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

HIROSHI OKUDA 0 TOSHIKI HORIUCHI 0 TOSHIKI MARUYAMA 0 MICHIAKI YAMASAKI 0 YOSHIHITO KAWAMURA 0 KOJI HAGIHARA 0 SHINJI KOHARA 0 0 HIROSHI OKUDA, Associate Professor, TOSHIKI HORIUCHI, Graduate Student , and TOSHIKI MARUYAMA, Undergraduate Student, are with the Department of Materials Science and Engineering, Kyoto University , Kyoto 606-8501, Japan . MICHIAKI YAMASAKI, Associate Professor, and YOSHIHITO KAWAMURA, Professor, are with the Department of Materials Science and Engineering, Kuma- moto University , Kumamoto 860-8555, Japan . KOJI HAGIHARA, Associate Professor, is with the Department of Adaptive Machine Systems, Osaka University , Suita 565-0871, Japan . SHINJI KOHARA, Principal Researcher, is with the Japan Synchrotron Radiation Research Institute , Sayo 679-5198, Japan . Contact 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. - precipitation case.[12] 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.[15] II. EXPERIMENTAL 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 h1 1 -2 0i 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 Pyrex* tubes at temperatures between 673 K and *Pyrex is a trademark of Corning Incorporation, Corning, NY. 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. RESULTS AND DISCUSSION Figure 1 gives the temporal evolution of small-angle scattering patterns obtained for the cast samples annealed at 673 K (400 C) up to 2 weeks. The scattering patterns show sharp diffraction peaks in two Debye rings and outer diffuse spots. The magnitudes of the scattering vector, q, for the sharp diffraction spots are about 4.01 and 4.86 nm 1, respectively. This spacing agrees with the distance between the stacking fault in 10H and 18R strufficffitures rffiffieported in the TEM works by Itoi et al.,[5] 2p3 2p3 Egusa et al.,[3] and other researchers for MgYZn alloys and also close to that of the reported MgAlGd alloy.[4] In the as-cast sample, the outer diffuse spot showed a well-defined sixfold pattern, showing that the diffraction comes from only one of the crystals in the sample. These spots agreed with the in-plane ordering of L12 clusters with structure formed in the segregation layer of the LPSO structure reported by Yokobayashi et al.,[4] as shown in the previous work.[15] As-cast samples used in the present work showed one or two sets of sixfold patterns. The present result, that the number of crystals that give a sixfold pattern is much smaller than that of crystals giving diffraction peaks of 10H and 18R as Debye rings, can be explained by two reasons. First, the in-plane ordering is much more sluggish than the formation of LPSO stacking, so that not all the crystals giving LPSO diffraction peaks are necessarily fully ordered in the in-plane directions. This idea is supported by the fact that the full-width at half-maximum (FWHM) of the diffraction spot for the in-plane ordering is much larger than those for the LPSO peaks for 10H and 18R, whose FWHM values are already equal or close to the resolution limit of the present small-angle scattering setup, about 50 nm, even for the as-cast sample. The second point is that in-plane order spots appear only when the X-rays transmit in the direction close to the c-axis of the sample. In contrast, LPSO diffractions are observed when the c-axis lay parallel to the detector, i.e., normal to the direction of X-rays. These points are discussed later. For longer annealing times, diffraction peaks from the 10H structure decreased and eventually disappeared after 1 week of annealing. The LPSO peaks are very sharp compared with the diffuse spot of in-plane ordering appearing around 6 nm 1. Figure 2 gives the change of the integrated intensity of 10H relative to that of 18R for isothermal annealing at 673 K and 773 K (400 C and 500 C). For both annealing temperatures, the integrated intensities of 10H structure monotonically decreased with time. This finding suggests that the transition from 10H to 18R during heat treatment is gradual. A first-principles calculation by Iikubo et al.[16] suggested that the stability of 18R and 10H at high temperatures is better than the 2H (hcp) structure when the effect of lattice vibration is taken into account for Mg. Although the calculation does not directly evaluate the stability of the present ternary alloys, this tendency is in good agreement with the present results that 10H did not show fast disordering or collapse to form hcp structure, but instead was slowly replaced by 18R structures. Therefore, the 10H structure is still more stable than hcp at the annealing temperatures in the present condition. The in-plane order spots became sharper with annealing time, suggesting that the domain size of the ordered region within the segregated layer increased.[15] Figure 3 Fig. 1Two-dimensional SAXS patterns of Mg85Y9Zn6 cast alloy: (a) as-cast and (b) annealed at 673 K (400 C) for 48 h and (c) for 2 weeks. The 10H peaks decreased with annealing time, and only 18R peaks were observed after 2 weeks of annealing at 673 K (400 C). Fig. 2Integrated (...truncated)