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 (ΔfH° at 300K) of the cubic L12–HfAl3-xZnx solid solution. A high-temperature direct drop calorimeter has been employed to synthesize and simultaneously determine the ΔfH° 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 HfAl3-xZnx 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 ΔfH° (kJ/mol-atom at 300K) for the L12–HfAl3-xZnx were determined: -37.1±2.0 (HfAl0.8Zn2.2 corresponding to Hf25Al20Zn55 at. %), -41.7±2.0 (HfAl1.2Zn1.8 corresponding to Hf25Al30.0Zn45.0 at. %), -45.1±2.0 (HfAl1.5Zn1.5 corresponding to Hf25Al37.5Zn37.5 at. %) and -48.5±2.0 (HfAl1.8Zn1.2 corresponding to Hf25Al45.0Zn30.0 at. %). For two pertinent binary intermetallic phases, the following ΔfH° values (in kJ/mol-atom) at 300K have been obtained: -31.8±3.0 for HfZn3 (unknown structure) and -37.0±2.0 for HfAl3 (tetragonal DO23 – type structure).