A novel type of core-shell structured secondary phase particles in Ge-addition modified Zircaloy-4 alloy subjected to β-phase region solution treatment

•Core-shell structural SPPs were frequently observed in Ge-modified Zr-4 alloy.•The “core ” and “shell” were separately identified as Zr3Ge and Zr(Fe,Cr)2 phases.•The ORs between SPP and matrix were [115]Zr3Ge||[112¯6¯]Zr(Fe,Cr)2||[112¯3]α−Zr.•The formation mechanism of such SPPs were discussed. Secondary phase particles (SPPs) in a Ge-addition modified Zircaloy-4 alloy subjected to β-phase region solution treatment was investigated in detail by transmission electron microscopy to acquire a better understanding on the microstructure of this new zirconium alloy. Result revealed that a novel type of SPPs with core-shell structure were frequently observed in this alloy, in which the “core” and “shell” structures were separately identified as primitive tetragoanl Zr3Ge phase and Zr(Fe,Cr)2 Laves phase with a hexagonal close-packed (HCP) structure. The orientation relationship between the Zr3Ge core, Zr(Fe,Cr)2 shell structures and the surrounding matrix were investigated and can be approximately described as [115]Zr3Ge||[112¯6¯]Zr(Fe,Cr)2||[112¯3]α−Zr. It was strongly suggested that the gradient of Ge concentration occurred during the subsequent β-phase region annealing affected the phase stability of Zr(Fe,Cr,Ge)2 nanophase in the as-cast Ge-addition modified Zr-4 alloy, thereby causing the formation of Zr3Ge/core-Zr(Fe,Cr)2/shell microstructures. The knowledge attained from this study, shedding new lights on the formation mechanism of these novel core-shell SPPs, would also greatly benefit the understanding on the formation of similar composite precipitates in other alloys.