Cyclic behavior of heat resistant nickel-base alloys EP741NP and EI698VD under strain-control loading

Low cycle fatigue behavior of nickel-base alloys EP741NP and EI698VD under strain-control loading with zero strain ratio was studied for a wide range of strain amplitudes and temperatures. Dependences of the stress amplitude, mean stress and plastic strain range on the number of cycles were analyzed. Three stages of the cyclic behavior were marked out: the first stage of non-steady behavior with hardening, or softening, or transition from hardening to softening; the second stage of steady hardening, softening or stability; and the third stage associated with a crack development. Qualitative and quantitative parameters are proposed that make it possible to determine a share of the first stage of cyclic instability in the total cyclic durability and the nature of the material behavior in the first and second stages proceeding from the analysis of the dependence of the range of plastic deformations on the cycle number. The absence of cyclic stability was demonstrated for both alloys almost in the whole range of testing. It was shown, that the contribution of the first stage to the fatigue life (total durability) can be up to 30% and the greater the strain range, the greater the contribution. The temperature dependence of the character of cyclic behavior of materials is revealed. Alloy EP741NP is softened at the first and second stages at room temperature and is liable to hardening at elevated temperatures 300, 450, and 600°C, whereas alloy EI698VD is hardened at the first stage at 20 and 400°C, but inclines to softening at 650°C. At the second stage alloy EI698VD inclines to softening at 20 and 650°C and is liable to hardening at 400°C. With strain ranges 0.6 and 0.7%, both alloys can be considered cyclic stable at the second stage over the entire temperature range