α-Fe中氦泡极限压强的分子动力学模拟

Molecular dynamics simulation of helium bubble ultimate pressure in α-Fe

为了深入认识α-Fe中氦泡冲出位错环的微观机制,有必要研究α-Fe中氦泡冲出位错环时的极限压强特性.本文建立金属-氦泡的立方体型代表性体积单元模型,针对8种不同初始半径的球形氦泡,以初始氦空位比为变量,开展分子动力学模拟,得到了各模型中位错环开始形成时的氦泡极限压强和临界氦空位比.研究结果表明:对于无量纲半径介于2—10的氦泡,冲出位错环时的氦泡极限压强和临界氦空位比均随着氦泡初始半径的增大而非线性减小;基体中氦泡冲出位错环时的临界氦空位比具有明显的尺寸效应;初始时刻(0 ps),在经过立方体型模型中心的横截面上,氦泡周围Fe原子阵列的剪应力集中和最大剪应力出现在对角线与氦泡边界交点(即45°)处,并且关于横截面上平行于边的两条对折线对称分布,剪应力集中区的范围和最大剪应力均随着初始氦空位比的增大而增大;位错环冲出方向对应最大剪应力方向.本文的研究加深了对金属中氦泡物理特性的认识,为后续分析氦泡对材料宏观物理和力学性质的影响奠定了有益的基础.

In order to understand further the micro-mechanism of helium bubble punching out of the dislocation loop in α-Fe,it is necessary to study the ultimate pressure characteristics of helium bubble punching out of the dislocation loop.In this paper,a cubic representative volume element(RVE)model of the metal-helium bubble is established.For eight types of spherical helium bubbles with different initial radii,molecular dynamics simulations are carried out with the initial helium-to-vacancy ratio serving as a variable and the ultimate pressure of helium bubble and the critical helium-to-vacancy ratio at the beginning of dislocation loop formation in each model are obtained.The results show that for helium bubbles with dimensionless radius ranging from 2 to 10,both the ultimate pressure and the critical helium-to-vacancy ratio of helium bubble punching out of the dislocation loop decrease nonlinearly with the increase of initial helium bubble radius.The relationships of the ultimate pressure and the critical helium-to-vacancy ratio with the initial radius of helium bubble are fitted respectively according to the simulation results and the fitted two equations are in good agreement with the results of previous theoretical studies.The critical helium-to-vacancy ratio required for helium bubble punching out of the dislocation loop in α-Fe has an obvious size effect.For the helium bubble in the late nucleation stage(e.g.helium bubble with an initial radius of 0.81 nm)and non-ideal gas stage(e.g.helium bubble with an initial radius ranging from 1.00 nm to 2.50 nm),the critical helium-to-vacancy ratios for punching out of the dislocation loop are the same as the initial helium-to-vacancy ratio corresponding to the peak pressure point of helium bubble.But for early or middle nucleation stage,such as helium bubble with an initial radius of 0.50 nm,the critical helium-to-vacancy ratio for punching out of the dislocation loop is about 13.46%greater than the initial helium-to-vacancy ratios corresponding to the peak pressure points.At the initial moment(0 ps),in the cross section passing through the center of cubic RVE,the shear stress is concentrated,and the maximum shear stress of Fe atom array around the helium bubble is located at the intersection points(i.e.at 45°)of diagonal and helium bubble boundary,and it is distributed symmetrically with respect to the double fold lines of the cross section parallel to the sides.Both the range of shear stress concentrating area and the maximum shear stress increase with the initial helium-to-vacancy ratio increasing.The dislocation loop’s punching direction corresponds to the direction of the maximum shear stress.The research in this paper can deepen the understanding of the physical properties of helium bubbles in metals and lay a useful foundation for the subsequent analyzing of the effects of helium bubbles on the macroscopic physical and mechanical properties of materials.