Molybdenum and its Alloys
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- Category: Molybdenum knowledge
- Published on 15 August 2013
- Written by 陈桂兰
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Molybdenum and copper alloys are considered candidate materials for the heat-sink system of the divertor of ITER. The design which includes graphite tiles on molybdenum substrate has been prefered for the physics stage of the divertor, ref [1]. The main advantage of such a solution is the similarity of the coefficients of thermal expansion for molybdenum and graphite, which permits significant reduction in the levels of thermal stresses. A high level of strength, high resistance to radiation swelling, and good thermal conductivity are among the advantages of using molybdenum alloys.
The principal disadvantage of molybdenum-base alloys is a tendency to low-temperature embrittlement. Therefore it was necessary to find a solution to reduce the ductile to brittle transition temperature (DBTT) and to provide a minimal shift of the DBTT following low-temperature neutron irradiation.
It has been known for a long time that Mo-Re alloys provide satisfactory properties arising from the rhenium addition to molybdenum. Mo-Re alloys at 3-7% Re concentrations exhibit a significant increase in ductility in low-temperature tests, ref [2]. Due to the refining effects of Re on the microstructure the strength of welded joints of Mo-Re alloys is improved significantly, mainly because of the grain size reduction in the heat affected zone at ~5 % Re, ref [3].
Fundamental studies in the 1950's on the mechanisms controlling the use of pure molybdenum revealed that cold working was effective in improving fabricability and that small additions of reactive elements increased the elevated temperature capability by raising the recrystallization temperature. A Mo-0.5Ti-0.08Zr (TZM) alloy is representative of this class of materials.
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