Microstructure and mechanical properties of two-phase Cr–Cr2Nb, Cr–Cr2Zr and Cr–Cr2(Nb,Zr) alloys

The microstructures and mechanical properties of binary and ternary Cr-based alloys containing Nb, Zr, or both Nb and Zr, have been studied in both the as-cast and annealed conditions. The level of alloying in each instance was targeted to lie below, or approximately at, the maximum solubility in chromium. The as-cast microstructures of these alloys consisted of Cr-rich solid solution surrounded by small amounts of interdendritic Cr–Cr2X eutectic structure. Annealing at 1473 K resulted in solid-state precipitation of the Cr2X Laves phase in the Cr–Nb and Cr–Nb–Zr alloys, but not in the Cr–Zr alloys. The binary Cr2Nb phase consisted of an extensively twinned ({111}<112> twins) C15 structure whereas the presence of Zr modifies its appearance substantially; the twinned C15 structure persists. Oxides were occasionally present and their compositions were qualitatively determined. Vickers hardness primarily depended upon the volume fraction of the Cr2X Laves phase present. Age hardening due to solid-state precipitation of Cr2X Laves phase within the Cr-rich matrix was observed in the Nb-containing alloys. The room temperature bend strength of the alloys was strongly affected by the presence of grain-boundary Cr2X phase. It is considered that porosity as well as oxides in the alloys also lowers their bend strength.