Thermal stability of a nanocrystalline HfNbTiZr multi-principal element alloy processed by high-pressure torsion

The microstructural stability of a nanocrystalline HfNbTiZr multi-principal element alloy was studied by annealing the material in a differential scanning calorimeter (DSC). The as-received sample was obtained by severe plastic deformation using the high-pressure torsion (HPT) technique. Samples were annealed to the characteristic temperatures of the DSC thermogram, namely up to 740, 890 and 1000 K, and the microstructure of the heat treated specimens was analyzed. It was found that the heating up to 740 K yielded a decomposition of the initial body-centered cubic (bcc) phase into two bcc phases with a lower and a higher lattice constant. During annealing up to 890 K, a Zr/Hf-rich hexagonal-close packed (hcp) phase started to nucleate. Heating the sample up to 1000 K resulted in the formation of a nanocrystalline Zr/Hf-rich bcc phase beside the Zr/Hf-rich hcp and the Nb-rich bcc phases. Beside the phase transformation a recovery of the severely deformed defect structure and a grain coarsening were also observed which was accompanied by a reduction of the hardness above 740 K.