Boosting biocompatibility and mechanical property evolution in a high-entropy alloy via nanostructure engineering and phase transformations

High-entropy alloys (HEAs), as multi-component materials with high configurational entropy, have garnered significant attention as new biomaterials; still, their low yield stress and high elastic modulus need to be overcome for future biomedical applications. In this study, nanograin generation is used to enhance the strength and phase transformation is employed to reduce the elastic modulus of a biocompatible Ti-Zr-Hf-Nb-Ta-based HEA. The alloy is treated via the high-pressure torsion (HPT) process, leading to (i) a BCC (body-centered cubic) to ω phase transformation with ${\left[10\overline{1}\right]}_{\omega }//{\left[01\overline{1}\right]}_{\mathrm{BCC}}$ and ${\left[2\overline{1}1\right]}_{\omega }//{\left[\overline{1}2\overline{1}\right]}_{\mathrm{BCC}}$ through a twining mechanism, (ii) nanograin formation with a mean grain size of 20 ± 14 nm, and (iii) dislocation generation particularly close to BCC-ω interphase boundaries. These structural and microstructural features enhance hardness, increase tensile strength up to 2130 MPa, achieve tensile elongation exceeding 13 %, reduce elastic modulus down to 69 GPa and improve biocompatibility. Additionally, the HEA exhibits improved anodization, resulting in a homogenous distribution of oxide nanotubes on the surface with a smaller tube diameter and a higher tube length compared to pure titanium. These remarkable properties, which are engineered by the generation of defective nanograins and the co-existence of BCC and metastable ω phases, highlight the potential of HEAs treated using severe plastic deformation for future biomedical usage, particularly in the orthopedic sector.