The Oxidization Behavior of Zr–2.5Nb(wt%) Alloy in Dry Air

Abstract

The oxidation behavior of Zr–2.5Nb alloy at 500 °C, 600 °C and 700 °C from 0.5 to 24 h in a tubular furnace with dry air was systematically investigated by scanning electron microscope, transmission electron microscope and nanoindentation techniques. The results indicate that there is an oxidation rate transition related to grain morphologies of the oxide layer. It showed that the equiaxed grains are good for the oxidation in pre-transition, while the columnar grains are unfavorable to the oxidation in post-transition. The oxide layers first grow with small equiaxed grains and then, become columnar grains. Both t-ZrO2 and m-ZrO2 were identified in the equiaxed grains, while only m-ZrO2 was identified in the columnar grains. Some sub-oxides with different morphologies were observed in the substrate at the beginning of oxidation and were not found after long time oxidation. The occurrence of the oxygen-assisted eutectoid phase transformation of βZr → αZr + βNb results in the decomposition of βZr phase and the precipitation of βNb phase in the microstructure near oxide layer/substrate interface. As the oxide layer thickens, its nano-hardness firstly increases and then decreases, while its adhesion increases gradually. The present study suggests that thicker oxide layers with limited microcracks may also be suitable for the bearing surface of artificial joints.