Free energies of formation of WC and W2C, and the thermodynamic properties of carbon in solid tungsten

The activity of carbon in the two-phase regions W + WC and W + W2C has been obtained from the carbon content of iron rods equilibrated with mixtures of metal plus carbide powders. From this activity data the standard free energies of formation of WC and W2C have been calculated to be ΔG0/sub f/(WC) = -- 10,100 + 1.19T +- 100 cal/mode (--42,300 + 4.98T +- 400 J/mole) (1150 to 15750K), ΔG0/sub f/(W2C) = --7300 -- 0.56T +- 100 cal/mole (--30,500 -- 2.34T +- 400 J/mole) (1575 to 16600K). The temperature of the eutectoid reaction W2C = W + WC was fixed at 1575 +- 50K. Using available data for the solubility of C in solid W, the relative partial molar free energy of C in the dilute solid solution was calculated to be Δanti G/sub C//sup alpha/ = 23,000 -- [0.68 -- R ln X/sub C//sup alpha/]T +- 3000 cal/mole (96,200 -- [2.85 -- R ln X/sub C//sup alpha/]T +- 12,600 J/mole). The heat solution of C in W obtained was Δanti H/sub C//sup alpha/ = 23,000 +- 5000 cal/mole (96,200 +- 20,000 J/mole) and the excess entropy for the interstitial solid solution, assuming that the carbon atoms are in the octahedral sites, Δanti S/sub C//sup alpha/(xs,i) = --1.5 +- 2 cal/deg-mole (--6.3 +- 8 J/deg-mole)