Development of a compact compression test stage for synchrotron radiation micro-Laue diffraction measurements of long-period stacking-ordered phases in Mg–Zn–Y alloys

Ternary Mg–transition metal (TM)–rare earth (RE) alloys with long-period stacking-ordered (LPSO) structures, which are formed by the synchronization of structure modulation and the chemical enrichment of TM and RE atoms, have attracted much attention as lightweight structural materials because of their extraordinary mechanical properties such as high strength and high ductility.^1–6) These excellent mechanical properties of the alloys are believed to be provided by the LPSO phase through its unique deformation behaviors referred to as "kink deformation".⁵⁾

Synchrotron radiation (SR) micro-Laue diffraction (MLD) measurement is a novel technique to visualize grain boundaries with 10-µm-order spatial resolution.^7,8) MLD can reveal internal grain boundaries, which cannot be observed by the electron backscatter diffraction (EBSD) technique. This feature is very important for studying the deformation behavior.

To investigate the deformation behavior of the LPSO-phase Mg polycrystal, we developed a compact compression test stage for SR-MLD measurements, which can compress a small sample with a size of 0.3 × 0.3 × 1.0 mm³. The loading can be changed from 0 to 100 N. Using the compression test stage, SR-MLD experiments were performed at the white X-ray diffraction beamline BL28B2 of SPring-8. We succeeded in visualizing the change in the grain boundaries with increasing compression loading.

Figures 1(a) and 1(b) show photographs of our newly developed compact compression test stage for SR-MLD measurements, manufactured by VIC International. Here, Fig. 1(a) shows an overall image of the compression test stage and Fig. 1(b) shows a close-up image around the sample position. The compression loading can be changed from 0 to 100 N by changing the length of the spring, which pushes a flat indenter to keep the sample stress constant. The length of the spring is controlled by a stepping motor.

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MLD measurements were performed at BL28B2 of SPring-8. BL28B2 can provide a white X-ray microbeam, which is necessary for MLD measurements.⁹⁾ Figure 2 shows a photograph of the setup for MLD measurements in the BL28B2 experimental hutch. A white X-ray microbeam with a size of 10 × 10 µm² was formed by an incident slit. The compact compression test stage was mounted on XYZ translation stages equipped with a multiaxis diffractometer. The MLD pattern was detected by a CMOS flat panel sensor (Hamamatsu Photonics C7942CA-02), which has a large photodiode area (100 × 100 mm²) with a pixel size of 0.1 × 0.1 mm². The sample position was scanned two-dimensionally using translation stages with a step size of 10 µm to obtain a set of MLD patterns from the sample area. Compression stress was changed from 0 to 204.1 MPa. To visualize the grain boundaries, we plot the differential intensity between adjacent MLD patterns. This technique is called grain boundary imaging by the Laue pattern (GILP).¹⁰⁾

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The samples used in this experiment were prepared from a directionally solidified (DS) Mg₈₅Zn₆Y₉ alloy polycrystal with a single 18R-type LPSO phase, whose solidification direction was highly oriented along the $\langle 11\bar{2}0\rangle$ direction,⁵⁾ as confirmed by diffraction measurements at BL02B1 of SPring-8. A rectangular sample with a size of 0.3 × 0.3 × 1.0 mm³ whose longitudinal direction was parallel to the $\langle 11\bar{2}0\rangle$ direction was cut from the DS polycrystal. Because the grain size of the DS polycrystal was more than 0.1 mm in the transverse direction and more than 2 mm in the longitudinal direction,⁵⁾ the sample consists of a few grains.

Figure 3 shows grain boundary images of the sample obtained at different compression loadings. We can see two boundaries clearly in Fig. 3(a), indicating that the sample mainly consists of three grains. This is reasonable because the DS polycrystal consists of large grains with a size of more than 0.1 mm in the transverse direction and more than 2 mm in the longitudinal direction.⁵⁾ With increasing loading, the sample height decreased gradually up to a loading of 166.3 MPa, and then rapidly decreased at 170.1 MPa. The images in Figs. 3(m)–3(o) are considerably different from the others. This is because plastic deformation began at a loading of 170.1 MPa; thus, the grain boundaries cannot be clearly observed owing to the lack of spatial resolution.

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On the other hand, a slight change in contrast in part was observed at a loading of 121.0 MPa, as indicated by an arrow in Fig. 3(i). The part spreads gradually as the loading stress is increased to 166.3 MPa, which is near the yield stress. Because the contrast change means grain deformation, this is expected to be related to the generation of kink deformation. Figure 4 shows an optical micrograph of the sample after compression at 204.1 MPa. We can see many bird's-beak-like images, which are a typical feature of kink deformation.⁵⁾

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Note that the contrast changes in Figs. 3(i)–3(l) were observed mainly in one grain. This is interesting when considering the deformation behavior of the LPSO-phase polycrystal.

Figure 5 shows the stress–strain curve of the sample, obtained from the grain boundary images. Because the image resolution is 10 µm, the strain resolution is limited to 1%. Although the strain resolution is unsatisfactory, the obtained stress–strain curve is very similar to that measured for an LPSO-phase DS polycrystal with a size of mm order.¹¹⁾ This confirms that the present measurement is reasonable even though the sample size is small.

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In summary, we developed a compact compression test stage for SR-MLD measurements to investigate the deformation behavior of Mg–Zn–Y alloys with an LPSO phase. This stage can compress a small sample with a size of 0.3 × 0.3 × 1.0 mm³, and the loading can be changed from 0 to 100 N. Using the compression test stage, SR-MLD experiments were performed on a DS Mg₈₅Zn₆Y₉ alloy polycrystal with a single 18R-type LPSO phase. By using the GILP technique, we succeeded in revealing the change in the grain boundaries with increasing compression loading. This will provide important information for studying the deformation behavior of the LPSO-phase Mg alloys.

Acknowledgments