Effect of La doping on impurity segregation and fracture behavior of tungsten-based heavy-alloy composites

The effects of La doping on impurity segregation and fracture behavior of tungsten heavy-alloy composites (78W-17Mo-2.5Ni-2.3Fe-0.2Mn) were studied. The La-doped composites were prepared by liquid phase sintering a powder mixture at 1510 degrees C for 1.5 hours under a hydrogen atmosphere. The microstructure and composition were analyzed by scanning electron microscopy/energy dispersive spectroscopy x-ray diffraction, auger electron spectroscopy, electron microprobe, and transmission electron microscopy. It was found that the interfacial segregation of oxygen is the main cause of embrittlement of tungsten heavy-alloy composites, and such embrittlement can be suppressed by adding the rare earth element La. Microstructural analysis showed that there are two stable new microphases in La-doped composites; an orthorhombic LaMnO3 and a small amount of Mn3O4. The formation of these oxides increases the interfacial bonding strength of tungsten (W)-matrix interfaces by reducing the segregation of impurity elements to these interfaces, and hence greatly improves the mechanical properties of the composite. Fractographic observation in the La-doped composites showed that the tensile fracture morphology changes from the brittle intergranular fracture of W grains and separation along the W-matrix interfaces into a ductile fracture appearance with the transgranular cleavage of W grains and some ductile rupture of the matrix phase.