Solid-State Phase Transformation Explains the Mixed Crystallographic Character of Zr(Nb,Fe)₂ Nanoprecipitates in Zr-2.5Nb

Zirconium alloys have widespread applications in nuclear energy, with Zr-2.5Nb commonly being used as pressure tube material in reactors. Their microstructure encompasses intermetallic nanoprecipitates (NPs) and solutes that significantly impact their behavior in corrosive environments and irradiation. Hence, we analyze the crystal structure of Zr((Zr,) Nb,Fe)₂ NPs using transmission electron microscopy (TEM), electronic density functional theory (DFT) calculations, and finite element analysis (FEA). Our findings unveil a mixed c14 and c15 Laves phase structure within the NPs and provide an explanation through the syncroshear mechanism. Through thermodynamic analysis, we evaluate the electronic, vibrational, and strain contributions to the free energy of the NPs. Our results indicate that the c15 structure is energetically favored at temperatures below 600 K, while the c14 structure prevails at higher temperatures. We provide an explanation for the observed coexistence of these structures in the NPs based on two key insights: (1) During annealing at high temperatures, the energetically favorable c14 NPs form, and (2) as the alloy cools, a partial phase transition to the c15 structure occurs, constrained by kinetic limitations. Furthermore, our study reveals that the NP/α-Zr interface is likely to be incoherent due to the considerable stresses involved. This finding is consistent with high-resolution transmission electron microscopy (HRTEM) micrographs, which demonstrate the presence of an incoherent interface.