On the existence and the crystal structure of Ni4W, NiW and NiW2 compounds

The existence of the previously reported intermetallic phases NiW and NiW2 is refuted on the basis of new experiments involving arc-melted samples. A careful study of the literature shows that these two phases were previously evidenced only in sintered alloys and in diffusion couples, both techniques being very prone to surface contamination. It is shown that the phase previously reported as NiW is in fact the low carbon containing ternary carbide Ni6W6C. The second phase NiW2 is most probably the higher ternary carbide Ni2W4C. This study raises also the question of the existence of the equiatomic phase appearing in the Fe–W phase diagram. A crystal structure Rietveld refinement of the intermediate phase Ni4W is also given in the present work, evidencing large substitutional disorder.