The aging response of zirconium-tin alloys

The potential for precipitation hardening in binary Zr-Sn alloys has been examined. Transmission and scanning electron microscopy, hardness measurements, optical microscopy and X-ray diffraction were used to study alloys with the following nominal compositions: (a) 1.5%, 5%, 7% and 9% Sn, produced from Zr of iodide purity, and (b) 8% Sn, made from sponge purity Zr. Following solution treatments in the α- or β-phases, little or no precipitation response was observable under most aging conditions, including times for up to 1000 h at temperatures in the range 700 K to 1000 K. The only evidence for significant precipitation was in Zr-8% Sn, where recrystallization was induced by aging material that had been β-quenched and then cold-worked. With this treatment, precipitation proceeded in a discontinuous manner by the formation of particles probably Zr4Sn, at the grain boundaries during recrystallization. The resulting volume fraction was more than an order of magnitude less than expected from the phase diagram. It is concluded that the solvus line for the α-Zr(Sn) solid solution may be incorrect and the solubility of Sn is higher than previously believed. In addition, it is evident that the diffusional kinetics for phase decomposition are very sluggish. The potential for precipitation strengthening in Zr-rich Zr-Sn alloys by thermal aging is poor. (orig.)