Grain-growth behavior and low-pressure oxygen compatibility of an Ir-0.3 wt.% W alloy

Abstract The grain-growth behavior in a low-pressure oxygen-containing atmosphere of an Ir-0.3 wt.% W alloy (designated DOP-26) currently used for cladding of the 238-plutonium oxide fuel in radioisotope thermoelectric generators (RTGs) was studied. A small amount of added thorium improves the impact properties of this alloy partly by segregating to grain boundaries where it provides strengthening and partly by forming Ir 5 Th precipitates in the matrix which refine grain size. However, a previous preliminary study showed that, in an atmosphere containing a low partial pressure of oxygen (on the order of 1 mPa), the dissolution of the Ir 5 Th precipitates and diffusion of thorium to the surface to form ThO 2 was thermodynamically favorable, leading to anomalous growth of near-surface grains. In this study, specimens were exposed to an oxygen partial pressure of 1.3 mPa at temperatures of 1230, 1280 and 1330 °C for times up to 3000 h. The grain-growth behavior in a vacuum of 0.7 mPa was also determined as a comparison. The results substantiated the earlier study and showed that anomalous growth of near-surface grains does occur in this alloy. The results also indicated a marked difference in the kinetics of grain growth in the vacuum versus low-pressure oxygen atmospheres.