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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