The embrittling effect of C and S trace impurities in W-Ni-Fe heavy alloy has been studied. Impact property is controlled primarily be the strength of the tungsten-matrix interfaces.
In order to find the embrittlement by carbon and sulfur, small amount of WC and FeS powders have been added to 90W-7Ni-3Fe alloy specimens. All the specimens have been sintered in hydrogen atmosphere at 1460$^\circ{C}$ and heat-treated in nitrogen atmosphere at 1000$^\circ{C}$ for 30 minutes and quenched.
Tensile elongation of standard specimen without any additives do not change with sintering time, but the specimens to which compound has been added show low elongation but with sintering time the elongation increases that of strandard specimen. Impact energy of standard specimen is high, but the specimens which contained carbon and sulfur are lower in impact energy for various sintering times. Dimples of the matrix phase ovserbed in fracture surfaces of the standard specimens result from high interface strength. In contrast, carbon and sulfur of trace amount lower the interface strength and cause the material to fail at very low degrees of deformation. In the specimen to which WC or FeS has been added, tungsten grain-matrix interfacial decohesion is found to be the principal fracture mode by SEM fractography.
From the thermodynamical analysis, addition of $WO_3$ is expected to reduce carbon in the specimen. But sulfur is not expected to be effectively eliminated by the addition of $WO_3$. These predictions are confirmed by the obserbation of the fracture surfaces.
