Abstract <p>The parameters of the high-temperature background of internal friction of the dispersion-strengthened polycrystalline alloy VZh171 (KhN29K28VMT) are studied. It is found that the transition of the studied alloy from the elastic to the viscoelastic state occurs upon reaching the temperature limit <i>T</i>_cr = 791 K in at least two stages. The first stage determines the migration of crystalline structure point defects along the grain boundaries at low deformation in the low-temperature region, which corresponds to the diffusion activation energy of 0.51 eV. The second stage determines the detachment and movement of dislocations from impurity atoms, as well as the occurrence of grain boundary sliding in the high-temperature region, which requires an activation energy of 0.86 eV. The relative decrease in the shear modulus in the temperature range of 300–890 K was ≈0.15. To determine the temperature limit, a formula is proposed that allows calculating the temperature <i>T</i>_cr on the basis of the data on the activation parameters of the high-temperature background of internal friction. A general thermodynamic condition for the transition of a material from an elastic to a viscoelastic state corresponding to high-temperature deformation mechanisms is formulated. It is shown that the temperature boundary determining the beginning of a material transition from an elastic to a viscoelastic state can be used to diagnose the heat-resistant properties of nitrided alloys as an additional criterion.</p>