  Stacking fault energy (SFE) plays an important role in deformation mechanisms
and mechanical properties of face-centered cubic (fcc) metals and alloys. In
metastable fcc alloys, the SFEs determined from density functional theory (DFT)
calculations and experimental methods often have opposite signs. Here, we show
that the negative SFE by DFT reflects the thermodynamic instability of the fcc
phase relative to the hexagonal close-packed one; while the experimentally
determined SFEs are restricted to be positive by the models behind the indirect
measurements. We argue that the common models underlying the experimental
measurements of SFE fail in metastable alloys. In various concentrated solid
solutions, we demonstrate that the SFEs obtained by DFT calculations correlate
well with the primary deformation mechanisms observed experimentally, showing a
better resolution than the experimentally measured SFEs. Furthermore, we
believe that the negative SFE is important for understanding the abnormal
behaviors of partial dislocations in metastable alloys under deformation. The
present work advances the fundamental understanding of SFE and its relation to
plastic deformations, and sheds light on future alloy design by physical
metallurgy.
