A constitutional diagram of the system TiC−HfC−“MoC”

Abstract

The system TiC−HfC−“MoC” was investigated by means of melting point, differentiothermoanalytical, X-ray diffraction and metallographic techniques on hotpressed as well as melted alloy specimens. A constitutional diagram from 1500°C through the melting range was established.

Investigation of the (Hf, Mo)C system (isopleth: HfC_0.98−“MoC_1.0”) showed a small miscibility gap within the cubic monocarbide solution (δ) [Tc=1630°C, (HfC)_0.45(MoC)_0.55]. The miscibility gap interacts with the solvus curve with a monotectoid-like decomposition reaction at 1575°C, (HfC)_≈0.35(MoC)_≈0.65.At temperatures below 1630°C, phase equilibria within TiC−HfC−“MoC” are characterized by a large miscibility gap connecting the TiC−HfC and HfC−MoC boundary systems. Additions of “MoC” to TiC−HfC alloys decrease the critical temperature (1780°C); additions of TiC to HfC−“MoC” alloys raise the critical temperature (1630°C). No maximum type ternary critical point or saddle point was found to occur.

Isothermal sections were prepared at 1500°C and 1650°C. At temperatures above 1960°C (μ-MoC+C⇄δ-MoC) a complete solid solution (δ-B 1) is formed within TiC−HfC−“MoC”. The melting behaviour (liquidus projection of TiC−HfC−“MoC”) shows flat melting temperatures in the “MoC” corner but extremely heterogeneous melting near the TiC−HfC boundary.

Isothermal sections have been calculated assuming regular solutions.