纯Mo〈110〉对称倾侧晶界的分子静力学模拟研究

Molecular Statics Simulation of 〈110〉 Symmetric Tilt Grain Boundary in Pure Mo

Mo的固有脆性是限制Mo及Mo基合金作为包壳材料应用的关键性问题,其脆性的一个重要来源是Mo本身强度较低的晶界。为系统理解纯Mo的晶界结构,本文采用分子静力学方法,对32个〈110〉对称倾侧晶界的晶界能及空位形成能进行研究,并讨论空位在晶界上的形成规律。计算结果表明,晶界区域相较于完美晶体更易形成空位,且晶界上空位形成能与晶界能密切相关,越稳定的晶界越不易形成空位。空位在不同晶界结构上的形成规律有显著差异,在倾侧角小于等于58.992°的晶界结构中,空位易在C结构单元内形成;对于倾侧角介于58.992°与144.364°之间的晶界结构,空位易在“屋顶”结构中形成;对于倾侧角大于144.364°的晶界结构,空位易在Z型结构中形成。此外,通过4种势函数对32个晶界结构进行计算的结果表明,在晶界能与空位形成能方面,4种势函数无显著差异,但通过光谱近邻分析势计算得到的一部分晶界结构具有更高的不对称度。

Mo and its alloys were selected as candidate structural materials for high-temperature applications because of many attractive properties like outstanding heat-conducting capabilities and high-temperature strength.However,the inherent embrittlement of Mo at and below ambient temperature severely limits the application of molybdenum and Mo-based alloys as cladding materials.This weak point is generally considered to be caused by several factors,and one of the important factors is that the Mo grain boundary itself has low strength and is easy to become the preferred location for crack nucleation and propagation.In order to systematically understand the grain boundary structure of pure Mo,the grain boundary energy and vacancy formation energy of 32〈110〉symmetric tilt grain boundaries were studied by molecular statics method,and the formation regularity of vacancy at grain boundaries was discussed.For grain boundary energy,the results calculated by four inter-atomic potentials are generally higher than previous study but the trend is nearly the same.Only a few grain boundaries,such asΣ3(111)andΣ3(112)grain boundaries,are inconsistent with the trend.For vacancy near the GB,the vacancy formation energy calculated by MEAM and ADP potential functions are 2.95 eV and 2.97 eV respectively,which are in agreement with results calculated by density functional theory(DFT).However,the results calculated by EAM and SNAP are slightly lower,which are 2.65 eV and 2.58 eV.For the formation regularity of vacancy,the results show that vacancies are more easily formed in the grain boundary than in the perfect crystal,and the vacancy formation energy in the grain boundary is closely related to the grain boundary energy,and the more stable the grain boundary is,the less prone to form vacancies.There are significant differences in the formation regularity of vacancies in different grain boundary structures.In the grain boundaries with tilt angle less than or equal to 58.992°,vacancies are easy to form in the C structure units.For the grain boundaries with tilt angle between 58.992°and 144.364°,vacancies are easily formed in the“roof”structure.For the grain boundaries with tilt angle greater than 144.364°,vacancies are easily formed in the Z structure.In addition,the results of 32 grain boundary structures calculated by four inter-atomic potentials show that there is no significant difference between the four potentials in terms of grain boundary energy and vacancy formation energy,but some grain boundary structures relaxed by SNAP potential function have higher asymmetry.