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

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 Zr ₃Ge phase and Zr(Fe,Cr) ₂ Laves phase with a hexagonal close-packed (HCP) structure. The orientation relationship between the Zr ₃Ge core, Zr(Fe,Cr) ₂ shell structures and the surrounding matrix were investigated and can be approximately described as [ 115 ] Zr 3 Ge | | [ 11 2 ¯ 6 ¯ ] Zr ( Fe , Cr ) 2 | | [ 11 2 ¯ 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 Zr ₃Ge/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.