Abstract: The Zr-2.5Nb alloy with a fine microstructure consisting of α laths was successfully prepared by electron beam melting (EBM). The thermal oxidation behaviors and kinetics of the as-built, and the EBM-built and hot isostatically pressed (HIPed) Zr-2.5Nb materials in a temperature range of 450-600 °C were investigated and compared with those of the alloy prepared by conventional casting and forging. It was found that the oxidation kinetics of the as-built and the forged materials followed the parabolic rate law during isothermal oxidation at 550 °C, but the HIPed materials exhibited a parabolic-to-linear kinetic transition, suggesting that the larger grain sizes enhanced the oxidation. The oxide layers of all materials were composed of a large fraction of monoclinic zirconia phase (m-ZrO₂) and a small fraction of tetragonal zirconia phase (t-ZrO₂), and transformed from t-ZrO₂ to m-ZrO₂ with increasing oxidation time. The surface hardness of the as-built, the forged and the HIPed materials increased from 215, 204, and 188 HV before oxidation to 902, 1070, and 1137 HV after oxidation, respectively. The cross-sections of the materials showed the presence of micropores and microcracks inside the oxide layers with thicknesses ranging from 4 to 8 µm. With the oxidation temperature of 600 °C and oxidation time duration of 3 h, a dense black m-ZrO₂ oxide layer with smooth surface and 902 HV hardness was obtained on the EBM as-built Zr-2.5Nb materials.

Key words: Zr-2.5Nb alloy, Electron beam melting, Thermal oxidation, Oxidation behaviors and kinetics, Grain size