1. Introduction
Mg alloys are expected as lightweight structural materials because of their high specific strength. In particular, Mg alloys with a long-period stacking order (LPSO) structure as the strengthened phase have gained attention because they simultaneously exhibit light weight, high strength and high heat resistance [1]. Various systems have been systematically investigated since the formation of the LPSO phase in Mg-M (transition metal)-RE (rare earth element) alloys was confirmed [2–8]. The stability and mechanism of the LPSO structure have been studied using first-principles calculations [9, 10] and cluster variation methods. Iikubo et al. calculated the free energy of Mg as the dominant factor in the stability and formation of the LPSO structure, and reported that the relative stability of the 14H and 18R structures increases compared to the 2H structure, which is hcp, at high temperatures [11]. In addition, Masumoto and Iikubo et al. proposed that the wide spinodal region of the added elements in Mg is the cause of LPSO structure stabilization based on cluster variation methods [12, 13]. According to these studies, stacking faults and phase separation were considered as the driving forces for nucleation in the formation of LPSO structures, although experimental validation of these models has been difficult. Recently, Okuda et al. performed in situ small-angle X-ray scattering (SAXS) intensity measurements using synchrotron radiation on the formation process of microstructures including LPSO structures in MgZnY alloys, which form LPSO structures similar to MgCoY alloys [14–18]. In situ SAXS measurements are very useful because they provide a non-destructive and real-time assessment of changes in internal nanostructures. It is revealed that the transformation process of LPSO is not due to spinodal decomposition or stacking fault driven, but is a new hierarchical transition. However, the detailed structure of the MgZnY system has been clarified among the combinations of additive elements forming the LPSO structure, but much remains uncertain for other systems.
In MgCoY cast alloys, 15R, 12H 21R LPSO structures have been observed, as well as LPSO structures where two different stacking structures coexist [19–21]. Intermetallic compounds have also been studied under different heat treatment conditions, and Mg3(Co,Y), MgCoY4 and Mg24Y5 (β) have been reported to coexist with LPSO [22–24]. On the other hand, the process of microstructure formation in MgCoY alloys remains to be elucidated, as the tendency of mixing enthalpy is different from that of Zn, Cu and Ni, and furthermore, structural differences have been reported, such as clusters in the segregation layer that are different from the L12 structure, and the competition and coexistence phase relationship between the LPSO structure and intermetallic compounds is complex [25]. In MgCoY casting alloys, the formation of large crystallising and precipitating compounds occurs during the casting process. The evaluation of phase transformation processes in this alloy system, which exhibits complex changes in phase stability depending on temperature, is precluded by the long diffusion treatment time required for the disappearance of the large precipitated phases. This is particularly the case at low temperatures, where meaningful discussion of phase stability is therefore not possible for cast alloy. The objective of this study is to clarify the microstructure formation process of MgCoY amorphous ribbon by in situ SAXS and wide-angle X-ray scattering (WAXS) measurements using synchrotron radiation and STEM observations.
3. Results and Discussions
Figure 1 shows BF-STEM images and EDX analysis results at various temperatures. At 533 K, some Y rich clusters of about 20 nm and high-density clusters of 1 nm were present. These clusters are so small that they are not observed as spots in the selected-area electron diffraction (SAED) pattern. Figure 2 show the WAXS profile at 533, 573 and 643 K, and the black and white circles indicate the peak of MgO and MgO2, respectively. Although an oxide peak is observed at 533 K, the broadening of the overall WAXS profile indicates that no crystallisation has occurred. When the temperature was increased to 573 K, Y and Co rich precipitates of about 100 nm were observed in addition to the Y rich precipitates of about 20 nm, as shown by the black arrows in BF-STEM in Fig. 1(b). At this temperature, some crystallisation was confirmed from the SAED pattern. At 643 K, where the temperature is further increased, four phases can be observed: Y rich precipitates of about 20 nm, Y and Co rich precipitates of about 100 nm, a Mg matrix phase in which Y and Co are distributed throughout the crystal grains, and a Mg matrix phase in which nothing is enriched. The results of elemental analysis of each phase are presented in Table 1. Co was significantly enriched in Y and Co rich precipitates of about 100 nm. The SAED pattern at 643 K shows that the amorphous material is fully crystallised. A stacking structure as shown in Fig. 3 was observed in the Y and Co distributed Mg matrix phase at 643 K. SAED patterns and high-resolution images show no long-period periodicity in this stacking structure, which is considered to be a stacking defect with aperiodic clustering, which was also observed in the MgZnY amorphous sample.
Table 1 EDS quantitative analysis of the Mg
83.8Co
6.7Y
9.5 alloy at 643 K. The positions shown in the table correspond to the numbers in the BF-STEM image in
Fig. 1(c).
Figure 4 shows the change in the SAXS intensity during heating. The dashed red line shows the intensity change during the temperature increase at a rate of 10 K/min up to the target temperature, followed by the intensity change during isothermal holding for 90 min. The black dashed line shows the intensity during the 30 min hold, the same as in the TEM observation sample. As shown in Fig. 4(a), in the sample heated to 533 K, changes started to appear around 480 K during the heating process, characterised by a broad peak at q = 5 nm−1 and a stable structure that hardly changed during isothermal holding. In the case of MgZnY, this diffuse peak is the inter-cluster distance corresponding to the two-dimensional arrangement of clusters in the stacking faults. However, the very similar scattering profiles in the present alloys reveal the origin of a completely different microstructure in which the Y rich clusters are stably distributed at a distance of about 1 nm from the TEM images. In the sample heated to 573 K (Fig. 4(b)), no difference from the 533 K sample was observed during the heating process, but a very strong peak at q = 6.5 nm−1 and a weak peak at q = 7.9 nm−1 appeared at a holding time of 5 min. Peaks were identified based on the relationship between q and spacing d.
| \begin{equation}
q = \frac{4\pi \sin \theta}{\lambda} = \frac{2\pi}{d}
\end{equation}
| (1) |
where θ is the diffraction angle and λ is the wavelength. The peak at q = 7.9 nm−1 is d = 0.795 nm from eq. (1), which is in good agreement with d(110) = 0.794 nm for Mg24Y5(β), and this peak may correspond to the formation of Mg24Y5. For the peak at q = 6.5 nm−1, d is 0.967 nm, which is three times the Mg lattice constant aMg = 0.321 nm, indicating the formation of a 3a-period ordered structure. This 3a-period ordered structure was assumed to be β′′, the precursor of Mg7Y (β′), which will be discussed later. In the sample heated to 643 K (Fig. 4(c)), the 3a-period ordered structure (peak at q = 6.5 nm−1), which was also observed at 573 K, appeared at 600 K during the temperature increase process. A broad peak around q = 4 nm−1 and a peak at q = 4.9 nm−1 were subsequently observed when 643 K was reached. Okuda et al. [17, 18] identified the peaks at q = 4 and q = 4.8 nm−1 as 18R and 10H LPSO structures, respectively, through SAXS measurements of MgZnY alloys. However, the q = 4 nm−1 peak in the present alloy is attributed to a mixed-period stacking structure with predominantly six periods (12R, 18R), since the peak is broad and no long-period periodicity was observed in the SAED pattern. On the other hand, no 10H LPSO structure was observed by TEM, despite the sharp peak at q = 4.9 nm−1. Therefore, this peak also corresponds to a mixed-period stacking structure with mainly five periods (10H and 15R). A peak at q = 5.4 nm−1 appeared after 30 min of holding at 643 K. For this peak, d was 1.14 nm from eq. (1). The calculated d of q = 5.4 nm−1 is in good agreement with the lattice spacing (1.11 nm) of the Y rich atomic plane (010) of Mg7Y(β′), which identifies this peak as the β′ phase [26]. In addition, the origin of the small-angle scattering in the small q region below 5 nm−1 can be attributed to surface oxidation-induced roughness and grain boundary segregation. Furthermore, the pronounced change during the temperature increase process is due to grain boundary segregation and its disappearance. In the initial stages of crystallisation, occurring at a temperature of approximately 480 K, a portion of the solute is observed to segregate to the grain boundaries. This phenomenon is attributed to the formation of α-Mg in supersaturated solid solution, as reported by Kim et al. [27]. Additionally, the scattering intensity of crystallites below 5 nm−1 with concentration contrast due to grain boundary segregation is observed to increase, as documented by Okuda et al. [15]. Subsequently, the grain boundary segregation and the corresponding scattering intensity disappear when the Y segregated at the grain boundary form clusters on the stacking faults.
The microstructure formation process of MgCoY amorphous ribbons was discussed using TEM observations and in situ SAXS measurements. Figure 4 shows the correspondence between the peaks in Fig. 4(c) and the microstructure. Figure 5 shows the correspondence between the peaks in Fig. 4(c) and the microstructure. Peaks A and B in Fig. 4(c) are considered to correspond to the stacking structure based on the SAXS intensity analysis, but they could not be distinguished from the TEM image. Peaks C, D, and E in Fig. 4(c) correspond to the β′, β′′ and β phases, respectively, and correspond to the square Y rich precipitates in Fig. 5. In this temperature range, the β′′, β′ and β phases coexist.
Figure 6 shows the microstructure formed during the heating and holding processes. The DSC profile during heating is also shown. By increasing the temperature from the amorphous state, Y clusters as shown in Fig. 1(a) form around 480 K. The Y clusters were ordered into the β′′ phase, observed as a strong peak at q = 6.5 nm−1 in Fig. 4(b), with increasing temperature or holding above a certain temperature. Subsequently, a transition through the β′ phase (q = 5.4 nm−1) to the β phase (q = 7.9 nm−1) was observed, although the direct formation of the β phase without following the β′ phase was also observed, as shown in Fig. 4(b). It is assumed that this was due to the inhomogeneity of the amorphous ribbon specimen. At 643 K, when the Mg matrix phase was fully crystallised, the formation of stacking structures corresponding to the peaks around q = 4 and q = 4.9 nm−1 was also observed. Although no peaks related to Co were observed in the SAXS measurements, it was assumed that most of the Co in the sample was present as MgCoY4, based on the fact that Co is hardly soluble in Mg and the composition of the Y and Co rich precipitates in Fig. 1(c) is similar to MgCoY4 based on the results of elemental analysis (Table 1, position 1). In MgZnY alloys, an L12 ordered cluster is formed at low temperatures, which subsequently transforms continuously into the LPSO phase [14]. Conversely, in MgCoY alloys, the formation of dense precipitated nanoclusters Mg24Y5, which are distinct from the LPSO structure, and the absorption of Co into another stable phase, MgCoY4, are observed in the low temperature region immediately after crystallisation. This distinctive microstructural transformation on the low-temperature side is the defining feature that differentiates MgZnY alloys from MgCoY alloys. In terms of the relationship between the DSC profile and microstructure of the formation, the first DSC peak around 480 K corresponds to amorphous crystallisation and the second peak around 590 K corresponds to the β′′ phase or MgCoY4 precipitation. The broad peak around 640 K has a similar morphology to the peak attributed to the LPSO structure in the DSC profile of the MgZnY alloy. The broad peak around 640 K in the DSC profile is considered to correspond to a stacking structure with a mixed period of five or six periods forming around 640 K. Another important result is that the temperature needs to be raised above a certain temperature to crystallise from the amorphous state, as crystallisation of the Mg matrix phase and Y clusters did not progress at 533 K, even if the sample was held for a long time.
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