GRAIN-GROWTH BEHAVIOR AND LOW-PRESSURE OXYGEN COMPATIBILITY OF AN IR-0.3 WT.PERCENT W ALLOY

The grain-growth behavior in a low-pressure oxygen-containing atmosphe re of an Ir-0.3 wt.% W alloy (designated DOP-26) currently used for cl adding of the 238-plutonium oxide fuel in radioisotope thermoelectric generators (RTGs) was studied. A small amount of added thorium improve s the impact properties of this alloy partly by segregating to grain b oundaries where it provides strengthening and partly by forming Ir,Th precipitates in the matrix which refine grain size. However, a previou s preliminary study showed that, in an atmosphere containing a low par tial pressure of oxygen (on the order of 1 mPa), the dissolution of th e Ir5Th precipitates and diffusion of thorium to the surface to form T hO2 was thermodynamically favorable, leading to anomalous growth of ne ar-surface grains. In this study, specimens were exposed to an oxygen partial pressure of 1.3 mPa at temperatures of 1230, 1280 and 1330 deg rees C for times up to 3000 h. The grain-growth behavior in a vacuum o f 0.7 mPa was also determined as a comparison. The results substantiat ed the earlier study and showed that anomalous growth of near-surface grains does occur in this alloy. The results also indicated a marked d ifference in the kinetics of grain growth in the vacuum versus low-pre ssure oxygen atmospheres.