Microstructure and mechanical properties of two-phase Cr-Cr2Nb, Cr-Cr2Zr and Cr-Cr-2(Nb,Zr) alloys

The microstructures and mechanical properties of binary and ternary Cr-base d alloys containing Nb, Zr, or both Nb and Zr, have been studied in both th e as-cast and annealed conditions. The level of alloying in each instance w as targeted to lie below, or approximately at, the maximum solubility in ch romium. The as-cast microstructures of these alloys consisted of Cr-rich so lid solution surrounded by small amounts of interdendritic Cr-Cr2X eutectic structure. Annealing at 1473 K resulted in solid-state precipitation of th e Cr2X Laves phase in the Cr-Nb and Cr-Nb-Zr alloys, but not in the Cr-Zr a lloys. The binary Cr2Nb phase consisted of an extensively twinned ({111} <1 12> twins) C15 structure whereas the presence of Zr modifies its appearance substantially; the twinned C15 structure persists. Oxides were occasionall y present and their compositions were qualitatively determined. Vickers har dness primarily depended upon the volume fraction of the Cr2X Laves phase p resent. Age hardening due to solid-state precipitation of Cr2X Laves phase within the Cr-rich matrix was observed in the Nb-containing alloys. The roo m temperature bend strength of the alloys was strongly affected by the pres ence of grain-boundary Cr2X phase. It is considered that porosity as well a s oxides in the alloys also lowers their bend strength. (C) 1999 Elsevier S cience S.A. All rights reserved.