Intermediate temperature embrittlement of one new Ni-26W-6Cr based superalloy for molten salt reactors

Ni-26W-6Cr based superalloy is considered a potential structure material for the molten salt reactors due to its high strength and good compatibility with the fluoride salt. In the present work, the temperature dependence of the tensile behavior of the alloy was studied by tensile tests in the temperature range of 25-850 degrees C. This alloy exhibited a good ductility at RT and 450 degrees C, a ductility minimum from 650 to 750 degrees C and an intermediate ductility at 850 degrees C. TEM and EBSD characterization was performed on specimens tested at three typical temperature points (RT, 650 degrees C and 850 degrees C) to determine the deformation and fracture mechanisms accounting for the intermediate temperature embrittlement. At RT, the grain boundaries can accommodate enough dislocations to provide compatibility of the sliding between adjacent grains, then M6C carbides act as crack origins and cause the fracture. In case of 650 degrees C, the grain boundaries cannot withstand the local stress even if only a small number of dislocation pile-ups exist. The premature cracks at grain boundaries impede the development of plastic deformation from single slips to multiple ones and cause the low ductility. If tested at 850 degrees C, the fracture process is retarded by the dynamic recovery and local dynamic recrystallization at crack tips. (C) 2016 Elsevier B.V. All rights reserved.