纳米结构材料由于其特殊的结构使其表现出许多传统材料所无法比拟的优异性能，如高强度、良好的摩擦磨损性能等。但同粗晶材料相比绝大多数纳米结构材料的塑性均很低。研究如何同时提高纳米结构材料的强度和塑性、探索纳米结构材料的结构-性能关系和塑性变形机制是近年来材料科学研究领域的热点。已有研究表明，纳米结构材料的力学性能同位错-晶界的相互作用密切相关。然而，由于实验手段所限到目前为止位错-晶界的相互作用机理尚未澄清。
位错与孪晶界的相互作用是位错-晶界相互作用的一个特例，本工作通过分子动力学方法模拟了面心立方晶体中螺位错与平行的共格孪晶界的反应过程。模拟结果表明，该过程分三个阶段完成：1）扩展位错受力运动到孪晶界附近，2）扩展位错在孪晶界上合并成全位错，3）全位错分解并在新的滑移面上运动。在第三阶段，位错存在两条可能的反应路径：a）位错在孪晶界上分解并沿孪晶界运动，b）位错穿透孪晶界，在孪晶中的滑移面上分解、运动。路径的选择依赖于两条路径上新的肖克莱不全位错的形核能垒。本工作根据Peierls模型给出两个无量纲参数，用于描述位错在不同路径上形核遇到的阻力。此外，还研究了该过程中系统的能量变化；但由于预加载应变能的影响，无法得到能垒的信息。
为了进一步研究位错-孪晶界的反应机理，本工作使用微动弹性带方法（Nudged Elastic Band method, NEB），在没有预加载应变的情况下，确定了位错-孪晶界反应的最小能量路径（Minimum Energy Path，MEP）及其能垒。利用简单的模型，结合线性弹性理论对得到的能量拟合，给出了位错-孪晶界间相互作用的表达式。进一步分析表明：
a） 当位错-孪晶界相距较远时，两者的相互作用来自两部分：1）孪晶界两侧由于晶体取向不同导致的弹性模量不匹配，2）应力应变场在穿越孪晶界时由原子属性导致的畸变；
b） 当位错-孪晶界距离较近时，由于位错核心区的应变很大，如果孪晶界所能承受的最大剪切应变较小，会导致孪晶界屈服，孪晶界对位错的排斥将变成吸引。
此外，本工作还使用微动弹性带方法测量了面心立方晶体中形成层错缺陷的最小能量路径，并与传统的刚性位移方法得到的层错形成能曲线作了对比。结果表明微动弹性带方法是一种在给定始末状态条件下确定反应最小能量路径的有效方法，对于研究材料变形过程（如孪生）有一定的意义。
其他摘要
Nanostructured materials usually exhibit superior properties such as high strength and enhanced tribological properties as compared to their coarse-grained counterparts. However, experimental results indicated that most nanocrystalline materials have a very limited ductility, which severely limits their practical utility. Consequently, much attention has been concentrated on the relationship between the microstructures and the deformation mechanisms.
It has been well established that the mechanical properties of nanostructured materials closely depend on the interactions between the lattice dislocations and grain boundaries, but many details of these interactions are not yet understood. To elucidate the physical mechanisms, a simple but illustrative case is studied by means of molecular dynamics (MD) simulations: the interaction between a screw dislocation and coherent twin boundaries (CTBs) in Al and Cu. The interaction process is observed to be in three steps: the dissociated dislocation is first constricted to the CTB, and then the partials recombine into a full dislocation, at last it re-dissociates. Depending on the material and the applied strain, during the last stage a screw dislocation on the coherent twin boundary may either dissociates within the boundary plane (cross slip) or it may propagate into the adjacent twin grain by cutting through the boundary (slip transmission). Which one of these two interaction modes/paths applies seems to depend on the material dependent energy barrier for the nucleation of Shockley partial dislocations. Two dimensionless parameters based upon Peierls’ model are used to describe the re-nucleation resistance encountered during the third stage. The system energies have also been illustrated, from which the energy barriers can be hardly extracted due to the pre-loaded strain.
MD simulations in combination with Nudged Elastic Band (NEB) method are used to examine energies required to impinge a screw dislocation on a CTB. The minimum energy paths of dislocation-CTB interaction processes are determined. The repulsive forces are described by a function of distance between the dislocation and the twin boundary based on our NEB data and the classical dislocation theory. At large distances, we find that the dislocation-CTB interaction is characterized by repulsive force which can be attributed to both the elasticity mismatch and distortion (shift and rotation) of deformation fields across the twin boundary; At short distances, the interaction is significantly influenced by the shear strength of the CTB: relatively low CTB shear strength can induce close range attractive force and cause slip to be absorbed into the twin plane.
The NEB method is also used to examine energy required to form and to remove a stacking fault in Cu and Al respectively. The results were compared with the general stacking fault energy curve calculated with rigid displacement method. The NEB method is proved to be an efficient technique for finding the minimum energy path between an initial state and a final state of a transition.
修改评论