非晶合金因展现出一系列优异的力学、物理以及化学性能，在基础科学研究及高新技术领域中已展现出重要的学术价值和应用潜力。 然而， 由于本征的长程无序结构， 非晶合金在室温下的塑性变形往往高度局域化，限制在宽度为数十纳米的带状区域内， 形成纳米尺度的剪切带。 剪切带的涌现以及快速扩展往往导致材料塑性失稳，发生宏观脆性灾变破坏。这严重制约了非晶合金作为结构材料的应用前景。 因此， 厘清非晶合金塑性失稳机制， 探明剪切带涌现的物理过程，是固体力学、凝聚态物理、材料科学领域广泛关注的核心科学问题。

  本质上，非晶合金中剪切带的涌现是一个远离热力学平衡状态的多时空尺度耦合的复杂非线性过程。 空间上，本征的无序结构会引起变形以及动力学行为的不均匀性。时间上，涵盖原子振动、 局部结构重排、塑性流动等多个速率过程。这些事件均具有各自的特征空间和时间尺度，他们的关联耦合推动非晶合金内部的变形局部化行为以及剪切带的涌现。 极度的时空局域性以及各个尺度事件之间的强耦合作用是理解剪切带涌现机制的关键挑战。此外，由于非晶合金本征的无序原子排列特征，基于有序结构的经典凝聚态理论很难有效地刻画非晶合金的塑性失稳过程。因此， 对非晶合金无序结构的表征以及探索变形局部化的结构起源是认识剪切带涌现以及塑性失稳机制的又一关键挑战。

  为此，本文针对非晶合金的剪切带涌现机制这一关键科学问题，基于原子尺度认识的角度，探索了塑性失稳的结构起源、揭示了剪切带涌现的物理图像、澄清了剪切带的软化机制、建立了剪切带涌现与玻璃转变相关性。为实现非晶合金剪切带行为的表征、预测和控制奠定了理论基础。 取得的具体结果包括：

  剪切带涌现的结构起源： 首先， 通过对不同化学组分的非晶合金的剪切带行为开展原子尺度模拟，揭示了剪切带敏感性对局部化学组分不均匀性的强烈依赖行为。 体系化学短程序水平较高的样品剪切带敏感程度更强，更容易形成剪切带。该工作从局部化学组分的角度出发，丰富了对非晶合金剪切带涌现结构起源的认识。 进一步，将对剪切带涌现结构起源的认识从体系水平拓展至原子尺度。 通过知识驱动以及数据驱动等策略定义了多种新型的原子尺度结构缺陷。这些结构缺陷的定量表征为全面认识非晶合金剪切带涌现的结构起源提供了有效的结构模型。此外，这些结构指标还能准确地预测非晶合金的动力学行为，从而建立了复杂体系领域长期寻求的结构-性能关联。

  剪切带涌现的时空序列： 在外载激励下，结构缺陷处的原子倾向于参与塑性重排事件。剪切带涌现过程内蕴三种基本团簇运动： 剪切、体胀、旋转，这三种团簇运动的强耦合使得剪切带涌现过程的原子尺度物理图像仍不明朗。 基于对无序介质变形场的细致分析，我们提出了耦合仿射和非仿射变形信息的理论模型—两项梯度模型，成功解耦了非晶合金剪切带涌现过程内蕴的剪切、体胀、旋转。在此基础上，将经典剪切转变区的概念进一步细分为剪切主导区、体胀主导区、旋转主导区。通过追踪这三个原子团簇运动的演化规律，给出了非晶合金剪切带涌现的原子尺度时空序列，揭示了体胀主导初始塑性事件以及三个运动从一致分布到间隔分布转变的物理图像。此外，结合极值理论与逾渗分析发现了非晶合金剪切带涌现过程的临界幂律标度规律，指出该过程是一个定向的逾渗行为。

  剪切带涌现的软化机制： 非晶合金剪切带的涌现过程伴随材料的软化行为，关于剪切带涌现的软化机制存在长期广泛的国际争议，即剪切带是由热诱导的还是由构型软化引起的。 基于大量的分子动力学模拟，我们在原子尺度讨论这个问题。 通过将局域温度定义为热软化指标，将原子构型温度定义为表征构型软化引起的结构失稳参量，进而定量刻画剪切带演化过程中热软化与构型软化的时空演化。大量的构型温升表明非晶合金剪切带是由构型软化主控，此过程热温度的影响甚小。构型软化发生在剪切带涌现之前，是剪切带形成的根本原因，而热软化事件发生在剪切带涌现之后，是剪切带形成带来的结果。基于此，提出非晶合金剪切带涌现过程遵循构型软化主控，热软化辅助的机制。

  剪切带行为与玻璃-液体转变一致性： 在上述对剪切带涌现结构场、变形场描述的基础上，进一步通过表征非晶合金剪切带涌现过程的动力学、热力学以及微观结构等演化特征， 揭示了其与玻璃-液体转变过程的相似性。厘清了力载荷和温度载荷在控制非晶合金的动力学行为、热力学特征以及结构演化等方面具有相似的效应。本质上，非晶合金剪切带的涌现可以看作是外力作用引起的玻璃-液体转变过程。

  Owing to plenty of outstanding mechanical, physical and chemical properties, and a wide range of potential structural and functional applications, metallic glasses are of great significance in both scientific research and engineering technology. However, since the atomic packings of metallic glasses are short-range order but long-range disorder, the deformation localization is easily accessible in metallic glasses, leading to the emergence of nanoscale shear bands at room temperature. The nucleation and rapid propagation of shear bands often cause plastic instability as well as catastrophic fracture with very limited ductility, thus severely impeding the further engineering application of metallic glasses as a kind of structural materials. In this connection, it is extremely attractive and imperative for the broad communities of solid mechanics, condensed matter physics and materials science to clarify the mechanism of plastic instability and shear banding emergence in metallic glasses.

  In essence, shear banding emergence of metallic glasses is so far considered as a complex nonlinear process which is far away from the state of thermodynamic equilibrium. Such process involves multiple characteristic events, which are strongly coupled with various temporal and spatial scales. On the one hand, the inherently disordered atomic packing will cause the inhomogeneity of deformation and dynamic behavior in metallic glasses. On the other hand, the initiation of shear band covers processes such as atomic vibration, local structural rearrangement and plastic flow. All of these events have their own spatially and temporally characteristic scale. And it is their coupling motion that drives the severely strain localization and the formation of the shear band. Such extreme localization in spatial and temporal field as well as the strong coupling of these characteristic events are thus one of the main challenges, which causes understanding the mechanism of shear banding emergence in metallic glasses remains an issue yet to be resolved. In addition, due to the lack of either long-range translational or rotational symmetry in metallic glasses, it is rather difficult to use the research paradigm which is based on ordered structure to describe the plastic behavior in metallic glasses. Therefore, establishing an intuitive one-to-one structure-property relationship and thus exploring the structural origin of strain localization is another key challenge to understand the shear banding emergence mechanism of metallic glasses.

  In this connection, to address the issue of clarifying the microscopic mechanism of shear banding emergence in metallic glasses, especially tracing back to the possible atomic scale, we carried out a series of researches and have obtained innovative progress accordingly. The achieved advances in this dissertation involves exploring the structural origin of plastic instability, revealing the physical image of shear band emergence, clarifying the softening mechanism of plastic instability, and establishing the intriguing correlation between shear band emergence and glass transition. These lay a theoretical foundation for the characterization, prediction and control of shear banding behavior in metallic glasses. The specific obtained results are briefly listed below:

  The structural origin of shear banding emergence: First of all, atomic simulations are used to explore the shear band manner in metallic glasses with distinct chemical compositions. It is revealed that the susceptibility of shear band is strongly dependent on the inhomogeneity of local chemical compositions. Here, the emergence of shear band is more likely to take place in glass samples with higher level of chemical short-range order. This provides, in the perspective of local chemical components, the explanation of structural origin of shear bands in metallic glasses. Furthermore, the understanding about the structural origin is traced back to atomic scale. we propose and define a variety of new structural indicators of metallic glasses, namely integrated glassy defect and structural Shannon entropy, based on the data-driven and physics-informed strategy, respectively. Such structural indicators can quantitatively characterize the atomic-scale environment in disordered materials, and thus provide an effective structural model benefiting the understanding of the structural origin of the shear banding emergence in metallic glasses. In addition, these structural representations are also capable of detecting the dynamical behavior in glasses, which indicates the establishment of long-sought structure-property relationship in the community of complex systems.

  The spatiotemporal sequence of shear banding emergence: Atoms in glass defects are prone to participate in plastic rearrangement under external stimuli. Three elementary cluster motion: shear, dilatation and rotation are highly entangled in the process of shear banding emergence. Such strong coupling among these cluster motions causes the atomic-scale physical image of shear banding emergence remain elusive. Based on the careful analysis of deformation field in disordered media, a theoretical protocol, namely two-term gradient model, which covers both affine and non-affine components of deformation, is proposed to decode the three types of elementary local atomic motion – shear, dilatation, and rotation. On the basis of this decoupling framework, the broad concept of the shear transformation zone is further demonstrated comprehensively as the more exact shear-dominated zone, dilatation-dominated zone and rotation-dominated zone. By tracing the evolution of these three atomic motions, the atomic scale spatiotemporal sequence of the shear band emergence in metallic glasses are carefully observed. Evidences show the critical physical images during strain localization which include that dilatation is the dominant mode at the activation of initial plastic units and that the evolution of shear, dilatation and rotation exhibits a transition from synchronous motion to separate distribution at the onset of the shear band. In addition, the integration of extreme value theory and percolation analysis points towards the critical power-law scaling nature underlying shear banding emergence. It is pointed out that the plastic instability of metallic glasses can be characterized as a directional percolation process.

  The softening mechanism of shear banding emergence in metallic glass: The emergence of shear band is accompanied with the material softening behavior. There is a longstanding and widespread international debate on the softening mechanism of shear band in metallic glasses, that is, whether shear bands are induced by heat or by configurational softening. Based on the molecular dynamics simulations, we address this issue at atomic level. Here, the local thermal temperature attributed to thermal effect and configurational temperature induced by athermal structural disordering are defined as the quantitative indicators of thermal softening and configurational softening, respectively. In this connection, the spatial and temporal evolutions of thermal softening as well as configurational softening during shear banding emergence are uncovered. A large amount of configuration temperature rises that commensurate with glass transition temperature gives the direct evidence that configurational softening is the dominant instability mode, whereas the shear banding emergence shows little thermal responses. It is also found that configurational softening takes precedence over shear banding, while evident thermal softening lags behind strain localization. Therefore, the root cause of shear banding is conceivably configurational softening, while the subsequent thermal softening is more like the consequence of shear band. On the basis of these evidences, it is proposed that the material weakening mechanism of plastic instability process in metallic glasses follows configuration-softening dominant, yet thermal-softening assisted mode.

  Towards commonality between shear banding and glass-liquid transition: On the basis of the demonstration of structure and deformation filed of shear band, we further uncover a series of robust commonalities between shear banding and the glass-liquid transition, including the dynamical behavior, thermodynamic evolution and structural origin. It is pointed out that external force and thermal effect have similar effects on the dynamics, energy state and structural evolution of metallic glasses. Such observations indicate that shear banding in metallic glasses can be regarded as one kind of glass-liquid transition induced by external loading.