Constitution of ternary titanium-tungsten-carbon alloys

Abstract

The ternary alloy system Ti-W-C was investigated by means of melting point, differential-thermoanalytical, X-ray diffraction, and metallographic techniques on hot-pressed and heat-treated, as well as melted, alloy specimens and a phase diagram from 1500 °C through the melting range established. Above 2530 °C, titanium monocarbide and the isomorphous cubic high temperature phase in the W-C binary system are completely miscible and the solid solution has a congruent (maximum-type) melting point of 3130 °C at the approximate composition (Ti 0.54W 0.44)C 0.75. Below 2530 °C, the tungsten exchange in the cubic monocarbide is temperature-dependent and decreases to about 50 at.% at 1400 °C. The titanium exchange in W 2C is less than 3 at.% at 1500 °C and reaches a maximum of 8 at.% at 2680 °C. In addition to the ternary congruent melting point, four reaction isotherms occur in the system: Two are pseudobinary eutectics, monocarbide + graphite at 3030 °C, and monocarbide + tungsten-rich metal alloy at 2700 °C; peritectic melting of the binary WC results in a class II reaction L + C⇄cubic monocarbide + WC (hex) at 2760 °C, and a fourth isotherm at 2680 °C is associated with a ternary eutectic between metal, monocarbide ( B1), and W 2C. Pseudobinary eutectic melting between the monocarbide and the metal phase is directly attributable to the large stability difference between titanium and tungsten carbide, and the measured tie line distribution in the two-phase range metal + monocarbide is in accordance with estimates from known thermodynamic values for the binary boundary phases. The phase behavior in pseudobinary monocarbide solutions is largely determined by the relative lattice size of the boundary carbides, except for systems ( M, W) C 1 − x ( M = Ti, Zr, Hf, V). which show extra stabilization due to bonding effects.