Second-phase-dependent grain refinement in Ti-25Nb-3Mo-3Zr-2Sn alloy and its enhanced osteoblast response.

Ti-25Nb-3Mo-3Zr-2Sn (TLM) substrates, which consist of pure β phase and duplex α+β phases were achieved by different heat treatment. Different substrates with and without α phase were subjected to surface mechanical attrition treatment (SMAT) for 5 and 30 min, respectively. Investigated by transmission electron microscopy (TEM), it is found that the content and morphology of α phase in the TLM substrates play crucial roles in nanocrystallization of the alloy. During SMAT, the substrates composed of duplex α+β phases are much easier to nanocrystallized than that composed of pure β phase, and the duplex-phase substrate containing 35 vt.% α needles is more inclined to grain refinement than those substrates containing 27 vt.% α cobbles and 31 vt.% α needles. Accompanied with the nanocrystallization in the surface layers of the duplex-phase substrates, the stress induced α-to-β phase transition occurs. In addition, employing hFOB1.19 cells, the behaviors of osteoblasts on the unSMATed and as-SMATed surfaces were evaluated by examining the morphology and viability of the cells. It shows that the SMAT-induced grain refinement in the surface layer of the alloy can significantly improve the osteoblast response. Our study lays the foundation for nanostructuring β titanium alloys to be used as biomedical implants.