Identification of the segregation layer and its effects on the activated sintering and ductility of Ni-doped molybdenum

Previous literature has shown that the sintering temperature of molybdenum compacts can be decreased significantly through activated sintering by adding a small amount of nickel. This nickel doping, however, reduces the ductility of the material. A number of studies have postulated mechanisms to explain this enhanced sintering and embrittlement, but little direct evidence has been reported to date. In this study, the use of a field emission transmission electron microscope and a scanning Auger electron microscope has identified a delta-NiMo intermetallic compound film, about 2 nm thick, at the grain boundaries. An experiment focusing on the Mo/Ni-Mo diffusion couple shows that the delta-NiMo layer at the grain boundaries serves as a short-circuit diffusion path and causes the activated sintering of Ni-doped molybdenum. This compound is, however, intrinsically brittle and thus is the root cause of the embrittlement problem of Ni-doped molybdenum. (C) 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.