Changes in phase composition of Zr-Fe-V getter after hydriding and vacuum dehydriding cycles

The stability and phase composition of the commercial non-evaporable getter SAES St 707 (70% Zr, 24.6% V, and 5.4% Fe) during hydriding and vacuum dehydriding cycles were investigated. Structure and phase composition were studied using X-ray diffraction (XRD) with Co Kα and 57Fe Moessbauer spectroscopy (MS). The samples were hydrided during annealing in pure hydrogen at 550 oC for 15 min and dehydrided in vacuum at 550 oC for 15 min. This procedure was applied 1, 5 and 10 times in the same furnace without a contact with an ambient atmosphere (air) between the cycles. Finally, the samples were removed from the furnace to an ambient atmosphere where XRD and MS experiments were performed. Cubic C15 Laves phase and α-Zr were determined from XRD measurements in the samples which were annealed in vacuum during the last heat treatment step. ZrH2 and ZrV2H3.6 phases were identified in the samples which were in the hydrogen charged state after the last step of the heat treatment. In the samples after 5 and 10 cycles a small amount of cubic and monoclinic ZrO2 was found. These oxides nucleated already during the first step of the heat treatment in vacuum and their amounts increased by subsequent annealing steps. In situ XRD measurements during heating showed a structure amorphisation due to hydrogen absorption.