Development of Microstructures of Long-Period Stacking Ordered Structures in Mg85Y9Zn6 Alloys Annealed at 673 K (400 °C) Examined by Small-Angle X-Ray Scattering
Development of LPSO structure and in-plane ordering during annealing the Mg85Y9Zn ternary alloy sample at 673 K (400 °C) was examined by synchrotron radiation small-angle scattering/diffraction measurements. By examining the first diffraction peaks for 18R, 10H, and in-plane order spot, the growth kinetics of in-plane order domain and the transition from 10H into 18R were discussed. The domain growth of in-plane order was characterized by small domain with little correlation between neighboring segregation layers.
KeywordsSAXS Pattern LPSO Phase Debye Ring Phase Diagram Study LPSO Structure
Long-period stacking ordered (LPSO) structures having periodic modulation in composition and stacking fault have drawn attention due to their mechanical performance as new light metal alloys[1,2] and their unique structure and mechanical properties,[3, 4, 5] in particular, their characteristic deformation mechanism.[6,7] Although the details of LPSO structure having several periodicities, namely, 10H, 14H, 18R, and 24R, were examined by electron microscopy,[1, 2, 3, 4, 5, 6, 7] there are many aspects yet to be examined from the viewpoints of formation kinetics and phase stability/phase diagram. Earlier phase diagram studies give LPSO structures as a phase with a limited composition range as a single-phase region. However, the works made with electron micrographic observations suggest a rather wider range for the 18R phase region,[3,9,10] although it is not necessarily identified as a single-phase region under equilibrium state. Recent experimental phase diagram studies extended in a wider composition region do not yet explain the microstructural changes reported by these transmission electron microscopy (TEM) works. To discuss the microstructures in this viewpoint, it is important to know if the microstructures of the alloy we are examining are described by a simple two-phase or single-phase model that we usually adopt for the precipitation case. Many electron microscopic results[3,13,14] suggest fluctuations in periodicities of LPSO structures, degree of in-plane orders and segregation, leading to a conclusion that formation kinetics of LPSO microstructure is not as simple as described by a simple two-phase precipitation model.
To determine phase diagram and microstructural characteristics of LPSO in MgYZn alloys, therefore, we need to examine the more quantitative characteristics of the microstructure. In the present work, small-angle X-ray scattering (SAXS)/diffraction was used to examine the temporal evolution of the diffraction peaks corresponding to the first peaks of LPSO and in-plane ordering appearing in the SAXS region, as described in our previous work.
Polycrystalline cast ingot and directionally solidified ingot were used in the present work. The composition of the samples is Mg-9 at. pct Y-6 at. pct Zn. The cast ingot (hereafter referred to as cast sample) exhibited polycrystalline microstructures. The directionally cast sample showed[6,7] large and faceted grains with a thickness up to a hundred micrometers and length of a couple of millimeters with a preferred orientation of 〈1 1 -2 0〉 in the growth direction (hereafter referred to as the DS sample). The volume fraction of the LPSO phase is reported to be 90 pct or more for the present composition. The sample was heat treated under vacuum in sealed Pyrex1 tubes at temperatures between 673 K and 773 K (400 °C and 500 °C) and polished down to the thickness used for the measurements, and the samples annealed at 673 K (400 °C) were examined in detail. Small-angle scattering measurements were made at BL6A of Photon Factory with a wavelength of 0.15 nm and at BL04B2 of SPring8 with a wavelength of 0.03 nm.
3 Results and Discussion
With the preceding discussion on the SAXS pattern of DS samples, let us return to the question of why only one or two sets of sixfold order spots appeared while more than 50 diffraction spots of 18R were observed in the as-cast polycrystalline samples. The number of observed 18R LPSO peaks for an exposure with a two-dimensional detector was between 50 and 120, and that for a sixfold pattern was one or two. The FWHM of the measured order spot was 2.5 nm−1 and that calculated for the 18R LPSO diffraction spot corresponding to a domain thickness of 50 nm was 0.1 nm−1, which is close to the present SAXS resolution and of the same order of magnitude as the measured data. When only the solid angle of acceptance defined by the FWHM of the spots determines the number of allowed diffraction, as schematically shown in Figure 6, the solid angle for the 18R peak, 1.5 × 10−1 sr, is about 5 times larger than the solid angle for the sixfold order spot. Therefore, there is still a gap of a factor of 10 between the ratio of the number of 18R diffraction peaks to the number of order spots and the expected ratio. Some of the discrepancy may be explained by an effect of warpage or mosaicity of the 18R grains. Figure 1 shows that the shape of some of the LPSO peaks elongated along the ring about a couple of times, keeping the FWHM in the radial direction small. As schematically shown in Figure 6, this warpage increases the acceptance angle to satisfy the Bragg condition for 18R diffraction by several times for some of the diffraction spots when Δq < δ′. On the other hand, as shown in Figure 5(c), the number of in-plane ordering should increase with overlapping twofold order spot conditions. Therefore, the number of order spot sets that in-plane ordering appears is still less than expected. This result implies that not all the LPSO grains giving well-defined 18R diffraction are ordered in the in-plane direction, and it agrees with the SAXS pattern that for 2 weeks of annealing, the number of sets of order spot increased while the FWHM of the spot decreased.
Small-angle scattering/diffraction measurements were applied to examine the microstructures and their evolution of Mg85Y9Zn6 polycrystalline cast alloys during annealing at 673 K (400 °C) and compared with as-cast directionally solidified alloys. The temporal change of the cast sample showed that development of in-plane order is far slower than that of LPSO order, and the correlation of the order structures between the neighboring segregation layers is still slower. Although the 18R and 10H LPSO structures developed in plates whose thickness was larger than the resolution limit of present SAXS measurements, even in the as-cast state, the in-plane ordering was much slower with a domain size of typically 2.5 nm and grew to 7.2 nm after annealing for 2 weeks when all the 10H structures were transformed into 18R structure. A 90-degree rotation of sixfold pattern for the directionally cast sample gave twofold elongated order spots and 18R LPSO diffraction perpendicular to the order spot, suggesting that there was little coherent interference between neighboring order domains. A temporal change of statistical nature of the 18R diffraction spot suggests that the lattice constant in the stacking direction has distribution stable during transformation from 10H to 18R, keeping the average atomic layer distance in the c direction constant.
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Part of the present work was supported by a grant-in-aid for scientific research on Innovative Areas, “Synchronized Long-Period Stacking Ordered Structure,” from the Ministry of Education, Science, Sport and Culture, Japan (Grant No. 23109005). Small-angle scattering measurements were made under proposal numbers 2012G178 at Photon Factory, KEK and 2012A1186, 2012B1434 at SPring8.