Effect of Fe on the Hall-Petch relationship of (CoCrMnNi)100-x Fex medium-and high-entropy alloys

In the present study, a systematic investigation on the effect of Fe content on the Hall-Petch coefficient of (CoCrMnNi)(100-x) Fe-x (x = 20, 40, 50, and 60) medium- and high-entropy alloys (M/HEAs) was carried out. The cold-rolled alloys were annealed at 900 degrees C and 1000 degrees C between 3 min and 10 h for recrystallization. Scanning electron microscope with a backscattered detector was used to obtain micrographs of recrystallized specimens for grain size calculation. Tensile testing was used to evaluate the mechanical properties of the alloys. The microstructure showed that regardless of the alloy composition, the grain size was approximately similar when subjected to the same heat treatment condition. Moreover, all the alloys obeyed the classical Hall-Petch relationship. Friction stress (solid solution, SS strengthening) decreased with an increase of Fe content, which was attributed to weak lattice distortion caused by the reduction of the atomic size misfit. The Hall-Petch coefficient, which represents grain boundary (GB) strengthening, also decreases as the Fe content increases. A linear relationship between intrinsic stacking fault energy and Hall-Petch coefficient was found not to exist. However, it is proposed that the monotonic decrease of the Hall-Petch coefficient is directly related to the unstable stacking fault energy (gamma(USFE)). As a result, an increase of Fe content in (CoCrMnNi)(100-x) Fe-x alloy system leads to an increase of gamma(USFE), which in turn weakens GB strengthening (Hall-Petch coefficient). Moreover, HEAs and MEAs with higher Fe content tend to have low yield strength due to weak contributions from both SS and GB strengthening. Therefore, to design superior MEAs and HEAs with enhanced strength, the choice of principal elements and their respective contents is imperative for an optimized contribution from both SS and GB strengthening mechanisms.