Ab initio tensile tests applied to bcc refractory alloys

Refractory metals exhibit high strength at high temperature, but often lack ductility. Multiprinciple element alloys such as high entropy alloys offer the potential to improve ductility while maintaining strength, but we do not know a priori what compositions will be suitable. A number of measures have been proposed to predict the ductility of metals, notably the Pugh ratio and the Rice-Thomson D-parameter, among others. Here we examine direct ab initio simulation of deformation under tensile strain, and we apply this to a variety of Nb- and Mo-based binary alloys and to several quaternary alloy systems. Our results exhibit peak stresses for elastic deformation, beyond which defects such as lattice slip, stacking faults, transformation, and twinning relieve the stress. The peak stress grows strongly with increasing valence electron count. Correlations are examined among several physical properties, including the above-mentioned ductility parameters.