Abstract: Nanoparticles are extensively introduced to improve the mechanical, physical, and chemical properties of alloys. In the present study, the underlying nano-refinement mechanisms of face-centered cubic Zr(Fe, Cr)₂ secondary phase particles (SPPs) that precipitated in Zircaloy-4 alloy under high-temperature compression were investigated in detail by utilizing high-resolution transmission electron microscopy (HRTEM) and conventional TEM techniques. The frequently observed Zr(Fe, Cr)₂ SPPs were incoherent with the matrix and exhibited brittle fracture behaviors without measurable plasticity. HRTEM observations revealed two mechanisms underlying the nano-refinement of incoherent micro-sized SPPs via localized shear fracture on $\{11\bar{2}}_{SPP\}$ and nanoprecipitate-assisted bending fracture, respectively. The latter was, for the first time, found to occur when the movements of large SPPs were blocked by nanometer-sized SPP during alloy deformation. Accordingly, two force models were proposed to visualize their potential nano-refinement processes. The knowledge attained from this study sheds new light on the deformation behaviors of Zr(Fe, Cr)₂ SPPs and their associated size refinement mechanisms under high-temperature compression, and is expected to greatly benefit the process optimization of zirconium alloys to achieve precipitate nano-refinement.

Key words: Secondary phase particles, Nano-refinement mechanism, Shearing and bending fracture modes, Zirconium alloys, High-temperature compression