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 τ1-TbAl2Si2, τ2-Tb2Al3Si2, τ3-TbAlSi, τ5-TbAl0.15Si0.85 and τ6-Tb6Al3Si have been confirmed, while the compound τ4 (Tb2AlSi2, crystal structure oI10-W2CoB2) 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 τ4-Tb2AlSi2 and τ5-TbAl0.15Si0.85 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(Al1−xSix)3 (0 ≤ x ≤ 0.13), Tb(Al1−xSix)2 (0 ≤ x ≤ 0.02), Tb5(AlxSi1−x)3 (0 ≤ x ≤ 0.08), Tb5(AlxSi1−x)4 (0 ≤ x ≤ 0.10), Tb(AlxSi1−x) (0 ≤ x ≤ 0.10), Tb(AlxSi1−x)2-b (0 ≤ x ≤ 0.24) and Tb(AlxSi1−x)2−a (0 ≤ x ≤ 0.15). The Al and Si richest part of the system is characterized by the coexistence of three phases (Al), (Si) and TbAl2Si2 (τ1); 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) + TbAl2Si2 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.