Studies of the Thermal Fatigue of Superalloys to Obtain Data to Calculate the Life of Parts

The reported results of studying the thermal fatigue of superalloys by the Coffin method are analyzed after varying the plastic deformation in a cycle ε_pl by changing the constraint stiffness of free thermal deformation ε₀, which induces elastoplastic deformation Δε = ε_el + ε_pl in half-cycles equal to 0.6, 1.0, and 1.3%, according to the results of testing a ZhS6F superalloy. It is found that, in tests with Δε = 0.6%, plastic deformation ε_pl is absent, and elastic deformation ε_el completely compensates the entire constraint part of ε₀. In these tests, fracture cannot be considered as a result of thermal fatigue, since, according to the Coffin law, the existence of ε_pl is a necessary condition for it. In tests with full constraint ε₀ and Δε = ε₀ = 1.3% in both half-cycles, plastic deformation occurs, and the measured maximum compression stresses σ_comp and tensile stresses σ_t correspond to the values of the yield strength achieved in the temperature ranges of heating and cooling after removing ε_el of the preceding half-cycle. In tests with partial constraint of ε₀ (when Δε = 1.0%), the situation is ambiguous. In this case, the change in Δε with temperature includes plastic deformation, as TC maps show. However, the measured values of stresses σ_comp and σ_t are substantially lower than yield strength σ_y. The observed results and the existence of ε_pl cannot be explained because of the absence of required data. The results of the analysis carried out in this work show that the construction of the dependence of life N_f on ε_pl provided by the Coffin method using the data obtained in the tests considered above is impossible. Therefore, the reliability of numerous reference data on the dependence of N_f on ε_pl obtained before in identical tests on the same equipment needs a verification for many superalloys.