Experimental investigation of phase equilibria in the Mg-rich corner of Mg-Nd-Sc system

Phase equilibria in the Mg-rich corner of Mg–Nd–Sc ternary system was studied by using the equilibrated alloys method. The partial isothermal sections at 500 °C, 530 °C, and 550 °C were constructed by analyzing the phase constitution and the chemical compositions of the present phases in 19 alloy samples. The Mg3(Mg, Nd, Sc) phase exhibited noticeable ternary solubility introduced by the solid solution of Mg and Sc in the Mg3Nd lattice. (Mg_hcp) showed a narrow homogeneity range paralleling to Mg–Sc binary boundary, whereas Mg41Nd5 showed negligible ternary solubility at all three temperatures. The three-phase equilibrium region of (Mg_hcp) + Mg41Nd5 + Mg3(Mg, Nd, Sc) dominated the Mg-rich corner and was surrounded by three two-phase equilibrium regions constituted by any two of above three phases. The primary crystals and the solidification pathways of alloys in the Mg-rich corner were analyzed via the as-cast microstructure. The matrix phases of (MgSc_bcc) and/or (Mg_hcp) together with the precipitations of Mg3(Mg, Nd, Sc) and other ordering structures could provide additional space to optimize microstructure and properties of Sc-alloyed Mg–Nd alloys.


Introduction
Rare Earth (RE) Mg alloys have received much more attention due to high strength, improved creep resistance, and good deformability [1][2][3][4][5]. Nd and other RE elements are commonly utilized due to the pronounced precipitation strengthening [6][7][8] and the modest solid solution strengthening [9,10]. The most successful Mg alloys developed in this category are Mg-Nd-Y system a [7,11,12] and the modified versions by adding Tb, Er, Dy, Gd, etc. [1][2][3][4][5]. Sc element exhibits physical and chemical properties similar to RE elements but a lower density. The solubility of Sc in Mg lattice is wider than common RE elements. Alloying with Sc increases the melting temperature [13] and decreases the diffusivity of Mg-based solid solution [(Mg_hcp) hereinafter] [14], which would be in favor of solid solution strengthening and creep resistance [15]. The formation of ordered precipitates involving Mg and Sc may also improve the room-and high-temperature properties [16][17][18][19]. Therefore, the design of the ternary Mg-Nd-Sc even multi-component Mg alloys by adjusting the ratio of Sc to Nd is expected to enhance the effect of age-hardening in RE contained Mg alloys.
Phase diagram has served as a roadmap for optimizing composition, microstructure, and processing [20,21]. It conveys useful information on the solid solubility limit and the precipitated phase being equilibrium with the matrix. The binary phase diagrams of Mg-Nd [22], Mg-Sc [13,23], and Nd-Sc [24] have been experimentally studied [25][26][27] and assessed via the technique of computational thermodynamics [13,23,[28][29][30][31]. Different from Mg-Nd-Y system [7,32], the phase diagram data of the Mg-Nd-Sc ternary system are still limited. The objective of the present work is to experimentally determine the phase equilibrium relations in the Mg-Nd-Sc ternary system and construct the equilibrium isothermal sections at 500°C, 530°C, and 550°C by studying the equilibrium phase constitutions and the chemical compositions of equilibrium phases in alloys. The determined ternary phase boundaries, solid solubilities, and solidification pathways will provide the fundamental data to guide the composition optimization and phase selection in the works of solid solution strengthening and/or precipitation strengthening.

Experiments
19 ternary alloy samples were prepared to study the phase equilibria in Mg-Nd-Sc system and to construct the isothermal sections at 500°C, 530°C, and 550°C. The alloys were prepared from the binary master alloys of Mg-29.85 wt.% Sc and Mg-23.94 wt.% Nd together with pure Mg (99.99 wt.%) and pure Nd (99.9 wt.% Nd). Due to the high melting temperature of Nd, the pure Nd grains with a size smaller than f1 mm × 1 mm were utilized to facilitate the melting. The weighed raw materials were wrapped in inner tubes made of tantalum foils (twisted at both ends) and then sealed in outer tubes made of quartz under argon atmosphere. The two-layer sealed tubes were heated up and held at 950°C for 4 h for melting in a muffle furnace. To promote chemical composition homogeneity, the tubes were turned upside down every 1 h. The ingots with good quality were resealed in the agron backfilled quartz capsules and annealed at 500°C, 530°C, and 550°C for 720 h, 672 h, and 240 h, respectively. The alloy samples after annealing were quickly pulled out from the furnaces and quenched into ice water to retain the microstructure and composition distribution at the annealing temperatures. 4 The phase present in the annealed alloys were examined by powder X-ray diffraction (XRD) using a Rigaku Smartlab X-ray diffractometer and Scanning Electron Microscopy (SEM) using JEOL JSM-6510. XRD was performed with Cu Kα radiation at 40 kV, 200 mA, in the range of 2θ from 10°to 80°, and a rate of 4°min −1 . The chemical composition of the alloys and that of the individual phases were detected using Energy Dispersive x-ray Spectroscopy (EDS) and Electro-Probe Microanalyzer (EPMA) equipped on JEOL JXA-8230, respectively. The acceleration voltage was set 15 kV and the beam current was 20×10 -8 A. The chemical composition was detected in different areas of a sample repetitively, thus the mean values were of statistical significance. Tables 1-3 summarize the actual chemical compositions of the alloys and the identifications of individual phases as well as the chemical composition of each phase.    molecular formula thus is expressed as Mg 3 (Mg, Nd, Sc). The Mg 41 Nd 5 phase shows negligible ternary solubility in Mg-Nd-Sc ternary, which is different from other Mg-RE-RE(RE-like) system [33][34][35]. There is only one three-phase equilibrium region of (Mg_hcp)+Mg 41 Nd 5 +Mg 3 (Mg, Nd, Sc) present in the Mg-rich corner. It is surrounded by three two-phase equilibrium regions constituted by any two of above three phases. These twophase equilibria are illustrated by the green tie-lines. The chemical compositions of (Mg_hcp) phase in 1#∼4# are varying with the chemical compositions of alloys, while that of Mg 41 Nd 5 is almost fixed. The XRD results have demonstrated the same phase constitution of (Mg_hcp)+Mg 41 Nd 5 in all these 4 samples. The reasoning also applies to alloy samples 10#∼13# in the two-phase equilibrium of (Mg_hcp)+Mg 3 (Mg, Nd, Sc). In   equilibrium in alloy 10# and the alloy 10# was in a three-phase equilibrium region. However, the presence of (MgSc_bcc) phase can not be confirmed since the positions of the diffraction peaks of (MgSc_bcc) phase coincide with some diffraction peaks of (Mg_hcp) and Mg 3 Nd. The chemical compositions of (Mg_hcp) matrix are varying in samples 10#, 11#, and 13#. It deviates from the principle that the chemical composition of individual phases in the three-phase equilibrium triangle would be fixed, independent of alloy compositions. Therefore, we can reason that the alloys 10#, 11#, and 13# must be located in the two-phase equilibrium region of (Mg_hcp)+Mg 3 (Mg, Nd, Sc). The light grey blocks may be introduced by the diffusional transformation between the undissolved pure Sc (in hcp structure) and the Mg in (Mg_hcp).

Isothermal sections at 530°C and 550°C
The phase equilibria displayed on the isothermal sections of 530°C and 550°C are similar to 500°C(see figures 4 and 5). A liquid phase is present at 550°C in a narrow three-phase equilibrium region close to the Mg-Nd boundary binary. The (Mg_hcp), Mg 41 Nd 5 , and Mg 3 (Mg, Nd, Sc) remain present at both temperatures. The  region of (Mg_hcp) is shrinking along the Mg-Sc boundary with increasing temperature, whereas that of Mg 3 (Mg, Nd, Sc) phase is subjected to slight expansion towards the Mg-rich corner. The chemical composition of (Mg_hcp) in the three-phase equilibrium triangle of (Mg_hcp)+Mg 41 Nd 5 +Mg 3 (Mg, Nd, Sc) is also left shifting towards the Mg-rich corner, from ∼23 at.% Sc at 500°C to ∼18 at.% Sc at 550°C. Figure 6 shows the XRD patterns and SEM images of 14#, 6#, and 12# alloys after annealing at 530°C. In comparison with figure 1, the increased temperature leads to growth and full crystallization of individual phases. In particular, the history of peritectic solidification for the 6# alloy in the three-phase equilibrium region has been completely erased.
Most of (Mg_hcp), Mg 41 Nd 5 , and Mg 3 (Mg, Nd, Sc) grains show perfect crystallization of convex polygon shapes. The collision and coalesce of Mg 41 Nd 5 grain boundaries are clearly shown. The morphology indicates a near-equilibrium state. Figure 7 illustrates SEM images and the XRD patterns of 14#, 1#, 6#, and 19# alloys after annealing at 550°C. 1# and 14# alloys have the same phase constitution in the XRD patterns but display different morphologies in SEM observation. The morphology of 14# alloy exhibits the characteristics of as-cast microstructure after long-term annealing, indicating a full re-melting and re-solidification in this alloy. Therefore, alloys 14# is in equilibrium with the liquid at 550°C. In contrast, the coarse grains of (Mg_hcp) and Mg 41 Nd 5 are clearly shown in alloy 1#, indicating an equilibrium of (Mg_hcp)+Mg 41 Nd 5 . Different from the polygon-shaped phase boundary in the Mg-rich alloys, the phase boundaries of (Mg_hcp) and Mg 3 (Mg, Nd, Sc) in 12# alloy at 530°C and 19# alloy at 550°C show irregular shape. These two alloys with chemical compositions relatively distant from the Mg corner are difficult to fully attain phase equilibrium due to the relatively high thermodynamic stability and the low diffusion coefficients of element in Mg 3 (Mg, Nd, Sc) phase.

Primary crystallization and solidification pathways
In addition to the study of isothermal phase equilibria, a furnace cooling was employed to study the nearequilibrium solidification behaviors. The as-cast microstructure can convey rough but useful information on the primary crystallization and solidification pathways.  Figure 9 shows the SEM images of the as-cast 1# and 4# alloys which depict different solidification pathways. The as-cast image of 1# (figure 9(a)) illustrates the solidification pathway of the primary crystallization of (Mg_hcp) followed by the eutectics of L→(Mg_hcp)+Mg 41 Nd 5 . The microstructure of 4# in figure 9(b) is consist of primary crystal of Mg 3 (Mg, Nd, Sc), peritectic layer Mg 41 Nd 5 , and eutectics of (Mg_hcp)+Mg 41 Nd 5 . It's worth noting that the Mg 12 Nd phase commonly found in as-cast Mg-Nd alloy hasn't been detected in our experiment. The small cooling rate in the furnace may suppress the formation of Mg 12 Nd. In addition, an alloy Mg 92.6 Nd 7.2 Sc 0.2 (denote X in figure 8) was prepared to verify the existence of Mg 41 Nd 5 primary crystallization. Figure 10 shows the as-cast microstructure and XRD pattern. The alloy has undergone the primary crystallization of Mg 41 Nd 5 followed by the eutectics of L→(Mg_hcp)+Mg 41 Nd 5 ( figure 10(a)). Other than the conventional lamellar eutectics, the coarse blocks of the (Mg_hcp) phase are observed in figure 10. The formation of the block (Mg_hcp) phase is probably triggered by the divorced eutectic mechanism considering the much smaller size of Mg 41 Nd 5 primary crystallization region than (Mg_hcp). The small cooling rate in the furnace can further facilitate the growth of (Mg_hcp) blocks. The experimental discoveries indicate that the above three alloys follow the solidification pathway similar to Mg-Nd binary alloys since their chemical compositions are close to the Mg-Nd boundary binary. Figure 11 depicts a complex as-cast microstructure of 13# which combines solidification reactions and solid states phase transformations. The coarse dendrites of the black phase with convex boundaries imply the nucleation and growth of them from the liquid phase. The chemical composition of the black phase is detected Mg 83.5 Nd 0.8 Sc 15.7 , far away from the alloy composition. A large number of particles are dispersedly precipitated in the black matrix. All these experimental phenomena indicate the occurrence of element partitioning and diffusional solid states phase transformations after primary crystallization. The white phase is identified as Mg 3 (Mg, Nd, Sc) due to the chemical composition of Mg 78.3 Nd 19.4 Sc 2.2 . The needle-like precipitates are observed inside the Mg 3 (Mg, Nd, Sc) phase. The XRD pattern has demonstrated four phases in this as-cast alloy, i.e. bcc_A2, hcp, Mg 3 Nd, and an unindexed phase with fcc structure. We were unable to tell fcc from MgSc_bcc via detecting the chemical composition of phases due to the limit of beam spot size in EPMA. The fcc and MgSc_bcc phases have not been labeled in figure 11(a). Due to the bcc_A2 phase is stable at high temperature in Mg-Sc rich alloys, we can infer that the 13# alloy has crystallized into (Mg_bcc) phase firstly, then the complex phase transformations during the furnace cooling have introduced the other phases. The verification and rationalization of the transformation pathways in this alloy are still open.

Conclusions
In summary, the phase equilibria in Mg-Nd-Sc system have been studied via analyzing phase constitution and phase chemical composition in annealed alloys. The isothermal sections at 500°C, 530°C, and 550°C have been constructed by assessing all the experimental data in this work. Mg 41 Nd 5 shows negligible ternary solubility in  comparison with noticeable ternary solubility of Mg and Sc in Mg 3 Nd lattice. The structure homogeneity range of Mg 3 (Mg, Nd, Sc) phase slightly increases with the increase of temperature. The matrix phases of bcc and/or hcp together with the precipitations of Mg 3 (Mg, Nd, Sc) and other ordering structures provide an improved space for microstructure and composition modulation. The results of this work offer an overview of the phase diagram for composition design in the Sc alloyed Mg-Nd alloys.