On The Sintering Of Molybdenum With Two Liquid Phases

The liquid phase sintering (LPS) of Molybdenum-base alloys containing 80 vol% Mo and 20 vol% Cu and Ni as sintering aids was investigated. Sintering experiments and subsequent microstructural analysis together with equilibrium and non-equilibrium thermodynamic calculations elucidate the densification behavior and the microstructure of Mo-Cu-Ni alloys. LPS is dominated by the liquid miscibility gap in the Mo-Cu-Ni system. Surrounding the Mo-grains during sintering, a Cu-rich and a Ni-rich liquid phase with moderate and high solubility for Mo respectively, generate fully dense and ductile composites via the solution and reprecipitation mechanism. The rates of densification and microstructural coarsening increase significantly with the amount of Ni-rich liquid, i.e. with the solubility of the binder for Mo.Upon cooling, the two immiscible liquids solidify into a dual-phase binder comprising a Cu- and a Ni-rich fcc (Cu,Ni) solid solution respectively. Precipitation of the brittle MoNi (delta) phase, which was reported in previous studies, can be suppressed provided that (i) sintering happens at temperatures where delta, which develops during heat-up of the elemental Mo-Cu-Ni powder blend, dissolves in the liquid phases and (ii) cooling from sintering is sufficiently fast to suppress its reprecipitation.