Hot Rolling Effects on the Microstructure and Chemical Properties of NiTiTa Alloys

In the present paper, a Ni-37Ti-15Ta (wt.%) shape memory alloy was analyzed in as-cast and hot-rolled conditions, by differential scanning calorimetry, scanning electron microscopy coupled with energy-dispersive spectroscopy, x-ray diffraction and electrochemical tests into a simulated body fluid (SBF). Both processing states experienced reversible martensitic transformations, with lower temperatures of reverse transformation at hot-rolled specimen, where the amount of secondary phases suffered contrary variation tendencies: β-Ta solid solution increased and (Ti,Ta)2Ni intermetallic compound decreased. Hot rolling contributed to the increase in internal stresses which caused a slight decrease of 2θ angle and to the stress-induced formation of B19’ martensite from B2 parent phase. The electrochemical corrosion test, performed in an experimental SBF, emphasized the superior corrosion resistance of as-cast specimen, since it had a lower amount of internal stresses. The corrosion compounds, mostly oxides, were completely removed by ultrasonic cleaning.