Low-cost surface modification of a biomedical Zr-2.5Nb alloy fabricated by electron beam melting

摘要

The Zr-2.5Nb alloy with a fine microstructure consisting ofαlaths 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 in-vestigated 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-ZrO_(2))and a small fraction of tetrago-nal zirconia phase(t-ZrO_(2)),and transformed from t-ZrO_(2)to m-ZrO_(2)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 materi-als showed the presence of micropores and microcracks inside the oxide layers with thicknesses ranging from 4 to 8μm.With the oxidation temperature of 600°C and oxidation time duration of 3 h,a dense black m-ZrO_(2)oxide layer with smooth surface and 902 HV hardness was obtained on the EBM as-built Zr-2.5Nb materials.