FORMATION AND STABILITY OF FE-RICH PRECIPITATES IN DILUTE ZR(FE) SINGLE-CRYSTAL ALLOYS

The formation and stability of Fe-rich precipitates in two alpha-Zr(Fe ) single-crystal alloys with nominal compositions I, 50 parts per mill ion by atom (ppma) Fe, and II, 650 ppma Fe, have been investigated. Op tical microscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were used to examine the characteristics of Fe-rich precipitates. The SEM and TEM micrographs showed that in as-g rown alloy II, Zr2Fe precipitates were located at ''stringers.'' Preci pitates were not observed in as-grown alloy I. Annealing treatments be low 700-degrees-C, for alloy I, and 820-degrees-C, for alloy II, resul ted in the diffusion of excess Fe (above the a-phase solution limit) t o the free surface with the subsequent formation of Zr3Fe precipitates in both alloys. Dissolution of Zr3Fe surface precipitates of alloy I (annealing above the solvus) left precipitate-like features on the sur -faces. Zr2Fe precipitates in as-grown alloy II were readily dissolved by beta-phase annealing.