报 告 一：Computational designs and experimental verifications of binary Mn-B compounds

主 讲 人：牛海洋 博士生 （材料加工模拟研究部）

报 告 二：Preparation and characterization of superhard AlB₂-type WB₂ coatings

主 讲 人：蒋春磊 博士生 （材料表面工程研究部）

时    间：10月29日（周二） 15：00—17：00

地    点：师昌绪楼 403 会议室

报告一简介：

In combination with the variable-composition evolutionary algorithm calculations and the first-principles calculations, we have systematically searched for all the stable compounds and their crystal structures in the extensively investigated binary Mn-B system. Our results have uncovered four viable ground state compounds, with Mn₂B, MnB, MnB₄ and previously never reported MnB₃ and two metastable compounds, MnB₂ and Mn₃B₄. Our calculations demonstrated that the early characterized mC₁₀ structure of MnB₄ showed dynamical instability with large imaginary phonon frequencies and instead, a new mP₂₀ structure is predicted to be stable both dynamically and thermodynamically, with a considerable energy gain and no imaginary phonon frequencies. The new MnB₃ compound crystallizes in the monoclinic mC₁₆ structure which lies 3.2 meV/atom below the MnB (oP₈) ↔ MnB4 (mP₂₀) tie-line at T = 0 K. Furthermore, these proposed phases have been verified by our annealed samples after the arc-melting synthesis and the corresponding powder XRD measurements.

报告二简介：

WB₂ nanocomposite coatings were synthesized by dc magnetron sputtering. The morphology, chemical composition, chemical bonding state, microstructure, hardness, and tribological properties were investigated. The columnar-structured coatings are dense and uniform. The as-deposited coatings were found to have a metastable AlB₂-type structure characterized by X-ray diffraction and further confirmed by high-resolution transmission electron microscopy. Coatings exhibit superhardness about 43.2 ± 5 GPa (taken from the plateau region) obtained by nanoindentation. In addition, the steady-state friction coefficient μ = 0. 23 and wear rate K = 6.5 × 10^-6 mm³/Nm were obtained under dry sliding condition in ambient environments. Those characters indicate AlB₂-type WB₂ coatings have high potential application as superhard and low wear coatings. Moreover, the AlB₂-type WB₂ can hardly be prepared using the conventional powder metallurgy technology without high pressure, and this study proposes a new practical approach to prepare it.

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