Characterization of phases in the Zr–Nb–Fe ternary system at the Zr–Nb rich side of the phase diagram

Abstract The formation and transformation behavior of Zr–Nb–Fe ternary phases were studied in a ternary alloy with 25 at% Fe, and Zr-2.5 wt% Nb (also ∼2.5 at% Nb) pressure tube material that has a nominal Fe composition in the range ∼0.1 at% to 0.2 at%. For the ternary alloy, the evolution of the phase constitutions and corresponding lattice parameters from the as-cast to the annealed condition were measured by both X-ray and TEM diffraction, combined with crystallographic structural modeling. In the as-received material the two major phases are a Ti2Ni structured face centered cubic phase (FCC) and a hexagonal close-packed (HCP) C14 Laves phase. The alloy also contained a small fraction of metastable βNb/Zr phase containing a low concentration of Fe (∼1 at%). In addition to the aforementioned ZrNbFe phases, heat-treatment of the ternary alloy at temperatures below the ZrNb monotectoid temperature (∼580 °C and 500 °C) resulted in decomposition of the meta-stable βNb/Zr phase into HCP precipitates and BCC βNb precipitates. These precipitates were different from the as-received bulk HCP phase in composition but had similar lattice parameters, and showed similar electron diffraction patterns to the majority bulk HCP phase. A compositional analysis using known standards showed that the best stoichiometry assignment for the bulk HCP phase and the equilibrium phases that formed from decomposition of the metastable βNb/Zr phase is Nb(Zr,Fe)2. The HCP phase variant produced during heating below the monotectoid temperature was also produced upon heating of the Zr-2.5Nb pressure tubing. Detailed analysis of the HCP phase formed from the decomposition of the βNb/Zr phase that is dilute in Fe shows that it has a polytype structure consisting of both FCC and HCP structures.