Characterization of Lattice Defects in Refractory Metal High‐Entropy Alloy HfNbTaTiZr by Means of Positron Annihilation Spectroscopy

Abstract

Lattice defects in refractory metal high‐entropy alloy HfNbTaTiZr are investigated using positron lifetime spectroscopy combined with coincidence Doppler broadening (CDB) spectroscopy. Annealing of HfNbTaTiZr alloy at 1000 °C results in full recovery of defects and formation of single‐phase random solid solution characterized by the bulk positron lifetime of 141 ps. This value is in accordance with the theoretical estimation. Quenching of the alloy from high temperature prevents recovery of thermally equilibrium vacancies and enables to retain a high concentration of vacancies in the sample at ambient temperature. For single vacancies in the HfNbTaTiZr alloy, a characteristic positron lifetime of 212 ps is measured, which is significantly shorter than the value estimated by theoretical calculations. It indicates that there is considerable ion relaxation around vacancies. The CDB spectroscopy investigations reveal that vacancies in HfNbTaTiZr alloy are not distributed randomly but are preferentially coupled with Hf solutes.

Elucidating Primary Degradation Mechanisms in High-Cycling-Capacity, Compositionally Tunable High-Entropy Hydrides

“…Random distribution of Ti, V, Nb, and Cr atoms around vacancy seems to be surprising since it has been demonstrated earlier that vacancies in MPEAs/HEAs are preferentially located in the vicinity of atoms with large radii. 48,49 This latter result was, however, obtained for thermally equilibrium vacancies formed in the samples by quenching from high temperatures. Nevertheless, if vacancies are very mobile at high temperatures and can easily find the lowest energy sites in the lattice, vacancy clusters in hydrides studied in the present work were introduced by hydrogeninduced plastic deformations at room temperature.…”

“…Previous results on as-cast MPEAs/HEAs demonstrated the presence of two components in the positron lifetime spectra. 48,49 The shortlived component can be attributed to free positrons (not trapped at defects), while the longer one represents the contribution of positrons trapped in defects, either in dislocations introduced by cutting from ingots 43 or in vacancies introduced by quenching from elevated temperatures. 49 In contrast, the as-cast TiVNbCr alloy shows only one component from free positrons, indicating that this material contains a low density of defects and virtually all positrons are annihilated in the free state.…”