N. I. Medvedeva

Ternary site preference energies, size misfits and solid solution hardening in NiAl and FeAl

The electronic structure of NiAl and FeAl doped with Ti, V, Cr, Mn, Fe, Co, Y, La and Zr additions in both sublattices has been investigated with the local density linear muffin-tin orbital (LMTO) method. The peculiarities of chemical bonding for both undoped and with some ternary additions in NiAl and FeAl were analyzed using the LMTO-Green function method. The preferred sites for ternary additions were found and the sensitivity of site preference energies to crystal relaxation effects was investigated. It was shown that the relaxation has an elastic nature and the values of size misfit parameters for substitutional impurities were estimated. The experimental data concerning solid solution hardening were reanalyzed and the additional contributions to hardening besides the conventional size misfit mechanism were shown to be of importance for Ti ternary additions.

Carbon stabilized A15 Cr3Re precipitates and ductility enhancement of Cr-based alloys

Abstract A new metastable A15 phase in the Cr–Re system is predicted based on full-potential linear muffin-tin orbital (FLMTO) calculations of its electronic structure and cohesive properties. While A15 Cr3Re is unstable as a bulk compound, we demonstrate that nonstoichiometry and carbon impurities play a dominant role in stabilizing this phase and may lead to the occurrence of A15 type particles in Cr-based alloys. Thus, these A15 precipitates may scavenge carbon impurities and prevent the formation of brittle carbides and, additionally, provide an obstacle for dislocation motion. The formation of small close-packed particles appear to explain the improvement of both ductility and strength and is most likely the mechanism of the “rhenium effect” in Cr (W, Mo) based alloys.