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为了研究纳米多晶铁的拉伸性能,本文采用泰森多边形方法[12]建立了周期性边界条件下的纳米多晶模型,包括16个具有随机晶向的晶粒。首先建立晶粒尺寸为5.62 nm的多晶模型,第一步是生成一个边长为40 a(a = 2.8553 Å,为晶格常数)的立方体空间,并在该空间按照晶格结构生成铁原子;第二步撒入16个点,每个点代表一个晶粒,然后在周期性边界条件下将该空间中的铁原子划分给这16个晶粒,具体操作是把空间中的铁原子划分给离它距离最近的那个点;第三步是给每个晶粒重新设定随机晶向,这样就建立了具有16个晶粒的多晶模型。
为了消除晶粒的晶向对机械性能的影响,对晶粒尺寸为5.62 nm的模型按各向等比例放大,但不改变每个晶粒的晶向,如此分别形成晶粒尺寸为8.44, 11.24, 13.36以及14.46 nm的模型。纳米多晶铁的晶粒尺寸用16个晶粒的平均直径来表示。由于模型中晶粒的晶向为随机分布,因此形成的晶界为理想条件下多晶的一般晶界。本文采用软件Ovito[13]分析模型的晶体结构,模型的晶内区域为体心立方结构,晶界区域为非体心立方结构,具体参数如表1所列。多晶模型晶界区域的原子数比例与晶粒尺寸近似成反比,即:28.58:19.21:14.51:12.25:11.09
$ \approx \frac{1}{5.61}:\frac{1}{8.35}:\frac{1}{11.05}:\frac{1}{13.09}:\frac{1}{14.46} $ 。表 1 掺氦前纳米多晶模型的参数(a表示晶格常数,为2.8553 Å)
晶粒尺寸/nm 模型体积/Å3 模型原子数 晶界原子比例/% 5.62 (40×a)3 123 822 28.58 8.44 (60×a)3 422 583 19.21 11.24 (80×a)3 1 008 485 14.51 13.26 (95×a)3 1 692 218 12.25 14.46 (105×a)3 2 286 822 11.09 考虑到实验所制备的纳米材料的晶界区域通常存在空位,因此本文的掺氦纳米多晶模型是在初始模型的晶界区域随机引入替代氦原子。参考结构材料在实际服役过程中产生的氦原子分数 (约0.2%) 以及前人对晶界氦原子的相关研究[7, 14],本研究中氦的原子分数为0.5%,这样可以在较大的应变范围内观察到明显的裂纹演化过程,便于追踪裂纹产生与微观结构变化之间的联系。为了后文的表达简洁清晰,用“晶界氦原子(GB He)”代表位于晶界区域的替代氦原子。
本文采用Lammps程序[15]模拟在室温拉伸载荷作用下纳米多晶铁的微观结构与机械性能。模拟过程如下:首先对初始模型进行能量最小化弛豫(以11.24 nm的多晶铁为例,驰豫后的模型如图1所示),然后使模型在NPT(Nose/Hoover isobaric-isothermal) 系综下升温至300 K,接着使模型沿X轴方向以5×10−8/s的应变率发生拉伸形变,Y与Z方向则保持零压。拉伸模拟过程中采用的相互作用势包括Fe-Fe[16],Fe-He[17]与He-He[18]。拉伸模拟结束后,采用Lammps程序Diffraction模块[19]计算形变过程中纳米多晶铁的XRD谱。
Molecular Dynamics Simulation Study of the Effect of Grain Size on the Mechanical Property of Nano-polycrystalline Iron Doped with He
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摘要: 采用分子动力学方法研究了晶粒尺寸对掺氦纳米多晶铁机械性能的影响。在拉伸形变过程中,纳米多晶铁将产生裂纹与晶格畸变,通过模拟XRD谱探索二者之间的联系。拉伸模拟结果显示,由于晶界氦原子的引入,峰值应力将显著减小。另外,在拉伸模拟中观察到,沿晶裂纹的产生与长大随着晶界氦原子的引入而增强。研究结果表明,晶界氦原子能够促进沿晶裂纹的产生,而裂纹的大小和数量随着晶粒尺寸的增加而增加。在拉伸过程中,纳米多晶铁的XRD谱显示,{200}峰与{211}峰发生了明显的分裂。在应变范围6%~10%之间,与掺氦前相比,掺氦后纳米多晶铁的XRD谱中副峰的衍射角较高。随着晶粒尺寸的增加,副峰峰位逐渐向高衍射角移动,且裂纹萌生逐渐增强。这说明XRD谱显示的晶格畸变与沿晶裂纹所致的机械性能退化密切相关。Abstract: Molecular dynamics(MD) simulations were performed to investigate the effects of grain size on the mechanical properties of nano-polycrystalline iron doped with helium(He). Simulated X-ray diffraction(XRD) was used to explore the relationship between the generation of cracks and the distortion of crystal structural in nano-polycrystalline iron during tensile deformation. The simulation results show that the peak stresses are obviously decreased due to the introduction of He atoms into the grain boundaries of nano-polycrystalline iron. In addition, it is observed that the generation and growth of intergranular cracks are significantly enhanced by He atoms which are distributed at grain boundary(GB) regions during tensile simulation. The results suggest that the intergranular cracks are promoted by GB He atoms, and the size and number of cracks increase with the increasing grain size of nano-polycrystalline iron. The separation of peak {200} and {211} are significantly observed in the XRD patterns during loading. The diffraction angle of subpeaks is higher in the XRD patterns of nano-polycrystalline iron with GB He than in that without He during the strain range from 6% to 10%, and the diffraction angle of subpeaks and the growth of cracks increase with the increasing grain size. It is demonstrated that the degradation of mechanical property caused by intergranular cracks has close relationship with the change of structural distortion obtained by simulated XRD patterns in nano-polycrystalline iron.
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Key words:
- nano-polycrystalline Fe /
- simulated XRD /
- grain size /
- intergranular crack /
- He effect
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图 5 (在线彩图) 晶粒尺寸为11.24 nm的多晶铁,其应变从6%逐渐增加到9%,沿晶裂纹随应变的增加以及氦原子的聚集而扩展
红色为氦原子,铁原子按照铁原子体积着色,着色条与图3中的一致。
表 1 掺氦前纳米多晶模型的参数(a表示晶格常数,为2.8553 Å)
晶粒尺寸/nm 模型体积/Å3 模型原子数 晶界原子比例/% 5.62 (40×a)3 123 822 28.58 8.44 (60×a)3 422 583 19.21 11.24 (80×a)3 1 008 485 14.51 13.26 (95×a)3 1 692 218 12.25 14.46 (105×a)3 2 286 822 11.09 -
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