Solid solution softening in bcc Mo alloys: Effect of transition-metal additions on dislocation structure and mobility

The electronic origin of solid solution softening (SSS) in bcc molybdenum alloys was investigated in the framework of a combined approach that includes atomistic dislocation modeling with first principles parametrization of interatomic interactions. The softening additions are found to locally change the chemical bonding which results in a decrease of the generalized stacking fault (GSF) energy and atomic row shear resistance. Using the atomic row model, we show that the isotropic core of the screw dislocation in Mo tends to a "split" (planar) core under alloying with softener solutes (Re, Os, Ir, Pt). The generalized Peierls-Nabarro model for a non-planar core was employed to link the reduction in GSF energy with the enhancement of double kink nucleation and dislocation mobility. Our study appears to explain the experimental dependence of the alloying effect on the atomic number of the addition and to provide an understanding of the electronic reasons for SSS in Mo alloys.