Experimental investigation and thermodynamic description of the Fe–Hf–Nb system

According to solidification microstructure of as-cast alloys and phase constituents of annealed samples, the liquidus surface projection and phase relationships at 1373 and 1273 K in the Fe–Hf–Nb system were constructed. A ternary phase τ with AlCuHf-structure was found, and the composition ranges of Nb in τ at 1373 and 1273 K were ∼ 20.5–30.8 at.% and ∼ 20.8–30.2 at.%, respectively. Moreover, the maximum solubilities of Nb in FeHf2 was determined to be ∼ 16.9 at.% and ∼ 16.0 at.% at 1373 and 1273 K, and the maximum solubilities of Hf in μ was measured to be ∼ 4.6 at% and ∼ 3.3 at.% at 1373 and 1273 K. Fe2Hf and Fe2Nb with the same C14 structure formed a homogeneous solid solution from Fe2Hf to Fe2Nb at 1373 and 1273 K. Seven primary solidification regions in the liquidus surface projection and two three-phase regions and seven two-phase regions in the isothermal sections at 1373 and 1273 K were experimentally determined. On the basis of experimental data, the thermodynamic parameters of individual phases in the Fe–Hf–Nb system were optimized by CALculation of PHAse Diagram (CALPHAD) method. Four solution phases (liquid, bcc, fcc, and hcp) were described as the substitutional solution. The intermetallic phases FeHf2, λ1, λ2, λ3 and τ were modeled as (Fe)1(Hf,Nb)2, (Fe,Hf,Nb)2(Fe,Hf,Nb)1, (Fe)2(Hf)1, (Fe)2(Hf,Nb)1, and (Fe,Hf,Nb)1(Fe,Hf,Nb)1 by a two-sublattice model, respectively. μ was described as (Fe,Hf,Nb)1(Hf,Nb)4(Fe,Hf,Nb)2(Fe,Hf,Nb)6 by a four-sublattice model. A set of self-consistent thermodynamic parameters of the Fe–Hf–Nb system was obtained.