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

Abstract

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 W 2 C 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-Fe 2 N 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-Fe 2 N type as an intermediate. Taking the results obtained and literature data into consideration, we have corrected the phase diagram for the W − C system.

Neutron and x-ray diffraction study and symmetry analysis of phase transformations in lower tungsten carbideW2C

“…8,19,20,22, and 24͒ is given in Table I. Samples of W 2 C prepared by arc melting of W and WC mixtures with subsequent annealing at 1720-2120 K for 1.5 h and quenching to room temperature were studied 26,27 by the x-ray method. The structure was determined in W 2 C single crystals 0.2-0.5 mm in size taken from quenched samples.…”

“…The structure was determined in W 2 C single crystals 0.2-0.5 mm in size taken from quenched samples. Velikanava and co-workers 26,27 found that at a temperature of ϳ2120 K, the disordered lower carbide ␤-W 2 C transforms to the trigonal ordered phase -W 2 C. According to Ref. 27, the trigonal phase -W 2 C is thermodynamically stable at temperatures of 2120-1520 K.…”

“…27: two-phase ͑W+W 2 C͒ and ͑W 2 C+WC͒ samples, which were prepared by arc melting, were annealed at 1170 K for 850 h. The higher carbide WC was formed after lowtemperature annealing of ͑W+W 2 C͒ samples and tungsten W appeared after annealing of ͑W 2 C+WC͒ samples. Therefore, Kublii and Velikanova 27 assumed that at a temperature of about 1520 K, the ordered trigonal phase -W 2 C decomposes by an eutectoid reaction to tungsten W and the higher tungsten carbide WC, with the intermediate decomposition product being the metastable ordered orthorhombic ͑space group Pbcn͒ phase ␤Ј-W 2 C having the -Fe 2 N structure.…”

Structural, electronic properties and stability of tungsten mono- and semi-carbides: A first principles investigation

“…The peculiarities of the phase formation in a W-C system as a function of W/C content and temperature were studied by means of systematic X-ray, dilatometric and microstructural experiments, and six different individual W-C phases were established. Among them, there are two mono-carbides WC-hexagonal (h) and cubic (c) and four phases (a, b, g and e) of semi-carbide W 2 C, see reviews in [8,17,18]. Some theoretical efforts have been made to understand the electronic structure and the origin of properties of these materials such as hardness, bulk modules and stability.…”

Phase composition and microstructure of WC–Co alloys obtained by selective laser melting

“…Cobalt exists in two modifications: a-Co (hexagonal, space group P63/mmc) and b-Co (cubic, space group Fm-3m) [22,23]. In WC-Co alloys, the transformation of WC carbide to W 2 C, Me 6 C, and Me 12 C carbides occurs due to decreasing carbon quantity in the alloy [24,25]. In the course of obtaining WC-Co alloys, the formation of the face-centered cubic (fcc) b-Co-based supersaturated solid solution which contains a large amount of W and C is possible [20,26].…”