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Features and uniform description of I, II, and III stages of the creep in single-crystal superalloys

Features of the creep process in single-crystal nickel-based superalloys are studied in a wide range of temperatures and holding time for different crystallographic orientations. The results of experimental study of high-temperature creep obtained for different single-crystal superalloys are presented. The dominance of stage III of creep is observed for considered temperature range and loads. Uniform creep models describing I, II, and III stages, are proposed taking into account damage accumulation based on the Norton – Bailey relations and power law of evolution for the scalar damage measure of Kachanov – Rabotnov. A hierarchical sequence of creep models of various degrees of complexity is suggested depending on the necessity of taking into account stage I. A material model including six constants is sufficient to describe stages II and III. Simultaneous accounting of all three stages of creep can be carried out using model with nine constants. The assumption on the absence of damage at the first stage leads to a material model with ten constants. The entered additional tenth parameter characterizes the duration of the first stage. Identification methods for the parameters of the introduced models based on using the least-squares method and Nedeler – Mead method for solving the problem of minimizing the error functional are proposed. The results of verification of the proposed inelastic deformation models are presented for various nickel-based single-crystal superalloys. The standard deviation between the experimental and computation results for all the proposed creep models does not exceed 10%. This allows us to recommend the developed approach for estimating the level of irreversible accumulated strains and durability of the structural elements made of single-crystal superalloys.