Solute atmosphere around an edge dislocation in Al-based ternary alloys

We evaluated the solute atmosphere around a moving dislocation and the dragging stress due to the atmosphere in binary and ternary Al-based alloys in terms of a chemical potential gradient by modifying the method proposed by Yoshinaga et al. In ternary alloys, we analyzed formation of the complex solute atmosphere around a straight edge dislocation and the dragging stress in terms of a misfit parameter of a solute element (positive or negative) and an interaction parameter between solute elements (attractive or repulsive).     

Solute atmosphere around an edge dislocation in Al-based ternary alloys

We evaluated the solute atmosphere around a moving dislocation and the dragging stress due to the atmosphere in binary and ternary Al-based alloys in terms of a chemical potential gradient by modifying the method proposed by Yoshinaga et al. In ternary alloys, we analyzed formation of the complex solute atmosphere around a straight edge dislocation and the dragging stress in terms of a misfit parameter of a solute element (positive or negative) and an interaction parameter between solute elements (attractive or repulsive).     

Dislocation dynamics simulations of dislocation interactions and stresses in thin films

The dislocation interactions that stop threading dislocations (threads) during relaxation at increasing applied strains in single-crystal thin films are investigated using large-scale three-dimensional dislocation dynamics simulations. Threads were observed to stop via interactions with both threads and misfit dislocations (misfits). Both types of interactions were shown to depend on stress inhomogeneity. Low-stress regions enabled threads to stop in weak thread–misfit interactions even at high average film stresses. Threads were also concentrated in low-stress regions, which facilitated their interaction with other threads. Threads accumulated in thread–thread interactions, and stopped only temporarily in thread–misfit interactions. The mean free path for dislocation motion is shown to be accurately predicted from details of the inhomogeneous stress state arising from the applied strain and the misfit structure. These behaviors are analyzed to present a more complete picture of film strength, strain hardening and relaxation.     

Atomistic simulations of interactions between screw dislocation and twin boundaries in zirconium

Molecular statics was employed to simulate interaction between screw dislocation and twin boundaries (TB) in hexagonal close-packed zirconium. In the moving TB model, the interaction of a moving $\left\{10\overline{1}2\right\}$ TB with a static $1/3〈11\overline{2}0〉\left\{10\overline{1}0\right\}$ screw dislocation was investigated. Twinning dislocation (TD) nucleation and movement play an important role in the interaction. The screw dislocation passes through the moving TB and changes to a basal one with a wide core. In the moving dislocation model, a moving $1/3〈11\overline{2}0〉\left\{10\overline{1}0\right\}$ dislocation passes through the TB, converting into a basal one containing two partial dislocations and an extremely short stacking fault. If the TB changes to the $\left\{10\overline{1}1\right\}$ one, the moving $1/3〈11\overline{2}0〉\left\{10\overline{1}0\right\}$ prismatic screw dislocation can be absorbed by the static TB and dissociated into two TDs on the TB. Along with the stress–strain relationship, results reveal the complicated mechanisms of interactions between the dislocation and TBs.     

Mechanical and energetical analysis of molecular dynamics simulations of dislocation–defect interactions

In this paper, we present a method of analysis on the continuum scale of molecular dynamics simulations of dislocation–defect interactions. It is shown how the applied work can be decomposed into its elementary components: the elastic strain energy, the dissipated work against the Peierls stress and the curvature energy needed to enable the dislocation, pinned by the defect, to bow out. While the dissipated work is entirely extracted from the system in molecular statics, the elastic and curvature energies contribute to a large increase in the internal energy. The curvature energy is evaluated and compared with predictions of the line tension model. This analysis allows the calculation of the dislocation–defect interaction energy and the determination of the predominant features in the unpinning process.     

Quasi-four-dimensional analysis of dislocation interactions with grain boundaries in 304 stainless steel

The application of diffraction contrast electron tomography to dynamic experiments involving dislocation interactions with grain boundaries is demonstrated for the first time. Two applications are shown: the first is concerned with post-mortem analysis of dislocation interactions with grain boundaries and illustrates the usefulness of the tomography technique for defect analysis; the second is in conjunction with in situ straining experiments in which the dynamics of dislocation interactions with grain boundaries are observed directly and the resulting structure visualized three-dimensionally. The in situ straining experiments were conducted at room and elevated temperatures to determine the influence, if any, of thermal processes on the slip transfer mechanism. It was found that increasing the temperature lowers the barrier for dislocation absorption and emission from the boundary and increases the complexity of the interactions, but does not change the fundamental mechanisms governing slip transmission. Previous experimentally determined criteria for slip transmission across boundaries were extended to interactions involving partial dislocations, where it was found that the reaction continues to be governed predominately by reduction of the Burgers vector of the residual grain boundary dislocation left after slip transfer.     

The effect of dislocation density on the interactions between dislocations and twin boundaries in nanocrystalline materials

The interactions between dislocations and twin boundaries (TBs) are significantly affected by both intrinsic material properties and extrinsic factors, including stacking fault energy, the energy barriers for dislocation reactions at TBs, twin thickness and applied stress. In this study, dislocation–TB interactions in grains with different dislocation densities were investigated and we conclude that the dislocation density also affects the dislocation–TB interactions. In a twinned grain with a low dislocation density, a dislocation may react with a TB to fully or partially penetrate the TB or to be absorbed by the TB via different dislocation reactions. Alternatively, in a twinned grain with a high dislocation density, dislocations tangle with each other and are pinned at the TBs, thereby making it unfavourable for further dislocation reactions to mediate dislocation penetration across the TB. This leads to an accumulation of dislocations at the TBs, raising the local strain energy, which, in turn, is released by the activation of secondary twins by partial dislocation emissions from the other side of the TB.     

Solute concentrations and strains in nanograined materials

Taking advantage of both the Gibbs and McLean adsorption isotherms, we develop a Gibbs-approach-based adsorption isotherm for grain boundary (GB) segregation in nanograined (ng) polycrystals. An excess GB thickness is introduced to describe the excess of GB atomic volume in comparison with the atomic volume in lattice. The GB bulk modulus is determined with the excess GB thickness and a universal function. The newly developed adsorption isotherm is able to analyze simultaneously stresses, concentrations and their coupling behaviors in grains and GBs. Numerical calculations and plots are conducted to illustrate the theoretical analysis. The results show that the apparent solute concentration could be greatly enhanced in ng materials, due to a large grain boundary volume fraction and a considerable increase in the lattice concentration that is, in turn, boosted by the concentration-induced stresses.     

Solute–vacancy binding in aluminum

Previous efforts to understand solute–vacancy binding in aluminum alloys have been hampered by a scarcity of reliable, quantitative experimental measurements. Here, we report a large database of solute–vacancy binding energies determined from first-principles density functional calculations. The calculated binding energies agree well with accurate measurements where available, and provide an accurate predictor of solute–vacancy binding in other systems. We find: (i) some common solutes in commercial Al alloys (e.g., Cu and Mg) possess either very weak (Cu), or even repulsive (Mg), binding energies. Hence, we assert that some previously reported large binding energies for these solutes are erroneous. (ii) Large binding energies are found for Sn, Cd and In, confirming the proposed mechanism for the reduced natural aging in Al–Cu alloys containing microalloying additions of these solutes. (iii) In addition, we predict that similar reduction in natural aging should occur with additions of Si, Ge and Au. (iv) Even larger binding energies are found for other solutes (e.g., Pb, Bi, Sr, Ba), but these solutes possess essentially no solubility in Al. (v) We have explored the physical effects controlling solute–vacancy binding in Al. We find that there is a strong correlation between binding energy and solute size, with larger solute atoms possessing a stronger binding with vacancies. (vi) Most transition-metal 3d solutes do not bind strongly with vacancies, and some are even energetically strongly repelled from vacancies, particularly for the early 3d solutes, Ti and V.     

Solute segregation transition and drag force on grain boundaries

We investigate solute segregation and transition at grain boundaries and the corresponding drag effect on grain boundary migration. A continuum model of grain boundary segregation based on gradient thermodynamics and its discrete counterpart (discrete lattice model) are formulated. The model differs from much previous work because it takes into account several physically distinctive terms, including concentration gradient, spatial variation of gradient-energy coefficient and concentration dependence of solute–grain boundary interactions. Their effects on the equilibrium and steady-state solute concentration profiles across the grain boundary, the segregation transition temperature and the corresponding drag forces are characterized for a prototype planar grain boundary in a regular solution. It is found that omission of these terms could result in a significant overestimate or underestimate (depending on the boundary velocity) of the enhancement of solute segregation and drag force for systems of a positive mixing energy. Without considering these terms, much higher transition temperatures are predicted and the critical point is displaced towards much higher bulk solute concentration and temperature. The model predicts a sharp transition of grain boundary mobility as a function of temperature, which is related to the sharp transition of solute concentration of grain boundary as a function of temperature. The transition temperatures obtained during heating and cooling are different from each other, leading to a hysteresis loop in both the concentration–temperature plot and the mobility–temperature plot. These predictions agree well with experimental observations.     

溶质扩散控制层片共晶生长

基于平界面溶质扩散假设,改进界面物质守恒边界条件求得了层片共晶稳态生长的溶质边界层.结果表明,层片共晶组织生长过程中沿生长方向的溶质边界层与沿界面方向的溶质边界层相关.对CBr4-C2Cl6透明合金系的分析表明,共晶合金层片组织生长过程中界面处液、固相平均成分的差值随凝固速率增大而增大.合金成分偏离共晶成分时,界面处液相平均成分与共晶成分非常接近.在给定的凝固速率下,共晶相界面处液、固相平均成分差值与层片宽度的比值随合金成分的不同而不同.     

非平衡凝固过程中合金的溶质分凝和界面形态选择

对凝固过程中的非平衡问题,作了简要的讨论,并对非平衡态热力学在凝固过程中的应用作了介绍。凝固过程的非平衡性,决定了其溶质分凝和界面形态的选择。不同的凝固条件将获得不同的溶质分凝状况,凝固条件的变化又会使晶体的凝固过程随着凝固速率的增加经历从平面凝固向胞状晶、枝晶最终超高速平面凝固过程的转变。     

Solute strengthening from first principles and application to aluminum alloys

Alloys containing substitutional solutes exhibit strengthening due to favorable solute fluctuations within the alloy that hinder dislocation motion. Here, a quantitative, parameter-free model to predict the flow stress as a function of temperature and strain rate of such alloys is presented. The model builds on analytic concepts developed by Labusch but introduces key innovations rectifying shortcomings of previous models. To accurately describe the solute/dislocation interaction energies in and around the dislocation core, density functional theory and a flexible-boundary-condition method are used. The model then predicts the zero temperature flow stress, the energy barrier for dislocation motion, and thus the finite temperature flow stresses. The model is used to predict the flow stresses of various Al alloys. Excellent results are obtained for Al–Mg and Al–Mn. Al–Fe with ppm levels of Fe is not predicted well but, using experimental results for Fe, results for the quasi-binary Al–Cr–(Fe) and Al–Cu–(Fe) alloys agree well with experiments. The model is also consistent with the “stress equivalency” postulate of Basinski. This parameter-free model using first-principles input thus provides a basis for achieving the long-sought goal of computational design of alloys, within the context of solute-strengthening mechanisms.     

Solute trapping model based on solute drag treatment

A solute trapping model is developed based on a so-called solute drag treatment. By adopting a basic approach of phase-field models, and defining the free energy density in the interfacial region, a suitable interface shape function is introduced to derive the current model, in which the equilibrium and non-equilibrium interface behaviours can be described using a dimensionless parameter L (i.e. an important parameter in the present interface shape function). When applying the current model to Si-9%As (molar fraction) alloy with L=0.5, a good prediction of the steeper profile for high interface velocity, which is analogous to that using a phase-field model of DANILOV and NESLTER, has been obtained.     

微量溶质元素在金属晶界的偏聚

溶质元素在金属晶界的偏聚可以分为两类，即平衡偏聚和非平衡偏聚。本文回顾了这两类偏聚现象及其机理，着重讨论分析了有重要工程用途的三类典型的晶界偏聚：硼在奥氏体晶界的两类偏聚的实验规律及对硼钢淬透性影响；磷在奥氏体晶界的平衡偏聚及对钢回火脆性的作用，钢中的其他合金元素对磷的偏聚的影响；硼在多种金属间化合物的偏聚及对这些金属间化合物塑性的影响及其机理。     

低合金钢中Mo溶质原子的拖曳效应

对Mo在低合金钢和合金钢中的主要作用及对相变的影响进行了简要描述,重点分析了Fe-C-Mo合金中的Mo的偏聚行为及其对溶质原子拖曳效应的影响。Mo在晶界与界面存在明显的偏聚,Mo与C的相互作用形成了耦合的溶质原子拖曳效应。使Fe-C-Mo合金中的中温转变区域在TTT曲线中存在明显的港湾,在中温转变中存在不完全转变或者转变停滞现象。在港湾温度以下形成的铁素体（贝氏体）具有异常形态,被认为是由于棒状的亚结构激发形核形成的结果。Mo的碳化物析出后大大加速了退化铁素体的转变,强磁场可以加速Fe-C-Mo合金的中温转变和促进碳化物的析出。