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Phase Diagram Evaluations: Section II

Al-Fe-Tb (Aluminum-Iron-Terbium) V. Raghavan

Recently, Yanson et al. [2001Yan] studied the phase relationships in this system at 500 °C in a narrow region around 10.5 at.% Tb. Here, the structurally related phases of the types Th2Ni17, Th2Zn17, and TbCu7 occur close to one another.

Binary Systems The Al-Fe phase diagram [1993Kat] shows that the facecentered-cubic (fcc) solid solution based on Fe is restricted by a ␥ loop. The body-centered-cubic (bcc) solid solution ␣ exists in the disordered A2 form, as well as the ordered B2 and D03 forms and extends up to 50 at.% Al at the temperature of interest here (500 °C). In the Fe-Tb system [1993Oka], there are four intermediate phases. The Th2Ni17-type hexagonal phase Tb2Fe17 (denoted here 2: 17H) is stable at stoichiometric and at Fe-rich compositions and the Th2Zn17-type rhombohedral phase 2:17R is stable at Tb-rich compositions [2001Yan]. The other three phases are the Th6Mn23-type cubic phase Tb6Fe23, the PuNi3-type rhombohedral phase TbFe3, and the MgCu2-type cubic phase TbFe2.

Ternary Isothermal Section With starting metals of purity of not less than 99.9%, [2001Yan] melted about 65 alloy compositions in an arc furnace under Ar atm. Three series of compositions were prepared at 9.5, 10.5, and 11.5 at.% Tb and at 2.5 at.% Al intervals up to 50 at.% Al. Some samples were also prepared at 5 and 15 at.% Tb. The samples were given a final anneal at 500 °C for 720 h and quenched in water. The phase equilibria were studied by x-ray powder diffraction. At 9.5 at.% Tb, the 2:17H phase, the TbCu7 type hexagonal phase 1:7, and the 2:17R phase are stable in the

ranges of 0-18, 20.5-23, and 43-50.5 at.% Al, respectively. At 10.5 at.% Tb, they cover the concentration ranges of 0-18, 20.5-22.5, and 40-49.5 at.% Al, respectively. At 11.5 at.% Tb, only the 2:17R phase is stable and covers the composition range of 0-43.5 at.% Al. The influence of the geometrical factors on the structural stability of these phases was discussed by [2001Yan]. Due to the narrow range of the Tb content and the extensive line overlap in the x-ray patterns of the closelyrelated structures, the identification of the two-phase fields or of the presence of other closely-related structure variants such as the rhombohedral PrFe7 type could not be done. A single plot of the subcell parameters of all the three structures indicated a continuous increase in the a and c parameters with increasing Al content [2001Yan]. The partial isothermal section at 500 °C constructed by [2001Yan] is redrawn in Fig. 1. The ordered forms of Fe-Al bcc phase ␣ are not shown separately from ␣. The three phases 2:17H, 1:7, and 2:17R are “in equilibrium with the neighboring solid solutions ␣, Tb(Al,Fe)2 and Tb(Fe,Al)12 and an unidentified ternary phase,” [2001Yan]. As seen in Fig. 1, these equilibria are sketched schematically by [2001Yan] without labeling the phase fields. The details are not known.

References 1993Kat: U.R. Kattner and B.P. Burton: “Al-Fe (AluminumIron)” in Phase Diagrams of