Formation of the Fe23Zr6 phase in an Fe-Zr alloy

Stainless steel-zirconium alloys are being developed at Argonne National Laboratory for the containment and disposal of radioactive metal isotopes isolated from spent nuclear fuel. The nominal compositions that have been selected as waste form alloys are stainless steel-15 wt% zirconium (SS-15Zr) and zirconium-8 wt% stainless steel. To a first approximation, the composition and crystal structure of phases present in these alloys can be inferred from present Fe-Zr phase diagrams. However, many uncertainties exist in the understanding of the Fe-Zr binary, as evidenced by the differing phase diagrams presented by various authors. On the Fe-rich side, the main controversy is over the existence and crystal structure of the Fe23Zr6 (also known as Fe3Zr) phase. In this laboratory, Fe-9.8 at% Zr (15 wt% Zr) alloys were prepared and analyzed to resolve the controversy over the Fe23Zr6 phase. Alloys were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and neutron diffraction (ND). The major phases in as-cast alloys were α-Fe and Fe2Zr, whereas the major phases in alloys heat-treated near the eutectic temperature were α-Fe and Fe23Zr6. This paper discusses possible mechanisms for the transformation of the Laves intermetallic Fe2Zr to the Fe23Zr6 phase during high temperature annealing. An understanding of these mechanisms in the simpler Fe-Zr binary alloy will help in predicting the stability of phases in the SS-15Zr metal waste form alloy