Valence electron concentration as an indicator for mechanical properties in rocksalt structure nitrides, carbides and carbonitrides
Valence electron concentration as an indicator for mechanical properties in rocksalt structure nitrides, carbides and carbonitrides
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DOI：
10.1016/j.actamat.2018.04.033
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发表时间：
2018-06-15
期刊：
影响因子：
9.4
通讯作者：
Gall, Daniel
中科院分区：
文献类型：
--
作者：
Balasubramanian, Karthik;Khare, Sanjay V.;Gall, Daniel
采用第一性原理计算确定岩盐结构二元和三元过渡金属氮化物、碳化物和碳氮化物的力学性能，预测力学性能的统一指标：价电子浓度（VEC）。皮尤比和泊松比表明随着VEC的增加，延性增加，在临界VEC = 10时，脆性向韧性转变。碳氮化物和三元氮化物的计算C-44从VEC = 8时的164 +/- 12 GPa单调降低到VEC = 11时的-39 +/- 46 GPa，表明在VEC = 10.6时转变为机械不稳定性。类似地，平均各向同性弹性模量从VEC = 8的420 GPa略微降低到VEC = 10的388 GPa，但随后急剧降低到VEC = 11的98 GPa，而相应的硬度从25降低到12再降低到2 GPa。随着VEC的增加，整体软化归因于d-t（2g）轨道中电子密度的增加，其在剪切时重叠并导致C-44的减少。声子色散曲线，计算在0 K的二元氮化物和碳化物，显示虚频率VEC >= 10，表明动态稳定性不稳定性之间的过渡VEC = 9和10，这是小于临界VEC = 10.6的机械稳定性不稳定性转变。此外，机械稳定性增加的磁有序，但减少现场库仑排斥时，而温度和空位导致的幅度减少的C-44的稳定和不稳定的化合物，可能会导致增加的临界VEC的稳定性-不稳定性转变。总体结果表明VEC = 9和10之间的狭窄区域，其中岩盐碳氮化物是可延展的，但也表现出高硬度、机械和动态稳定性，因此预期表现出最高的韧性。(C)2018 Acta Materialia Inc.由爱思唯尔有限公司出版。保留所有权利。
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 C-44 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-t(2g) orbitals, which overlap upon shear and cause a decrease in C-44. 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 C-44 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. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.