Effect of deviations of composition from the quasi-binary section TiNi-TiCu on structural and phase transformations in rapidly quenched alloys

Methods of X-ray diffraction, transmission and scanning electron microscopy, and selected-area electron diffraction (SAED) have been used to study the phase and elemental composition and structure of alloys close to the stoichiometric Ti₅0Ni₂₅Cu₂₅ alloy. Based on the method of rapid quenching of the melt (free-jet melt spinning), alloys of the quasi-binary TiNi-TiCu section have been prepared, which in the initial as-cast state exhibited the thermoelastic martensitic transformations B ₂ ? B ₁₉ and related shape-memory effects. The chemical composition of the Ti_{50 + x} Ni₂₅Cu_{25 ? x} alloys was varied by changing titanium and copper concentrations within x ≤ ±1 at % (from Ti₄₉Ni₂₅Cu₂₆ to Ti₅₁Ni₂₅Cu₂₄). It has been established that quenching at a cooling rate equal to 10⁶ K/s leads to the amorphization of all the alloys under consideration. Heating to 723 K and higher leads to the devitrification of the alloy with the formation of a nanocrystalline or submicrocrystalline structure of the B2 austenite. The mechanical properties of these alloys have been measured in the initial amorphous state and in the polycrystalline martensitic state. It has been shown that, depending on the extent of the deviations of the alloy composition from the stoichiometry, which cause the decomposition of the alloys in the process of nanocrystallization, regular changes are observed in their mechanical properties and in the shape-memory effects. The kinetics of the processes of the devitrification of the alloys, as well of the forward and reverse martensitic transformations, have been studied, their characteristic temperatures have been determined, and a diagram of the dependence of the characteristic temperatures on the chemical composition of the alloys has been constructed.