GRAIN-GROWTH IN IR-0.3 PCT W ALLOYS

The fuel cladding material for radioisotope heat sources operating above 1300°C is currently an Ir-0.3 pct W alloy. The ductility of this material under high-temperature impact accident conditions is reduced by large grain sizes. Knowledge of grain size as a function of time and temperature is required to determine the effects of high-temperature exposure during fabrication, assembly, ground testing, and operation of the heat source. The average grain diameter of three different heats of Ir-0.3 pct W is reported as a function of time at 1583 K (1310°C), 1688 K (1415°C), and 1808 K (1535°C) up to 2000 h. Growth rates were measured both perpendicular and parallel to the sheet surface in longitudinal cross sections of sheet samples. In samples with Th, Al, Fe, Ni, and Rh dopant additions, the grain structure was elongated and grain growth was retarded. Grain size was measured by the line in-tercept method and the data were described by the grain growth equation, d 2 -d 0 2 =kt. An activation energy for grain growth was determined for each heat, in both the perpendicular and parallel directions, and was found to be consistent with the expected activation energy for self-diffusion in the alloy. Abnormal grain growth was not observed within the time and temperature ranges of this experiment. © 1979 American Society for Metals and the Metallurgical Society of AIME.