Fracture behavior of a Pt--30% Rh--8% W alloy between room temperature and 13000C

Pt--30 percent Rh--8 percent W alloys are being considered for structural applications in Radioisotopic Thermoelectric Generators (RTGs) for space nuclear power systems. To determine materials compatibility and structural properties thermally exposed tensile specimens were tested at temperatures between 23 and 13000C. The specimens had been exposed as control specimens or in contact with graphite or 238PuO2 (the radioisotopic fuel) at 900 and 11000C for 2000 and 8000 h. The fracture behavior was brittle and intergranular below 2500C and between 900 and 11000C, and ductile transgranular at 250 to 8000C and above 11000C. Auger Electron Spectroscopy of freshly fractured specimens revealed segregation of phosphorus up to 11 atomic percent at the grain boundaries which appears to be responsible for the low temperature brittle fracture. Exposure to 238PuO2 further embrittled this alloy at low temperatures. The high temperature brittle fracture behavior resembled that caused by liquid metals. (U.S.)