Ordering in the carbide W2C and phase equilibria in the tungsten — carbon system in the region of its existence

We have studied alloys in the W - C system with compositions 24 to 48 at.% C, melted in an arc furnace, homogenized and quenched from temperatures of 1450°C to 2650°C or cooled stepwise from a subsolidus temperature down to 850°C, and annealed for a prolonged period at this temperature or at temperatures close to it. We studied the specimens by x-ray single-crystal and powder diffraction, metallography, and differential thermal analysis. We have shown that the carbide W₂C exists in two modifications: an hexagonal phase with structure of type L′₃ crystallizes from the melt, which at T ≈ 1850°C transforms to an ordered hexagonal phase of the ε-Fe₂N type. The latter, for a carbon content of about 32 at.% and T ≈ 1250°C, also decomposes according to a eutectoid reaction to 〈W〉 and WC. This decomposition is accompanied by formation of a metastable phase of the ξ-Fe₂N type as an intermediate. Taking the results obtained and literature data into consideration, we have corrected the phase diagram for the W - C system.