Effect of rhenium on the brittle fracture threshold of molybdenum

Abstract

1. Addition of rhenium in quantities from 0.5 to 36% into molybdenum does not result in monotonic reduction of the metal cold brittleness threshold in its cast condition (Tbu), as this is usually suggested in technical literature [5, 6, 9]. 2. In introducing rhenium into molybdenum an increase is observed in the temperature range of transition of cast metal from plastic to brittle state. This increases Tbu and reduces Tbl with the exception of alloys containing 4–6% Re whose Tbu corresponds to Tbu of pure molybdenum. 3. An increase of Tbu in adding rhenium to molybdenum is connected with increased values of σ0·2 in the higher temperature region of the cold brittleness range, while a reduction of Tbl is caused by a marked weakening of the temperature sensitivity σ0·2. 4. The retention of plasticity down to lower temperatures with a high rhenium content (36%) is associated with the additional mechanism of strain-twinning. In alloys with a lower rhenium content (0.5–20%) twinning is not observed. 5. At all temperatures below Tbu the character of fracture of alloys, regardless of their rhenium content, is brittle, transcrystalline and differs little when the test temperature changes from Tbu to Tbl. An increase in plasticity during transition from Tbl to Tbu leads to the appearance in the fracture of traces of a preliminary plastic strain with the general brittle nature of fracture retained. At temperatures above Tbu the specimens bend with α=120°C without fracture, consequently, the Mo−Re alloys like pure molybdenum develop no tough fracture.