MS-4-P-3143 The study of Sn effect on recrystallization of hot rolled Ti-Nb-Ta-Sn biomedical alloy

Beta-titanium alloys are used as materials for bio-application. Their properties suitable for this purpose are low Young’s modulus, high strength, good corrosion resistance etc. These properties are attached with microstructure (phase composition, grain size etc.). These alloy exhibit very coarse grains in as-cast state. Therefore hot rolling (or hot forging) is desirable process step in order to obtain fine grained microstructure with desired mechanical properties. Fine grained microstructure is also suitable for subsequent cold deformation that may result in further improvement of mechanical properties. In this work hot rolling of-as-cast beta titanium alloys (type TiNbTaSn) was performed at temperatures between 900 and 1080°C in several steps. The total section reduction was about 50%. Subsequently rolled rods were solution treated at 850°C/0.5h/water quenched. The effect of Sn addition (6, 8 and 10 wt.% - marked 6Sn, 8Sn and 10Sn) has been studied by using light microscopy (LM) and electron microscopy (electron back scatter diffraction – EBSD).
The microstructure of hot rolled specimens consists mainly of very coarse and elongated grains of β-Ti (bcc) phase. These grains originate from as-cast grains. Inside these grains significant amount of low angle grain boundaries has been detected by EBSD analysis for all studied materials. This signs, that these coarse grains are deformed. Along their grain boundaries relatively fine grains occured in all studied specimens (e.g. Fig.1). This means that dynamic recrystallization took place during hot rolling on grain boundaries (places with higher accumulated deformation). Recrystallization was observed in solution treated 6Sn and 8Sn specimens, where fine equiaxed β-grains has been observed (Fig.2). In 6Sn specimen areas consisting of α”-martensitic phase are present (Fig3). These areas were not indexed during EBSD analysis (dark areas in Fig.2) This signs that MS (martensite start) temperature is lowered below room temperature by Sn addition, because no martensite has been observed in specimens 8Sn or 10Sn addition. In 10 Sn specimen both recrystallized and deformed grains has been observed (Fig.4). The recrystallization took place in this specimen only in highly deformed regions (original grain boundaries) during solution treatment, so the Sn hinders recrystallization in this type of alloys.