Valence electron concentration as an indicator for mechanical properties in rocksalt structure nitrides, carbides and carbonitrides

By K. Balasubramanian, S. Khare, D. Gall. Acta Materialia, 2018

249 citations recorded by the source corpus.

Abstract First-principles calculations are employed to determine the mechanical properties of rock-salt structure binary and ternary transition metal nitrides, carbides, and carbonitrides from groups 4 to 12, predicting a unified indicator for mechanical properties: the valence electron concentration (VEC). Pugh's and Poisson's ratios indicate an increasing ductility with increasing VEC, with a brittle-to-ductile transition at a critical VEC = 10. The calculated C44 of carbonitrides and ternary nitrides monotonically decreases from 164 ± 12 GPa at VEC = 8 to −39 ± 46 GPa at VEC = 11, indicating a transition to mechanical instability at VEC = 10.6. Similarly, the average isotropic elastic modulus decreases slightly from 420 GPa for VEC = 8 to 388 GPa for VEC = 10, but then steeply to −98 GPa for VEC = 11, while the corresponding hardness decreases from 25 to 12 to 2 GPa. The overall softening with increasing VEC is attributed to the increasing electron density in d–t2g orbitals, which overlap upon shear and cause a decrease in C44. Phonon dispersion curves, calculated at 0 K for binary nitrides and carbides, exhibit imaginary frequencies for VEC ≥10, indicating a dynamical stability-to-instability transition between VEC = 9 and 10, which is smaller than the critical VEC = 10.6 for the mechanical stability-instability transition. In addition, mechanical stability is increased by magnetic ordering but decreased when accounting for on-site Coulomb repulsion, while temperature and vacancies cause a reduction in the magnitude of C44 for both stable and unstable compounds, likely leading to an increase in the critical VEC for the stability-instability transition. The overall results indicate a narrow region between VEC = 9 and 10 where rocksalt carbonitrides are ductile but also exhibit a high hardness, mechanical and dynamical stability, and therefore are expected to exhibit the highest toughness.

Subfields: Materials Chemistry, Mechanical Engineering, Mechanics of Materials

View Paper: https://www.semanticscholar.org/paper/6a110a2dcc9b4dbe790267af5000c239e8f0d6c3

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Citations in the corpus

Phase stability and mechanical properties of novel high entropy transition metal carbides (2019, background): “[48], that the hardness of carbides decreases as a function of VEC from 8.”