High Temperature Phase Stability and Microstructural Characterization of D9 Stainless Steel-Zirconium Metal Waste Form Alloys

Abstract

Zirconium present in stainless steel-zirconium metal waste form (MWF) alloys form Ni–Zr and Fe–Zr intermetallic phases which act as a sink for radionuclide and improve resistance to localized corrosion as well as selective radionuclide leaching. The present study looks into the behavior of Zr intermetallics in MWF alloys with the variation of Zr content after heat treatments. Two MWF alloys of D9 SS (Ti modified 15Cr–15Ni–2.5Mo stainless steel) with 8.5 and 17 wt% Zr were heat treated at 1,323 K for 2 and 5 h and characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The stability of the Zr intermetallic compounds was examined by high temperature XRD. The results from XRD study showed the presence of NiZr, Ni5Zr, Ni7Zr2, FeZr2, and Fe3Zr peaks along with fcc Fe based solid solution. The MWF alloy with 17 wt% Zr exhibited α-ferrite peak in as-cast condition which was not observed after heat treatment. From the SEM micrograph the agglomeration of intermetallic phases was observed after heat treatment and the grain size of the intermetallic phases increased with duration of heat treatment. The high temperature XRD study revealed that all the intermetallic phases were stable up to 1,173 K and above this temperature Ni–Zr intermetallics started disappearing. However Fe–Zr intermetallics were stable till 1,373 K. The paper presents the high temperature phase stability of D9 SS-Zr MWF alloys.