OBSERVATION OF KINETICS OF GAMMA-ZIRCONIUM HYDRIDE FORMATION IN ZR-2.5NB BY NEUTRON-DIFFRACTION

Neutron diffraction was employed to observe an isothermal transformati on among zirconium hydrides in a commercial zirconium alloy. A specime n of Zr-2.5Nb, which contained about 200 mg deuterium/kg of alloy, was heated to 450 degrees C for almost 17 h, then cooled directly to 17 d egrees C and held at this temperature for an extended time. A series o f neutron diffraction patterns was collected during this thermal cycle . The diffraction patterns show that a small amount of the tetragonal gamma-phase zirconium hydride appears soon after cooling, along with a predominant quantity of the cubic delta-phase hydride. Over three sub sequent days at 17 degrees C, the amount of gamma-phase hydride increa ses while there is a corresponding reduction in the amount of delta-ph ase hydride. (C) 1998 Elsevier Science B.V. All rights reserved.