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

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