Experimental determination of phase stability of the cubic L12 HfAl3-xZnx phase

The present study is devoted to the experimental investigation of homogeneity range and heat of formation (Δ_fH° at 300 K) of the cubic L1₂–HfAl_3-xZn_x solid solution. A high-temperature direct drop calorimeter has been employed to synthesize and simultaneously determine the Δ_fH° of several alloys along the HfZn₃–HfAl₃ section (25 at. % Hf) whereas X-Ray Powder Diffraction (XRPD) and Scanning Electron Microscopy (SEM) paired with an EDS (Energy Dispersive Spectrometer detector) have been employed to characterize the samples. The performed analysis confirmed that the ternary HfAl_3-xZn_x alloys were nearly single phase in the range 1 ≤ x ≤ 2.24 having the cubic L1₂ structure; this in turn helps establish the trend of L1₂ lattice parameter (at room temperature) with composition. Thanks to the interpolation of our experimental data, the following values of Δ_fH° (kJ/mol-atom at 300 K) for the L1₂–HfAl_3-xZn_x were determined: -37.1 ± 2.0 (HfAl_0.8Zn_2.2 corresponding to Hf₂₅Al₂₀Zn₅₅ at. %), -41.7 ± 2.0 (HfAl_1.2Zn_1.8 corresponding to Hf₂₅Al_30.0Zn_45.0 at. %), -45.1 ± 2.0 (HfAl_1.5Zn_1.5 corresponding to Hf₂₅Al_37.5Zn_37.5 at. %) and -48.5 ± 2.0 (HfAl_1.8Zn_1.2 corresponding to Hf₂₅Al_45.0Zn_30.0 at. %). For two pertinent binary intermetallic phases, the following Δ_fH° values (in kJ/mol-atom) at 300 K have been obtained: -31.8 ± 3.0 for HfZn₃ (unknown structure) and -37.0 ± 2.0 for HfAl₃ (tetragonal DO₂₃ – type structure).