金属塑性变形机理与微观原子构型紧密相关。比如晶体塑性变形是由微观晶格缺陷引发的。而金属玻璃原子排列长程无序，无法通过观察原子排列找到缺陷，导致其塑性变形载体仍无定论。局域剪切转变区可解释金属玻璃缺陷变形过程，但其结构和起源还不明确。晶体和非晶金属的缺陷特征和塑性行为有哪些异同，针对晶体的研究方法能否有效应用于非晶金属，仍是热点问题。金属塑性变形的功热转化，是实验中的经典问题，旨在确定金属塑性变形后有多少能量储存在固体，有多少能量变成热，以验证相关塑性理论的可靠性。过去三十年有不少实验和理论针对产热来源，产热占比，留存冷功进行了深入研究，并发现缺陷是功热转化的基础，但缺少一定的微观物理图像和机制。本文主要采用理论结合分子模拟的方法，在微观上对金属塑性变形的缺陷识别以及功热转化进行研究。

在金属塑性变形缺陷识别方面，为了寻找晶体和非晶体金属缺陷的一些共性，从而预测剪切转变区的可能位置，本文从微观尺度研究了金属材料变形与局部非均匀性、原子位移的关系，分为以下三个层面：

(1)  晶体金属微观缺陷的局域刚度特征。分子模拟表明晶体中原子非仿射位移与内部非均匀结构相关，因此基于动力学矩阵提出等效局域刚度来表征晶体的非均匀性，并可有效识别晶格缺陷。

(2)  基于局域刚度梯度识别金属玻璃微观缺陷。简单剪切下研究了非仿射位移场与等效刚度梯度场的关系，只有满足特定形状的刚度梯度分布才容易形成剪切状非仿射位移。最终基于刚度梯度分布提出参数ξ来识别可能导致局域剪切变形的微观缺陷，并通过分子模拟进行验证。

(3)  不同加载条件下金属玻璃剪切转变区与体胀区的预测。基于准静态下原子的平衡方程，理论推导变形梯度张量与外界加载、刚度梯度以及模式因子之间的近似关系，最终提出静态结构张量来识别可能形成局域剪切转变区或体胀区的缺陷，并通过分子模拟结果加以验证。

在功热转化方面，采用分子模拟方法从微观上研究了塑性变形产热的来源和机理，功热转化率的变化规律及温度效应和应变率效应，并对宏观实验现象进行了解释。

微观上，热是畸变晶格在恢复正常晶格的时候，原子势能跌落产生的，因此缺陷类型决定产热密度。热转化率与温度和应变率相关。高温会降低位错的激活能，留下更多位错，增大留存冷功最终降低热转化率。高应变率会产生更多位错，也会导致更长的塑性流动区，但位错稳定性不会提高，位错最终消失，其能量耗散为热，导致热转化率升高。

模拟得到的热转化率范围与实验结果一致，同时也解释部分实验结果：(1) 热转化率两种初始趋势取决于位错形核和运动的时间顺序；(2) 初始温升滞后可能是由于热扩散速度小于位错滑移速度，断裂之后温度升高是因为还有塑性事件发生；(3) 实验中高应变率下位错能远低于留存冷功，是因为高应变率下留存冷功只有少部分在位错，大部分在自由表面和晶界。

进一步，对金属玻璃功热转化也做了初步探究。以铜锆非晶合金为例，不同于晶体，其塑性功对温度敏感，但对应变率不敏感，并且留存冷功与温度无明显关系。最终其热转化率与应变率无明显关系。

Mechanism of plastic deformation stems from atomic packing structure, as crystal plasticity is induced by micro-defects. Defects in metallic glasses (MGs) cannot be identified by packing structure for lacking of long range order, leaving plastic unit unsolved yet. Shear transformation zone (STZ) describes the deformation of defects, but incapable of revealing structure and origin of defects in MGs. What are the common points and differences between defects and plastic behavior of crystals and MGs, and whether methods for crystals can be effectively applied to MGs are still hot issues. The work-heat conversion of plastic deformation is a classic problem in experiments. Determining how much energy is saved as stored energy of cold work (SECW) and how much becomes heat could verify the reliability of related plasticity theories. In the past 30 years, many experiments and theories have been conducted on sources/proportion of heat production and forms of SECW, revealing defects are fundamentals of work-heat conversion, but short of micro-mechanism and physical images. Combined with molecular simulation and theories, this dissertation presents a systematic study of defects identification and work-heat conversion in metals.

In defects identification, to find commonalities between defects of crystals and MGs and to predict STZs, the relationship between deformation, inhomogeneity and atomic displacements is investigated from following three parts:

(1)    Local stiffness characteristics of crystal defects. Molecular simulations found that nonaffine displacements are related to internal heterogeneity. As a result, equivalent stiffness is proposed to characterize the inhomogeneity inside crystals and identify crystallographic defects.

(2)    Identification of defects in MGs by local stiffness gradient. Under simple shear, the relationship between stiffness gradient and nonaffine displacement is studied, showing stiffness gradient satisfying specific shapes prefers forming shear-like nonaffine displacements. Finally, based on stiffness gradient, a parameter ξ is proposed to identify defects which lead to STZs and verified by molecular simulations.  

(3)    Prediction of STZs and tension transformation zones (TTZs) under different loads: Based on the quasi-static atomic equilibrium equation, the approximate relationship between the deformation gradient tensor, the external loading, stiffness gradient and mode factor is theoretically derived, A structural tensor is finally proposed to identify the defects that may form STZs or TTZs and verified by molecular simulations.

In work-heat conversion, the micro-mechanism of plasticity-induced heating is studied by molecular simulation, illustrating effects of temperature and strain rate. Several macroscopic experimental phenomena are also explained.

Microscopically, heat is generated by drops of potential energy during the restore of lattice and dislocation propagation. As a result, density of heat is determined by types of defects. Heat generation is also related to temperature and strain rate. A higher temperature reduces the activation energy of defects, increasing the final density of dislocation and SECW, which decreases heat generation. A higher strain rate extends plastic flow regime and increases the defect density, which finally disappear with SECW dissipated into heat, which increases heat generation.

The simulated Taylor-Quinney coefficient (TQC) is matched with experiments. Meanwhile, several phenomena are explained: (1) Two initial trends of TQC are controlled by the sequence of dislocation nucleation and propagation. (2) The initial temperature rise lag may due to thermal diffusion slower than the speed of dislocation. Temperature rise after fracture is due to remaining plastic events. (3) SCEW calculated based on dislocation density is underestimated because most of SECW is stored in free surfaces and grain boundaries under high strain rate.

Preliminary simulations on plasticity-induced heating of MGs are also conducted. Taking Cu-Zr MG as an example, different from crystals, plastic work is sensitive to temperature rather than strain rate, and SECW has no obvious relationship with temperature. The final TQC of MGs is little influenced by strain rate.