Correlations between valence electron concentration and the phase stability, intrinsic strength, and deformation mechanism in fcc multicomponent alloys

Valence electron concentration (VEC) is a crucial parameter affecting the structure and mechanical properties of materials. Nevertheless, the influence of VEC in multicomponent alloys remains muddy. Here, the correlations between VEC and phase stability, intrinsic mechanical strength, and deformation mechanism are revealed by investigating six equiatomic $\mathrm{CoNi}M$ ($M$ = Ti, V, Cr, Mn, Fe, and Cu) alloys with VEC ranging from 7.7 to 10.0. As VEC increases, the ground-state structure evolves from bcc to hcp and ultimately to fcc. Both elastic moduli and ideal tensile strength increase initially and then decrease with increasing VEC, i.e., an inverted parabolic trend, which arises due to the orbital-filling effect of valence electrons. We highlight that a medium VEC ($\ensuremath{\sim}8.3$) has superior intrinsic strength at a full filling of the bonding state but an empty antibonding state. Furthermore, a strong link between intrinsic strength and the energy difference between bcc and fcc ($\mathrm{\ensuremath{\Delta}}{E}_{\mathrm{fcc}\ensuremath{\rightarrow}\mathrm{bcc}}$) is built. The stacking fault (SF) energy ${\ensuremath{\gamma}}_{\mathrm{sf}}$ roughly increases with VEC except for in CoNiCr; this correlation is inherited from the energy difference between fcc and hcp. The dependence of intrinsic deformation energy barriers on VEC follows that of $\mathrm{\ensuremath{\Delta}}{E}_{\mathrm{fcc}\ensuremath{\rightarrow}\mathrm{bcc}}$ since both are linked to the bonding strength. Depending on orientation, dislocation slip (SL) and SF can co-occur in the alloys with low VEC while SL and twinning (TW) coexist in those with high VEC. Both competition between SF or TW and SL and competition between SF and TW are dictated by ${\ensuremath{\gamma}}_{\mathrm{sf}}$. This work unveils relations between VEC and phase stability, intrinsic strength, and deformation mechanisms and could provide some useful information for designing novel multicomponent alloys.