using abrasive papers (#800, #1200, and #2400) followed by ethanol-based 3 mm and 1 mm diamond suspensions. A 0.04 mm colloidal silica suspension (OPS) was used for the final polishing. X-ray pole figure measurements were made on a Rigaku D Max 2500 X-ray diffractometer. The ð0002Þ, ð1010Þ, ð1011Þ, ð1012Þ and ð1120Þ incomplete pole figures (maximum tilt angle: 70 ) were measured by the Schulz reflection method. The Berkeley Texture Package BEARTEX software was used for the pole figure calculations and plotting. The ex-situ EBSD maps were measured in the plane normal to the ND via JEOL (JSM-7100F) field emission scanning electron microscope (FE-SEM) equipped with TSL data acquisition software. The central region of the ND plane was examined at different punch strokes (PSs) under the ex-situ mini-V-bending test by scanning an area of 100 mm 100 mm at a step size of 0.25 mm. The cross-section EBSD specimens of E-form and AZ31 Mg alloys were measured for sub-surface deformation twinning behavior on a FE-SEM JEOL7001F system equipped with an Oxford NordlysNano (sensitive camera) and 5 forescatter detectors (FSD) by scanning the area of 100 mm 100 mm at a step size of 0.1 mm. Furthermore, to explain the deformation mechanism occurring in non-indexed regions, a high-resolution analysis technique, transmission EBSD (t-EBSD) measurement, was performed using Carl Zeiss MERLIN (FE-SEM) equipped with a transmission kikuchi diffraction (TKD) mode in a Bruker QUANTAX EBSD system at a step size of 20 nm. The t-EBSD specimens were fabricated via a focused ion beam (FIB) technique using a FEI Helios NanoLab™ 600 DualBeam (FIB/SEM) system. For EBSD analysis, data with a confidence index (CI) of more than 0.1 were used. 2.3. Surface roughness measurements
2. Experimental methods
A 3D confocal laser scanning microscope (CLSM) (VK-X100) was used to measure the surface relief/roughness along with 3D height images on the ND plane of as-polished and deformed specimens at different PSs during mini-V-bending testing of E-form and AZ31 Mg alloys. The 3D height images of the deformed specimens were captured at 100 magnification. The average surface roughness ðRa Þ was quantified using 3D height images by selecting three random areas (100 mm 100 mm), which covered an area that totaled 534 mm 712 mm for the measurements. More details about the measurement procedure are provided elsewhere [36]. The height profiles were obtained by drawing a measurement line on the height images along the RD. Multiple equidistant measurement lines were drawn to eliminate the variations and to obtain the actual height profile.
2.1. Materials and mini-V-bending test specimens
3. Results and discussion
In the present study, we used hot-rolled E-form® (Mg-Al based Easy-formable alloy) and AZ31 Mg alloys in sheets with an initial thickness of 1.2 mm. These Mg alloy sheets were supplied by POSCO Mg Inc. and fabricated via twin roll strip casting technology. The Eform had a relatively weaker basal texture compared with AZ31. The dimensions of the specimens for the mini-V-bending tests were 10 mm (rolling direction, RD) 6 mm (transverse direction,
3.1. Crystallographic texture and ex-situ mini-V-bending test Fig. 1(a) and (b) show the ð0002Þ basal pole figures (PFs) measured by X-ray diffraction on the ND plane of as-rolled sheets of E-form and AZ31 Mg alloys, respectively. Fig. 1(c) shows the schematic for the experimental procedure of the ex-situ mini-V-bending test and the insets showing the mini-V-bending jig and FE-SEM
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Fig. 1. The (0002) basal pole figures measured by X-ray diffraction in the ND plane of as-received rolled sheets of (a) E-form, (b) AZ31 Mg alloys. (c) Schematic for the experimental procedure of the ex-situ mini-V-bending test and the insets showing the mini-V-bending jig and FE-SEM (field emission scanning electron microscope) images for different PSs.
images at different PSs. The differences in the initial crystallographic textures between E-form and AZ31 Mg alloys can be observed in Fig. 1(a) and (b). AZ31 shows the typical strong basal texture that represents the c-axis of grains aligned mostly parallel to the ND plane. However, the E-form showed either a weaker or an off-basal texture and exhibited a relatively low basal pole intensity compared with that of the AZ31. The values for the maximum intensity of the E-form and AZ31 Mg alloys were 3.91 and 6.23, respectively. 3.2. Microstructure evolution via ex-situ mini-V-bending test We conducted an ex-situ mini-V-bending test to directly observe the microstructural evolution under different PSs. Fig. 2(a) and (b) show the FE-SEM images observed on the ND plane of as-received and deformed specimens at different PSs of E-form and AZ31 Mg alloys, respectively. The probable sites for crack initiation are marked with white circles. It should be noted that as PS increased, surface relief became more distinct and microcracks began to appear in some areas on the surface of the deformed specimens.
The as-received E-form Mg alloy showed a random distribution of Mg-Al base precipitates in the matrix as shown in Fig. 2(a). Several experimental studies showed that precipitates had a strong effect on the formation of deformation twinning in Mg alloys [37e40]. However, the effect is expected to vary depending on the precipitate type, size, and morphology. In general, the precipitates present in the matrix attribute to a large number of smaller deformation twins [38,39]. Ghaghouri et al. [39], however, found that if twins are much thinner than the precipitates, twin growth can be arrested by large precipitates. Therefore, the precipitates present in the E-form could partially contribute to the evolution of deformation twins during mini-V-bending. This phenomenon will be further studied in detail to determine the role of Mg-Al based precipitates in contributing to deformation and fracture behaviors during mini-Vbending. Specimens deformed at lower PSs had a relatively smaller degree of surface relief compared with specimens deformed at higher PSs. The corresponding ND-IPFs of E-form and AZ31 Mg alloys deformed at different PSs are shown in Fig. 3(a) and (b), respectively, which show the evolution of the microstructures under ex-
Fig. 2. FE-SEM surface images measured on the surface (ND plane) of as-received and deformed specimens at different PSs under the ex-situ mini-V-bending test of (a) E-form and (b) AZ31 Mg alloys. White circles are marked on the probable crack initiation sites under the ex-situ mini-V-bending test.
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Fig. 3. EBSD (electron-backscatter diffraction) IPF (inverse pole figure) maps of (a) E-form and (b) AZ31 Mg alloys at different PSs under the ex-situ mini-V-bending test. Black and white circles are marked on the probable sites for the twin nucleation and crack initiation, respectively, under the ex-situ mini-V-bending test.
situ mini-V-bending testing. The average grain sizes in as-received E-form and AZ31 Mg alloys were 14.5 and 8.12 mm, respectively. In Fig. 3, the TBs and microcracks were highlighted with black and white circles, respectively. The limited strain accommodation due to dislocation slip along the c-axis was expected to promote deformation twinning during mini-V-bending, which would result in the formation of TBs to maintain the strain compatibility between the neighboring grains. However, it should be noted that localized deformation zones must be generated to accommodate an external strain with a high magnitude while TBs can accommodate the low-magnitude external strains [20,41]. The strain accumulation in localized deformation zones enhances the stress concentration and the resultant formation of microcracks. However, the evolution of the twin nucleation in the grains of E-form (G5 and G8) and AZ31 (G1 and G2) Mg alloys at different PSs showed that TTWs further grew following the twin nucleation and tended to accommodate the in-grain deformation without increasing deformation in the matrix. The low CI (<0.1) or non-indexed regions shown in the IPF maps was due to either microcracks or surface relief. However, significant surface relief was observed in the grain boundaries (GBs) and TBs regions after PSs of 0.70 and 0.875 mm, respectively, which also contributed to non-indexed regions. The color-coded Schmid factor (SF) maps were plotted for basal D E hai (f0001g 1120 ) slip of the undeformed E-form and AZ31 Mg alloys shown in Fig. 4(a). To obtain the SF maps, the stress states for mini-V-bending using the stress tensors given in Ref. [30] were applied. The SF maps clearly show the higher average SF for the basal hai slip in E-form Mg alloy compared with that of AZ31 Mg alloy. This result could be attributed to the higher ductility and deformation twinning in E-form, which was influenced by the initial texture. The corresponding IQ (image quality) maps of undeformed and deformed specimens at PSs of 0.35 and 0.525 mm are shown in Fig. 4(b)e(d), respectively for the E-form and AZ31 Mg alloys. The TBs and high-angle grain boundaries (HAGBs) are plotted on the IQ maps. It is very difficult to identify the twin boundaries of all thin CTWs and DTWs because of the high portion of localized deformation regions generated by higher PSs. The grains residing on the free surface of the deformed specimens at
higher PSs were subjected to heterogeneous crystal rotation due to dislocation slip and twinning, which resulted in the formation of surface relief. Therefore, the stress state near the specimen surfaces under mini-V-bending can be evolved to a relatively low level compared with that in the bulk state, which is considered to be a result of the insufficient generation of CTWs and DTWs with relatively high CRSSs. A detailed description of this phenomenon is given in the later part of this paper in the analysis of the crosssection near the free surface. The TBs of TTWs were present in both Mg alloys while CTWs and DTWs were rarely observed. However, while plotting the twin boundaries, we considered D D E E f1012g TTW (86 1210 ±5 ), f1011g CTW (56 1210 ±5 ) and all six f1011g f1012g DTW variants as given in Ref. [42]. Fig. 5(a) shows the average for the kernel average misorientation (KAM) evolution at different PSs during the ex-situ mini-Vbending test. The KAM quantifies the average misorientation around a measurement point or a kernel and all its surrounding or nearest-neighbor points. In this study, we used the 3rd nearest neighbor to define the measuring point inside a grain and misorientations critical value of 5 . The results indicated that the AZ31 Mg alloy had a relatively low average KAM compared with that of the E-form Mg alloy. However, this difference can be attributed to a high SF for the basal a slip in the E-form Mg alloy, which resulted in relatively higher average KAM values. Fig. 5(b) represents the twin boundary lengths of TTWs and CTWs measured at different PSs during the ex-situ mini-V-bending test. The results clearly showed a large number of TTWs in the E-form compared with that of the AZ31. Fig. 5(b) also indicates that TTWs were influenced by the grain size distributions as well as by the initial textures. After neglecting the annealing twin boundaries in the as-received microstructures, the lengths of the twin boundaries were highest at a PS of 0.35 mm in both Mg alloys. However, the E-form specimen with a relatively large grain size and a weaker initial basal texture had a higher twin boundary length at different PSs than the AZ31 Mg alloy specimen. It should be noted that a significant level of surface relief and localized deformation regions contributed to the difficulty in measuring the exact twin boundary at higher PSs. As a result, we assumed that after a PS of 0.35 mm the total twin
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Fig. 4. (a) Schmid factor (SF) maps correlating the basal hai slip for undeformed E-form and AZ31 Mg alloys. IQ (image quality) maps showing the evolution of twin boundaries in Eform and AZ31 Mg alloys at different PSs under mini-V-bending: (b) undeformed, (c) 0.35 mm, and (d) 0.525 mm.
Fig. 5. (a) Development of average kernel average misorientation (KAM) and (b) twin boundary lengths at different PSs under the ex-situ mini-V-bending test. (c) Punch force vs punch stroke and (d) slope of the punch force corresponding to each of the PS curves for E-form and AZ31 Mg alloys under the mini-V-bending test at a punch speed of 20 mm/s.
boundary length was underestimated. Fig. 5(c) shows the load versus PS curves measured with a punch speed of 20 mm/s via mini-
V-bending set-up for E-form and AZ31 Mg alloys. The E-form showed better bendability than the AZ31 in terms of maximum PS,
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while the strain-hardening rate was slightly higher in AZ31 than in the E-form. However, these results seemed attributable to the initial crystallographic texture [8]. The slopes of the load versus punch stroke curves were plotted to determine the level of PS at which microcracks occurred simultaneously. Fig. 5(d) shows the slopes of the punch forces corresponding to the PSs. As the PSs increased, the slope decreased rapidly beyond the peak of the slope, and the slope was not changed significantly. The region in which a sudden change in the gradient occurred, ignoring the slight gradient change, is enlarged in Fig. 5(d). A change in slopes at a PS of about 0.6 mm was observed in both E-form and AZ31 Mg alloys. E-form and AZ31 Mg alloys showed a tendency to decrease nonsteadily and steadily with increasing PSs after the sudden change in the slope, respectively. However, the change in slopes can be influenced by microstructural heterogeneity, which could contribute to the mechanisms of microcrack initiation. Therefore, the specimen for further cross-section analysis was determined by assuming that microcracks began to occur simultaneously in many areas at a PS of 0.6 mm. 3.3. Surface roughness analysis Further, 3D confocal laser scanning microscope height images were used to measure the evolution of surface relief at different PSs. Fig. 6(a) and (b) show the 3D height images of as-received and deformed specimens after a mini-V-bending test at different PSs of E-form and AZ31 Mg alloys, respectively. Deformation twins can induce surface relief via either nano-indentation [43] or uniaxial tension [41]. Therefore, surface relief seemed to increase due to the occurrence of deformation twinning and dislocation slip. Fig. 7(a) and (b) show the line profiles of surface relief along the RD for Eform and AZ31 Mg alloys, respectively, while Fig. 7(c) shows quantitative comparisons of the average surface roughness ðRa Þ of as-received and deformed specimens at different PSs via mini-Vbending. The surface relief of the E-form was higher than that of the AZ31, which could be due to a comparatively larger grain size and higher activity of deformation twins, as shown in Figs. 3 and 5(b). 3.4. Cross-section t-EBSD analysis The additional cross-sectional EBSD analysis was carried out to
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explain the exact deformation mechanism of Mg alloys during the mini-V-bending test. Fig. 8(a) and (b) show the high-resolution NDIPF and IQ maps measured at a PS of 0.60 mm (as determined in Fig. 5(d)) with a step size of 0.1 mm for E-form and AZ31 Mg alloys, respectively. The cross-sectional EBSD analysis position was 50 mm below the tension region. The specimens were mounted in a jig and prepared via standard polishing technique to examine the deformed microstructure. The TBs in the localized deformation zones could be responsible for the crack initiation in the tension region. The circles were marked on the IPF maps to highlight the different types of TBs such as TTWs, CTWs, and DTWs. It should be noted that TTWs and CTWs were observed in both Mg alloys. However, AZ31 Mg alloy had a relatively very low fraction of DTW's compared with that of E-form Mg alloy and it seemed that a PS of 0.60 mm was insufficient to activate the DTW's in the AZ31 Mg alloy. In order to investigate the deformation mechanism occurring in regions that were either non-indexed or had a low CI due to microcracks or surface relief, FIB milling technique was used to fabricate the cross-section specimens for a high-resolution t-EBSD analysis around the microcracks or surface relief of deformed specimens. Fig. 9(a) and (b) show FE-SEM images for E-form and AZ31 Mg alloys, respectively. The FE-SEM images include deformed surface morphologies at a PS of 0.875 mm, FIB milling areas of interest for TD1 specimen, and Pt protective layer coating deposition onto the cross-sections of t-EBSD specimens prepared by FIB. Fig. 10 shows the results of t-EBSD analysis measured on the cross-section specimens viz. TD1, TD2 and RD1 prepared by FIB milling. Here, TD1, TD2 and RD1 represent the FIB specimens taken on the TD and RD planes, respectively. High-resolution ND-IPF and pattern quality maps showing the evolution of TBs at a PS of 0.875 mm are shown in Fig. 10(a) and (b) for E-form and AZ31 Mg alloys, respectively. It should be noted that the occurrence of cavity/void/defect formations during the fabrication of very thin specimens (~1 mm) via FIB milling technique was responsible for the non-indexed region marked with a yellow circle in the E-form Mg alloy specimen pattern quality map. Therefore, it is clear from the cross-sectional t-EBSD analysis that only microcracks were unrelated to the non-indexing regions on the ND plane, but surface relief did make a significant contribution. The strain accumulation and evolution of twins on the ND plane resulted in surface relief during the mini-V-bending test. The
Fig. 6. 3-D laser scanning microscope height images for the bent surface (ND plane) of (a) E-form and (b) AZ31 Mg alloy specimens at different PSs under the mini-V-bending test.
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Fig. 7. Line profiles along the rolling direction (RD) for (a) E-form and (b) AZ31 Mg alloy specimens at different PSs. (c) Average surface relief on the bent surface (ND plane) of Eform and AZ31 Mg alloys under the mini-V-bending test.
Fig. 8. High-resolution ND-IPF and IQ maps for the specimens deformed at a PS of 0.60 mm (a) E-form and (b) AZ31 Mg alloys. EBSD was measured near the tension region and at a step size of 0.1 mm.
extent of surface relief might be correlated with the average grain size distributions and the crystallographic texture. The evolution of the TBs shows there is a considerable difference between the ND (Fig. 3) and the cross sections (TD and RD planes) (Figs. 8 and 10). The main deformation mechanisms in the E-form were shear localization by dislocation slip along with all types of twins while
CTW and DTW were the main deformation mechanisms in the AZ31. It should be noted that the activation of DTW in AZ31 was observed at a significantly higher PS of 0.875 mm. Therefore, it can be concluded that the activation of TTW in the E-form at the higher levels of deformation contributed to the relatively high formability at RT.
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Fig. 9. FE-SEM images showing the deformed surface morphology of bent specimens after PS of 0.875 mm during the ex-situ mini-V-bending test. The FIB milling area of interest for TD1 specimen is marked for the t-EBSD measurement. FE-SEM images showing Pt protective layer coating deposited onto the area of interest and cross-sectional t-EBSD specimen prepared by FIB for (a) E-form and (b) AZ31 Mg alloys.
Fig. 10. ND-IPF and pattern quality maps showing the evolution of TBs in FIB-prepared cross-section specimens viz. TD1, TD2 and RD1 following PS of 0.875 mm under the ex-situ mini-V-bending test for (a) E-form and (b) AZ31 Mg alloys.
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Fig. 11. Schematics of TBs in the as-received specimens and evolution of TBs in the E-form and AZ31 Mg alloys deformed at PSs of 0.35, 0.525 and 0.70 mm during the ex-situ miniV-bending test.
Fig. 11 shows the schematics of the evolution of TBs in the asreceived and deformed specimens at PSs of 0.35, 0.525, and 0.70 mm in E-form and AZ31 Mg alloys under ex-situ mini-Vbending testing. The schematics of the TBs evolutions were based on the observations from the IPF maps shown in Figs. 3 and 8. In the EBSD analysis, however, the TBs seemed to correspond to the localized deformation zones as favorable sites for crack initiation. This result promoted an understanding of the crack initiation mechanisms in the tension region or on the bent surface during mini-V-bending. At a PS of 0.35 mm, the TBs of TTW were observed in both Mg alloys, and the TBs of CTW appeared in the E-form Mg alloy. At PSs of 0.525 and 0.70 mm, the TBs of TTW and CTW were observed in both Mg alloys while the TBs of DTW were present only in the E-form Mg alloy at higher PSs. Furthermore, Fig. 12 shows a schematic illustration of the deformation mechanism and the evolution of the TBs based on the ND-IPF and pattern quality maps of the cross-sectional t-EBSD, as shown in Fig. 10. A schematic of the as-received specimen indicates that grains residing on the free surface of the as-received specimen were relatively flat. However, the schematic of the deformed specimens indicated that the free surfaces of the deformed specimens at higher PSs were subjected to heterogeneous crystal rotation due to dislocation slip and twinning, which resulted in the formation of surface relief. As a result, as the PSs increased, the grains residing on the free surface became less prone to stress concentration due to the surface relief, while the grains existing on the sub-surface remained relatively open to stress concentration, which is considered to be advantageous for the development of the twins. Such a trend is shown in Fig. 10, as well. This tendency demonstrates why the density of the twins
observed on the free surface during the ex-situ mini-V-bending test was underestimated, as shown in Figs. 3 and 5. Differences in the deformation twinning behavior resulted in different deformations and failure mechanisms for E-form and AZ31 Mg alloys under mini-V-bending. Moreover, with higher levels of PSs, these TBs were easily converted to a network of the twins, which resulted in a cluster of localized deformation regions that led to the crack initiation. Therefore, the strain accumulation at twin boundaries resulted in failures under larger levels of strain during mini-V-bending. 4. Conclusions The microstructure evolution in E-form and AZ31 Mg alloys was directly observed under ex-situ mini-V-bending. The following conclusions were obtained. 1) The effects of the initial crystallographic texture and average grain size distribution that affect the deformation behaviors and bendability were studied via EBSD analysis, which revealed how TBs that developed in the deformed grains contributed significantly to the localized deformation zones in Mg alloys under mini-V-bending. 2) The TBs in the localized deformation zones were responsible for the crack-initiation sites in the tension region. CTWs and DTWs were more prominent in the E-form Mg alloy compared with that seen in the AZ31 Mg alloy under mini-V-bending. 3) The grains residing on the free surface were less prone to stress concentration due to significantly higher surface relief and
Fig. 12. The schematic illustration of the deformation mechanism and the TBs evolution observed during the cross-sectional t-EBSD measurement as shown in Fig. 10.
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localized deformation regions, which was attributed to the difficulty in the measurement of the exact twin boundaries as PSs increased under the mini-V-bending process. As a result, the density of twins observed on the free surface during the ex-situ mini-V-bending test was probably underestimated. Acknowledgements This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (NRF2016M3C1B5906955) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A6A1030419).
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Microstructure evolution and deformation behaviors of E-form and AZ31 Mg alloys during ex-situ mini-V-bending tests Jaiveer Singh a, Min-Seong Kim a, Ji-Hyun Lee b, Hwanuk Guim b, Shi-Hoon Choi a, * a b
Department of Printed Electronics Engineering, Sunchon National University, 255 Jungang-ro, Suncheon, Jeonnam, 57922, Republic of Korea Electron Microscopy Research Center, Korea Basic Science Institute, Daejeon 34133, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 August 2018 Received in revised form 1 November 2018 Accepted 11 November 2018 Available online 13 November 2018
Microstructure evolution in E-form and AZ31 magnesium (Mg) alloys was studied via ex-situ mini-Vbending tests. Initially, the E-form and AZ31 Mg alloys had different crystallographic textures and average grain-size distributions. Direct observation of microstructural evolution during the ex-situ miniV-bending tests was experimentally observed via electron back-scatter diffraction (EBSD) technique. The EBSD results revealed how twin bands (TBs) developed at different punch strokes (PSs) in the deformed grains made a significant contribution to the localized deformation zones in both Mg alloys under the mini-V-bending process. Eventually, the TBs and grain boundaries (GBs) in the localized deformation zones were responsible for crack initiation sites in the tension region. At lower PSs, compression (CTW) and double (DTW) twins were more prominent in E-form than in AZ31 under the mini-V-bending process. High-resolution cross-sectional t-EBSD analysis showed that surface relief allowed the grains residing on the free surface to be less affected by stress concentration while the sub-surface grains were more affected by stress concentration, which promoted the development of twinning. © 2018 Elsevier B.V. All rights reserved.
Keywords: Mg alloys V-bending Twinning Plastic deformation EBSD
1. Introduction
deformations [8,9]. The activation of basal slip and f1012g tension twin (TTW) systems is insufficient to accommodate the large plastic
Magnesium (Mg) alloys are the lightest structural alloys available, and this property is used by the automobile/aircraft industry to replace denser materials such as steels, cast irons, copper alloys, and even aluminum alloys [1,2]. Mg has a hexagonal close-packed (hcp) crystal structure that affects the basic properties and deformation behavior [3,4]. The low symmetry crystal structure of hcp Mg has a limited number of active slip systems at room temperature (RT), which results in the poor ductility. Besides slip systems, active twin systems also play an important role in the plastic deformation behavior in Mg alloys. However, Mg alloys have several advantages that include excellent castability and high specific strength. Also, the addition of the appropriate alloying elements can also significantly improve the corrosion behaviors of Mg alloys [5e7]. The bending test is a typical forming method that is used to evaluate the formability of sheet metals [2,8]. However, Mg alloys exhibit poor bendability due to an insufficient number of active slip and twin systems at RT which are required to accommodate plastic
strains at RT. Therefore, the activation of non-basal slips and f1011g compression twin (CTW) systems are mandatory to accommodate the large deformations in order to improve the RT formability [3]. Methods have been developed to further improve formability at RT, and these include texture modifications via thermomechanical processing [10], pre-twinning [11e13], the addition of rare earth (RE) elements [14e17], and equal channel angular extrusion (ECAE) [18]. The plastic deformation behavior of Mg alloys during the bending process is significantly more complicated than the uniaxial loading. Recent studies have focused mainly on explaining the roles of deformation twinning mechanisms [5,19,20] and second-phase particles [21e23] affecting the fracture behavior of Mg alloys. A few experimental studies have reported the roles that slip/twin activities play in localized deformation and in crack regions [24e26]. Moreover, much effort has been focused on understand-
* Corresponding author. E-mail address: [email protected] (S.-H. Choi). https://doi.org/10.1016/j.jallcom.2018.11.138 0925-8388/© 2018 Elsevier B.V. All rights reserved.
ing the activation of CTW and f1011g f1012g double twins (DTW) in Mg alloys due to c-axis compression under bending [2,8,9,27e30]. The results of previous studies [19,20] have documented clear evidence of twin-sized microcracks parallel to the DTW. The activation of DTW plays a significant role in the crack
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initiation sites and in the propagation of cracks that occur along the TTW's boundaries [20]. The formation of micro-voids at the crack initiation sites could be attributed to microcracks in localized deformation regions. Kondori and Benzerga [21,31] described how plastic anisotropy in AZ31 alloys affects microcrack formation due to micro-void coalescence under triaxial loading. Furthermore, the second-phase particles at the grain boundaries, deformation twinning, and texture are also known to play important roles in the fracture behavior of RE-containing Mg alloys [31]. Three-point bending tests with in-situ EBSD have been performed for Mg alloys [8,27]. Jin et al. [8] focused on the deformation and fracture behaviors in the thickness direction of a specimen and showed that macro-scale strain gradients from tension to compression regions induce the heterogeneous evolution of both microstructure and texture under the bending process. Baird et al. [27] emphasized the generation of localized deformation bands in the thickness direction under three-point bending and discussed the possible mechanisms for such localized twin bands (TBs). The probable sites for crack initiation under the V-bending test are either in the normal direction (ND) plane or on the bent surface of a deformed specimen rather than in the sub-surface regions. Therefore, observation of the deformation and fracture behaviors on the surface of a deformed specimen is more effective than observation throughout the thickness direction. It is difficult to measure the direct observations of microstructure evolution, however, due to the occurrence of deformation twins and crack initiation sites on the surface of deformed specimens that is caused by conventional V-bending processes. Hence, in this work, an ex-situ mini-V-bending test using EBSD was focused on studying the effects of the initial microstructure and texture in order to explain the deformation and fracture behaviors on the surface where the cracks actually begin during mini-Vbending. Most of the previous in-situ/ex-situ research works have involved the study of microstructural changes under uniaxial tension or compression [32e35]. However, Yang et al. [34] showed that the strain path along with strong initial basal texture and grain size are factors that can significantly affect the amount of deformation twinning. In the present study, the microstructural evolution in E-form and AZ31 Mg alloys was directly observed for different PSs during ex-situ mini-V-bending. In addition, the objective of this work was to understand the effects that initial texture and deformation twinning can exert on failure behaviors. The deformation mechanisms due to microcracks and surface relief during mini-V-bending tests were also identified. The EBSD technique was effectively used to investigate the twin morphologies and localized deformation zones. High-resolution cross-section t-EBSD analysis was performed to investigate the role of stress concentration on grains residing at the free and sub-surface levels. Throughout this study, ex-situ mini-V-bending was an effective testing method for not only evaluating RT formability, but also for developing new Mg alloys with a desirable combination of mechanical properties and RT formability.
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TD) 1.2 mm (ND). The cut specimens were bent perpendicular to the RD at a speed of 20 mm/s in a mini-V-bending jig [28,30]. 2.2. Microstructure and crystallographic texture measurements The microstructural measurements of specimens deformed by ex-situ mini-V-bending were observed using the EBSD system. The specimens were polished via a standard metallography polishing procedure using abrasive papers (#800, #1200, and #2400) followed by ethanol-based 3 mm and 1 mm diamond suspensions. A 0.04 mm colloidal silica suspension (OPS) was used for the final polishing. X-ray pole figure measurements were made on a Rigaku D Max 2500 X-ray diffractometer. The ð0002Þ, ð1010Þ, ð1011Þ, ð1012Þ and ð1120Þ incomplete pole figures (maximum tilt angle: 70 ) were measured by the Schulz reflection method. The Berkeley Texture Package BEARTEX software was used for the pole figure calculations and plotting. The ex-situ EBSD maps were measured in the plane normal to the ND via JEOL (JSM-7100F) field emission scanning electron microscope (FE-SEM) equipped with TSL data acquisition software. The central region of the ND plane was examined at different punch strokes (PSs) under the ex-situ mini-V-bending test by scanning an area of 100 mm 100 mm at a step size of 0.25 mm. The cross-section EBSD specimens of E-form and AZ31 Mg alloys were measured for sub-surface deformation twinning behavior on a FE-SEM JEOL7001F system equipped with an Oxford NordlysNano (sensitive camera) and 5 forescatter detectors (FSD) by scanning the area of 100 mm 100 mm at a step size of 0.1 mm. Furthermore, to explain the deformation mechanism occurring in non-indexed regions, a high-resolution analysis technique, transmission EBSD (t-EBSD) measurement, was performed using Carl Zeiss MERLIN (FE-SEM) equipped with a transmission kikuchi diffraction (TKD) mode in a Bruker QUANTAX EBSD system at a step size of 20 nm. The t-EBSD specimens were fabricated via a focused ion beam (FIB) technique using a FEI Helios NanoLab™ 600 DualBeam (FIB/SEM) system. For EBSD analysis, data with a confidence index (CI) of more than 0.1 were used. 2.3. Surface roughness measurements
2. Experimental methods
A 3D confocal laser scanning microscope (CLSM) (VK-X100) was used to measure the surface relief/roughness along with 3D height images on the ND plane of as-polished and deformed specimens at different PSs during mini-V-bending testing of E-form and AZ31 Mg alloys. The 3D height images of the deformed specimens were captured at 100 magnification. The average surface roughness ðRa Þ was quantified using 3D height images by selecting three random areas (100 mm 100 mm), which covered an area that totaled 534 mm 712 mm for the measurements. More details about the measurement procedure are provided elsewhere [36]. The height profiles were obtained by drawing a measurement line on the height images along the RD. Multiple equidistant measurement lines were drawn to eliminate the variations and to obtain the actual height profile.
2.1. Materials and mini-V-bending test specimens
3. Results and discussion
In the present study, we used hot-rolled E-form® (Mg-Al based Easy-formable alloy) and AZ31 Mg alloys in sheets with an initial thickness of 1.2 mm. These Mg alloy sheets were supplied by POSCO Mg Inc. and fabricated via twin roll strip casting technology. The Eform had a relatively weaker basal texture compared with AZ31. The dimensions of the specimens for the mini-V-bending tests were 10 mm (rolling direction, RD) 6 mm (transverse direction,
3.1. Crystallographic texture and ex-situ mini-V-bending test Fig. 1(a) and (b) show the ð0002Þ basal pole figures (PFs) measured by X-ray diffraction on the ND plane of as-rolled sheets of E-form and AZ31 Mg alloys, respectively. Fig. 1(c) shows the schematic for the experimental procedure of the ex-situ mini-V-bending test and the insets showing the mini-V-bending jig and FE-SEM
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Fig. 1. The (0002) basal pole figures measured by X-ray diffraction in the ND plane of as-received rolled sheets of (a) E-form, (b) AZ31 Mg alloys. (c) Schematic for the experimental procedure of the ex-situ mini-V-bending test and the insets showing the mini-V-bending jig and FE-SEM (field emission scanning electron microscope) images for different PSs.
images at different PSs. The differences in the initial crystallographic textures between E-form and AZ31 Mg alloys can be observed in Fig. 1(a) and (b). AZ31 shows the typical strong basal texture that represents the c-axis of grains aligned mostly parallel to the ND plane. However, the E-form showed either a weaker or an off-basal texture and exhibited a relatively low basal pole intensity compared with that of the AZ31. The values for the maximum intensity of the E-form and AZ31 Mg alloys were 3.91 and 6.23, respectively. 3.2. Microstructure evolution via ex-situ mini-V-bending test We conducted an ex-situ mini-V-bending test to directly observe the microstructural evolution under different PSs. Fig. 2(a) and (b) show the FE-SEM images observed on the ND plane of as-received and deformed specimens at different PSs of E-form and AZ31 Mg alloys, respectively. The probable sites for crack initiation are marked with white circles. It should be noted that as PS increased, surface relief became more distinct and microcracks began to appear in some areas on the surface of the deformed specimens.
The as-received E-form Mg alloy showed a random distribution of Mg-Al base precipitates in the matrix as shown in Fig. 2(a). Several experimental studies showed that precipitates had a strong effect on the formation of deformation twinning in Mg alloys [37e40]. However, the effect is expected to vary depending on the precipitate type, size, and morphology. In general, the precipitates present in the matrix attribute to a large number of smaller deformation twins [38,39]. Ghaghouri et al. [39], however, found that if twins are much thinner than the precipitates, twin growth can be arrested by large precipitates. Therefore, the precipitates present in the E-form could partially contribute to the evolution of deformation twins during mini-V-bending. This phenomenon will be further studied in detail to determine the role of Mg-Al based precipitates in contributing to deformation and fracture behaviors during mini-Vbending. Specimens deformed at lower PSs had a relatively smaller degree of surface relief compared with specimens deformed at higher PSs. The corresponding ND-IPFs of E-form and AZ31 Mg alloys deformed at different PSs are shown in Fig. 3(a) and (b), respectively, which show the evolution of the microstructures under ex-
Fig. 2. FE-SEM surface images measured on the surface (ND plane) of as-received and deformed specimens at different PSs under the ex-situ mini-V-bending test of (a) E-form and (b) AZ31 Mg alloys. White circles are marked on the probable crack initiation sites under the ex-situ mini-V-bending test.
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Fig. 3. EBSD (electron-backscatter diffraction) IPF (inverse pole figure) maps of (a) E-form and (b) AZ31 Mg alloys at different PSs under the ex-situ mini-V-bending test. Black and white circles are marked on the probable sites for the twin nucleation and crack initiation, respectively, under the ex-situ mini-V-bending test.
situ mini-V-bending testing. The average grain sizes in as-received E-form and AZ31 Mg alloys were 14.5 and 8.12 mm, respectively. In Fig. 3, the TBs and microcracks were highlighted with black and white circles, respectively. The limited strain accommodation due to dislocation slip along the c-axis was expected to promote deformation twinning during mini-V-bending, which would result in the formation of TBs to maintain the strain compatibility between the neighboring grains. However, it should be noted that localized deformation zones must be generated to accommodate an external strain with a high magnitude while TBs can accommodate the low-magnitude external strains [20,41]. The strain accumulation in localized deformation zones enhances the stress concentration and the resultant formation of microcracks. However, the evolution of the twin nucleation in the grains of E-form (G5 and G8) and AZ31 (G1 and G2) Mg alloys at different PSs showed that TTWs further grew following the twin nucleation and tended to accommodate the in-grain deformation without increasing deformation in the matrix. The low CI (<0.1) or non-indexed regions shown in the IPF maps was due to either microcracks or surface relief. However, significant surface relief was observed in the grain boundaries (GBs) and TBs regions after PSs of 0.70 and 0.875 mm, respectively, which also contributed to non-indexed regions. The color-coded Schmid factor (SF) maps were plotted for basal D E hai (f0001g 1120 ) slip of the undeformed E-form and AZ31 Mg alloys shown in Fig. 4(a). To obtain the SF maps, the stress states for mini-V-bending using the stress tensors given in Ref. [30] were applied. The SF maps clearly show the higher average SF for the basal hai slip in E-form Mg alloy compared with that of AZ31 Mg alloy. This result could be attributed to the higher ductility and deformation twinning in E-form, which was influenced by the initial texture. The corresponding IQ (image quality) maps of undeformed and deformed specimens at PSs of 0.35 and 0.525 mm are shown in Fig. 4(b)e(d), respectively for the E-form and AZ31 Mg alloys. The TBs and high-angle grain boundaries (HAGBs) are plotted on the IQ maps. It is very difficult to identify the twin boundaries of all thin CTWs and DTWs because of the high portion of localized deformation regions generated by higher PSs. The grains residing on the free surface of the deformed specimens at
higher PSs were subjected to heterogeneous crystal rotation due to dislocation slip and twinning, which resulted in the formation of surface relief. Therefore, the stress state near the specimen surfaces under mini-V-bending can be evolved to a relatively low level compared with that in the bulk state, which is considered to be a result of the insufficient generation of CTWs and DTWs with relatively high CRSSs. A detailed description of this phenomenon is given in the later part of this paper in the analysis of the crosssection near the free surface. The TBs of TTWs were present in both Mg alloys while CTWs and DTWs were rarely observed. However, while plotting the twin boundaries, we considered D D E E f1012g TTW (86 1210 ±5 ), f1011g CTW (56 1210 ±5 ) and all six f1011g f1012g DTW variants as given in Ref. [42]. Fig. 5(a) shows the average for the kernel average misorientation (KAM) evolution at different PSs during the ex-situ mini-Vbending test. The KAM quantifies the average misorientation around a measurement point or a kernel and all its surrounding or nearest-neighbor points. In this study, we used the 3rd nearest neighbor to define the measuring point inside a grain and misorientations critical value of 5 . The results indicated that the AZ31 Mg alloy had a relatively low average KAM compared with that of the E-form Mg alloy. However, this difference can be attributed to a high SF for the basal a slip in the E-form Mg alloy, which resulted in relatively higher average KAM values. Fig. 5(b) represents the twin boundary lengths of TTWs and CTWs measured at different PSs during the ex-situ mini-V-bending test. The results clearly showed a large number of TTWs in the E-form compared with that of the AZ31. Fig. 5(b) also indicates that TTWs were influenced by the grain size distributions as well as by the initial textures. After neglecting the annealing twin boundaries in the as-received microstructures, the lengths of the twin boundaries were highest at a PS of 0.35 mm in both Mg alloys. However, the E-form specimen with a relatively large grain size and a weaker initial basal texture had a higher twin boundary length at different PSs than the AZ31 Mg alloy specimen. It should be noted that a significant level of surface relief and localized deformation regions contributed to the difficulty in measuring the exact twin boundary at higher PSs. As a result, we assumed that after a PS of 0.35 mm the total twin
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Fig. 4. (a) Schmid factor (SF) maps correlating the basal hai slip for undeformed E-form and AZ31 Mg alloys. IQ (image quality) maps showing the evolution of twin boundaries in Eform and AZ31 Mg alloys at different PSs under mini-V-bending: (b) undeformed, (c) 0.35 mm, and (d) 0.525 mm.
Fig. 5. (a) Development of average kernel average misorientation (KAM) and (b) twin boundary lengths at different PSs under the ex-situ mini-V-bending test. (c) Punch force vs punch stroke and (d) slope of the punch force corresponding to each of the PS curves for E-form and AZ31 Mg alloys under the mini-V-bending test at a punch speed of 20 mm/s.
boundary length was underestimated. Fig. 5(c) shows the load versus PS curves measured with a punch speed of 20 mm/s via mini-
V-bending set-up for E-form and AZ31 Mg alloys. The E-form showed better bendability than the AZ31 in terms of maximum PS,
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while the strain-hardening rate was slightly higher in AZ31 than in the E-form. However, these results seemed attributable to the initial crystallographic texture [8]. The slopes of the load versus punch stroke curves were plotted to determine the level of PS at which microcracks occurred simultaneously. Fig. 5(d) shows the slopes of the punch forces corresponding to the PSs. As the PSs increased, the slope decreased rapidly beyond the peak of the slope, and the slope was not changed significantly. The region in which a sudden change in the gradient occurred, ignoring the slight gradient change, is enlarged in Fig. 5(d). A change in slopes at a PS of about 0.6 mm was observed in both E-form and AZ31 Mg alloys. E-form and AZ31 Mg alloys showed a tendency to decrease nonsteadily and steadily with increasing PSs after the sudden change in the slope, respectively. However, the change in slopes can be influenced by microstructural heterogeneity, which could contribute to the mechanisms of microcrack initiation. Therefore, the specimen for further cross-section analysis was determined by assuming that microcracks began to occur simultaneously in many areas at a PS of 0.6 mm. 3.3. Surface roughness analysis Further, 3D confocal laser scanning microscope height images were used to measure the evolution of surface relief at different PSs. Fig. 6(a) and (b) show the 3D height images of as-received and deformed specimens after a mini-V-bending test at different PSs of E-form and AZ31 Mg alloys, respectively. Deformation twins can induce surface relief via either nano-indentation [43] or uniaxial tension [41]. Therefore, surface relief seemed to increase due to the occurrence of deformation twinning and dislocation slip. Fig. 7(a) and (b) show the line profiles of surface relief along the RD for Eform and AZ31 Mg alloys, respectively, while Fig. 7(c) shows quantitative comparisons of the average surface roughness ðRa Þ of as-received and deformed specimens at different PSs via mini-Vbending. The surface relief of the E-form was higher than that of the AZ31, which could be due to a comparatively larger grain size and higher activity of deformation twins, as shown in Figs. 3 and 5(b). 3.4. Cross-section t-EBSD analysis The additional cross-sectional EBSD analysis was carried out to
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explain the exact deformation mechanism of Mg alloys during the mini-V-bending test. Fig. 8(a) and (b) show the high-resolution NDIPF and IQ maps measured at a PS of 0.60 mm (as determined in Fig. 5(d)) with a step size of 0.1 mm for E-form and AZ31 Mg alloys, respectively. The cross-sectional EBSD analysis position was 50 mm below the tension region. The specimens were mounted in a jig and prepared via standard polishing technique to examine the deformed microstructure. The TBs in the localized deformation zones could be responsible for the crack initiation in the tension region. The circles were marked on the IPF maps to highlight the different types of TBs such as TTWs, CTWs, and DTWs. It should be noted that TTWs and CTWs were observed in both Mg alloys. However, AZ31 Mg alloy had a relatively very low fraction of DTW's compared with that of E-form Mg alloy and it seemed that a PS of 0.60 mm was insufficient to activate the DTW's in the AZ31 Mg alloy. In order to investigate the deformation mechanism occurring in regions that were either non-indexed or had a low CI due to microcracks or surface relief, FIB milling technique was used to fabricate the cross-section specimens for a high-resolution t-EBSD analysis around the microcracks or surface relief of deformed specimens. Fig. 9(a) and (b) show FE-SEM images for E-form and AZ31 Mg alloys, respectively. The FE-SEM images include deformed surface morphologies at a PS of 0.875 mm, FIB milling areas of interest for TD1 specimen, and Pt protective layer coating deposition onto the cross-sections of t-EBSD specimens prepared by FIB. Fig. 10 shows the results of t-EBSD analysis measured on the cross-section specimens viz. TD1, TD2 and RD1 prepared by FIB milling. Here, TD1, TD2 and RD1 represent the FIB specimens taken on the TD and RD planes, respectively. High-resolution ND-IPF and pattern quality maps showing the evolution of TBs at a PS of 0.875 mm are shown in Fig. 10(a) and (b) for E-form and AZ31 Mg alloys, respectively. It should be noted that the occurrence of cavity/void/defect formations during the fabrication of very thin specimens (~1 mm) via FIB milling technique was responsible for the non-indexed region marked with a yellow circle in the E-form Mg alloy specimen pattern quality map. Therefore, it is clear from the cross-sectional t-EBSD analysis that only microcracks were unrelated to the non-indexing regions on the ND plane, but surface relief did make a significant contribution. The strain accumulation and evolution of twins on the ND plane resulted in surface relief during the mini-V-bending test. The
Fig. 6. 3-D laser scanning microscope height images for the bent surface (ND plane) of (a) E-form and (b) AZ31 Mg alloy specimens at different PSs under the mini-V-bending test.
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Fig. 7. Line profiles along the rolling direction (RD) for (a) E-form and (b) AZ31 Mg alloy specimens at different PSs. (c) Average surface relief on the bent surface (ND plane) of Eform and AZ31 Mg alloys under the mini-V-bending test.
Fig. 8. High-resolution ND-IPF and IQ maps for the specimens deformed at a PS of 0.60 mm (a) E-form and (b) AZ31 Mg alloys. EBSD was measured near the tension region and at a step size of 0.1 mm.
extent of surface relief might be correlated with the average grain size distributions and the crystallographic texture. The evolution of the TBs shows there is a considerable difference between the ND (Fig. 3) and the cross sections (TD and RD planes) (Figs. 8 and 10). The main deformation mechanisms in the E-form were shear localization by dislocation slip along with all types of twins while
CTW and DTW were the main deformation mechanisms in the AZ31. It should be noted that the activation of DTW in AZ31 was observed at a significantly higher PS of 0.875 mm. Therefore, it can be concluded that the activation of TTW in the E-form at the higher levels of deformation contributed to the relatively high formability at RT.
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Fig. 9. FE-SEM images showing the deformed surface morphology of bent specimens after PS of 0.875 mm during the ex-situ mini-V-bending test. The FIB milling area of interest for TD1 specimen is marked for the t-EBSD measurement. FE-SEM images showing Pt protective layer coating deposited onto the area of interest and cross-sectional t-EBSD specimen prepared by FIB for (a) E-form and (b) AZ31 Mg alloys.
Fig. 10. ND-IPF and pattern quality maps showing the evolution of TBs in FIB-prepared cross-section specimens viz. TD1, TD2 and RD1 following PS of 0.875 mm under the ex-situ mini-V-bending test for (a) E-form and (b) AZ31 Mg alloys.
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Fig. 11. Schematics of TBs in the as-received specimens and evolution of TBs in the E-form and AZ31 Mg alloys deformed at PSs of 0.35, 0.525 and 0.70 mm during the ex-situ miniV-bending test.
Fig. 11 shows the schematics of the evolution of TBs in the asreceived and deformed specimens at PSs of 0.35, 0.525, and 0.70 mm in E-form and AZ31 Mg alloys under ex-situ mini-Vbending testing. The schematics of the TBs evolutions were based on the observations from the IPF maps shown in Figs. 3 and 8. In the EBSD analysis, however, the TBs seemed to correspond to the localized deformation zones as favorable sites for crack initiation. This result promoted an understanding of the crack initiation mechanisms in the tension region or on the bent surface during mini-V-bending. At a PS of 0.35 mm, the TBs of TTW were observed in both Mg alloys, and the TBs of CTW appeared in the E-form Mg alloy. At PSs of 0.525 and 0.70 mm, the TBs of TTW and CTW were observed in both Mg alloys while the TBs of DTW were present only in the E-form Mg alloy at higher PSs. Furthermore, Fig. 12 shows a schematic illustration of the deformation mechanism and the evolution of the TBs based on the ND-IPF and pattern quality maps of the cross-sectional t-EBSD, as shown in Fig. 10. A schematic of the as-received specimen indicates that grains residing on the free surface of the as-received specimen were relatively flat. However, the schematic of the deformed specimens indicated that the free surfaces of the deformed specimens at higher PSs were subjected to heterogeneous crystal rotation due to dislocation slip and twinning, which resulted in the formation of surface relief. As a result, as the PSs increased, the grains residing on the free surface became less prone to stress concentration due to the surface relief, while the grains existing on the sub-surface remained relatively open to stress concentration, which is considered to be advantageous for the development of the twins. Such a trend is shown in Fig. 10, as well. This tendency demonstrates why the density of the twins
observed on the free surface during the ex-situ mini-V-bending test was underestimated, as shown in Figs. 3 and 5. Differences in the deformation twinning behavior resulted in different deformations and failure mechanisms for E-form and AZ31 Mg alloys under mini-V-bending. Moreover, with higher levels of PSs, these TBs were easily converted to a network of the twins, which resulted in a cluster of localized deformation regions that led to the crack initiation. Therefore, the strain accumulation at twin boundaries resulted in failures under larger levels of strain during mini-V-bending. 4. Conclusions The microstructure evolution in E-form and AZ31 Mg alloys was directly observed under ex-situ mini-V-bending. The following conclusions were obtained. 1) The effects of the initial crystallographic texture and average grain size distribution that affect the deformation behaviors and bendability were studied via EBSD analysis, which revealed how TBs that developed in the deformed grains contributed significantly to the localized deformation zones in Mg alloys under mini-V-bending. 2) The TBs in the localized deformation zones were responsible for the crack-initiation sites in the tension region. CTWs and DTWs were more prominent in the E-form Mg alloy compared with that seen in the AZ31 Mg alloy under mini-V-bending. 3) The grains residing on the free surface were less prone to stress concentration due to significantly higher surface relief and
Fig. 12. The schematic illustration of the deformation mechanism and the TBs evolution observed during the cross-sectional t-EBSD measurement as shown in Fig. 10.
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localized deformation regions, which was attributed to the difficulty in the measurement of the exact twin boundaries as PSs increased under the mini-V-bending process. As a result, the density of twins observed on the free surface during the ex-situ mini-V-bending test was probably underestimated. Acknowledgements This research was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT (NRF2016M3C1B5906955) and the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2014R1A6A1030419).
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