Effect of impurities on the electronic structure and stability of the A15 phase of chromium

The cubic A15 phase is well-known as an important metastable phase for {\it bcc} transition metals: it was observed in thin films and as ultra-fine particles in Cr, Mo and W alloys and its formation in O and N atmospheres is found to be more preferable compared to the {\it bcc} structure. We present first-principles FLAPW\footnote{Wimmer, Krakauer, Weinert, and Freeman, PRB {\bf 24}, 864 (1981)} results on the electronic structure, elastic constants and stability of the A15 phase in Cr and discuss the stabilizing role of light impurities. At equilibrium, the total energy of the A15 structure is only 3 mRy higher than for {\it bcc} Cr, and the calculated A15 elastic moduli do not demonstrate any shear instability. The formation of stacking faulted (twin-related) structures and A15-based phases of Cr alloyed with substitutional Re, Fe, Ni and interstitial/substitutional O, N, C was investigated taking into account full structural optimization, and the most preferable structures were found. We discuss the effect of precipitates of such binary and ternary phases on the mechanical properties of Cr and suggest that the mechanism of the ``rhenium effect'' -- namely, the improvement of ductility and strength of {\it bcc} metals upon alloying with Re -- is connected with the presence of A15-type close-packed particles.