Hydride Precipitation Behavior with Hydrogen Concentration and Homogenization Temperature in Zr-2.5%Nb Pressure Tube Material

Abstract

The precipitation characteristics of hydrides in Zr-2.5% Nb pressure tube material were investigated for various hydrogen concentrations and homogenization temperatures. A surface hydride was formed using the electrolytic method and homogenized at various temperatures up to at 550 °C. The homogenization treatment was carried out at 300-550 °C over 240 hours, and the hydrogen concentration in the Zr-2.5% Nb alloy varied from 28 to 144 ppm. Hydrides were observed by optical microscopy and the precipitation parameters of the hydrides were analyzed by MATLAB, quantitatively. This is the first achievement of quantitative analysis of hydrides in Zr-alloys. The length of hydrides increased linearly with hydrogen concentration when homogenized at or below 400 °C. The area fraction of hydrides increased linearly with the hydrogen concentration regardless of homogenization temperature. The length and the ratio (length/thickness) of the hydrides decreased with increasing homogenization temperature at 350-475 °C in 81-86 ppm hydrogen. The changes in the precipitation behavior of the hydrides with homogenization temperature are considered to be due to microstructure changes, both the decomposition of β-Zr and grain growth of α-Zr. Key words: Zr-2.5%Nb alloy, homogenization, hydrogen concentration, hydride, microstructure