The investigation of as-cast Ti–18Nb–$x$Si alloys with silicon content from 0.6 to 1.2% wt. is carried out. The formation of silicides at various heat treatment conditions (temperature and time), their distribution, growth and dissolution dynamics, as well as the effect on hardness and biological properties are studied. The greatest precipitation of silicides (Ti, Nb)$_3$Si occurs as a result of eutectoid decomposition of solid solution at 800–900°C. On quenching at $\leq$ 1000°C temperature range the silicide particles with sizes > 0.05 $\mu$m are precipitating mainly at the grain boundaries and defects of the structure, which do not affect the movement of dislocations under plastic deformation. So at these temperatures the alloys do not strengthen, their hardness is quite low. An exposure leads to an increase in the size of silicides and a further decrease in hardness. At higher quenching temperatures due to the precipitation of more dispersed and hard Ti$_5$Si$_3$ silicides at defects inside the martensitic plates, there is an increase in hardness as a result of the balance between dispersion hardening and a decrease in solid solution hardening because silicon is removed from the solution during silicide formation. When the cells culturing $in vitro$ on the samples of Ti–18Nb–$x$Si alloys, it is shown that alloying with silicon increases their bioactivity. The greatest cell proliferation is observed for the Ti–18Nb–1.2Si alloy, and that may indicate the effect of silicides on the biocompatibility.