Equilibrium versus non-equilibrium stacking fault widths in NiCoCr

First principles calculations in the NiCoCr medium-entropy alloy predict a negative stacking fault energy (SFE) at T=0 K, implying an infinite Shockley partial dissociation distance. Many experiments at room temperature (RT) show however a finite dissociation. This discrepancy has been suggested due to solute strengthening that prevents the partial separation. Here, atomistic simulations in a model NiCoCr alloy having a negative SFE show that solute strengthening can limit partial separation at T=0 K but, the solute-induced barriers are easily overcome at RT and time scales of only 1 ns. Under experimental conditions (time scales of hundreds of seconds and longer), solute pinning is therefore insufficient to limit dissociation. Finite partial separations are thus presumably due to a positive stacking fault free energy at RT or short-range-order effects. It is argued here that the former is more likely than the latter.

Automated summary

First principles calculations predict a negative stacking fault energy in the NiCoCr alloy at 0K, indicating an infinite dissociation distance. However, experiments at room temperature show a finite dissociation, possibly due to solute strengthening. Atomistic simulations of a model NiCoCr alloy with a negative SFE reveal that solute strengthening can limit partial separation at 0K, but these barriers can be easily overcome at room temperature and shorter time scales. Therefore, solute pinning is insufficient to prevent dissociation under experimental conditions, suggesting a positive stacking fault free energy at room temperature or short-range-order effects.