Phase Equilibria in the Tm-Co, Tm-Fe and Tm-Co-Fe Systems

Phase diagrams of the ternary Tm-Co-Fe and boundary binary systems Tm-Co and Tm-Fe were constructed in the entire composition ranges using differential thermal analysis, powder x-ray diffraction, microscopic examination and electron probe microanalysis. The Tm-Co phase diagram was plotted for the first time. A new binary compound Tm₁₂Co₇ was found and its crystal structure was established. The Tm-Fe phase diagram was revised: all invariant temperatures were updated, and disagreements with previously reported data were resolved. The Tm-Co-Fe system was studied for the first time. The results are given in the form of liquidus and solidus projections, isothermal sections at 1200 and 1000 °C, and Scheil reaction scheme. Ternary compounds are not formed in the system. Instead, the isostructural phases of Tm-Co and Tm-Fe systems form continuous solid solutions (αCo,γFe), Tm₂(Co,Fe)₁₇, Tm(Co,Fe)₃, Tm(Co,Fe)₂ at all studied temperatures. The Tm₆Fe₂₃ phase dissolves up to 45 at.% Co. The solubility of Fe in Tm-Co compounds is much lower. Five three-phase regions are present on the solidus projection, which form from one eutectic and four transition (U) type invariant reactions. The boundaries of the primary crystallization regions of phases, types and coordinates of invariant and monovariant equilibria are determined. Isothermal sections differ from the solidus projection by: (i) the presence of wide areas of melt, resulting in the absence of Tm₃Co and Tm₁₂Co₇ compounds and (ii) the absence of TmCo₅ compound. As a result, there are two three-phase regions at 1200 and 1000 °C.