Influence of Silicon on the Structure and Hardness of Biomedical Ti–18Nb–4Zr–xSi Alloys in the As-Cast State and After Quenching

We study the biomedical Ti–18Nb–4Zr–xSi cast alloys with a silicon content of 0.5–1.5 wt.%. Quenching in water was carried out within the temperature range 900–1200°С with durations of holding equal to 5 min and 1 h. It is discovered that the procedure of heating of these alloys to the temperatures of quenching leads to the decay of nonequilibrium phases, and the silicon content is redistributed between the phases in a solid solution and silicides. Since almost all silicon is bound with zirconium and titanium and form silicides, the hardness of the Ti–18Nb–4Zr–xSi alloys mainly depends on the amount of silicides and constitutes 26–28 HRC. Moreover, its maximum values are reached for the alloys with eutectoid compositions containing 0.8–1.2 wt.% Si characterized by the most intense release of finely divided silicides. The increase in the silicon content of the hypereutectoid alloys leads to an increase in the sizes of silicides, as well as to the formation of larger amounts of the β-phase in the as-cast Ti–18Nb–4Zr–xSi alloys, and as a result, their hardness noticeably decreases. For low quenching temperatures (within the range 900–1000°C), the complete eutectoid destruction accompanied by the formation of relatively large stable (Ti, Zr)₃Si silicides leads to a decrease in hardness < 25 HRC. In the course of quenching of the as-cast Ti–18Nb–4Zr–xSi alloys at temperatures ≥ 1100°C, we observe the formation of more finely divided silicides, which increases the level of hardness of the eutectoid alloys up to 38–39 HRC. The observed changes in the parameters of the α″-phase demonstrate that, as a result of partial dissolution of silicides in the course of quenching at 1200°C, silicon passes into a solid solution and the amount of large silicides on the grain boundaries increases. Therefore, the level of hardness of the analyzed quenched alloys decreases.