Intra-granular alpha precipitation in Ti-Nb-Zr-Ta biomedical alloys

This article investigates the intra-granular precipitation of nanometer-scale α platelets in the β matrix of a complex quaternary β titanium alloy, Ti-35Nb-7Zr-5Ta (all in wt%), for orthopedic implant applications, during β-solutionizing/quenching/aging type heat-treatments. The role of metastable ω precipitates on the nucleation and growth of these α precipitates, has been specifically addressed by coupling transmission electron microscopy (TEM) and 3D atom probe (3DAP) tomography studies on this alloy. Athermal ω precipitates form in this alloy on quenching from above the β-transus temperature. On isothermal annealing at low temperatures (~400 °C), these ω precipitates coarsen, rejecting Zr into the adjacent β matrix as determined by 3DAP studies. Concurrently, the nucleation and growth of α precipitates is initiated at or near the ω/β interfaces, as determined by TEM studies. In addition to coherency strains induced by the ω precipitates, the local enrichment of Zr adjacent to these precipitates appears to play an important role in aiding the nucleation and growth of Zr-rich α precipitates in this alloy.