The dual effect of grain size on the strain hardening behaviors of Ni-Co-Cr-Fe high entropy alloys

It has been well documented that grain size plays a critical role in the strain hardening behaviors of metals and alloys. However, the influence of grain size on the strain hardening of high entropy alloys (HEAs) was not fully understood. Here, we report that the grain size not only affects the twinning-induced plasticity (TWIP) effect but also changes the dislocation-based deformation behaviors of face-centered-cubic (fcc) HEAs significantly. The strain hardening and deformation micro-mechanisms of NiCoCrFe and Ni₂CoCrFe were investigated using electron channeling contrast (ECCI) analysis. Our results showed that Ni₂CoCrFe exhibits a typical three-stage strain hardening behavior and NiCoCrFe shows the fourth stage at high strains due to the TWIP effect. For both NiCoCrFe and Ni₂CoCrFe, the increase of grain size leads to a transition of dislocation glide from wavy to planar mode, resulting in a low value and the recovery of strain hardening rate in stage II. The large-grain NiCoCrFe showed a higher strain hardening rate in stage IV due to the promoted deformation twinning. Combining the strain hardening behaviors of the TWIP-NiCoCrFe and the mechanically stable Ni₂CoCrFe, we showed that the grain size influences the stage II hardening through tuning dislocation glide mode and the stage IV by tailoring deformation twinning activity of the Ni-Co-Cr-Fe HEAs. The grain size just affects stages I and III slightly in the current cases. These findings will also provide some insights into the understanding of strain hardening behaviors in other face-centered-cubic HEAs.