Experimental investigation and thermodynamic optimization of the Fe–Ta–W system

The Fe–Ta–W system represents a crucial material foundation for nuclear engineering applications. However, detailed investigations of its phase relationships and thermodynamic calculations remain limited. In this study, the microstructures of all as-cast and annealed samples were examined by scanning electron microscopy (SEM), while the compositions of each sample and phase were evaluated by energy-dispersive spectroscopy (EDS). X-ray diffraction (XRD) was employed to determine the crystal structures of selected phases. The primary crystallization field of the Fe–Ta–W system was established by analyzing the primary phases and solidification paths of the as-cast samples. Four distinct primary solidification phases, including bcc(Fe), Fe 2 (Ta,W), Fe 7 (Ta,W) 6 , and bcc(Ta,W) were identified. Based on the annealing experiments, two three-phase regions Fe 2 (Ta,W) + Fe 7 (Ta,W) 6 #1 + bcc(Ta,W) and Fe 2 (Ta,W) + Fe 7 (Ta,W) 6 #2 + bcc(Ta,W) were observed at both 1373 and 1473 K, while two additional three-phase regions composed of bcc(Fe) + fcc(Fe) + Fe 2 (Ta,W) and bcc(Fe) + Fe 2 (Ta,W) + Fe 7 (Ta,W) 6 #2 were inferred at the same temperatures. Thermodynamic calculations for the Fe−Ta−W system were conducted by using the CALPHAD (CALculation of PHAse Diagram) method based on the experimental data obtained in the present study and from the literature. A set of self-consistent thermodynamic parameters for the Fe−Ta−W system was derived, and the calculated results showed good agreement with the experimental observations. The present study provides reliable thermodynamic parameters for the development of Fe-based and refractory high-entropy alloy databases.