The effect of stress state on rafting mechanism and cyclic creep behavior of Ni-base superalloy

A three-stepped specimen of Ni-base superalloy was designed in the paper to conduct cyclic creep tests under different stress levels. The experiment aimed at studying the effect of loading cyclic numbers and the stress state on the rafting mechanism and the cyclic creep behavior. The design of three-stepped specimen can help to simultaneously analyze the extent of microstructure evolution under different stress states, which included uniaxial states in gauge sections and multiaxial stress states in beveled sections. Metallographic observations revealed that both the rafting behavior and topological inversion behavior of γ/γ′ microstructure occurred during the cyclic creep tests. The specific connectivity number of the γ′ phase N_A (γ′) was introduced to assess the extent of these microstructure evolutions, which was found to be related to the loading cyclic numbers and stress levels. Then a parameter of normalized cyclic number T_r was proposed to consider the contribution of these two factors. The extent of microstructure evolution was observed increasing with the normalized cyclic number. And the relationship between evaluation parameters N_A (γ′) and T_r was proved to follow an asymptotic equation. The finite element model that established in the paper assisted to qualitatively analyze the rafting or topological inversion process. It successfully predicted the cyclic lives and acquired the normalized cyclic number T_r. The magnitude and distribution gradient of principal stress field in finite element model effectively account for rafting behavior under different stress states.