Strengthening in Al-, Mo- or Ti-doped CoCrFeNi high entropy alloys : A parallel comparison

In the current work, a parallel comparison of the influence of Al, Mo and Ti, on the microstructure and strengthening of the CoCrFeNi alloy was conducted. To achieve this, inconsistencies on variables including the extent of alloying, thermomechanical processing and property-evaluation method were avoided. Microstructurally, following cold-rolling, annealing of the 4 at.% Al-doped alloys at 800–1000 °C did not result in phase separation; nevertheless, that of the 4 at.% Mo- and Ti-doped alloys led to the respective formation of σ and η phase and, consequently, caused extra strengthening through the Orowan dislocation bypassing mechanism. Our systematic qualitative analysis and DFT calculations showed that Al and Ti are more effective than Mo in reducing the stacking fault energy (SFE) of the CoCrFeNi alloy, because they can induce more considerable deformation of electronic density, making the gliding of atomic layers easier. Following identical thermomechnical processing, Al-, Mo-, and Ti-doping causes different extent of solid solution strengthening and grain boundary strengthening. Mo causes the most pronounced solid solution strengthening but does not benefit the grain boundary strengthening; in contrast, the effectiveness of grain boundary strengthening is boosted by the doping Al and Ti. Current analyses support that Labusch instead of Fleischer mechanism is applicable to explain the differences in solid solution strengthening, and the observed differences in grain boundary strengthening arise from the different tendency of Al, Mo and Ti to reduce the SFE of CoCrFeNi. In addition, we determined the value of the dimensionless parameter ƒ in the Labusch model for CoCrFeNi-based alloys and observed a close relation between Hall-Petch slope and SFE. Although more in-depth studies are needed to provide full and mechanistic understandings, both these findings in fact presents significant values toward designing novel single-phase high-strength CoCrFeNi-based alloys through manipulating the solid solution and grain boundary strengthening by compositional tuning.