Microstructure stability and self-diffusion in the equiatomic HfScTiZr HCP multi-principal element alloy

Tracer diffusion of 44Ti, 46Sc and 89Zr in the hexagonal close-packed (HCP) HfScTiZr multicomponent alloy is investigated using the radiotracer technique. The microstructure stability is thoroughly examined by conducting prolonged heat treatments between 873 K and 1373 K to mimic the diffusion conditions. Electron microscopy analysis reveals that the alloy contains a mixture of two HCP phases with similar lattice constants, among which the major phase is enriched in Sc, whereas the minor phase is strongly enriched in Ti. The thermal stability is assessed by ab initio-informed calculations which support the existence of the two HCP phases. The Sc diffusion coefficients differ markedly between the HCP phases, while only marginal influence is found for the diffusion of Zr and Ti. The Arrhenius parameters are determined, i.e., the activation energy and the pre-factor. The diffusion properties are analyzed in correlation with the microstructure stability and the ab initio-informed thermodynamic results. The DFT-calculated mean squared atomic displacements, which represent the lattice distortions, are found to provide an appropriate parameter for predicting the diffusion trends of the individual elements. The phase analysis is further compared with existing CALPHAD-type predictions, and the discrepancies between the theoretical assessments and experimental observations are examined and discussed.