Nanostructured β-type titanium alloy fabricated by ultrasonic nanocrystal surface modification

•UNSM is a method that utilizes ultrasonic vibration to generate nanostructured surface.•The surface of β-type Ti-Nb-Ta-Zr (TNTZ) alloy was treated with the UNSM technique.•A gradient nanostructured layer was generated in the surface.•Top surface layer consisted of nanoscale lamellae with a width of about 60–200nm.•Hardness of treated surface was ∼385HV compared to 190HV for the untreated alloy. The surface of β-type Ti-Nb-Ta-Zr (TNTZ) alloy, which is a promising material for biomedical applications, was treated with the ultrasonic nanocrystal surface modification (UNSM) technique to enhance its hardness. As a result, a gradient nanostructured (GNS) layer was generated in the surface; the microstructure of the top surface layer consisted of nanoscale lamellae with a width of about 60–200nm. In addition, there were lamellar grains consisting of nanostructured subgrains having unclear and wavy boundaries. The treated surface exhibited a hardness value of ∼385HV compared to 190HV for the untreated alloy. It was further determined that highly dense deformation twins were generated at a depth of ∼40–150µm below the UNSM-treated surface. These deformation twins led to a significant work hardening effect which aided in enhancing the mechanical properties. It was also found that UNSM treatment resulted in the formation of micropatterns on the surface, which would be beneficial for high bioactivity and bone regeneration performance of TNTZ implants.