Diffusion mechanism of Zr-4 alloy at ultra-low temperature based on the α→β phase transition promoted by thermo-hydrogen treatment

Abstract

To achieve low-temperature robust diffusion bonding of Zr-4 alloys, thermo-hydrogen treatment (THT) has been conducted prior to the bonding process. After the THT, the bonding temperature can be reduced by 100 °C (achieving the same bonding strength of 200 MPa). The typical microstructure of hydrogenated Zr-4 alloy at room temperature was composed of precipitated hydrides (γ-ZrH, δ-ZrH_1.66, ε-ZrH₂) and α-Zr matrix. The in-situ XRD results at 550 °C∼700 °C indicated that the γ-ZrH δ-ZrH_1.66, and ε-ZrH₂ decreased gradually, the ζ-ZrH_0.25 and β_H-Zr formed with the rising temperature. DSC data indicated that the α→β phase transition temperature was decreased from 825 °C to 550 °C after the Zr-4 alloy was subjected to THT. According to the molecular dynamics (MD) results, the diffusion coefficient of Zr atoms in the β-Zr lattice was 1.36 × 10^–6 nm²·ps^-1 which was much higher than that in the α-Zr lattice (2.24 × 10^–8 nm²·ps^-1). After the phase transition, the vacancy and interstitial diffusion formation energy significantly reduced from 2.61 eV and 3.67 eV to -0.6 eV and -1.0 eV, corresponding to the abundant defects as shown in-situ TEM images of β (β_H) at 650 °C. The vacancy and interstitial diffusion activation energy of Zr atoms in β_H-Zr lattices were reduced from 3.22 eV and 3.92 eV to 1.3 eV and 0.1 eV. Thus, the hydrogen-induced phase transition and the formation of the β_H-Zr phase were confirmed as the key factors in achieving low-temperature diffusion bonding of Zr-4 alloys.