Understanding spinodal and binodal phase transformations in U-50Zr

Abstract

Engineered spinodal decomposition and spinodal precursors to precipitation hardening are effective approaches for tailoring material's properties and performance. These approaches leverage the spinodal and binodal nature of miscibility gaps. However, little is known about the chemical and crystallographic mechanisms controlling phase evolution across the spinodal curve and into the binodal regime. This study aims to reveal spinodal-binodal phase transformation in a binary U-50 wt.% Zr model alloy through coupled X-ray diffraction (XRD), in-situ transmission electron microscopy (TEM), and atom probe tomography (APT). The hexagonal ⍵-UZr2+x phase initially undergoes a spinodal decomposition into interconnected nanosized hexagonal Zr-rich and hexagonal U-rich domains at ~575 °C, near the hexagonal-bcc phase transformation. Between 600 and 800 °C, the microstructure evolution is dominated by the coarsening and chemical purification of decomposed bcc domains and a transition from spinodal to binodal nucleation. At 800 °C, the metastable Zr domains transfer to stable α-Zr phase and stay the same phase up to 1000 °C. On the contrary, metastable U domains transfer to orthorhombic α’-U firstly at 800 °C and then to tetragonal β-U at 1000 °C. Spinodal decomposed domain size is found to be linearly related to extrinsic specimen geometry. The effect of oxygen on spinodal decomposition is also discussed. In-situ TEM observations successfully captured the spinodal decomposition and the subsequent transition to binodal phase separation with temperature, providing critical evidence for an expanded miscibility gap, concerning both composition and temperature, on the U-Zr phase diagram.