Probing Nanoscale Phase Separation at Atomic Resolution within β-Type Ti–Mn Alloy: A Potential Candidate for Biomedical Implants

Abstract

Biocompatible β-type Ti alloys with high ultimate tensile strength (UTS) and yield strength are potential candidates for certain orthopedic and cardiovascular implants. Aiming for these applications, Ti alloy with 14 wt % Mn (Ti–14 Mn) as β-stabilizer was processed through thermomechanical treatment along with solutionizing and quenching, followed by 95% cold rolling, which resulted in ultrahigh UTS and yield strength. 3-[4,5-Dimethylthiazol-2-yl]-2,5-diphenyltetrazolimbromide assay with different cell lines suggests efficient cell growth on alloy surface without compromising biocompatibility. Cell adhesion and spreading assay show that cells are not only able to attach to the alloy surface but also able to spread and grow with normal morphology, which projects this material as a potential candidate for biomedical application. Previous studies on binary β-type Ti alloy systems treated with the above-mentioned processing route confirm the presence of nanoscale phase separation, which enhances its mechanical properties. To discover the same phenomena in the alloy of the present study, bright-field and high-resolution transmission electron microscopy (HRTEM) imaging experiments were performed and nanoscale contrast-modulated lamella regions were observed. Geometrical phase analysis on complex-valued exit wave, reconstructed using focal series HRTEM images, demonstrates that the lamella is a result of d-spacing modulation. Ab initio calculation indicates that d-spacing modulation with the same crystal structure occurs due to composition modulation and was proved by scanning transmission electron microscopy imaging coupled with quantitative energy-dispersive X-ray spectroscopy. Correlating contrast, strain, and composition modulation confirms nanoscale phase separation, which is the first report of this phenomenon in Ti–Mn alloy system.