Microstructure and corrosion properties of the as-cast Ti–Ni–Cu–Zr alloy

The purpose of this work is to study the structure, phase composition and electrochemical properties of the cast Ti–Ni–Cu–Zr alloy in physiological solutions. In the current study, the conventional casting procedure (arc melting) without additional thermo-mechanical post-treatments was employed to fabricate a quaternary Ti₃₅Ni₃₅Cu₁₅Zr₁₅ (at. %) alloy. Microstructural characterization of the grain structure, phase composition and chemistry were performed by XRD, SEM/EDS and TEM/EDS methods. Corrosion properties of the mechanically grinded and electrochemical polished alloy were evaluated in 0.9 wt. % NaCl and artificial saliva. Diffraction analysis has shown that during crystallization of the Ti–Ni–Cu–Zr ingot two isostructural B2 phases, exhibiting different chemical compositions, are formed. Also, the precipitates of Ti₂Ni-like and TiZr particles are revealed. It is supposed that a decomposition of the B2 phase into two solid solutions is occurred, most likely, due to a bulk diffusion and slow cooling of the supersaturated solid solution upon crystallization of the ingot from the liquid state. The solidification of the melt during casting is supposed to occurred via formation of the matrix B2 phases by eutectic and secondary crystallization mechanisms. An intense anodic oxidation precedes a crucial corrosion destruction of the material notwithstanding of the relatively low corrosion current (J_corr ~ 0.15 μA/cm²) and high polarization resistance. (R_p ~ 250 kΩ cm²). The as-cast alloy is not recommended for medical applications without additional surface treatments.