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.% we re 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 surroun ded by the eutectic structure having Cr2Nb and Cr(similar to 6% Nb) ph ases. The supersaturated Cr-rich solid solution in the cast alloys pre cipitated out secondary Cr2Nb (Laves-phase) particles upon annealing a t and above 900 degrees C. The studies by TEM and electron microprobe analysis indicated that the transition elements of Fe, Co, and Ni part ition strongly in the Cr2Nb-type Laves phase whereas rhenium and alumi num were only moderately enriched in the Laves phase. The Cr-Cr2Nb all oys with less than or equal to 12% Nb exhibited considerable plastic d eformation under compression tests. The yield strength of the alloys d epended strongly on the volume fraction of the hard Laves phase. The o xidation resistance also increased with this volume fraction. Among th e alloying additions, rhenium was the only element that substantially hardened the Cr-Cr2Nb alloys al room temperature and 1000 degrees C. T he hardening behavior is discussed in terms of partitioning and sublat tice occupation of the alloying elements as well as altering the volum e fracture of the hard Laves phase. Multilayer oxide products formed o n these alloys upon exposure to air of an elevated temperature. Little beneficial effect of any of the alloying additions on oxidation behav ior at 950 degrees C was found.