Abstract
Metastable ternary ceramic alloys with clustering tendencies are candidates for hard coating applications. In this work, mixing thermodynamics and structural parameters of ceramic Ti1−xAlxB2 alloys are investigated with theoretical first-principles based techniques. Lattice dynamics and temperature dependent phase stability are explored. The effect of lattice vibrations on the total free energy is investigated and found to not significantly affect phase stability at temperatures below 1200 K. The isostructural phase diagram is derived using both cluster expansion-based Monte Carlo simulations and a mean field approach. The phase diagram shows a miscibility gap that does not close at temperatures below the melting or decomposition temperatures of the constituent binaries TiB2 and AlB2. The lattice mismatch between phases in the system is small regardless of their composition even at elevated temperatures. These findings support the prospect of age hardening due to coherent isostructural decomposition, such as spinodal decomposition, in coatings of Ti1−xAlxB2 as diffusion is activated at elevated temperature.
Highlights
Boride compounds make up a class of ceramic materials, of which some exhibit exceptional properties useful for applications intended for tough environments
The 0 K curves are obtained from static density functional theory (DFT) without zero-point motion (ZPM) correction to the lattice parameters, which otherwise amounts to an increase of 0.5% or less
The other curves are obtained within the quasi-harmonic approximation (QHA) where ZPM is included
Summary
Boride compounds make up a class of ceramic materials, of which some exhibit exceptional properties useful for applications intended for tough environments. Transition metal borides, in particular, have high melting point, high hardness, and good thermal and electrical conductivity.. TiB2, is a well-studied ceramic compound that is known for its hardness and resistance to wear.. Vajeeston et al investigated 12 transition metal diborides and found TiB2 to be the strongest in terms of cohesive energy, as well as having the lowest value of heat of formation.. Vajeeston et al investigated 12 transition metal diborides and found TiB2 to be the strongest in terms of cohesive energy, as well as having the lowest value of heat of formation.6 This all relates to the remarkable properties of TiB2, such as its high hardness, corrosion resistance, and high melting point.
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