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-centeredcubic(fcc)HEAs significantly.The strain hardening and deformation micro-mechanisms of NiCoCrFe and Ni_(2)CoCrFe were investigated using electron channeling contrast(ECCI)analysis.Our results showed that Ni_(2)CoCrFe 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 Ni_(2)CoCrFe,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 TWIPNiCoCrFe and the mechanically stable Ni_(2)CoCrFe,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.

Examining the effect of the aging state on strength and plasticity of wrought aluminum alloys

A general rule of strength and plasticity was proposed for three typical wrought Al alloys(2xxx,6xxx,and 7xxx)subjected to different aging times.Investigations of the work-hardening processes and dislocation configurations in tensile and compressive testing reveal that this general rule arises because there is a common mechanism for these three kinds of wrought alloys whereby the tendency for cross-slip increases monotonously with aging time.By analyzing the strain hardening exponent and the stacking fault energy,it is demonstrated that the change in the dislocation slip mode is attributed mainly to the formation of second phases rather than to the matrix composition.Accordingly,a new work-hardening model was proposed for wrought Al alloys containing second phases and this explains the interaction between dislocations and second phases and other relevant experimental phenomena.This work is therefore beneficial for quantitatively investigating and optimizing the strength and plasticity of wrought aluminum alloys.