  Plasticity in body centered cubic (bcc) refractory metals are largely due to
the stress tensor induced either by solute or thermal activation. The mechanism
of the solute atom(s) residence causes instability in such metals. Earlier
research have considered the mechanism of oxygen (O) or carbon (C) in tungsten
(W), even though the major component of the environment is nitrogen (N). In
this article, the density functional theory (DFT) was employed to investigate
the thermodynamic stable site for an interstitial solute (N or O) in the bcc
refractory metals (Mo and Nb) by calculating the equilibrium and structural
parameters, dissolution energetics and volumetric strain. The dissolution
mechanism of all the relaxed solid solution structures were predicted to be an
exothermic reaction from the supersaturated cell to the low concentration (1.82
at.%) except for Mo-N solid solution. Convergence of volumetric strain was
observed at the low concentration of the solute. At this point, the solid
solution of Mo-N and Mo-O had a less measure of global stress (less distortion)
at the octahedral (o) site while that of Nb-N and Nb-O were at the tetrahedral
(t) site. This certainly shows why this two bcc-refractory metals in groups VB
(Nb) and VIB (Mo) of the periodic table exhibit different deformation
behaviours giving their difference in site preference stability.
