Effect of stress on the hydrogen solubility in the Zr2.5% Nb alloy studied by synchrotron light

This paper is the last of a series of works that resulted from X ray diffraction experiments performed in the line 1-ID at the Advanced Photon Source at Argonne National Laboratory, Illinois, USA in July 2008. Experiments were performed to study the hydrogen solubility in a zirconium alloy under externally applied stress, using a highly collimated beam and a CCD area detector located in transmission geometry. The study material is Zr2.5% Nb, an alloy used to fabricate pressure tubes for CANDU nuclear reactors. From the 2D diffraction images, the intensity, the radius and area under the Debye rings were analyzed as a function of temperature with the aim of studying the kinetic effect of the hydrogen solubility. The hydrides were identified as .- hydrides. The experiment allowed determining the temperatures of terminal solid solubility (TTSS) and the effect of external stresses on these temperatures. The values obtained under an applied tension of 225 MPa showed significant changes on TTSS with respect to those obtained for the same samples in the unloaded condition. Depending on hydrogen concentration (45 to 130 wppm) a reduction of the hysteresis between TTSS in dissolution and TTSS in precipitation of 20^oC to 30^oC was observed. The study of the spacing between basal planes (c-axis of the hexagonal cell) as a function of temperature showed that the tension causes a redistribution of the hydrogen atoms between different orientations of the grains of the microstructure. Precipitation starts at a higher temperature in those grains whose c-axis is under tensile stress, increasing the number of hydrides precipitated in such grains. The observed changes in the solubility temperature under stress can be explained in terms of an analysis of the energy required to accommodate the hydride precipitates into the metal matrix (author)