Effect of strain rate induced M23C6 distribution on cyclic deformation behavior: Cyclic hardening model

Abstract

Cyclic deformation mechanism of Sanicro 25 alloy was investigated under different strain rates (ε˙) at 700 °C. When ε˙ increased, the maximum stress amplitude σa decreased and the fatigue life Nf increased. The saturated hysteresis energy ΔWp first decreased and then increased with increasing ε˙. It is attributed to the variation of stress amplitude, which is related to the interaction of precipitates and dislocations. HAADF STEM analysis shows that the Cr23C6 precipitates were distributed around the NbCrN precipitate when ε˙ was less than 2 × 10−3 s−1. The local deformation mainly concentrated in the grain interior owing to the formation of the bowed dislocations. This structure can lead to an increase in the number of creep cavities owing to the increasing stress concentration around the Cr23C6 precipitates, which indicates that creep damage Dc increases. The Cr23C6 precipitates nucleated and grew at the interface between NbCrN precipitate and matrix at ε˙=4×10−3 s−1, which can lead to a small stress concentration. Therefore, it implies that the Dc decreases at the relatively high ε˙. Considering the interaction between dislocations, the interaction of precipitates and dislocations, and the interaction of dislocations and grain boundaries, a unified cyclic hardening model was established. In addition, for the interaction of precipitates and dislocations at ε˙=2×10−3 s−1, the effect of the precipitates with various sizes on the bowed dislocations was considered. The predicted maximum cyclic stresses are in good agreement with the experimental values.