The 500 °C isothermal section of the Tb-Al-Si system and thermal behavior of selected Al-rich alloys.

Phase relationships in the Tb-Al-Si system have been established and experimentally studied at 500 °C. The main experimental techniques used were scanning electron microscopy, electron microprobe analysis and X-ray powder diffraction. Some samples have been analyzed by differential thermal analysis. Six ternary compounds form in the system: the five ternary structures τ-TbAlSi, τ-TbAlSi, τ-TbAlSi, τ-TbAlSi and τ-TbAlSi have been confirmed, while the compound τ (TbAlSi, crystal structure oI10-WCoB) was not previously known in the literature for the Tb-Al-Si system, but its formation was known in other R-Al-Si (R: Dy,Ho,Er,Tm) systems. The two ternary phases τ-TbAlSi and τ-TbAlSi show a homogeneity range, at constant Tb content. A number of boundary binary phases dissolve the third element, at 500 °C, forming ternary solid solutions extending into the ternary field: Tb(AlSi) (0 ≤ x ≤ 0.13), Tb(AlSi) (0 ≤ x ≤ 0.02), Tb(AlSi) (0 ≤ x ≤ 0.08), Tb(AlSi) (0 ≤ x ≤ 0.10), Tb(AlSi) (0 ≤ x ≤ 0.10), Tb(AlSi) (0 ≤ x ≤ 0.24) and Tb(AlSi) (0 ≤ x ≤ 0.15). The Al and Si richest part of the system is characterized by the coexistence of three phases (Al), (Si) and TbAlSi (τ); this behavior is common to all the known R-Al-Si phase diagrams. The phases are involved in the ternary eutectic equilibrium L ⇄ (Al) + (Si) + TbAlSi close to the binary eutectic point of the Al-Si binary system. Experimental measurements confirm the ternary eutectic reaction at a temperature of 565 °C. [ABSTRACT FROM AUTHOR]