<p>Newly developed TiNiCuZr-based alloys are of significant interest for medical applications due to their ductility, corrosion resistance, and shape memory effects. Alloying the TiNi alloy with copper and zirconium enables the formation of two fundamentally different phase states: amorphous and crystalline under non-extreme changes in external conditions. The outer amorphous layer, formed in the medical TiNiCuZr alloy, may effectively protect the material surface from aggressive biological media and improve the corrosion resistance. In the current study, the ion-beam treatment of the TiNiCuZr shape memory alloy leads to the formation of an amorphous surface layer. The goal of the study is to reveal the mechanisms of radiation-induced phase transitions and amorphization of the near-surface layer in order to optimize the properties of TiNiCuZr alloys. For this, the comprehensive TEM and XRD study of the structural-phase state of the Ti₃₅Ni₃₅Cu₁₅Zr₁₅ (at. %) alloy subjected to the high-dose ion implantation was performed. The radiation-induced transformations of the matrix B2 phase in the near-surface zone were revealed. XRD analysis has shown that as-cast and ion-beam treated Ti–Ni–Cu-Zr alloys consist of two isostructural (B2 + B2') modifications of the matrix phase, exhibiting different lattice constants. It has been proposed that the decomposition of the B2 phase is related to bulk diffusion and slow cooling of the supersaturated solid solution during crystallization of the ingot from the liquid state. According to the TEM data, the ion-beam modification of the Ti–Ni–Cu-Zr samples leads to the solid-state amorphization of a thin (~ 100&#xa0;nm) surface layer. The issues of the texture formation and excess vacancies in the B2 superstructure after irradiation were discussed.</p> Graphical abstract <p></p>