  Developing ductile refractory BCC alloys has remained a challenge. The
intrinsic ductility (D) of an alloy is the ratio of surface energy ($\gamma_s$)
and unstable stacking fault energy ($\gamma_{usfe}$). Lowering the valence
electron concentration has been shown to improve the intrinsic ductility of
refractory alloys. However, Re has been widely used to ductilize W, contrary to
the low valency criteria suggested in the literature. Here we use density
functional theory to calculate the enthalpy of formation, $\gamma_{usfe}$ and
$\gamma_s$ of Group IV, V, VI elements and their 25 equiatomic binary alloys in
BCC crystal structure. We found that positive enthalpy leads to a considerable
reduction in $\gamma_{usfe}$ compared to composition averaged value, resulting
in improved intrinsic ductility. Enthalpy is maximum at the equiatomic
concentrations indicating the highly repulsive interaction between the alloy
constituents and vicer-versa. We found that the repulsive interaction between
the alloy constituents leads to a reduction in $\gamma_{usfe}$, making alloys
intrinsically ductile.
