Measuring the fracture toughness of molybdenum-0.5 pct titanium-0.1 pct zirconium and oxide dispersion-strengthened moly
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a refractory metal that undergoes no phase change for the bcc crystal structure from the ambient to the melting temperature (2610 ⬚C). The good hightemperature strength, creep resistance, low coefficient of thermal expansion, and high thermal conductivity are attractive properties for molybdenum-based alloys.[1] Molybdenum forms a volatile oxide at high temperatures at high oxygen partial pressures, which limits long-term use in air at temperatures ⱖ500 ⬚C. However, molybdenum is widely used where the high-temperature strength and creep resistance are needed for applications that involve short-term exposures in air (i.e., forging dies, metalworking tooling, and glass-melting furnaces) or long-term exposures in a controlled atmosphere (i.e., vacuum-furnace components). Molybdenum has also been considered for use in advanced nuclear power generation systems.[2,3] Metals with a bcc structure generally exhibit high levels of ductility and toughness at a homologeous temperature (T /Tm (Kelvin)) ⱖ0.3, while a greater tendency for brittle fracture is observed at homologeous temperatures ⬍0.2. The homologeous temperature for molybdenum alloys at room temperature is low (T /Tm ⱕ 0.10), and the ductile-to-brittle transition temperature (DBTT) is generally within 100 ⬚C of room temperature, which indicates that brittle fracture can be a concern at ambient temperatures. Since molybdenum is generally used for applications that are creep limited, where brittle fracture is less of a concern, B.V. COCKERAM, Senior Scientist, is with the Bechtel-Bettis Atomic Power Laboratory, West Mifflin, PA 15122-0079. Contact e-mail: [email protected] Manuscript submitted February 28, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
few data are available in the literature on the fracture toughness of molybdenum and molybdenum-based alloys.[4–13] A review of some fracture-toughness data for molybdenum and molybdenum-based alloys has been reported.[4] Most of the early work involved the use of Charpy specimens that were not precracked, the use of an electrodischarge machined (EDM) wire for precracking, or specimen sizes that were too thin for a valid measurement, which indicates that these values may not be conservative. Only recently has a method of compression precracking been developed to allow a more accurate and conservative measure of molybdenum fracture toughness.[4,5,7–10] Although some fracture-toughness data have been reported for molybdenum, most of the data have been measured only at room temperature. Little testing has been performed over a range of temperatures to define a transition temperature or DBTT. Two important molybdenum-based alloys are molybdenum - 0.5 pct titanium - 0.1 pct zirconium (TZM)[14] and oxide dispersion–strengthened (ODS) molybdenum.[15–18] The TZM alloy has a fine distribution of carbide precipitates that are rich in titanium and zirconium, which inhibits grain growth and recrystallization at high temperatures and results in good high-temperature strength. The ODS molybdenum has a fine distribution of lanth
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