冷拉拔珠光体钢丝因其独特纳米片层结构同时表现出高强度和良好延性，从而被广泛应用于飞机阻拦索、斜拉桥缆索、塔吊钢索、轮胎钢帘线等工程结构中。珠光体钢丝在极端服役工况下往往发生循环塑性变形而降低其使用寿命，而其对应的微观塑性变形与失效机理仍然是不明确的，尤其是铁素体/渗碳体界面处的原子尺度位错行为。本论文结合实验研究和分子动力学（MD）模拟方法，针对珠光体钢丝的微观塑性变形机理、疲劳力学性能与损伤失效行为等开展了系统性的研究工作，得到的主要结论如下：

（1）研究了冷拉拔珠光体钢丝（拉拔应变εd  = 2.34）棘轮变形过程中微观机制。棘轮应变随着循环次数增加呈现先快速增加后平稳的两阶段现象。渗碳体在棘轮变形早期发生快速分解。位错结构由初始的低密度位错线、位错缠结向高密度的位错胞、亚晶转换。初始塑性应变快速累积导致位错从铁素体/渗碳体界面形核发生快速增殖，棘轮应变快速增加。渗碳体分解产生的碳原子进入铁素体钉扎、阻碍位错运动，导致第二阶段的棘轮应变率为常数。渗碳体分解机制与碳原子-位错相互作用和渗碳体表面/体积比相关。

（2）采用分子动力学方法研究铁素体/渗碳体界面的疲劳力学性能与变形机理。我们设计拉压弹性、拉弹压塑性、拉压塑性三种循环变形模式来研究钢丝循环塑性机制。位错密度在前30次循环快速累积而后减少至稳定。界面发射位错主导塑性事件发生。形核位错扩展进入铁素体，部分位错在界面发生湮灭。当铁素体中位错形核速率等于湮灭速率，位错密度稳定导致稳定应力流动阶段。在高应变加载下，当珠光体中塑性应变超过临界值时，将从铁素体穿过界面往渗碳体发生滑移转化。渗碳体滑移转化破坏界面，是材料循环变形可能的失效机制。

（3）采用分子动力学模拟研究了纳米珠光体钢中反常的应变率诱导的脆性断裂-延性断裂-群体分层破坏转变。随着应变率增加，断裂模式开始从脆性裂纹扩展转变为延性裂纹尖端位错形核，最终转变为群体铁素体/渗碳体界面同时分层破坏。建立了关于温度和应变率的二维失效相图。通过在特定载荷下界面解理与裂纹尖端附近热激活的位错形核间的能量竞争，从物理上合理解释了脆-延-分层转变的机理。

（4）采用热处理工艺对钢丝力学性能进行优化。在325 ℃退火10-30分钟，钢丝强度可基本维持不变，但破断延伸率可提高为初始钢丝的三倍。钢丝强度维持主要归结于渗碳体分解碳原子的固溶强化作用；塑性提高是由于渗碳体再结晶导致片层被打破，减少位错在界面塞积。在更高温度退火下，钢丝强度降低主要由于原有片层结构完全消失，渗碳体晶粒发生粗化。钢丝强塑性优化平衡条件源于退火过程中球化渗碳体形成。

Cold-drawn pearlitic steel wires are used widely engineering structures such as aircraft arrester ropes, suspension bridge cables, tower crane ropes, and automobile tires, because they exhibit both high strength and appropriate ductility due to their unique nanoscale lamellar structures. Pearlitic steel wire endures cyclic plastic deformation in some extreme applications, which reduces its service life. However, the micro-plastic deformation and failure mechanisms of these important materials remain elusive, especially, there is little understanding of the atomic-scale dislocation behaviors at the ferrite-cementite interface. In this paper, we have systematically studied the micro-plastic deformation mechanisms, fatigue mechanical properties and damage failure behaviors of pearlitic steel wires by combining experimental research and molecular dynamics (MD) simulations techniques. The main conclusions in the present paper are shown as follows:

(1) The microscopic mechanisms during ratchetting deformation of pearlitic steel wire with a true drawing strain of 2.34 are characterized. A two-stage evolution of ratchetting strain is observed, i.e. ratchetting strain keeps unchanged after a rapid increase. Cementite decomposes quickly at the beginning of the ratchetting loading. The dislocation patterns transform from initial low-density dislocation lines, dislocation tangles to high-density dislocation cells and subgrain with incrasing number of cycles. The rapid accumulation of initial plastic strain results in quick dislocation nucleation from the ferrite-cementite interface, leading to a rapid increase of ratchetting strain. Carbon atoms from cementite decomposition enter ferrite and hinder dislocation movement, resulting in a nearly constant ratchetting strain rate in the later stage. Cementite decomposition may depend on both carbon-dislocation interation and cementite surface/volume ratio.

(2) Dislocation nucleation and evolution at the ferrite-cementite interface of pearlitic steels under cyclic loadings are explored by using MD method. We design sophisticatedly three cyclic loading schemes to characterize the cyclic plastic mechanisms under different strain amplitudes corresponding to elasticity in both tension and compression, elasticity in tension but plasticity in compression, and plasticity in both tension and compression. Dislocation density in ferrite increases in the first 30 cycles during cyclic loadings, and then decrease to a stable value. The onset of plasticity of the interface is dominated by dislocation nucleation from the interface. After entering into the ferrite phase, some dislocations annihilate at the interface. When dislocation nucleation and annihilation rates become equal, dislocation density in ferrite remains stable and a steady-state of stress flow appears. At high applied strain level, slip tranfers across the interface from the ferrite to cementite phases once the plastic strain value of pearlite exceeds the critical values. Since it can destory the ferrite-cementite interface, slip transfer from ferrite to cementite may be a possible failure mechanism of pearlitic steels under cyclic deformation.

(3) An abnormal strain-rate-induced brittle-ductile-delaminated transition in nanoscale pearlitic steel is explored through extensive MD simulations. With increasing strain rate, the fracture mode transforms from brittle crack propagation, to ductile crack tip dislocation nucleation, and finally to an athermal instantaneous delamination of all the ferrite-cementite interfaces. A two-dimensional failure diagram of the nanoscale pearlitic steel is therefore established in terms of strain rate and temperature. The surprising transition is physically rationalized by a scenario of energetic competition between the mechanism of interface cleavage and the thermally activated dislocation nucleation in the vicinity of crack tip at specific loading conditions.

(4) Mechanical properties of cold-drawn pearlitic steel wire (εd  = 2.34) are optimized by heat treatment. When annealing for 10-30 minutes at 325 ℃, the tensile strength of steel wire nearly remains unchanged, while the elongation to failure is increased to three times than that of the initial steel wire. The increase of steel wire strength is mainly attributed to the solid solution strengthening effect of carbon atoms generated from cementite decomposition. The increase of ductility is due to the fact that cementite recrystallization causes the lamellae to be broken and reduces the dislocation pile-ups at the interface. After annealing at higher temperature, the reduction of tensile strength is mainly due to the complete disappearance of original lamellar structure and the coarsening of cementite grains. The optimized equilibrium condition for the strength and ductility of pearlitic steel wire is derived from the formation of spheroidized cementite during annealing.