Effect of iron additions on structure of Laves phases in NbCrFe alloys

Abstract

The effect of iron additions on the stability of various Laves phases in the NbCr base system has been examined. In the binary NbCr system a hexagonal (C14) Laves phase exists at temperatures above approximately 1860 K, and undergoes] a shear-like martensitic transformation to a face centered cubic (C15) Laves phase on cooling to room temperature. Addition of iron promotes formation of an additional phase, the dihexagonal (C36) Laves phase. In alloys with a lower iron content (below approximately 5 wt.%), the C36 phase forms at higher temperatures. In addition, the C15 phase begins to form on cooling to room temperature. In sharp contrast, in alloys with a higher iron content, only the C14→C36 phase transformation takes place in the entire temperature range studied ( RT 1473 K). These findings were rationalized on the basis of the effects of chemical energy change and elastic strain energy on magnitude of the activation energy for martensitic transformation.