Microstructural Characterization of AA6061 Versus AA6061 HIP Bonded Cladding–Cladding Interface

Microstructural characterization using scanning electron microscopy and scanning transmission electron microscopy (TEM/STEM) was carried out near the interface between the two AA6061 alloys that were hot isostatically pressed (HIP) to clad Zr laminated U-10 wt.% Mo metallic nuclear fuel. The HIP-bonded AA6061–AA6061 interface consisted of discontinuous layer of Mg2Si along with traces of fine MgO dispersoids and small precipitates of Al19(Fe, Cr, Cu)4MnSi2. To examine the presence of statistical variation, quantitative microscopy was also conducted, using several HIP’ed samples, to measure the relative linear density of the Mg2Si precipitates at the HIP bonded AA6061–AA6061 interface. In order to better understand the formation of discontinuous Mg2Si layer, solid-to-solid diffusion couple experiments were carried out using temperature and time relevant to HIP. The discontinuous Mg2Si layer was not observed in diffusion couples that were rapidly water-quenched, but those slowly cooled in air and in furnace developed the discontinuous Mg2Si. Presence of oxygen, confirmed by electron energy loss spectroscopy via STEM, at the interface would be the potential driving force for the migration of Mg and Si atoms, where Mg would preferentially react with oxygen to form MgO, and excess Mg would react with Si to form Mg2Si during cooling. Faster cooling after HIP may minimize the formation of excessive Mg2Si.