Fluence- and thickness-dependent microstructure evolutions in Ti-Zr-Hf-Nb-Ta high entropy alloy thin films

Abstract

Refractory Ti-Zr-Hf-Nb-Ta high entropy alloy (HEA) attracts numerous attention owing to its ductility at room temperature and biomedical application. Extensive work on mechanical properties of this alloy has been done in the bulk alloy, however, there is limited work dedicated to the investigation in thin film. In this work, a series of Ti-Zr-Hf-Nb-Ta thin films with the deposition power from 60 W to 360 W and film thickness of ∼1.5–2.0 µm are prepared. The film morphology, microstructure and nano-mechanical properties can be controlled by the deposition power and film thickness. With increasing deposition power, the amorphous fraction experiences a process of first decreasing and then increasing, and finally decreasing for 2.0 µm-thick film (saturation for 1.5 µm-thick film). The competition between adatom diffusion, available time and elevated temperature during deposition decides the phase selection in various deposition power range. At low deposition power range, adatom diffusivity is dominant, leading to the increased crystallinity with power. At intermediate power range, available time for atomic rearrangement controls phase selection, causing the regaining of amorphous phase. The elevated temperature effect is responsible for re-devitrification at high deposition power range. Furthermore, thicker film possesses higher crystallinity than thinner counterpart, and the crystallinity difference between ∼1.5 µm- and ∼2 µm-thick films at the same deposition power depends on atomic fluence. Also, continuous strengthening from 60 W- to 360 W-thin film are observed, which can be attributed to improved adhesion between nanocolumns at low deposition power range and bombardment effect at high deposition power range. This work sheds a comprehensive and profound light on deposition power-structure and film thickness-structure correlations.