Neutron diffraction study of the precipitation and dissolution of hydrides in Zr-2.5Nb pressure tube material

Abstract

The precipitation and dissolution of zirconium hydrides in Zr-2.5Nb pressure tube material have been tracked directly by neutron diffraction as a function of temperature. The differences between the critical temperatures for the onset of precipitation ( T SSP) and the completion of dissolution ( T SSP) are about half as great as those differences found by methods that involve continuous temperature scanning. The solvus line obtained during heating agrees with equations that fit earlier results by Kearns and by Slattery, except in the early stages of dissolution, when large hydride precipitates are present in the matrix. On cooling, the solvus line is shifted by about 20°C towards higher temperature, compared to that of Slattery for Zr-2.5Nb. The γ-phase hydrides persist at room temperature, through a number of thermal cycles in which the specimen was soaked at temperatures over 400°C for several hours, then cooled at rates between 0.1°C/min and 2.5°C/min. Hydrides in a zirconium matrix appear to require a time scale on the order of hours to reach equilibrium during precipitation and phase transformations.