Tungsten heavy alloys are industrial materials used in many applications including balancing weights and collimators for high energy beams. From a theoretical perspective, W-(Fe, Ni, Cu) alloys have served as a model system for the liquid-phase sintering of metals over many dec-ades. Interestingly, despite the striking similarities between W and Mo, the assistance of liquid phases has not been used to densify technical Mo-alloys, apart from “non-sintering” mecha-nisms such as Cu-infiltration of pre-sintered, porous Mo-skeletons. It is the intention of this presentation to elucidate the constitutional fundamentals for the liquid phase sintering (LPS) of dense and ductile Mo-alloys with a Mo content > 70 wt%., by (i) investigating the constitution of binary and ternary Mo-systems using the Thermocalc® software and (ii) comparing the results with microstructural and analytical observations.
The assessment of the thermodynamics of LPS-Mo uses both equilibrium computations and the Scheil-Gulliver-solidification scheme. These calculations reveal that Cu-Ni additions are most favorable to sinter Mo with a liquid phase. Equilibrium phase diagrams show a high solubility for Mo in the Ni-Cu melt but also a pronounced tendency for the formation of the intermetallic MoNi-phase (δ-phase). In contrast, Scheil-Gulliver simulations, which assume infinite diffusivity in the liquid and zero diffusivity in the solid, indicate that the δ-phase does not precipitate dur-ing cooling. Furthermore, a liquid miscibility gap, comprising a Ni- and a Cu-rich melt, exists in the composition range of interest.
Microstructural and analytical investigations in the Mo-Cu-Ni system by light and scanning elec-tron microscopy, XRD, EDX, EBSD and DSC on specimens compacted by liquid phase sintering confirm that intermetallic phases do not form at cooling rates on the order of 10 K/s. Instead the two immiscible liquids, which are present at the sintering temperature (1400°C), solidify into two fcc-phases which constitute the binder between the almost spherical Mo particles.
The experimental and theoretical study of the Mo-Cu-Ni system reveals that all essential crite-ria for LPS, i.e. chemical compatibility, solubility and wetting of Mo by the liquid binder, are fulfilled. The liquid sintering in the Mo-Cu-Ni-system leads to the desired, fully dense micro-structure consisting of spherical Mo particles embedded in a matrix of fcc-Cu,Ni-binder phases.