Abstract
Among transition metal carbides and nitrides, zirconium, and hafnium compounds are the most stable and have the highest melting temperatures. Here we review published data on phases and phase equilibria in Hf-Zr-C-N-O system, from experiment and ab initio computations with focus on rocksalt Zr and Hf carbides and nitrides, their solid solutions and oxygen solubility limits. The systematic experimental studies on phase equilibria and thermodynamics were performed mainly 40–60 years ago, mostly for binary systems of Zr and Hf with C and N. Since then, synthesis of several oxynitrides was reported in the fluorite-derivative type of structures, of orthorhombic and cubic higher nitrides Zr3N4 and Hf3N4. An ever-increasing stream of data is provided by ab initio computations, and one of the testable predictions is that the rocksalt HfC0.75N0.22 phase would have the highest known melting temperature. Experimental data on melting temperatures of hafnium carbonitrides are absent, but minimum in heat capacity and maximum in hardness were reported for Hf(C,N) solid solutions. New methods, such as electrical pulse heating and laser melting, can fill the gaps in experimental data and validate ab initio predictions.
Highlights
The carbides and nitrides of zirconium and hafnium provide a reservoir of very effective refractory materials, including some compounds that have been predicted to have the highest melting points [1].Ti, Zr, and Hf belong to group IVb of transition metals
The state of the art calculation of phase diagrams (Calphad) method [70,71,72,73] allows for the calculation of phase equilibria in multicomponent systems above room temperature at atmospheric pressure, providing that Gibbs free energy functions for all phases from room temperature to the melting point and above can be derived from observed phase equilibria and thermodynamic data
A Calphad optimization of Zr-N system was performed by Ma et al [24] using experimental data from Domagala et al [90] for Zr-rich part and enthalpies of formation estimated from ab initio computations
Summary
The carbides and nitrides of zirconium and hafnium provide a reservoir of very effective refractory materials, including some compounds that have been predicted to have the highest melting points [1]. Melting temperatures for carbides and nitrides of zirconium and hafnium are higher than those for Ti, they are studied less and their applications are currently limited. During the last 20 years, Lengauer et al [37,38,39,40,41] provided comprehensive reviews and new experimental data on and based on more than 2000 original research reports published before 1980. Our focus is on rocksalt (δ-phase) (oxy)carbonitrides: monocarbide and mononitrides of Zr and Hf, their solid solutions, and oxygen solubility limits They are the only compounds in these systems that melt without decomposition and are traditionally considered most interesting for applications. We review the structure and thermochemistry of rocksalt (oxy)carbonitrides and list recommendations for future experimental and computational works
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