纳米单晶α—Fe功能材料的力学性能及破坏机制

A study on mechanical properties and failure mechanism of nano-scale functional materials made of monocrystalline α-Fe

基于修正的镶嵌原子势建立了单晶α-Fe一维纳米线、二维纳米超薄膜和三维纳米固体3种纳米功能材料的物理模型，并采用分子动力学方法研究了其力学性能和变形破坏机制,模拟表明自由表面是影响纳米材料力学行为和睦能及破坏机制的重要因素之一。受自由表面断键影响，纳米线和纳米薄膜存在初始应力，系统平均原子能量高于纳米固体；纳米线和纳米薄膜的破坏从表面晶格开始向内部扩展，而纳米固体的拉伸破坏从内部晶格开始。研究得到α-Fe单晶2．87nm×2．87nm纳米线、2．87nm厚的纳米超薄膜和三雏纳米晶固体的屈服强度、拉伸断裂强度和初始拉伸刚度（弹性模量）及可承受变形。纳米金属功能材料的拉伸本构关系与宏观多晶金属相似，存在软化阶段，无明显应力强化过程，与实验结果吻合。

Numerical models for nano-wire, nano-film and nano-bulk made of monocrystalline α-Fe were proposed based on the modified embedded atomic potential, and the mechanical properties and failure mechanism of the nano-scale functional materials were studied using molecular dynamics method. Simulation shows that free surfaces take great effect on the mechanical behavior and failure mechanism of nano-scale materials. In nano-wire and nano-film, the average atomic energy is much higher than that in nano-bulk, and there are initial stresses without external loading. Failure of nano-wire and nano-film behaves as the expansion of lattice defect from external lattices to the interior ones, but that of nano-bulk starts from interior lattices directly. The yielding strength, fracture strength, elastic modulus and fracture strain of nano-wire （cross section size 2.87nm × 2.87 nm）, nano-film （thickness 2.87 nm） and nano-bulk were obtained and analyzed according to the simulation results. The constitutive curves of nano-scale metallic functional materials subject to uni-axial tension are similar to that of macro polycrystalline metals. An evident soften period presents but no harden process appears in the curves describing stress-strain response of nano-materials, which keeps favorable accordance to experimental results.