金属材料的强度、塑性和热稳定性是其关键的性能指标。多晶材料的强度会随着晶粒尺寸的减小而显著提升 (Hall-Petch关系) 。但是，当晶粒尺寸细化至纳米尺度后，会牺牲材料的应变硬化能力，引起应变局部化，严重降低材料的塑性。当晶粒尺寸细化至几纳米 (临界尺寸以下) 时，材料的强度反而开始降低，出现反Hall-Petch关系。这是由于大量晶界的存在，使纳米晶材料蕴含大量过剩自由能，处于热力学不平衡状态，基于位错的变形机制转变为晶界主导，材料易发生软化。因此在纳米晶结构材料中强度-塑性、强度-热稳定性之间的矛盾关系以及细晶强化手段失效等问题亟待解决。本文通过分子动力学模拟方法结合实验方法，针对纳米晶强度与塑性的矛盾问题，研究了两类含有纳米尺度结构金属合金的强韧化机理：一是包含纳米晶的非均匀晶粒中熵合金，二是含有极小尺寸结构(化学短程序，晶格畸变)的中熵合金，以及引入共格纳米析出相后的多相耦合强化机理。针对强度与热稳定性的矛盾问题，研究了影响受限晶体 (一种极小尺寸结构) 稳定性的控制因素及其力学性能与晶粒尺寸的关系，主要结果如下：

(1) 发现在变形初期晶粒逐步屈服和Masing hardening现象发生，从而引起线弹性后显著的应变硬化。在变形后期两种非均匀晶粒结构 (不同晶粒尺寸范围) 中均发现了HCP相变和多级纳米孪晶的形成。这两种变形机制能够显著提高材料的应变硬化能力。同时发现，HCP相变是由同一晶界相邻滑移面上的大量内禀层错同时形核和扩展形成的。

在两种非均匀晶粒结构中，单轴拉伸加载模式下大晶粒与小晶粒之间存在明显的拉伸应变分配，并且这种拉伸应变分配随着大晶粒与小晶粒之间晶粒尺寸比的增加而增大。正是由于这种拉伸应变分配导致的几何必需位错的积累，实现了非均匀变形诱导的额外应变硬化能力，从而实现高强度和大塑性。

(2) 通过分子动力学模拟方法研究并揭示了CoCrNi中熵合金中晶格畸变、局部化学有序和纳米析出相诱发的耦合强化效应和相应的变形机制。研究表明，晶格畸变和局部化学有序均存在强化效应，且晶格畸变的强化效应大于局部化学有序。此外，局部化学有序度越高，强化程度越高。两种结构均通过减缓位错滑移速度，使位错线呈现波浪形构型，实现强化效应。此外，晶格畸变与局部化学有序使位错滑移的平均速度降低，并且波浪形结构的弯曲程度随着局部化学有序度的增加而增加。变形过程中，激活的位错线微段更容易从有序的Co-Cr原子对区域中脱离，然后传播到周围区域。因此，位错线微段逐步脱钉机制诱导的粗糙位错路径是含有局部化学有序和晶格畸变的中熵合金最主要的强化机制。

与单一强化机制相比，晶格畸变、局部化学有序与纳米析出相共同作用可以产生额外的耦合强化效应。晶格畸变、局部化学有序和纳米析出相对阻碍刃位错的滑移具有耦合效应，并且这种耦合效应随着局部化学有序度的增加而增加。从纳米析出相中脱钉的过程中发现，位错通过一系列微段的向前滑动以间歇方式传播。因此刃位错从纳米析出相的脱钉过程被纳米微段逐个滑移的运动特性显著延迟，导致额外的耦合强化。

(3) 通过时效处理在(FeCoNi)₈₆Al₇Ti₇合金中引入了剧烈晶格畸变，化学短程序以及纳米共格析出相，研究其强韧化机理。力学性能方面，对比FeCoNi基体，(FeCoNi)₈₆Al₇Ti₇合金由于引入了更加显著的晶格畸变，获得了强度-塑性的同步提升。相比未时效(FeCoNi)₈₆Al₇Ti₇合金，时效处理获得了双异质结构合金 (共格纳米析出相和晶粒尺寸异构)，具有更好的拉伸性能。研究表明，共格纳米析出相引入后，变形后的平均几何必需位错密度提高，说明共格纳米析出相进一步增加了晶粒尺寸异构样品的非均匀性，产生了更显著的异质变形诱导硬化效应。其次在拉伸变形过程中L1₂共格纳米析出相是可变形的，既可阻碍位错运动，也可以通过变形避免产生应力集中。因此位错与L1₂纳米析出相之间的交互作用提供了额外的应变硬化，使时效态合金表现出更好的拉伸性能。

(4) 以纯金属Au作为研究对象，利用分子动力学模拟研究了控制受限晶体转变的关键因素，及热稳定性和力学性能的尺寸效应。通过对具有不同孪晶界密度的极小尺寸晶粒模型热弛豫发现，孪晶界密度是控制受限晶体转变的关键因素。当孪晶界密度达到一定阈值后，才会发生受限晶体转变。这与目前实验研究中只能在低层错能纯金属中获得受限晶体的结果吻合。对力学性能和热稳定性的研究发现，当受限晶体结构晶粒尺寸为4.9 nm时，在接近熔点的环境温度中依然保持稳定，且在传统临界晶粒尺寸以下仍能持续强化。这是因为受限晶体中晶界原子的平均能量低于传统晶粒中晶界原子的平均能量，且晶界原子数量低于传统晶粒。总的来说，受限晶体中的过剩自由能低于传统纳米晶，因此其热/机械稳定性更高。

Strength, ductility and thermal stability are key parameters for metallic materials. Strength can be significantly increased by grain refinement, especially down to nanoscale. While strength elevation by grain refinement is at the expense of strain-hardening capability, causing early strain localization and resulting in a dramatic reduction in ductility. Further grain refinement below the critical size will cause a strength softening, resulting in an inverse Hall-Petch relationship. At the same time, nanostructured metals are thermally unstable due to the presences of a large number of grain boundaries and a large amount of excess free energy. Therefore, the trade-off between strength and ductility, the dilemma between strength and thermal stability, and the strengthening limitation by grain reinforcement in nanostructured materials become urgent problems to be solved. In this thesis, the strengthening/toughening mechanisms of two types of extremely fine size structures were investigated by molecular dynamics simulations and experimental studies to address the trade-off problem between strength and ductility: The first type of structures are the heterogeneous grain structures with nanograins, and the second type of structures are the extremely fine size structures in medium/high entropy alloys (chemical short range order, lattice distortion). Moreover, the coupled strengthening mechanisms after the introduction of coherent nanoprecipitates in the second type of structures were also revealed. The key physical parameters for the preparation of constrained crystal structures and the relationship between their mechanical properties and grain size were investigated, with respect to the dilemma of strength and thermal stability, and the strengthening limitation by grain refinement. The main findings are summarized as follows:

(1) Grain-to-grain yielding, also known as Masing hardening, was observed in the early stages of deformation during the molecular dynamics simulations for the CoCrNi medium entropy alloy with heterogeneous grain structures, resulting in strong strain hardening right after the linear elasticity stage. In the later stages of deformation, HCP phase transformation and formation of hierarchical deformation nanotwins were found in both heterogeneous grain structures. Both deformation mechanisms can contribute to the strain hardening and large tensile ductility. The HCP phase was observed to be formed by the simultaneous nucleation and extension of a large number of intrinsic stacking faults on adjacent slip planes at the same grain boundary.

In the two heterogeneous grain structures with different size ratios between small grains and large grains, significant tensile strain partitioning between large grains and small grains in heterogeneous grain structures, and this tensile strain partitioning becomes more pronounced as the grain size ratio between large grains and small grains increases. Thus, high yield strength and large tensile ductility can be achieved in heterogeneous grain structures since geometrically necessary dislocations and hetero-deformation-induced hardening can be induced due to this strain partitioning.

(2) Coupled strengthening effects and corresponding atomic deformation mechanisms among lattice distortion, local chemical ordering and nano-precipitates have been investigated and revealed by molecular dynamics simulation. Both lattice distortion and local chemical ordering have an effect on strengthening, and the effect of lattice distortion on strengthening is greater than that of local chemical ordering. Moreover, the higher the degree of local chemical ordering, the higher strengthening effect. Both structures lead to a strengthening effect by slowing dislocation slip and inducing a wavy dislocation line. Moreover, lattice distortion and chemical local ordering reduce the average dislocation slip velocity and the bending degree of wavy dislocation lines increases with higher degree of local chemical ordering. It is observed that activated nanoscale segments are more readily un-pinned from regions with stronger Co-Cr local chemical ordering and then propagate to regions without such local chemical ordering. Thus, the strengthening mechanism of local chemical ordering and lattice distortion can be mainly attributed to the roughened pathway for dislocations by the progressive un-pinning of nanoscale segments.

Compared to each strengthening effect, additional coupled strengthening effects can be induced for the structures with lattice distortion, local chemical ordering and nanoprecipitates, and this additional coupled strengthening effect increases with increasing local chemical ordering. The coupling effect on obstructing edge dislocation slip was also observed for the structures with lattice distortion, local chemical ordering and nanoprecipites, and this coupling effect also increases with increasing local chemical ordering, resulting in coupled strengthening. During un-pinning from the nanoprecipitates, dislocations are found to propagate through a series of forward slips (one at a time, in an intermittent manner) of nanoscale segments. Thus the un-pinning process of the edge dislocations from the nanoprecipitates is significantly slowed down by the kinematic manner of the intermittent nanosegment sliding, leading to additional coupling strengthening.

(3) Severe lattice distortion, chemical short-range ordering and coherent nano-precipitates were introduced in a (FeCoNi)₈₆Al₇Ti₇ medium entropy alloy, and the corresponding strengthening and toughening mechanisms were revealed by experiments. A better synergy of strength and ductility is obtained in the (FeCoNi)₈₆Al₇Ti₇ medium entropy alloy due to the introduction of more significant lattice distortion, as compared to the FeCoNi medium entropy alloy. Even better tensile properties can be achieved in the dual-heterogeneous structures (heterogeneous grain structure and nano-precipitates) by further aging treatment. First, severer heterogeneity can be obtained by the introduction of nanoprecipitates into heterogeneous grain structures, thus higher density of geometrically necessary dislocations and higher hetero-deformation-induced hardening can be induced after tensile deformation. Second, the L1₂ nanoprecipitates are deformable. Thus, the dislocation slip can be obstructed by the L1₂ nanoprecipitates on one hnad, and severe stress concentration can be avoided by the deformation of the L1₂ nanoprecipitates on the other hand. Moreover, extra strain hardening can be induced and better tensile properties can be achieved due to the interaction between the dislocations and the L1₂ nanoprecipitates.

(4) Using pure gold as a model material, the key factors for the occurrence of constrained crystal transitions and the size effects on their thermal stability and mechanical properties were investigated by molecular dynamics simulations. It was found that the density of twin boundary is the key factor for the occurrence of constrained crystal transitions by thermal relaxation of models with different densities of twin boundary. The constrained crystal transition can occur only when the twin boundary density reaches a critical threshold, which is in consistent with the experimental fact that constrained crystals can only be prepared in pure metals with low stacking fault energy. A study of the mechanical properties and thermal stability of the constrained crystal structure found that the recrystallisation temperature was close to the melting point at a grain size of 4.9 nm, and continued to strengthen below the traditional critical grain size. The atom number of grain boundary is lower, the average free energy at grain boundaries and the total free energy are also lower in constrained nanocrystals as compared to the conventional nanograins, resulting in higher thermal/mechanical stability.