Specifics of crystallization of amorphous TiNiCu alloys with high concentrations of copper

Ribbons of alloys of the quasi-binary TiNi–TiCu system with 25, 30, 35, and 40 at% copper were fabricated by planar flow casting at a cooling rate of 10$^6$ K/s. The thickness of the ribbons was 30–50 $\mu$m, and their width was 10–20 mm. The structure and phase transformations of alloys were studied using electron microscopy, X-ray diffraction, and differential scanning calorimetry. It was found that the alloys with 25 and 30 at% Cu in the initial state have an amorphous-crystalline structure, which undergoes one-stage polymorphic crystallization of the amorphous state with the formation of austenitic phase $B$2 under heating in a calorimeter. When cooled to room temperature, phase $B$2 undergoes a martensitic transformation to orthorhombic phase $B$19. It was demonstrated that the alloys with 35 and 40 at% Cu become amorphous when quenched and undergo two-stage (primary and eutectic) crystallization under heating. The resulting structure is two-phase (tetragonal phase $B$11 (TiCu) with a small fraction of phase $B$2). An increase in the concentration of copper leads to a reduction in the onset temperature of crystallization.