The effect of prolonged natural aging on the structure and characteristics of martensitic transformations in three-component titanium nickelide alloys doped with niobium, vanadium, silicon and copper was studied by X-ray analysis of polycrystals. Titanium nickelide products are used in medicine as implants and are implanted into the human body for many years and decades, which entails the requirement for the immutability of the material's characteristics over a long period of operation in order to avoid tragic consequences. It was found that an increase in the duration of isothermal aging of alloys at room temperature from 3 to 6 years to 45 years had an effect on the structural and phase state of each alloy studied. Alloys Ti46Ni46Cu8, Ni51Ti45Si4, Ti46Ni46Nb8, Ti44Ni44Nb12 exhibit a rather weak change in the crystalline state. The alloys of the TiNi-V system (Ti48Ni48V4 and Ti46Ni46V8) in naturally aging samples exhibit the greatest instability at the crystal structure level, which raises the question of the utmost attention to the safety of using titanium nickelide products and alloys based on it for medical purposes.
$^1$M.V. Lomonosov Moscow State University, Faculty of Physics, Solid state physics department\
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