Phase Equilibria of the Al–Co–Er System at 400°C and 600°C

The 400°C and 600°C isothermal sections of the Al–Co–Er system were studied assisted with X-ray diffraction (XRD), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) techniques. 18 three-phase fields were identified in the 400°C isothermal section. The maximum solid solubilities of Al in Co 3 Er and Co 2 Er were 13.93 at.% and 16.13 at.%, respectively. Whereas the maximum solid solubilities of Co in Al 2 Er, Al 2 Er 3 and AlEr 2 were 6.93 at.%, 6.65 at.%, and 6.49 at.%, respectively. And the solid solution range of λ is from 22.22 at.% Al to 44.44 at.% Al. While the 600°C isothermal section included 20 three-phase fields. The maximum solid solubilities of Al in Co 17 Er 2 and Co 7 Er 2 were 10.17 at.% and 10.24 at.%, respectively. Whereas the maximum solid solubilities of Co in Al 2 Er and Al 2 Er 3 were 3.63 at.% and 2.01 at.%, respectively.

crystallographic data provided in Pearson's Handbook. In 1993, Wu et al. [27] reevaluated the phase diagram of the Co-Er binary system evaluated by Okamoto and Massalski [28] . In 2009, Wang et al. [29] thermodynamically assessed the Co-Er binary system on the basis of the experimental data on the thermodynamic properties and phase equilibria. The authors found that the calculated results agreed with the experimental data.

Experimental
More than 70 alloy samples were prepared for the determination of the isothermal sections of the Al-Co-Er ternary system at 400°C and 600°C. The raw materials, Al, Co, and Er (99.99% purity), supplied by China Materials Technology Co., Ltd., were smelted to abtain the experimental alloy samples. The weight of each sample was about 6 g. The composition of each sample was designed based on the existing phase diagram information, and the amount of each starting material was obtained by weighing with an analytical balance. The samples were arc-smelted in a vacuum consumable electrode arc furnace under an argon atmosphere in a water-cooled copper crucible, and the alloyed samples were smelted together with Ti as an oxygen getter to prevent oxidation during smelting. To ensure the uniformity of the samples, each button sample was turned and remelted at least three times during smelting, with the weight loss not exceeding 1%. The obtained button alloy samples were sealed in silica capsules backfilled with high-purity argon to resist oxidization and then annealed in a tube furnace at 400°C for 2880 hours and 600°C for 2160 hours. Afterward, the alloy samples were immediately immersed in ice water to quench and cool to room temperature.
The microstructures of these alloy samples were investigated via EPMA (JEOL JXA-8530F) after the samples were polished. The total mass of all the elements in each phase ranged from 97% to 103%, and the standard deviation of the measured concentration was ± 0.5 at.%. The phases in the alloy samples were identified using a Rigaku D-max/2500 X-ray diffractometer operated at 40 kV and 200 mA. The phase-identification results were analyzed using the Jade 6.0 program, and the diffraction patterns were obtained. The nominal chemical compositions of 35 alloy samples and the chemical compositions of each individual phase at 400°C obtained via EPMA are summarized in Table 2.

Isothermal Section at 600°C
According to the EPMA data and XRD patterns in Table 3, the isothermal section of the Al-Co-Er ternary system at 600°C was established (Fig. 13).
Alloys #B21 and #B24 were respectively located in the following adjacent three-phase equilibrium fields: AlCo2Er6+AlCo2Er2+Al5Co4Er11 and AlCo2Er2+Al5Co4Er11+λ (Fig. 10), with both alloys containing two same phases: AlCo2Er2 and Al5Co4Er11. Phase Al5Co4Er11 has not yet been reported in the literature; thus, the exact atomic ratio and the lattice parameters of the phase need further study. As shown in Fig. 11(a, c, e), one distinct three-phase field occurred in each of alloys #B25, #B27, and #B30. According to the XRD pattern analysis (Fig. 11b, d, f), alloys #B25 and #B27 were located in the following three-phase fields: Al2Er3+Al3Co3Er14+λ and Al2Er3+Al3Co3Er14+Al3CoEr2, respectively. Because of the tiny spot of the dark phase in alloy #B30, the third phase could not be confirmed, however, according to the peripheral phase regions and phase law, #B30 is assumed to be in the three-phase field Al2Er3+Al3Co3Er14+AlEr2. Figure 12(a) shows the three-phase microstructures of AlCo+Co7Er2+Co17Er2 of alloy #B22, and the XRD result is shown in Fig. 12(b). According to the SEM, EPMA, and XRD data of alloys #B28 (Fig. 12c, d) and #B34 (Fig. 12e, d), the alloys were respectively located in the AlCo+Co7Er2+Co3Er and AlCo2Er+AlCo+Co3Er three-phase fields.
Based on the above analysis and the EPMA data in Table 3, the isothermal section of the Al-Co-Er ternary system at 600°C was established (Fig. 13). As shown in Fig. 13, 10 ternary intermediate compounds were detected in the Al-Co-Er system at 600°C: Al19Co6Er2, Al9Co3Er2, Al12Co4Er3, λ (a solid solution including AlCoEr and Al4Co2Er3), AlCo2Er, Al3CoEr2, AlCo2Er2, AlCo2Er6, Al3Co3Er14, and Al5Co4Er11. Six of these compounds (Al19Co6Er2, Al12Co4Er3, AlCo2Er, AlCo2Er6, Al3Co3Er14, and Al5Co4Er11) have not been previously reported. The maximum solid solubilities of Al in Co17Er2 and Co7Er2 were 10.17 at.% and 10.24 at.%, respectively, Whereas those of Co in Al2Er and Al2Er3 were 3.63 at.% and 2.01 at.%, respectively. Moreover, the solubility range of AlCo was from 44.76 at.% Al to 50.94 at.% Al.

Figure 13
Isothermal section of Al-Co-Er system at 600°C

Conclusions
The isothermal sections of the Al-Co-Er ternary system at 400°C and 600°C were determined via EPMA and XRD. Eight ternary intermediate compounds were detected in the Al-Co-Er system at 400°C, which included five ternary intermediate compounds that have not been reported in the literature (Al19Co6Er2, Al12Co4Er3, AlCo2Er, AlCo2Er6, and Al3Co3Er14). Furthermore, 18 three-phase equilibrium regions and 6 two-phase equilibrium regions were detected in the isothermal section of the Al-Co-Er system at 400°C. Ten ternary intermediate compounds were detected in the Al-Co-Er system at 600°C, among which six have not been reported in the literature (Al19Co6Er2, Al12Co4Er3, AlCo2Er, AlCo2Er6, Al3Co3Er14, and Al5Co4Er11). In addition, 20 three-phase equilibrium fields and 6 two-phase equilibrium fields were determined in the isothermal section of the Al-Co-Er system at 600°C. The solution range of the solid solution phase λ was confirmed to be from 22.22 at.% Al to 44.44 at.% Al.  Table captions   Table 1 The intermetallic compounds have been reported in literature for the Al-Co-Er system along with crystal structure data