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Neutron and x-ray diffraction study and symmetry analysis of phase transformations in lower tungsten carbide W 2 C

2007; American Physical Society; Volume: 76; Issue: 17 Linguagem: Inglês

10.1103/physrevb.76.174115

1550-235X

A. S. Kurlov, A. I. Gusev,

Metallurgical and Alloy Processes

The literature data on the crystal structure of disordered and different ordered phases of the lower tungsten carbide ${\mathrm{W}}_{2}\mathrm{C}$ are contradictory. In this context, the symmetry analysis of all possible superstructures of the carbide ${\mathrm{W}}_{2}\mathrm{C}$ is performed and the physically possible sequence of phase transformations in ${\mathrm{W}}_{2}\mathrm{C}$ carbide is established. Atomic and vacancy ordering in the lower tungsten carbide ${\mathrm{W}}_{2}\mathrm{C}$ with the basic hexagonal structure of the ${L}^{\ensuremath{'}}3$ type is studied by the neutron and x-ray diffraction methods. It is found that the trigonal $\ensuremath{\epsilon}\text{\ensuremath{-}}{\mathrm{W}}_{2}\mathrm{C}$ phase (space group $P\overline{3}1m$) is the only ordered phase of the lower tungsten carbide over a wide temperature interval of $\ensuremath{\sim}2300--1370\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Trigonal phase $\ensuremath{\epsilon}\text{\ensuremath{-}}{\mathrm{W}}_{2}\mathrm{C}$ is formed at a temperature of $\ensuremath{\sim}1900--2300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ most likely by the mechanism of the second-order phase transition. The disorder-order phase transition channel and the structure of the trigonal carbide $\ensuremath{\epsilon}\text{\ensuremath{-}}{\mathrm{W}}_{2}\mathrm{C}$ are determined. The carbon atom distribution function is calculated for the trigonal $\ensuremath{\epsilon}\text{\ensuremath{-}}{\mathrm{W}}_{2}\mathrm{C}$ superstructure. The distribution of carbon atoms in the trigonal $\ensuremath{\epsilon}\text{\ensuremath{-}}{\mathrm{W}}_{2}\mathrm{C}$ phase is described by two long-range order parameters ${\ensuremath{\eta}}_{15}$ and ${\ensuremath{\eta}}_{17}$. It is shown that the lower tungsten carbide does not undergo solid-phase decomposition to W and WC over the investigated temperature interval from $2300\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1370\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The phase diagram of the W-C system is refined considering data obtained for the $\ensuremath{\epsilon}\text{\ensuremath{-}}{\mathrm{W}}_{2}\mathrm{C}$ phase.