Effects of alloy additions on the microstructure and properties of Cr-Cr2Nb alloys

Alloying additions of Ni, Co, Fe, Al and Re at levels up to 16 at.% were added to Cr-Cr2Nb alloys containing 5.6-17 at.% Nb for the study of their microstructure, oxidation behavior, and mechanical properties. These alloys contain patches of primary Cr-rich solid solution surrounded by the eutectic structure having Cr2Nb and Cr(similar to 6% Nb) phases. The supersaturated Cr-rich solid solution in the cast alloys precipitated out secondary Cr2Nb (Laves-phase) particles upon annealing at and above 900 degrees C. The studies by TEM and electron microprobe analysis indicated that the transition elements of Fe, Co, and Ni partition strongly in the Cr2Nb-type Laves phase whereas rhenium and aluminum were only moderately enriched in the Laves phase. The Cr-Cr2Nb alloys with less than or equal to 12% Nb exhibited considerable plastic deformation under compression tests. The yield strength of the alloys depended strongly on the volume fraction of the hard Laves phase. The oxidation resistance also increased with this volume fraction. Among the alloying additions, rhenium was the only element that substantially hardened the Cr-Cr2Nb alloys al room temperature and 1000 degrees C. The hardening behavior is discussed in terms of partitioning and sublattice occupation of the alloying elements as well as altering the volume fracture of the hard Laves phase. Multilayer oxide products formed on these alloys upon exposure to air of an elevated temperature. Little beneficial effect of any of the alloying additions on oxidation behavior at 950 degrees C was found.