The effects of microstructural control on the mechanical behavior of Cr2Nb based alloys

Microstructural evaluations and mechanical testing of Laves-phase alloys based on Cr₂Nb were examined in order to optimize microstructural and mechanical properties via secondary processing techniques and thermo-mechanical treatment at temperatures up to 1600°C. At ambient temperatures, single-phase Cr₂Nb alloys are very hard and brittle due to the complicated crystal structure (C-15). The following results were revealed through examination of the Cr-Cr₂Nb two-phase region: (a) with increasing amounts of the soft chromium-rich phase, the compression strength and hardness decrease: (b) the annealing treatments studied provided the best break-up of the coarse/brittle eutectic structure in the 94 at.% Cr - 6 at.% Nb (CN-7) alloy; (c) two different anneals, 1 hour (h) at 1600°C + 4 h at 1200°C and 4 h at 1550°C + 2 days (d) at 1200°C, lead to a substantial improvement in the room-temperature compressive ductility over previous annealing treatments (3d at 1100°C). In addition. Hot Isostatic Pressing (HIPping) + annealing led to a substantial refinement of the brittle eutectic structure in the CN-7 (Cr-6 at.% Nb) composition. HIPping led to only a marginal reduction in casting defects and a refinement of the eutectic structure in the CN-4 (Cr-12 at.% Nb) composition. A combination of hot forging and annealing is promising in refining the brittle eutectic structure in the CN-7 composition. Tensile properties were found to be sensitive to the morphology of the Cr₂Nb Laves phase, even after HIPping to break up the interconnected Cr₂Nb phase. Solid state precipitation of Cr₂Nb particles was found to occur when annealing at temperatures well below the eutectic temperature (1100-1200°C) and these particles were found to be fairly resistant to coarsening, even after 30 days at 1200°C.