Effect of Ta on Microstructures and Mechanical Properties of NiTi Alloys

Microstructures and mechanical properties of NiTiTa alloys with Ta contents of 5, 10 and 15 at.% were studied. It is found that, above the solubility limit of Ta, NiTiTa alloys consist of β-Ta, (TaTi)₂Ni, Ti₄Ni₂O oxide and NiTi matrix. (TaTi)₂Ni phase has a tetragonal crystal structure. It is brittle and broke up during processing. Ta in solution largely increases the phase transformation temperatures, while the extra Ta that forms secondary phases affects the phase transformation through its influence on residual internal energy. The volume fraction of β-Ta increases with increasing Ta content. Its ductility appears to be limited by tramp oxygen absorption. Although Ta addition largely improves the radiopacity of NiTiTa alloys, their future in medical applications may be impeded by the potential negative influences of brittle phases on ductility and fatigue performance.