Structural Features of High-Entropy HfTaTiNbZr Alloy Fabricated by High-Energy Ball Milling

Abstract

The HfTaTiNbZr high-entropy alloy (HEA) of equimolar concentration is fabricated from powder components by high-energy ball milling (HEBM) and spark plasma sintering (SPS). The treatment of the initial powders was performed in a planetary high-energy ball mill for 20, 40, 60, and 90 min. It is shown based on the studied surface morphology, microstructure, and phase composition of HEA samples that the multicomponent powder mixture Hf + Ta + Ti + Nb + Zr is subjected to substantial structural variations during HEBM. Starting from X-ray diffraction analysis (XRD) data, it is established that mill treatment for 20 min leads to the formation of a Hf-based solid solution with an fcc structure (Fm3m). Subsequent HEBM for 40 min promotes the formation of a Ta-based solid solution with a bcc structure (Im3m). Peaks of Hf-based and Ta-based solid solutions in the diffraction pattern completely merge after 60-min treatment, forming one common asymmetric peak in an angle range of ~35°–51°. It is revealed that the formation of the HEA with the HfTaTiNbZr composition and bcc structure is observed after 90-min HEBM. The material has a homogeneous structure according to scanning electron microscopy (SEM) data, and the results of energy-dispersive analysis showed that initial elements Hf, Ta, Ti, Nb, and Zr are uniformly distributed in the material bulk. The powders formed after 90-min HEBM are sintered at t = 1150 and 1350°C for 10 min. The results of XRD, SEM, and energy-dispersive spectrometry of SPS-consolidated high-entropy alloys at t = 1350°C show that the material preferentially consists of one phase with the bcc structure and a small amount of Hf2Fe and ZrO. The hardness of the sintered HEA material (10.7 GPa) exceed the hardness of the material consolidated from the mixture of initial elements (6.2 GPa) by a factor 1.8. Density of the samples sintered from initial and HEA powders at t = 1350°C is 9.49 g/cm3 (95.8%) and 9.87 g/cm3 (99.7%), respectively.